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1 Dimensions of Precision Agriculture The management of agricultural production is undergoing a change, both in philosophy and technology. Until recently, agricultural managers have generally made decisions regarding fields based on average conditions within those fields, with data that was often sparse and qualitative in nature. Soil fertility was deter- mined by compositing soil cores into a single sample that was intended to best describe conditions across a field. Field scouting for crop condition or pest infes- tations was done at a few locations within the field, and observations often have been more qualitative than quantitative. For the most part, whole fields have been considered to be the basic agricultural production units, and have been managed for the mean condition or, in the case of pest management, managed intensively to overcome variability within that field. Historically, a desire to improve production efficiency and farm income has stimulated interest in innovative technologies. Advances in technology, as well as other factors such as farm policy have contributed to increases in the size of individual farmsteads and fields within a farmstead. With this larger scale of operation, the potential for the individual to effectively manage variability by observation and experience has declined precipitously. In addition, as individual farm fields increased in size, within-field variability has generally increased. A major feature of todayâs precision agriculture is that it allows producers to man- age previously unmanaged variability as well as the increased variability result- ing from increased field size. In other words, precision agriculture will allow several geographic units currently being managed as a single entity (a field) to be addressed as individual decision-making units. Managers will be able to respond to the distinctive agronomic characteristics that exist within the subunits, in con- trast to todayâs approach of addressing the average needs of several units or ex- treme conditions in parts of the field, such as pest outbreaks in small patches. 16
DIMENSIONS OF PRECISION AGRICULTURE 17 The incorporation of information technologies into agricultural production practices began in the mid-1980s and has increased sharply in recent years. While the use of information in agricultural decision making is not new, agriculture is experiencing a vast increase in the amount of information available, and in the timeliness and means by which information can be collected, analyzed, and used to manage inputs and outcomes of agricultural practices. The application of new information technologies in agriculture is known by several terms, including pre- cision agriculture, precision farming, and site-specific management. A variety of definitions have been offered for the concept of integrating information technolo- gies with agronomic practices. Most authors have focused on the ability to obtain data and to vary production inputs on a subfield basis. While this is an important aspect, there are other geographic scales at which information can be obtained and used to facilitate site-specific management. The committee chose to view precision agriculture broadly, adopting the following definition: Precision agriculture is a management strategy that uses information tech- nologies to bring data from multiple sources to bear on decisions associated with crop production. A key difference between conventional management and precision agricul- ture is the application of modern information technologies to provide, process, and analyze multisource data of high spatial and temporal resolution for decision- making and operations in the management of crop production. Advances in the technologies will be an evolutionary process and they will continue to be adapted for agricultural decision making. Precision agriculture has three components: capture of data at an appropriate scale, interpretation and analysis of that data, and implementation of a manage- ment response at an appropriate scale and time. Each particular manageable fac- tor has its own scale of variability. Area-wide management of insects and weather forecasting for crop management decisions are examples of variables that are managed at a scale larger than the individual field. Other factors like soil fertility and pest distributions can vary significantly at the subfield level and over the growing season. Therefore, it is natural and important to perceive precision agri- culture in terms of finer spatial or temporal units of decision making. PRECISION AGRICULTURE AND AGRICULTURAL MANAGEMENT Advances in information technology and their application in crop produc- tion, which are labeled as precision agriculture in this report, are creating the potential for substantial change in management and decision making in agricul- ture. The word potential in the previous sentence is critically important. The vari- ous technologies and practices that will make up tomorrowâs precision agricul- ture are only emerging, being tested and refined, and implemented or rejected
18 PRECISION AGRICULTURE IN THE 21ST CENTURY today. This process is further enhanced by the dynamic nature of advances in information technology. A capability that is technically or economically unfea- sible can become feasible as a result of a technological innovation occurring well outside the arena of agricultural technology development or agricultural research. Thus the process by which precision agriculture is adopted could be fragmented and discontinuous. Therefore, it is impossible to specify the precise dimensions and characteristics of the precision agriculture of the future. Precision agriculture could materially affect on-farm decision-making pro- cesses that depend on implied knowledge gained by observation and experience. While its precise dimensions continue to evolve, the following features character- ize most precision agriculture applications in use or under development: â¢ Data capture tends to be electronic, automated, and relatively inexpensive. â¢ Data capture can occur more frequently and in more detail. â¢ Information, either captured as a part of field operations or purchased externally, can be considered separate input into the production operation. (It is also a feature of integrated pest management and sustainable agricul- ture concepts.) â¢ Data interpretation and analysis can be more formal and analytical. â¢ Scientific decision rules are applicable to actual farming operations. â¢ Implementation of the response can be more timely and more site specific. â¢ Performance of alternative management systems can be quantitatively evaluated. The long time lags between input decision making, application of inputs, and observation of yields in crop production systems make it difficult to evaluate decision-making effectiveness. The chance for misinterpreting results is further heightened when inputs and outcomes are observed rather than measured. The difficulty of learning in such settings is not constrained or unique to farmers. Considerable research has documented that human decision making is more likely to suffer bias and misinterpretation when (1) feedback loops are long between the time the decision is made and the outcome occurs and (2) cause/effect linkages are not simple (Einhorn, 1980; Hogarth and Makridakis, 1981). These two char- acteristics apply to traditional crop production settings. The uncertainties associated with the rapid evolution of information tech- nologies and the dynamics of the process of adopting precision agriculture repre- sented a significant challenge in the preparation of this report. However, these same uncertainties provided considerable excitement and a sense of mission for the project. Tomorrowâs precision agriculture will be significantly affected by actions in the public and private sectors today. The focus of this committee, therefore, was not on predicting a single future. Rather, members chose to recognize the uncertainties inherent in the future evo- lution of precision agriculture and to emphasize possible paths and the implica- tions of those paths. Further, the study recommendations define key actions that
DIMENSIONS OF PRECISION AGRICULTURE 19 society can undertake to extend the dimensions of precision agriculture where they are deemed most desirable. GEOGRAPHIC CONTEXT: SCALES IN THE SPATIAL SPIRAL Agricultural production systems vary in many ways, including scale of op- eration, commodities produced, and philosophical approaches to management. Current production systems draw on diverse approaches and knowledge bases. For any approach, information technologies will play an increasingly important role in agricultural production and natural resource management. This impact will be felt directly through the coupling of newly acquired information with recently developed tools for agricultural production, on-demand products and services, and increased access to information and services. A number of scales characterize crop production systems of today. These scales might be viewed as a continuum ranging from individual plants in a field to plant populations, fields, farmsteads, and regions. Others have used this Lewin- Kolb model of hierarchies as an organizational structure to study complex issues such as pesticide regulation and diversity in agroecosystems (Olson et al., 1995). Consider this continuum in the form of a spatial spiral ascending from the sub- field to national geographical levels (Figure 1-1). As we move up the spiral we Communication Threads FIGURE 1-1 Scales in a spiral. A number of scales characterize crop production systems of to- day. In precision agriculture, an unprecedented amount of spatial Country and temporal data may become available at the individual plant, Ecoregion farm, and regional scales. At each scale various processes will influence crop production. A County goal is to determine an optimal scale for data collection and Farmstead management response. Commu- nication technologies will pro- vide connecting threads up and Field down the spatial spiral. Population of plants Individual plant
20 PRECISION AGRICULTURE IN THE 21ST CENTURY move from individual plants to fields and regions. Fresco (1995) underscores the need to relate phenomena or outcomes to processes occurring at both higher and lower level scales. The goal is to determine an optimal scale at which each pro- cess is to be studied, one in which variability is minimal. For example, if plant population is dependent upon small-scale variation in soil physical and chemical properties, then varying seeding rates may require information and hardware ca- pable of rate changes every few centimeters. Such information may reside locally in a nearby computer in a farmhouse. At a wider scale, real-time weather infor- mation collected from locally placed weather stations may provide irrigation or area-wide pest management information in a timely manner to improve decision making for a field, farmstead, or county. Communication technologies will provide connecting threads up and down the spatial spiral. Telephone, high-speed digital lines, and wireless communica- tions are needed to link the various levels together. For example, digital data could be collected by an on-the-go yield monitor in a combine, sent via a wire- less cellular link to the operatorâs home computer, and retrieved via a high- speed Internet connection by an agricultural chemical dealer. The dealer may then add the yield data to a nutrient management analysis and send recommended fertilizer application rates for various subfield units back to the farm operatorâs computer. Different scales of assessment are being used to investigate aspects of crop- environment systems. Scale can be considered for both information sources and management actions. Depending on the situation, data from different scales may be combined and used to determine management actions at another scale. For example, a producer deciding what crop and variety to plant (field scale) may consider the available forward contracting prices (national or global scale), the availability of custom field operations (farm scale), and a field map of soil wa- ter-holding capacity (subfield scale). With precision agriculture methods, such decisions can be made with more objective data. Some of the uncertainty factors can be reduced with the information technologies of precision agriculture, al- though the extent to which this will be feasible and of value to the grower is not clear. Information technologies permit the modern producer to obtain detailed spa- tially explicit information at the scale of entire farms but with information suffi- cient for efficiently managing the land at the fine scale. Most of the new precision agriculture technologies can be used to disaggregate informationâfor example, to characterize soil, yield, nutrients, and water variation within fieldsâas well as to assemble regional information. Perhaps the ultimate disaggregation would be to look at agricultural fields as a collection of individual plants. The extent to which data are disaggregated or reassembled for different spatial units depends on the nature of the management problem and the resolution of the data gathering techniques. Decision makers will need to consider the spatial heterogeneity of the area being managed and the relative value of the information. (A brief review of
DIMENSIONS OF PRECISION AGRICULTURE 21 the impact of information technologies on current management decision processes can be found in Chapter 2.) Subfield Management The potential for individually managing small areas, whose size is deter- mined by local characteristics and crop value, is one of the most enticing aspects of precision agriculture. The ability to repeatedly locate a specific site and mea- sure agronomic characteristics provides an opportunity to optimize management throughout the production area. Subdividing a field into small management units may improve both the economic and environmental sustainability of crop pro- duction systems. The earliest advocates of precision agriculture took the approach that man- agement decisions should be based on soil characteristics, assuming that similar soil series could be managed as homogeneous units. Subsequent research showed that for many soils, nearly the same nutrient variability exists within the mapped soil series as among them (Sadler et al., 1995). Even precise management based on variability of the physical and chemical properties within soil types may or may not be sufficient for optimal management of crop production activities. As producers try to manage smaller areas, the law of limits comes into play more strongly. For any given site, from year to year, the most limiting factors to crop growth can change from nutrient or moisture availability (deficit or excess), to disease or insect pests, to weather factors. In fact, the limiting factor may change within the growing season as the crop matures and its needs change. For improved decision making, managers must be aware of the limiting factors for each subfield unit and be able to modify management at that scale. The determi- nation of the most limiting factors is currently both difficult and expensive, and these costs are considered by decision makers. All of these concerns point to the need for analytical systems and technologies that can determine the important factors and decision-support systems that can use available data. Some management factors exhibit a relatively small amount of variability. For example, levels of less mobile soil nutrients (i.e., potassium and phosphorus) may exhibit little variation in crop response within some fields that have received heavy fertilizer applications for many years. These crops may be subject to greater variabil- ity from other influencesâsuch as weather, nitrogen, diseases, and insectsâparticu- larly if the time frame for assessing the performance of a method is short (i.e., a single growing season). Similarly, technologies that work well for one cropping system or biophysical setting may not work in another. Efficacy testing should be done for a variety of settings and systems and over several growing seasons. Beyond Subfield Management It is unarguable that an individual growerâs precision agriculture data has substantial additional value when combined above the subfield level with similar
22 PRECISION AGRICULTURE IN THE 21ST CENTURY data from other production operations. Management strategies consistent with our definition of precision agriculture are currently practiced, and new strategies will be developed that address spatial and temporal variability at the scale of the whole field and larger. While this report focuses on subfield level precision agri- cultural practices, a discussion of two key larger-scale strategies follows. Data Warehousing Large amounts of spatially referenced data on individual fields are, or soon will be, generated by yield monitors, real-time and remote sensors, on-the-ground sampling and observation by producers and consultants. This site-specific infor- mation will have value for use within individual fields in ways discussed in Chap- ter 2, but will also have value when combined with data on the same variables collected for nearby fields. Seed, chemical, and machinery agribusiness, among others, are assisting growers in data collection and interpretation. In a number of cases, agribusiness is providing financial assistance so growers will share data with the agribusiness itself. Several companies have promoted a concept of data aggregation which permits growers as well as an agribusiness free access to participantâs data. Still others have promoted concepts of data collection in which data could be purchased by third parties. Many growers have expressed opposi- tion to any of their data being shared with others. However, most growers do agree that there is economic value in the learning that results from data sharing and that may increase the likelihood of vertical integration of agricultural opera- tions. Though it is unlikely that a commercial interest will freely share informa- tion to which they have purchased rights and made further investments, other groups may see benefits from voluntary sharing. Grower clubs such as Practical Farmers of Iowa have been successful models of farmer-directed research in which land grant or private sector consultants act as facilitators in planning and implementing research trials. The idea is for a number of growers to implement similar practices of interest in their farm operation (i.e., row-spacing, herbicide dose and timing, cultivar selection) in statistically sound on-farm experiments (Stroup et al., 1993). In these clubs, data are openly shared to identify desirable practices in local growing areas. Imagine the same grower clubs now sharing spatially referenced data from experiments where growers agree to apply similar agronomic practices. The potential to create locally derived recommendations from locally collected data is a fascinating prospect. In effect, a version of this vision is in practice today with the private crop consultant. By working with numerous growers, the consultant is afforded the opportunity to observe how diverse recommendations can affect crop fitness, yield, and production efficiency in farming enterprises as small as several acres to those that extend over thou- sands of acres. Such an approach would require growers to openly share data with fellow producers.
DIMENSIONS OF PRECISION AGRICULTURE 23 Landscape Analysis There are opportunities to link management decisions at various levels to improve soil and water quality. The National Research Councilâs report on Soil and Water Quality (National Research Council, 1993) described the inherent links between farming systems and the landscape. Management practices to improve input use efficiency and reduce erosion can improve the quality of the surround- ing watershed. The Committee on Long-Range Soil and Water Conservation rec- ommended use of landscape buffer zones that connect farms and fields, provide widespread protection to waterways, and prevent soil degradation. Focusing on the impact of within-field production practices on adjacent ecosystems changes the unit of analysis to the landscape scale for studies on agricultural nonpoint sources of pollution. Landscape analysis considers effects of farming practices on larger areas than a specific crop field. Coordinating information at various levels could enhance protection of the environment. For instance, tracking production practices across a watershed could be useful in targeting areas with soil and water quality problems (National Research Council, 1993). Regional Management The appropriate scale for management will vary according to the factor most limiting to productivity. Manageable factors such as soil fertility or weed compe- tition may vary significantly at a subfield level, thus input use can be based on subfield units. However, there may be more utility in managing other factors at a field or farm level. For example, because insects migrate over areas larger than a field, monitoring their movements on a regional scale may be appropriate. Ac- quiring other regional data also may improve the accuracy of the decision-mak- ing process. Information provided to producers that is regional in nature, can have a di- rect impact on local management decisions. Evapotranspiration is typically moni- tored using networks of weather stations that cover large areas. Regional data also interacts with more site-specific data that producers can incorporate into their decision making. The California Irrigation Management Information Sys- tem (CIMIS) is a computerized crop weather information system that producers can access by modem or the Internet to obtain hourly and daily weather condi- tions. Producers combine regional evapotranspiration data and local soil- and crop-specific coefficients for their fields to determine the daily water use and water demand of their farms (see Box 1-1). It is unclear how to appropriately use data collected at different spatial scales together to help make better decisions. There are significant statistical and mod- eling issues to be addressed. Precision agriculture will greatly increase the amount and perhaps the availability of geographic data snapshots for many cropping fields, which will increase the demand for these analytical techniques.
24 PRECISION AGRICULTURE IN THE 21ST CENTURY BOX 1-1 California Irrigation Management Information System California has more than 10 years of experience operating the Califor- nia Irrigation Management Information System (CIMIS), a computerized crop weather information system that growers can access by modem or the Internet to obtain hourly and daily weather conditions. The first five weather stations went on-line for research from May 30th to June 7th, 1982. Stations number one and two were installed in Fresno County, numbers three and four were installed in Santa Cruz County, and num- ber five was installed in Kern County. By the end of 1982, 27 stations were operating. After three years of research and testing, CIMIS was made available to the general public on July 1, 1985 (Eching et al., 1995; Eching and Moellenberndt, in press). Ninety CIMIS weather stations are now in use throughout California, with information generated from a num- ber of sensors at each site which are directly linked to a computer. The stations are ground referenced with latitude, longitude, and elevation readings. CIMIS is an excellent example of current technology that provides information on crop water requirements. Growers use the CIMIS weather system and soil- and crop-specific coefficients for their fields to deter- mine the daily water use and water demand of their farms. Vendors may combine these data with data from other sources and provide specialty products tailored to weather information needs for specific crops. CIMIS is operated by the State Department of Water Resources in cooperation with the University of California, local water districts, and various agencies. The information gathered at each site includes maxi- mum, minimum, and average air temperatures and relative humidity read- ings. Data are also collected on precipitation, evapotranspiration, dew point, vapor pressure, average soil temperature, wind speed and run, and solar radiation. Evapotranspiration data represent water loss from soil evaporation and crop transpiration and referenced to water use for a healthy grass; values must be multiplied by a crop coefficient developed for various growth stages. Evapotranspiration data are used as an aid in irrigation scheduling. Growers and consultants use the information to maintain crop water-use budgets by comparing how much water has been applied to a
DIMENSIONS OF PRECISION AGRICULTURE 25 BOX 1-1 Continued field with how much water the crop is using each day. Water use can be projected and water can then be ordered from the local irrigation district for delivery to the field before the crop depletes the available water in the soil. The crop water-use information does not take into account the appli- cation efficiency of various irrigation systems, however, nor does it calcu- late the leaching requirement for salt-affected soils. Information from CIMIS weather stations used for assessing crop water requirements is widely disseminated through various means of communication. Farmers in the San Joaquin Valley can listen to the radio for daily early morning agricultural reports that include evapotranspira- tion values and crop coefficients for numerous crops. The information is supplied to the radio station by agricultural consultants as a service to the industry. A CIMIS report is part of a weekly newspaper (Ag Alert) published by the California Farm Bureau Federation in Sacramento. The weekly refer- ence evapotranspiration information is shown in a histogram, along with comparison data from the corresponding week of the previous year and an average year. Growers with computers and modems can access daily and weekly evapotranspiration data directly from CIMIS, through several sites via the Internet, or from the Agri-Tech Information Network main- tained at California State University-Fresno. Growers can call a contact at the University of California-Davis for crop coefficient information. Growers and businesses that subscribe to the Data Transmission Network (DTN) satellite information service on-line can access daily and monthly CIMIS weather data for all 90 operating stations in the state. The computer hardware and satellite dish are owned by the company provid- ing the service, so there is no need for individuals to invest in expensive computer equipment. All levels of producers, regardless of farm size, have many ways to access the CIMIS weather information. Crop water-use data are avail- able for the current season and from historical databases, some of which go back to 1982. The major efforts made by the California agricultural industry in disseminating CIMIS evapotranspiration data should be used as an example of how to saturate a production region with important information which has been shown to aid decision making.
26 PRECISION AGRICULTURE IN THE 21ST CENTURY BOX 1-2 The Crop Consultant of Tomorrow Itâs early Friday morning in late June. John pours his first cup of cof- fee, turns on his computer, and reviews the list of fields he will visit today. With the click of his mouse, he opens a client list and downloads weed, insect, and nutrient application maps created by his farmer clients as they cultivated their corn fields late yesterday afternoon. At the same time, satellite images of crop greenness are downloaded for 12 fields. These images complement others collected earlier this year and in pre- ceding years. When John reads these images into his geographic infor- mation system, he extracts information about pest risk with several deci- sion tools for pest management and nutrient use efficiency. John transfers the information from his kitchen computer to the lap-top in his pickup truck. Before heading out the door, he reviews the maps of each of his fields to determine how to best use his three crop scouts that day. On- the-go sensing supplemented by smart or directed sampling is a very important part of Johnâs management efficiency plan and has resulted in timely crop management decisions which would otherwise have been missed. After visiting each of the 12 fields, John sits with his farmer client and reviews summary maps of variability in crop moisture, canopy clo- sure, and pest pressure. John knows the best decisions are made when their collective wisdomâhis and the farmerâsâis aided by the new types of information. John knows his clients have diverse opinions and man- agement philosophies. Some want little help from advanced information technologies whereas others value the added information. ENABLING TECHNOLOGIES A fascinating aspect of precision agriculture is that a single technology is not being undertaken to improve a single practice. Instead, across the crop-produc- tion sector of the United States, precision agriculture is emerging as the conver- gence of several technologies with application to several management practices. However, every technology is not necessarily required or applicable for every practice on all crops, and development and enhancement of several of the poten- tially relevant basic technologies are being driven by forces outside of the agri- cultural sector. Thus it is difficult to develop a generally accepted view of the dimensions of precision agriculture. Every area of information technologyâmi- croelectronics, sensors, computers, telecommunicationsâis in an evolutionary process of continuous improvement. As these introductions take place, some prod- ucts will become economically feasible for agricultural applications. In Box 1-2, describing a vision of tomorrowâs crop consultant is considered. According to
DIMENSIONS OF PRECISION AGRICULTURE 27 BOX 1-2 Continued Later that summer, John and his co-workers turn their attention to calibrating yield monitors on his clientsâ combines. Data logging devices in the combine cab are simultaneously tested for accuracy and ease of operation. In this way, Johnâs clients are able to collect yield maps while logging spatially referenced data and notes about weed patches, insect damage, and other concerns in the field. These new maps are transmit- ted through wireless communication to Johnâs office. After harvest is com- pleted, John visits one of his clients for postharvest evaluation of the growing season. The field maps, data from other Internet sources (i.e., weather data), and the cumulative collective wisdom make for a con- structive discussion. Because of Johnâs information-intensive approach, several management successes and problems become evident that may otherwise have gone undetected. For example, one cultivar significantly outperformed two others grown in the same and adjacent fields. They also note that weed problems were less severe with this cultivar. By de- tecting and treating the weeds during harvest the farmers can skip the preemergence treatment the following year. Their discussion continues. A new concept emerges from Johnâs business: the value of shared infor- mation. A subset of his 23 clients agree to share insights gained from this new information-intensive approach to farming. Later in the autumn, a group of 11 growers meet to discuss their successes and challenges. Through their discussion they learn that certain cultivars consistently outperformed others and some were less tolerant to herbicides. Several producers comment that the information helps them to plan better sched- ules for harvesting and for use of shared machinery. this vision, while many precision agriculture technologies are available for use, individual producers will assemble those technologies that address given man- agement issues in their particular production systems. The following discussion provides a broad overview of the precision agriculture technologies and practices that are or soon will be available. For more detailed information, the reader may want to access additional literature sources such as Pierce and Sadler (1997); Robert et al. (1995); and Robert et al. (1996). Research and development of many technologies used in precision agricul- ture have occurred outside the agricultural community. In the past century, of course, other developments such as the internal combustion engine, electrical power, telephone, and weather satellites produced outside of agriculture have been introduced to the agriculture sector. Precision agriculture technologies such as the global positioning system (GPS), geographic information systems (GIS), and remote sensing have their core constituencies outside agriculture. Crop and
28 PRECISION AGRICULTURE IN THE 21ST CENTURY soil sensors operating on farm machinery, variable-rate fertilizer applicators, and yield mapping systems are technologies that have been developed within the ag- riculture sector by private industry. Other economic sectors have supported the research and development of some of these technologies, which is a financial benefit to agriculture. Precision agriculture involves the integration of these in- formation technologies with agronomic knowledge. Georeferenced Information Georeferencing refers to relationships among data based on their geographic locations. This spatial emphasis implies a new way of looking at agricultural information and site variability. Although spatial variability has always been rec- ognized, data comparisons have often been made without specific information on site location, yielding qualitative results. Comparisons of data detailed from spe- cific locations which are obtained from specifications by using various precision agriculture methods will be one of the important new techniques that can improve farm management. The value of a database for precision agriculture practices increases when the data layers are spatially referenced to each other. Co-registration of data will become critically important as management units get smaller and as more precise field data (location precision from submeter to a few centimeters) become avail- able. It is expected that data referenced to physical location will allow different types of information to be compared and quantitatively analyzed at multiple loca- tions. For example, physical properties of soil core samples collected from a field could be compared with other spatially explicit data available for decision mak- ing, such as characteristics of the mapped soil unit and topography, yield monitor data, and irrigation, nutrient, or pesticide applications recorded during variable- rate applications. Global Positioning System The Global Positioning System (GPS) is a system of satellites emitting elec- tronic signals that can be received by mobile field instruments sensitive to the transmitting frequency. Positioning is achieved through the use of simultaneously received satellite transmissions from four or more satellites above the horizon. With a constellation of 24 satellites, any location on earth can have four or more satellites in view for 24 hours each day. By referencing the satelliteâs exact loca- tion and the time the signal takes to travel between the transmitter and the re- ceiver, the location of the receiver can be determined by triangulation. Use of the GPS receiver allows latitude and longitude coordinate informa- tion to be associated with data obtained from a specific site on the field. The GPS can also be used to provide navigational guidance, enabling a producer to revisit a spot in the field and check the efficacy of management decisions. The GPS is an
DIMENSIONS OF PRECISION AGRICULTURE 29 essential field component for most mapping-based precision agriculture and other measurements of field characteristics that would be used to determine product application maps. Even for operations with real-time sensing and control of in- puts, GPS positioning will be valuable. If the sensed parameters and application rates are recorded and georeferenced, these data can be included in the manage- ment database. Adoption of GPS and other spatial referencing technologies will have a widespread impact on data collection and analysis. The positions provided by GPS receivers currently are not sufficiently accu- rate for dynamic real-time precision agriculture uses. Various errors, including those introduced by the U.S. Department of Defense (DOD) for security purposes (selective availability), contribute to the inaccuracy (National Research Council, 1995c). The present system under selective availability has an accuracy of about 100 meters. However, technical solutions are available to improve the position- ing accuracy. A technique known as differential correction is widely used to re- move the effects of the error sources. Position error is determined by using one or more fixed base stations to compare the calculated position with the stationâs known location. By combining the error values with the GPS signals, position accuracy can be improved to about 2 meters or less. The augmented positioning is known as differentially corrected GPS (DGPS). These corrections can be made either by software in a postprocessing operation or by hardware for real-time positioning. Most precision agriculture operations require the availability of real- time positioning, necessitating the transmission of the differential correction sig- nals to the GPS receivers in the field. Differential correction procedures are cum- bersome, prone to signal loss, or expensive depending on the method used to generate and transmit the differential correction signal. Commercial applications of georeferencing systems will grow dramatically over the next decade, both in agriculture and other industries. Some commercial businesses offer real-time differential correction services in space-based or land- based networks to their subscribers. Many of these providers are focused on non- agricultural industries and so do not adequately cover rural areas with their sig- nals. The U.S. government provides differential correction signals through Coast Guard beacon signals, but access to these signals is limited to areas near navigable waterways (coastline and rivers). The Russian government continues to operate its GLObal Navigation Satellite System (GLONASS) which could augment basic ca- pabilities of GPS. Since GPS and GLONASS use different time standards and coor- dinate systems, these differences will need to be corrected by combined receivers (National Research Council, 1995c). Receivers that use techniques such as carrier phase tracking (Real Time Kinematic) offer higher accuracy, but have higher costs. Several other factors can limit the application of GPS in precision agricul- ture. Time delays for updating signals may limit the utility of DGPS for on-the-go sensing, particularly for high-speed operations such as aerial applications. Sys- tem inaccuracies make data collected along a crop row appear to suddenly shift, creating map displays that do not match actual travel paths. Signals can be se-
30 PRECISION AGRICULTURE IN THE 21ST CENTURY verely degraded by moderately inclement weather conditions, foliage and elec- tromagnetic radiation. Position data are not always available at the one-second frequency that is expected, so data gaps are created (data dropouts). Increasing the accuracy and reliability of GPS will increase its acceptance by producers and its utility for geographic referencing in precision agriculture applications. Geographic Information Systems and Mapping Software Digital geographic data that can be stored, analyzed, integrated, and displayed in different representations, form the core of precision agriculture. The software packages used to handle such data, Geographic Information Systems (GIS), are available with a wide range of capabilities and costs, but all are able to graphi- cally display georeferenced data. Although a single data layer (i.e., yield data) can be mapped with the use of less-sophisticated software, more complex rela- tionships (i.e., temporal patterns or multivariate comparisons) are best performed with full-function GIS packages. The data layers derived from combinations of raw data can generate information about spatial variability among factors in crop production. It is expected that adequately co-registered data will be quantitatively analyzed through the use of geostatistical and other procedures. Available GIS software ranges from simple map display systems to fully functioning systems capable of analyzing and integrating complex spatial data- bases. Some data can be stored as polygons within which the attributes (i.e., soil types) are considered to be homogeneous. Data can also be stored in a uniform array of grid cells or pixels with homogeneous attributes, which is the format used for remote sensing images and U.S. Geological Survey digital elevation maps. Most fully functioning GIS programs today can be converted between these formats, which has made it easier to combine data from different sources. Among the most important roles for GIS are the database functions for farm record keeping and for comparing management decisions, yields, pest activity, groundwater quality, and other concerns related to past and current practices. GIS can store farm records of inputs and outputs in a spatial array. For instance, data on crop rotation, tillage, nutrient and pesticide applications, yield, soil type, roads, terraces, or drainage tiles can be stored in a GIS. Data layers can be derived from digital orthophotography. GIS will enhance other components of precision agri- culture such as yield monitoring and farm-based research (i.e., crop modeling and efficacy testing) as well as provide better record keeping for producers. Such software has the potential to integrate all types of precision agriculture informa- tion, interface with other decision support tools, and output (printed or electronic maps) that can be used in precision applications. A key to realizing the promise of a dynamic GIS will be development of connections between the relational data- base and the decision support system. A disadvantage of the current generation of geographic information systems is the complexity of the software and the steep learning curve involved in using and interpreting spatial data in a valid and robust
DIMENSIONS OF PRECISION AGRICULTURE 31 way. The limitation with some commercial software is that spatial relationships among data layers often cannot be rigorously quantified; only visual relationships can be made. This situation is rapidly changing as several vendors are developing fully functional GIS programs intended for use on PCs. This should lead to soft- ware and hardware systems that are more user friendly and less expensive. In addition, firms are emerging in the marketplace that can provide GIS services or software tools to growers and field consultants. There is an urgent need to make fully functional GIS easier for nonspecialists to learn and use in order to transfer this technology to the agricultural community. GIS can be used with a spatially distributed process model as the basis for subsequent decisions on precision agricultural practices such as variable-rate ap- plications. Several classes of models should be considered as part of the suite of tools for precision agriculture. Yield Mapping Systems Yield mapping systems record the relative spatial distribution of yield while the crop is being harvested. These systems collect georeferenced data on crop yield and characteristics such as moisture content. The resulting maps can dra- matically illustrate the areas of yield variability from either natural processes or agricultural practices. Because yield is a primary factor in most management de- cisions, precise yield maps are desired to confirm spatial treatment decisions. Yield monitors have been developed for only a few crops, primarily cereal grains. Reliable monitors for vegetables, fruits, cotton, and other high-value crops are currently under development but are not yet widely available. Yield is more difficult to monitor for fruit or vegetable crops that are harvested manually or repeatedly. Use of machine-mounted yield monitors currently is limited to crops that are mechanically harvested in a single pass, such as potatoes, sugar beets, and processing tomatoes. Other techniques such as remote sensing may provide alternatives to yield monitors. The use of precision agriculture techniques in non- grain crops may be limited by the lack of appropriate yield monitoring systems. Since 1992, grain yield mapping has been done by using mass flow and moisture sensors to determine grain mass and using GPS receivers to record posi- tion. Yield monitors measure wet grain flow, grain moisture, and area harvested to determine moisture-corrected yield per acre. Because the mass-flow measure- ments are made in the combineâs clean-grain conveying system, there is a shift in harvester position between the point where the grain is actually cut and the loca- tion of the machine where it is measured. This shift results in dynamic inaccura- cies that currently cannot be completely removed by subsequent data processing. Field totals (with recommended calibrations) are considered more accurate than are small subfield yield measurements. Although yield monitors have been pro- moted widely, further yield monitor refinements are needed to improve their ac- curacy for precision agriculture applications.
32 PRECISION AGRICULTURE IN THE 21ST CENTURY Variable-Rate Technologies Precision agriculture was pioneered by domestic U.S. industry, beginning with the conception and implementation of Variable-Rate Technology (VRT). VRT applicators spatially vary the application rates of agricultural inputs such as seed, fertilizer, and crop protection chemicals. VRT systems include specialized controllers that vary specific material flow rates, even multiple product rates si- multaneously, in response to a desired change in the local application rate (on- the-go). VRT systems can be designed in different ways depending on the prod- ucts to be applied and the source of the information utilized to specify local rates. Present commercial VRT systems are either: (1) Map-based, requiring a GPS/DGPS georeferenced location system and a command unit that stores an application plan of the desired application rate for each location within the field, or (2) Sensor-based, which does not require a georeferenced location system, but includes a dynamic command unit that specifies application through real-time analysis of soil and/or crop sensor measurements, for each lo- cation within the field as it is encountered. Historically, VRT methods were introduced by industry during the mid- 1980s. Dry nitrogen, phosphorus, and potassium fertilizer application rates were simultaneously varied on commercial spreader applicators based on a predeter- mined map strategy (developed from earlier data collection such as photographi- cally derived soil maps or grid sampling). Farmer-owned machinery has been equipped with VRT for fertilizer applications requiring a standard liquid blend. In this case, product application rates are based on soil properties measured in real-time. Limited use has been made of sensor-based VRT by commercial appli- cators to date. Herbicide application responsive to soil organic matter (Gaultney and Shonk, 1988) is the singular exception (McGrath et al., 1990). Commercially available sensors employed for VRT include those responsive to organic matter, cation exchange capacity (CEC), topsoil depth, soil moisture, soil nitrates, and crop spectral reflectance. Proponents of real-time sensor-based VRT application have observed that soil and crop conditions are more variable than measurements obtained from current map based methodologies. Optimal crop management results are not expected from current GPS/DGPS/GIS method- ologies which are limited to one sample and one control change per second. The application of nitrogen fertilizer in response to measurements of side-dress soil nitrates and CEC (Colburn, 1991) and the application of nitrogen fertilizer in response to wheat nitrogen status as detected by spectral reflectance (Stone et al., 1996) are two examples of on-the-go sensing based VRT which do not rely on GPS/DGPS or GIS systems. Real-time sensors offer some benefits over map-based techniques for VRT. Real-time sensing is a direct and continuous measure of the attribute of interest
DIMENSIONS OF PRECISION AGRICULTURE 33 thus allowing the user to reduce the amount of unsampled area in a given applica- tion. In map-based applications, maps are based on a limited number of samples thus creating the potential for errors in estimating conditions between sample points. An additional uncertainty is associated with GIS due to the temporal dis- connection that occurs when samples are mapped at some point in time and a response is made at some later time. In the case of dynamic variables such as soil nitrogen content or pest distributions, significant change in the amount and distri- bution of the attribute of interest can take place during that time (Sudduth et al., 1997; Wollenhaupt et al., 1997). Sensor based VRT is employed on Midwest farm equipment to: â¢ Vary anhydrous ammonia application in response to soil type variations. â¢ Vary planting population in response to soil CEC and topsoil depth variations. â¢ Vary herbicide rates in response to soil organic matter variations. â¢ Vary starter fertilizer in response to soil CEC variance. â¢ Vary nitrogen fertilizer at side-dress time in response to soil CEC, topsoil depth, and soil nitrate levels. Map-based VRT is employed in the high-volume commercial (contracted) application of phosphorus and potassium fertilizers and lime using high-flotation applicators. Map-based variable-rate application systems for farm tractor use are widely available for liquid fertilizers, anhydrous ammonia, herbicides, and seeds. Map-based VRT controls for water and fertilizer are also available for center pivot irrigation systems. Because of the additional capital and maintenance expense for high volume, pneumatic or liquid material control systems in high-flotation VRT, application costs are higher than for conventional floater application technology. Floater VRT application of granular fertilizers is typically $2 to $3 per acre higher than non- VRT applications. Costs for upgrading tractor-mounted application controllers to add VRT ca- pability are often nominal. Upgrading a controller to allow for automated adjust- ment of application rates is a minor technical departure, representing only a soft- ware/hardware interface. However, the producer must also have a computer that manages GIS data and sends rate change commands to the controller, and a GPS/ DGPS receiver. Such a system can be assembled by more technologically sophis- ticated producers. In other cases, a VRT system may be more complex and costly, incorporating multiple chemical injection hardware and GIS/GPS/DGPS systems as an integrated, dealer-installed unit. Regardless of the type of VRT system uti- lized by a grower, implementation of a map-based VRT system requires full con- sideration of all related costs, including data acquisition, the GIS and GPS/DGPS to create and execute application maps, and the often time-consuming intellectual capital investment in learning how to successfully use all components of the tech- nology.
34 PRECISION AGRICULTURE IN THE 21ST CENTURY The cost of obtaining and interpreting soil test information on which to base floater or tractor-based application rates is a limiting factor in the site specificity of map-based VRT. Soil samples normally are acquired at a rate of one sample per 2.5 acres to reduce costs for collection and analysis. In an Illinois test, fertil- izer requirements based on 2 grid sizes were compared to uniform application rates. With a grid size of 0.156 acre, recommended fertilizer rates decreased dramatically resulting in a fertilizer savings of $18.00 per acre compared to $0.25 per acre savings with a 2.5 acre sized grid. The cost to collect the samples on the more detailed grid, however, far exceeded any savings in fertilizer costs (Illinois Agri-News, 1996). One key to improving the efficacy of map-based VRT is the development of additional cost-saving, higher sampling density sensor method- ologies. Groundbased Sensors Basic research is needed to investigate soil and crop processes applicable to development of ground-based sensing systems. Sensors offer the opportunity to automate collection of soil, crop, and pest data at a level of intensity not economi- cally feasible with manual sampling and laboratory methods. Fields are highly heterogeneous. Increased sampling will result in accurate characterization of within-field variability. Improvements to VRT and crop modeling are expected to advance rapidly with a higher spatial density of measured soil and crop param- eters. Sensors are needed that are fast, efficient, and can assess factors important to crop production. Moran et al. (1996) concluded that the information from ground-based sen- sors is needed for soil organic matter, soil moisture, cation exchange capacity, nitrate nitrogen, compaction, soil texture, salinity level, weed detection, and crop residue coverage. These parameters as well as soil pH, and availability of phos- phorus and potassium cannot be ascertained by remote-sensing technology. More- over, the use of real-time ground-based sensors provides the grower control over timing of data acquisition not possible with satellite or aircraft sensing techniques. Sensors have been developed or are under way to measure soil and crop conditions including soil organic matter, soil moisture content, electrical conduc- tivity, soil nutrient level, and crop and weed reflectance (Sudduth et al., 1997). Continuous, real-time electrochemical soil chemical constituent sensors are cur- rently available for nitrate measurement and are dedicated to specific application in corn side-dress applications. A real-time acoustic soil texture sensor and a real- time soil compaction tester are also under development at Purdue University (Liu et al., 1993; Morgan and Ess, 1996). Some important real-time indexes may be determined by their relationships to other variables rather than by direct determination. Soil conductivity is appro- priate for concurrent real-time assays of salinity, soil moisture, organic matter, cation exchange capacity, soil type and soil texture. Recently, this work was ex-
DIMENSIONS OF PRECISION AGRICULTURE 35 tended to non-saline soil methods in combination with electrochemical constitu- ent sensing which separates components of direct contact conductivity (Colburn, 1997). Conductivity component analysis is employed for georeferenced data gath- ering and analysis by several commercial companies as well as for VRT in midwest crops. Apparent soil conductivity using electromagnetic methods is an indicator of clay content, depth to claypan, soil water content, hydraulic charac- teristics, productivity (Kitchen et al., 1996), and as a promising substitute for yield monitoring (Jaynes et al., 1995). For immobile constituents (i.e., phosphorus and potassium), industry has not yet chosen to introduce real-time sensors. In some cases, phosphorus and potas- sium levels in the corn belt states where VRT was first used, are very high, and field availability has been found to exceed producer needs for the current crop year and the near future. In other regions, such as western states, lower availabil- ity of immobile nutrients is common. For these nutrients, discontinuous nutrient sensor mapping methods have the potential for gathering and analyzing soil samples in separate field operations. Three systems are under development by government and academia which automatically extract and analyze soil samples for phosphorus, potassium, and nitrates (Adsett and Zoerb, 1991; Birrell, 1995; Morgan and Ess, 1996). There exists the potential for a vast increase in the timeliness and amount of information if additional means of data collection and analysis become avail- able. Sensors will play an important role in supporting technology for precise applications of nutrients, pesticides, and other inputs. Only a few commercial sensors are available today. Efforts continue by both private companies and the public sector to develop real-time sensors for additional agricultural indexes. Basic research in the sensors arena is fundamental to an improved understanding of the variations in site-specific crop production in a wide variety of regional production systems. Remote Sensing Remote sensingâthe acquisition of information from remote locations such as an airplane or satelliteâis a potentially important source of data for precision agriculture. In the long term, remote sensing could provide numerous forms of information, both spatially and temporally. However, improvements are needed in the analytical products and delivery systems if remote sensing is to meet its promise for precision agriculture. For more than 30 years remote sensing has been envisioned as a valuable source of information for crop management. The pioneering research of Colwell (1956) showed that infrared aerial photography could be used to detect loss of vigor of wheat and other small grains resulting from disease. Although much research and development was directed at large-area crop inventory applications of satellite data in the 1970s (MacDonald and Hall, 1980), much less attention
36 PRECISION AGRICULTURE IN THE 21ST CENTURY BOX 1-3 Remote Sensing Vegetation Indexes One of the earliest digital remote sensing analysis procedures devel- oped to identify and enhance the vegetation contribution in an image was the vegetation index (VI), a ratio created by dividing the red by the near- infrared spectral bands (Tucker, 1979). The basis of this relationship is the strong absorption (low reflectance) of red light by chlorophyll and low absorption (high reflectance) in the near-infrared by green leaves. A form of this ratio, in digital and map formats, is one of the principal data prod- ucts that will be provided to producers for crop assessment. Dense green vegetation produces a high ratio, while soil, plant litter, and geologic min- erals have low ratio values, thus yielding a maximum contrast (Baret and Guyot, 1991; Huete et al., 1994; Verstraete and Pinty, 1996; Verstraete et al., 1996). A number of related indexes have been developed that minimize the effects of atmospheric and/or soil variation. The Normalized Difference Vegetation Index (NDVI), the ratio of the difference between the red and near-infrared bands divided by their sum, is the most widely used VI (Huete and Tucker, 1991; Kaufman and Tanre, 1992). Although, these indexes correlate to various plant parameters linked to the leaf area, it has been hard to determine precisely what plant property is being sensed (Baret and Guyot, 1991; Myneni et al., 1995; Pinty et al., 1993). The ratios correlate most closely with the fraction of absorbed incident photo- synthetically active radiation, and for this reason the indexes can be in- puts to models for estimating evapotranspiration and crop growth (Asrar et al., 1984; Myneni and Williams, 1994; Sellers, 1985). Although many other band combinations and analyses could provide important additional information for agriculture, these VIs will be the most widely used be- cause they are easy to produce and closely associated with particular crop processes. has been directed at crop management applications. Satellite data have not had spatial resolution, temporal frequency, and delivery times sufficient for the needs of production agriculture. In addition, supporting technologies and infrastructure have not been available. Nevertheless, the understanding of crop spectral and radiometric relationships gained from past research is relevant to crop manage- ment applications (Bauer, 1985). Jackson (1984) described the potential for remote sensing in crop manage- ment, and stressed that it is critical to provide frequent coverage, rapid data deliv- ery, spatial resolution of 5 to 20 meters, and integration with agronomic and
DIMENSIONS OF PRECISION AGRICULTURE 37 meteorological data into expert systems. These points were reiterated by Moran et al. (1997) in a recent review of the potential of remote sensing to acquire information for identifying and analyzing site-soil spatial and temporal variabil- ity within fields. In the past 10 years there have been rapid advances in acquiring and process- ing multispectral imagery with multispectral video by using digital cameras from aircraft. This approach has the flexibility of aerial photography acquisition and the advantage of digital multispectral imagery (Moran et al., 1996; Pearson et al., 1994). Although most planning and effort are going into the development of sat- ellite systems, aircraft-acquired imagery may continue to be needed when ex- tremely high resolution imagery is required. Aircraft platforms also provide an opportunity for developing and testing new sensors (i.e., thermal infrared and hyperspectral sensors) for future satellite systems. A sequence of remotely sensed images over time can provide information about crop growth and the spatial variation within fields. Detailed spatially dis- tributed multitemporal information, in visual form, is not readily obtainable from conventional crop management systems or from site-specific crop management methods. Remotely sensed images (i.e., color infrared aerial photographs or mul- tispectral images acquired from satellites or airplanes) show spatial and spectral variation resulting from soil and crop characteristics. These images show the state or condition of fields when the images were acquired. One of the most useful aspects of remote sensing is its ability to generate images showing the spatial variation in fields caused by natural and cultural factors. This information is not limited by sampling interval or geostatistical interpolations (Moran et al., 1997). Images acquired at different times during a season can be used to determine changes such as growth rates and condition. These data, in turn, can be compared with data from previous years and may be helpful in predicting yield. Commercial interest is growing in the potential of remote sensing to contrib- ute to site-specific crop management, particularly as precision agriculture tech- niques are being developed and the possibility of routine, frequent acquisition of remote sensing data by satellites seems likely. Several earth-observing satellites are scheduled for launch over the next decade by governments and private indus- tries. By 2005, 40 or more land observation satellites are expected be available (Stoney, 1996). Many of these satellites will acquire imagery with spatial resolu- tions ranging from 1-3 meters for panchromatic images to 3-15 meters for multi- spectral imagery. Others will have resolutions of 10-30 meters but with addi- tional spectral bands, including thermal infrared on LANDSAT-7. Still other systems will collect radar data at varying resolutions. These sensors have promise for many types of measurements beyond identifying crop type, including moni- toring crop stresses and condition, soil properties, and moisture. A major research challenge is the development of robust image analysis methods for agriculture, and a major educational need is training satellite data providers to meet agricul- ture needs.
38 PRECISION AGRICULTURE IN THE 21ST CENTURY BOX 1-4 Contemporary Remote Sensing Technology The technologies that can contribute to site-specific crop manage- mentâremote sensing, the global positioning system, yield monitors and mapping, geographic information systems, variable-rate applica- tion technology, computers, and electronic communicationâare cur- rently converging. Rapid growth in precision agriculture is stimulating renewed interest in developing remote sensing, especially from satel- lites, for crop management applications. Imagery acquired from con- tinuously orbiting satellites operated by commercial companies will enhance the possible applications and utility of remote sensing, and farmers will not have to contend with the challenges of collecting photo- graphs. Fritz (1996) suggests that despite high development costs, sat- ellite systems will be cost competitive with aerial imaging systems. He indicates that per unit of coverage, satellite imagery may be only one- half the cost of aerial imaging. The changes in U.S. policy resulting from the 1992 Land Remote Sensing Policy Act and the 1994 Presidential Directive on LANDSAT Remote Sensing Strategy specifically encourage commercial system de- velopment and operation and have led to several companies developing plans to launch satellite systems in 1997 through 1999. The new imaging satellites will acquire panchromatic (1- to 3-meter spatial resolution) and multispectral (4- to 15-meter resolution) imagery over swaths of 6 to 30 kilometers. At least two companies are targeting agriculture and preci- sion farming as either the primary application or as a major target of their planned marketing and sales efforts. Remote sensing products could play an important role in site-specific crop management, and there is also excellent market potential for the acquisition, processing, and delivery of remote sensing information. Per- haps no other application of remote sensing requires data so often over such large geographic areas. However, infrastructure to meet this re- quirement is not currently in place. Widespread application and success- ful adoption of remote sensing data products are not likely until such an infrastructure is developed; cadres of people who understand the rela- tionships between crop-soil properties and remote sensing are especially important. Similarly, more information and study on integration and use of spatial information in crop management is needed as well as opportu- nities for training in the use of spatial information. It will be very important for systems and data products to be based on crop producer needs, and for provisions to be made for farmers and others to develop an under- standing of remote sensing.
DIMENSIONS OF PRECISION AGRICULTURE 39 Crop Production Modeling A broad range of spatially explicit crop response models is needed to evalu- ate the efficacy of precision agriculture methods and provide the basis for precise recommendations. Many models for predicting how crops respond to climate, nutrients, water, light, and other conditions already exist, yet most of these do not include a spatial component appropriate to precision agriculture applications (Sadler and Russell, 1997). GIS can provide the means to run the model continu- ously across an extensive area using data that reflect continually varying condi- tions. Time series and other temporal analyses can aid in predicting final crop yield. Current models may be extended to account for spatial effects, such as edge effects along field boundaries. In the ecological and biometeorological literature, however, several spatially explicit models have been developed to predict hourly, daily, and annual rates of evapotranspiration and photosynthesis, and several spa- tially distributed hydrologic models predict surface and subsurface flows. Meso- scale climate models can resolve cells as small as 5 to 10 kilometers for predict- ing weather conditions. Pests are not dispersed evenly throughout the environment. To the extent that the factors influencing their spatial distribution are understood, their dispersion and potential for damage can be modeled. GIS can be used for spatially variable data for these factors. As with crop response models, a distinct pest model can be run continuously across a landscape, using GIS to input data to the model and display results (loosely coupled model), or a spatially explicit model can be cre- ated within the GIS software (tightly coupled model). GIS can provide the basis for multiscalar effects, for example, incorporating results of a regional pest pres- sure model into a system for generating within-field recommendations based on locally variable conditions. A crop growth model could be used as a decision aid for determining differ- ent yields based on varying plant populations, which could help a producer de- cide when to plant or replant areas within a field based on plant population data and risk factors for various soil types. Having to make a decision to replant a field that is in a questionable condition is perhaps the hardest decision a producer faces. Any information to aid such decisions and reduce risk would be valuable. In many crop production areas, landscape factors can cause dramatic varia- tions in yield. Landscape elements affect many properties relevant to plant growth, including soil texture, soil organic matter, and temperature. Landscape morphol- ogy affects soil moisture available to crops by its influence on drainage and catch- ment area. Soil surveys typically do not have sufficient resolution to capture this variability in enough detail to support precision recommendations; even field- based sampling on a regular grid may miss relevant soil-landscape features. Strati- fying sampling density on the basis of landscape features may be more cost effec- tive and informative than a simple grid. GIS allow users to create and manage digital elevation or digital terrain models created by photogrammetric methods
40 PRECISION AGRICULTURE IN THE 21ST CENTURY (analysis of stereo pairs of aerial photographs) with new techniques using interoferometric radar or by continuous three-dimensional coordinate measure- ments with in-field equipment. Precise recommendations can be made to the ex- tent that the relationships are understood between soil properties and surface morphology (i.e., slope, slope length, aspect, curvature, landscape position, catch- ment area, and drainage) derived from digital elevation or digital terrain models. Crop models do not offer a panacea for problem solving; they are limited in their ability to simulate various parts of a biological system. Most of the crop and pest models available or developed to date were not designed to be used for man- aging spatial and temporal variation. It is not clear whether a predictive model, an explanatory model, or a hybrid approach will be more appropriate for precision agriculture. Alternatively, data mining and other techniques may be used to ex- tract valuable information from large amounts of stored data. However, crop modeling is currently an important tool for gaining a theoretical understanding of a crop production system. Decision Support Systems Decision support systems (DSS) are used in agriculture for tactical, strategic, and policy-level decision support. Because producers are continually faced with making tactical decisions, such tools are becoming increasingly useful on the farm. However, few DSS are in general use by agricultural producers today, in part due to difficulty in use and limited information providedâfrom their point of view. They have been used to aid in decisions that are complicated by large amounts of information and data. A simple conceptual diagram of a DSS is shown in Figure 1-2 (Petersen et al., 1993). Data collected by a consultant, obtained through a weather forecasting service, or acquired through a sensing operation are analyzed and linked with appropriate decision rules that identify actions to assist in producer decision making. DSS rules are not developed to make a single recommendation but rather to provide decision makers with choices; decision support systems should be seen as sources of valuable tactical information. As is the case for crop modeling and current management recommendations, DSS have been developed for whole fields, and subfield variation has been largely ignored. Although subfield tactical decisions have been practiced by producers for many years (i.e., rouging, spot- spraying or rope-wicking residual weeds, or spot-treating chinch bugs in sor- ghum), most management practices are implemented for whole fields. The relationship between the scale of an operation and the resolution and variability of sample data used in a DSS is important. To demonstrate this point, consider the appropriateness of using DSS in two sites with widely differing char- acteristics. The variation in the assessed attribute used in the DSS is high at one site and low at the other. The DSS may be adequate for whole-field decisions at the site with low variability but not appropriate for the site with high variability.
DIMENSIONS OF PRECISION AGRICULTURE 41 FIGURE 1-2 Conceptual diagram of a decision support system. Tracing the steps in the figure, information can be viewed as flowing from the environment via instrumented or human sensors as data to a database. The information as data is analyzed and manipulated for storage or transmission to a user as part of a decision process. The information pro- cessed for a decision results in an action to be executed within the environment. After the action is carried out, the environment is again monitored to begin a new cycle of informa- tion flow. Thus, information flows to and from the environment in an endless loop that begins with sensing and ends with action. A DSS integrates expert knowledge, manage- ment models, and timely data to assist producers with daily operational and long-range strategic decisions. SOURCE: Petersen et al., 1993. Reprinted with permission; copyright 1993, Agronomy Society of America, Crop Science Society of America, and Soil Science Society of America. The site with high variability may require a DSS in which other attributes are assessed or the whole field is subdivided to overcome the variation. Assessing the relationship between attribute variation and DSS performance has been largely ignored in relation to pest management and only superficially addressed regard- ing soil fertility. Similarly, decision support systems do not address the problem of spatial heterogeneity. This is true for weed management DSS such as HERB, WeedSOFT, and PC-Plant Protection, and for insect and disease management programs; irrigation and crop selection programs are all whole-field based. Re- searchers recently combined weed management DSS with spatial weed infesta-
42 PRECISION AGRICULTURE IN THE 21ST CENTURY tion maps to determine the value of spatial information in pest management. In these simulations, pest density at individual locations in fields was used for the infestation level input to the DSS. Lindquist et al. (in press) found that a treatment map based on spatial information (800 observations) was a great improvement over use of the mean field population. The simulation indicated that, on average, herbicide use would be reduced by 30 percent to 40 percent with such an ap- proach. Christensen et al. (1996) also found herbicide reductions of 30 percent to 40 percent when they mapped weed populations in several cereal grains in Den- mark. In each case spatial data were used to run an economic-threshold-based DSS. Although such simulations show that subfield management could lead to sig- nificant changes in management practices, numerous questions remain unan- swered. First, the issue of risk of improper decisions is a real concern to consult- ants and producers. DSS have only recently begun to be used for many large acreage crops. Their slow adoption has partly resulted from concern over risk of nonperformance. Consultants are providing a service to a client and are concerned that the client be pleased with the outcome of their service, and the producer is concerned about the real agronomic impact of uncontrolled pests and the social implications of infested fields. Another concern is that the long-term effect of spatial management on infestation level and distribution is largely unknown. Seed production by uncontrolled plants and egg or cyst production by insects and nema- todes may result in infestations growing or in spatial orientations changing in ways that make GIS maps less valuable. Such concerns require studies to assess these longer-term impacts on precision agriculture. There is also the question of the extent to which a knowledge base exists for subfield decisions. For example, relatively little is known about the suitability of crop cultivars for specific soil types or cultivar-fertility-pesticide interactions. Little is known about the interactions between agronomic practices and their en- vironment at the subfield scale. A solid knowledge base will become more impor- tant as a foundation for more information-intensive practices. Additionally, as the complexity of databases in DSS grows, the inputs needed to initiate these applica- tions will also grow. For example, two years ago, the University of Nebraska released a weed management DSS that required little information on soil type. In the most recent release, the user can determine the potential risk to ground and surface water contamination from pesticide use, but the user must be familiar with the specific soil type in that field. Also program developers will be chal- lenged to make these decision aids easy to use. In the example, county soil maps are being incorporated in the new version of WeedSOFT; the user will find the field on the county map and click on the location and the DSS will do the rest. To develop the needed database, researchers will need to approach param- eterization used to aid decision making in a new way. Rather than restricting data collection to a handful of research station field trials, researchers will have to find a way to use producersâ fields as laboratories. Harnessing spatially referenced
DIMENSIONS OF PRECISION AGRICULTURE 43 data collected on individual farmsteads makes it possible to set parameters for data sets within localized areas. Such an approach would allow DSS to incorpo- rate local parameters, which has not been possible due to the cost of parameter- ization and of programming expertise. It is likely that future development and maintenance of decision support systems will be accomplished through land grant, Agricultural Research Service, consultant, producer, and other information ser- vice provider consortia. LOOKING TO TOMORROW Information technologies have the potential to provide considerable amounts of useful information for decision making in precision agriculture. A suite of tools will be used to assess and manage agronomic factors important to crop production. For these new tools to function properly, however, they will need to be user friendly for producers and consultants. Information technologies will pro- duce enormous data sets on crops and their interactions with their environment. The challenge remains as to how to convert these data into useful suggestions to aid in the decision-making process for the producer.