Freshwater is an essential input for agriculture that uses significant quantities of the U.S. water supply. It is projected that by 2050, average farm yields will need to double in major cereal systems in order to meet expected increases in food demand (Tilman et al., 2011). Freshwater is a finite resource; thus, the necessary increases in crop agricultural productivity (yields) can only be met with a significant increase in water-use efficiency. In addition, there are competing interests for water, including energy production, domestic and industrial needs, recreation, and maintaining environmental quality. In combination with population growth and increasing extreme weather events, these factors have already resulted in significant changes for U.S. agricultural water use. Some arid regions (e.g., west Texas) have reached a tipping point with low aquifer storage unable to meet agricultural water demands, forcing the use of high-salinity groundwater (Uddameri and Reible, 2018). Additional long-tail risks to the agricultural water supply could come from energy extraction activities (such as hydraulic fracturing) or carbon sequestration activities, which may contaminate subsurface freshwater supplies and make them unusable for agriculture without costly treatment (Vengosh et al., 2014). Sustainable intensification of agriculture and the associated need for sufficient freshwater to produce food will require a shift in water sources, treatment, use, reuse, and management.
This chapter describes the challenges in ensuring the availability of freshwater and optimizing the efficiency of water use in agricultural settings
along with the scientific opportunities and gaps to overcome the challenges. There are tremendous near-term opportunities to improve water-use efficiency1 and water productivity2 through new technologies and systems-level approaches. The opportunities include (1) better use of spatial-resolution data and data science, (2) improving plant and soil properties to increase water-use efficiency, and (3) optimizing water use and reuse through systems-level management approaches and implementation of controlled environments. Improving water-use efficiency will require simultaneously applying various water-saving and water-optimizing approaches. Finally, the chapter identifies research and societal barriers that may impede progress in increasing water-use efficiency. As with the rest of this report, policies and regulations that might be needed to promote the use of these water-saving approaches are not addressed.
Crop production is water intensive, and crop productivity is dependent on the availability of water. Crop agriculture constitutes an estimated 80 percent of national consumptive water use in the United States (USDA-ERS, 2018), largely through irrigation. In 2012, irrigated farms accounted for approximately half of the total value of crop sales on 28 percent of U.S. harvested cropland (USDA-ERS, 2018). In 2010, total irrigation withdrawals averaged 115 billion gallons per day, whereas total water withdrawals for direct use in livestock production and aquaculture averaged only 2 and 10 billion gallons per day, respectively (USGS, 2016). Water stress is the largest contributor to U.S. crop loss, and low water availability affects approximately 45 percent of U.S. land surfaces (DeLucia et al., 2014). Continued access to freshwater for irrigated high-value agriculture will be critical for meeting future food demands.
While irrigation practices have improved over the past several decades—with shifts from flood to spray to pivot to drip irrigation that incrementally decrease water consumption—water productivity and water-use efficiency are well below what is achievable. Further decreases in water use to promote more sustainable crop agriculture will require a combination of revolutionary new approaches, such as second-generation drip irrigation combined with sensors and data analytics, improved and regionally based weather and seasonal climate forecasts, plants engineered to be more water
1 Water-use efficiency is defined in this report in the hydrological context; it is the ratio of the volume of water used productively (Stanhill, 1986). This is the percentage of water supplied to the plant that is effectively taken up by the plant, that is, for example, not lost to drainage or bare soil evaporation.
2 Water productivity is the yield in production per unit of water used.
efficient, smarter soils, and alternative sources of water that are accessible. This section describes important obstacles that need to be overcome in the coming decade.
2.1 Agricultural Productivity Is Dependent on Freshwater
For many regions in the United States, the current methods of agricultural water use are unsustainable. For example, groundwater aquifers store rainwater for the future, but in some locations and during periods of prolonged drought, groundwater is extracted at a faster rate than it is recharged. This has caused substantial decreases in the groundwater levels in the Central Valley of California and in the Kansas High Plains (Ogallala) aquifer in the Midwest. This has also resulted in greater pumping costs and increasingly saline waters for agriculture. The Ogallala aquifer is responsible for over 90 percent of irrigation water in the Central High Plains. Many regions of the aquifer are already depleted, and from 1960 to 2010, about 30 percent of the storage had already been consumed. It is estimated that at current use rates, as much as 80 percent of the stored water will have been consumed by 2060 (Steward et al., 2013; see also Figure 6-1). The use of water-intensive crops in regions where water is scarce (e.g., almonds and alfalfa in California), and shifts from snow-fed water systems to rain-fed water systems as the climate warms (Pederson et al., 2011), will also stress water-poor regions. This will limit agricultural productivity in the region if the area must revert back to reliance on only rain-fed agriculture. Advances in data science, sensing, modeling, water-use efficiency, and systems-level management practices can be applied to achieve sustainable water use that meet the needs of agriculture.
2.2 Spatial, Temporal, and Climate Variability
A major challenge for better water management is planning and preparedness for the high levels of spatial and temporal variability of conditions that affect water-use efficiency, such as climate variability. Man-made climate change is projected to increase the intensity of storms as well as the number and duration of dry spells, increasing uncertainty in water availability at weather timescales. Though subseasonal to seasonal forecasts are becoming more skillful, current seasonal-scale forecasts are still inaccurate (~60 percent certainty) and this can negatively impact yields (Brown and Lall, 2006). This is in part due to limited predictability of some of the phenomena underlying subseasonal to seasonal forecast predictability, such as natural modes of variability and elements of external forcing. There is also limited publicly available, spatially resolved data on water use for irrigated agriculture, such as total amounts pumped to and applied to crops. This
hinders understanding of water use and prevents making informed decisions about improving water-use efficiency. Greater reliability of forecast can help improve water management decisions, and improve operating efficiency of crops (Block and Goddard, 2012). In addition to precise use of water in agriculture, spatially resolved data on vegetative health, soil properties, nutrient concentrations, water quality parameters, and water quantity are needed to construct accurate prediction models for water-use efficiency. The data will need to be collected at a higher spatial and temporal resolution than is currently collected, especially for remote sensing methods such as satellite data.
There are a number of research opportunities that could be transformative for reducing water use in agriculture, with many opportunities based on increasing water-use efficiency in crops. However, in some systems such as controlled environment agriculture (CEA), there is the potential to simultaneously increase both water-use efficiency and water productivity. Water use is fundamentally a challenge that cuts across many components of the food system, and the opportunities below are considered in the context of other chapters in this report, for example, in developing drought-, saline-, and flood-tolerant crops; engineering soil to improve its water-holding capacity; or creating animal housing facilities with higher water-use efficiency. The following opportunities have the greatest near-term potential to improve water-use efficiency in crops: (1) prescriptive analytics for maximizing water-use efficiency, (2) improved resiliency of plants with respect to water use and soils with respect to water-holding capacity, and (3) optimized water use and reuse through alternative water sources and CEA.
3.1 Prescriptive Analytics
Prescriptive analytics leverage an understanding of system behavior to prescribe (or proactively determine) the changes needed to operate optimally. The ability to cost-effectively monitor water demand and manage water supply at high spatial and temporal resolutions presents an opportunity to improve water-use efficiency for crop agriculture. To accomplish this will require ubiquitous data collection and seamless integration of data types and analytics across vastly different temporal and spatial scales (e.g., the Global Agriculture Monitoring project [CGLAMR, 2018]). Sensors on unmanned aerial vehicles that can move and measure at different locations, on-ground sensors, and satellite data can also be used together to provide higher resolutions of multiple parameters. The development of scalable and cost-effective technologies (e.g., sensors) would improve water-use management by allowing for more precise monitoring of moisture content at high spatial resolution over vast scales. These technologies could be combined with improved remote sensing capabilities at higher spatial resolutions to monitor for soil moisture, groundwater levels, or plant health. Water pricing and policy will need to provide the incentives required to promote adoption of these water-saving technologies (Olen et al., 2016).
Improved data collection, management, and sharing about water availability (quantity and quality), water demand, and water use at high spatial and temporal scales will be important for improving water-use efficiency in agriculture. These data can support ecohydrology-based approaches to water management. For example, improved seasonal climate forecasts
can be used to predict future water quantities and the viability of various cropping systems. As the number of plants with different traits increases (see Chapter 2), that could also enable farmers to select the most climate-appropriate varieties of those crops (such as drought-tolerant cultivars). In addition, enhanced prediction of extreme and disruptive events as well as inclusion of more components of the Earth system in subseasonal to seasonal forecast models will better enable water use and management (NASEM, 2016).
A better understanding of the interactions between agroecosystems, natural ecosystems, and the built environment can help to improve water-use efficiency and water productivity. The ability to collect on-ground sensor data at high spatial resolution provides an opportunity to better understand the coupled nature of agroecosystems and natural ecosystems, and to refine physics-based (mechanistic) models describing system behaviors. Understanding the role of sociological and economic factors in water use for agriculture has the potential to further improve water management practices using integrated systems approaches (Stuart et al., 2015). This ability will help to limit unintended consequences to the environment, human activities, and the economy that may result from reducing water use.
3.2 Soil and Plant Resiliency
Water-use efficiency can be improved through better soil management practices and by engineering soil properties to maximize water availability to plants. Promising opportunities include manipulating the soil microbiome to improve water use by plants (especially under drought or salt stress conditions), increasing soil carbon through additions of engineered materials, and improving soil management (e.g., no till). There are also opportunities to improve water-use efficiency through real-time monitoring of plant health.
The role of the soil and plant microbiomes in affecting water availability to plants is largely unexplored. However, this ability could be exploited through development of new approaches to controlling evaporation from soils (a portion of water consumption from evapotranspiration) while enhancing water uptake by plants, particularly during periods of salt or drought stress (Marasco et al., 2012). Any improvements to promote water availability to plants at higher soil tension would increase water efficiency in soils. However, these advances cannot diminish other important functions of soils (see Chapter 5): storing organic carbon and other nutrients, filtering contaminants and pathogens, provision of nutrients, biodiversity habitat, and hydrological buffer.
McBratney, 2017). Cost-effective methods to add natural or engineered carbon to soil coupled with best practices to manage water retention in soils (cover crops and no-till agriculture) provide a method to improve water-use efficiency (Turner, 2004). Better understanding of the optimal properties and sources of carbon to add to soils could lead to better engineered soils with respect to water-use efficiency (e.g., moisture-controlled water availability).
Spatially resolved real-time monitoring of plant health may also improve precision watering. Plant nanotechnology can provide methods to better monitor plant health status (Giraldo et al., 2014). Embedding nanomaterials in sentinel plants provides a signal that can be monitored remotely, potentially allowing the plants to communicate their health status with respect to water and nutrient availability. These data along with soil moisture content and other measurements will help to deliver water only where it is required.
3.3 Controlled Environments and Alternative Water Sources
Water-use efficiency can be improved by decentralizing parts of food production (e.g., fruits and vegetables) and better coupling it with the built environment. CEA offers systems-level opportunities to increase the sustainability of agriculture by providing resource-efficient farming systems with respect to water and nutrient use. For example, vertical farming of lettuce in controlled environments uses approximately 10 percent of water compared to traditional crop production in Yuma, Arizona (Barbosa et al., 2015). Fish production in recirculating aquaculture systems or aquaponics systems is highly efficient with respect to water use and enables reuse of nitrogen (waste) from fish production for plant production. These different types of CEAs also provide consistent year-round yields, fewer challenges with pests and disease, and the potential for high nutrient-use efficiency (Bregnballe, 2015; Harwood, 2017). These systems are currently used for producing fish protein as well as high-value crops such as vegetables, fruits, and herbs. Improvements in sensor technology, robotics, and artificial intelligence to monitor nutrient levels and plant health make controlled environments feasible and attractive. The integration of CEA within the built environment offers an additional opportunity to recover resources (e.g., water or nitrogen) from domestic or industrial sources, making this alternative more sustainable.
4.1 Lack of Accurate Methods and Forecasts
A better understanding of the flows of water in the agroecosystem at high spatial and temporal resolution can be used to develop more accurate models for predicting and improving water-use efficiency. Deploying sensor networks will require development of small, biodegradable, inexpensive, energy-efficient disposable sensors. They need to provide depth-resolved soil moisture or nutrient levels to allow for more precise irrigation and nutrient applications. In planta sensors are needed to monitor plant health status with respect to water requirements, potentially at the scale of single plants.
Data on crops, soils, water, climate, and more are collected at multiple spatial and temporal scales using different techniques. Different types of data are stored in different databases, and even for a particular variable may be reported in different units by different sources. Better methods are needed to harmonize disparate datasets, to cross-reference data collected at different spatial scales, and to verify data and model veracity.
Data-driven analytics for Internet-of-Things–enabled water use-efficiency decision support systems will benefit from efficient cloud computing and local analysis due to the difficulty in mobilizing and storing high volumes of data. Automated methods for quality control of data and improved communications between sensors to activate them only when required will be essential. For example, a sensor upstream could communicate with a sensor downstream to prompt it to begin to collect data at a higher rate when spikes of salinity or nutrients are expected (McGuinness et al., 2014). “Smarter” sensors to reduce the data collection and processing demands and methods to distribute the processing of data across all of sensor platforms (e.g., Edge Computing) will also be needed for real-time monitoring and responses to needs or perturbations in the system.
Several research questions need to be addressed:
- How can we develop and deploy sensor networks at high spatial and temporal scales to optimize water use at the system level?
- How can we integrate ground-based and remote sensing data to best manage water use?
- How can we improve computing speeds, data analytics, and data sharing to provide weather forecasts at high spatial and temporal resolution (<100 m)?
- How can we improve relevance and thus use of seasonal climate forecasts?
4.2 Lack of Understanding of Plant and Soil Property Impacts on Water-Use Efficiency
There is a growing body of literature indicating that the soil microbiome changes in response to water availability (e.g., Manzoni et al., 2012), yet, there has been less emphasis on identifying how to manipulate the soil microbiome to enhance water availability and water-use efficiency. Significant improvement in water-use efficiency can also be achieved by controlling the stomatal opening in plants through genetic modification or using plant growth regulators (Moshelion et al., 2015; Glowacka et al., 2018). However, a limited understanding of the genes and plant physiology that control water uptake and transpiration, as well as drought and salinity resistance, hinders rapid selection of plants that are more water efficient, enabling the use of lower-quality water for agriculture. Improved understanding of how to control the rate of stomatal closing (e.g., in periods of shading) could be used to decrease transpiration without altering biomass production, food production, or nutritional content. Systems biology approaches to modeling water flows in agroecosystems may provide new approaches to water management.
While increasing soil carbon has the potential to improve water-use efficiency by lowering evaporation or drainage losses, the magnitude of the increases in water-use efficiency that this may provide is under debate. The mechanisms behind the effects of carbon on water availability to plants and water-use efficiency require more careful assessment (Minasny and McBratney, 2017).
Plant sensor technologies have great potential to monitor water-use efficiency. However, advances will require the development of appropriate devices (e.g., disposable biodegradable sensors) and practical methods to emplace nanomaterials into plants. The impacts of these sensors on plant health and food quality will also need to be determined.
Several research questions remain to be addressed:
- What are the roles of the soil microbiome and plant traits in controlling water-use efficiency?
- What are efficient methods to increase soil carbon content to improve water-holding capacity of soils?
- How can plant and soil sensors be used together with weather forecasts, plant selection, and other technologies to optimize water use?
4.3 Need for Alternative Water Sources, Controlled Environments, and Sustainable Management
Alternative sources of water are available for agriculture, including treated wastewater, stormwater runoff, and water from energy production (produced water). It remains to be determined if contaminants in these waters (e.g., organic chemicals, heavy metals, salts, and radioactive elements) pose a significant risk to agriculture production, especially if they are deployed in the long term. In general, there is poor understanding of how to manage water systems for multiple purposes, which may ensure the availability of water for agriculture at the time it is needed and improve environmental quality and social welfare (Schoengold and Zilberman, 2007). The system-level consequences of using alternative sources of water for agriculture rather than through the existing treatment and disposal options have not been determined on a broad scale in the United States.
Water savings in CEA are proven; however, there are gaps in understanding of how these approaches affect overall system sustainability. The market potential and extent of cropping systems that can be utilized in CEA need to be determined. Currently, CEA and recirculating aquaculture systems are energy intensive so better energy management strategies are needed. The cost of the various CEA systems must also decrease to become competitive with traditional agriculture and imports. Also, the primary socioeconomic drivers that would promote acceptance of CEA need to be determined. The optimal approaches for lighting and cooling systems, temperature control, feed, disease management, and resilient systems (for aquaculture) and the optimal plant and fish traits for controlled environments still need to be determined to make these systems scalable, economically viable, and sustainable. There are still gaps related to coupling CEA systems to the other elements of the built environment to close the loop on nutrient and water use.
Alternative sources of water and controlled environments can improve water-use efficiency and water productivity. However, several research questions must be addressed to realize these opportunities:
- What alternative water sources and system-level management practices are available for agriculture?
- How can controlled environments be sustainably designed to lower water use and increase water productivity?
5.1 Water Policies and Pricing
Adoption of water-saving opportunities and water management practices will require new policies and realistic water pricing for agriculture as well as the political will to enforce policies and prices across states and international boundaries. This includes revising “water rights” to encourage climate-appropriate crop choices and to optimize overall system performance with respect to water use. It may also require significant upgrades to water infrastructure to meet future water demands for agriculture. Water storage practices, interbasin transfers, and water distribution systems are currently inadequate to meet future water demands for agriculture. The lack of adequate water storage increases the vulnerability of the agricultural system to droughts, especially in arid regions.
5.2 Competing Demands for Water
Agriculture is the largest water consumer in the United States, but energy production also requires access to vast amounts of water. Given that agriculture is a high-volume, low-economic-value user of water, competing demands for water with higher economic value (such as energy production) may ultimately decrease the amount of water available for agriculture. This could prevent achieving the goal of using only renewable water for agriculture.
5.3 Limited Resources for Interdisciplinary Teams
Improving water-use efficiency in agriculture will require the simultaneous development and integration of multiple technologies and technological approaches from different research communities (see Box 6-1). Successful integration of these technologies will require adequate research funding for highly interdisciplinary teams focused on the opportunities described in this chapter. Currently, there are limited opportunities for the necessary convergence of disciplines needed to tackle these complex problems.
5.4 Public Acceptance of New Technologies for Water-Use Efficiency
Many new opportunities to improve water productivity will require new policies as well as consumer acceptance to implement direct water reuse, multiple-use scenarios, use of impaired waters, and integrated agroecosystems and natural ecosystems. New technologies (e.g., genetic modifications of plants using CRISPR-Cas9 or nanotechnology for soil and plant
sensors) are often viewed with skepticism and fear, which can be magnified when they are applied to food. Public and stakeholder engagement at all stages of technology development is needed to promote technology adoption. Research is needed to better understand the factors that would affect public acceptance of alternative water sources and new technologies.
5.5 Lack of a Systems Approach
A systems approach to water management is needed to mitigate the potential for unintended consequences to the natural ecosystem and built environments. Proposed interventions need to account for their impacts at large spatial scales (e.g., watershed, basin, or interbasin scales). The interactions between the elements of agroecosystems need to be better elucidated (see Chapter 8), and the development of appropriate models and systems-level analytics is needed in conjunction with technology advances for increasing water-use efficiency.
Ensuring availability of water for agriculture is essential to meeting future demands. Emerging sensing technologies can provide real-time information about water availability and demand at high spatial and temporal resolution. Advances in data science and better integration of data and models will vastly improve the ability to predict water needs and optimize water-use efficiency. A better understanding of the soil properties affecting water availability and the physiological factors affecting plant water use—coupled with technologies that can modify soil, microbiome, and plant properties with respect to water use—will further enhance water-use efficiency. The use of controlled environments for some agriculture could meet the goal of making water use for agriculture sustainable. Realizing these opportunities will require a systems approach to water use and will require implementing a combination of these strategies simultaneously. Some high-priority opportunities for improving water-use efficiency include
- Increasing water-use efficiency by implementing multiple water-saving technologies across integrated systems;
- Lowering water use through applications of prescriptive analytics for water management;
- Lowering water demands by improving plant and soil properties to increase water-use efficiency; and
- Increasing water productivity by use of controlled environments and alternative water sources.
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