Optimizing Irrigation for Agricultural Water Management: Scientific Principles

Dr. John Letey

Professor Emeritus, University of California, Riverside

Introduction

The basic principles of irrigation are quite simple. However, the practical application of these principles can provide complications. Crops are exposed to energy originating from the sun, and/or wind. Much of the energy is dissipated by evaporating water from leaf surfaces, which is commonly referred to as transpiration. Water may also be evaporated from the soil surface and the combination of evaporation from leaf and soil surfaces is evapotranspiration (ET). Water transpired from the leaf surface is replaced by extracting water from the soil and transporting it through the root and stem system.


The soil serves as a reservoir from which the plant extracts water to meet the demands of transpiration. The quantitative storage capacity depends on both the type of soil and the rooting depth of the crop plants. Soil texture and structure which result in large pore sizes, have lower storage capacity than soils with smaller pore sizes. More deeply rooted crops have a higher storage capacity as compared to shallow rooted crops. Irrigation is the practice of recharging the storage capacity that has been depleted by ET when natural precipitation is not adequate to meet the ET demands. The time to irrigate is usually before the soil is too dry for water to be extracted by the plant at a rate to meet the transpiration rate. If the rate of water movement to the leaf surface is less than the rate of transpiration, the plant responds by closing stomata to reduce the water loss. Carbon dioxide (CO2) used for plant photosynthesis enters the plant through these same stomata. Therefore, closure of the stomata reduces water loss but also reduces CO2 intake and therefore reduces the rate of photosynthesis. Thus the plant has a dual mechanism for protecting itself under a limited water supply. It reduces transpiration, thus attempting to maintain turgidity. At the same time it reduces photosynthesis, which would otherwise increase plant surface area causing greater interception of energy and more transpiration.


Numerous studies have reported that total dry matter production in plants is linearly related to ET. This observed result is consistent with the mechanism that the plant uses to protect itself against inadequate water. However, the part of some crops that is sold is not linearly related to total dry matter production. For these crops the marketed product might be achieved or possibly increased by allowing the soil to dry to a level reducing transpiration at specific time periods. Dr. Goldhamer will present the application of this concept.


The combination of a soil and crop rooting system that has a low storage capacity dictates the use of smaller quantity, more frequent irrigations; whereas a larger storage capacity allows less frequent though higher quantity irrigations.


All irrigation waters contain some dissolved salts. Plants transpire pure water causing the soil solution to become concentrated with salts as transpiration proceeds. Because of this effect, occasionally irrigation must not only recharge the storage capacity, but additional water may be necessary to leach excessive salts from the root zone. The concentration of salts in the irrigation



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Agricultural Water Management: Proceedings of a Workshop in Tunisia Optimizing Irrigation for Agricultural Water Management: Scientific Principles Dr. John Letey Professor Emeritus, University of California, Riverside Introduction The basic principles of irrigation are quite simple. However, the practical application of these principles can provide complications. Crops are exposed to energy originating from the sun, and/or wind. Much of the energy is dissipated by evaporating water from leaf surfaces, which is commonly referred to as transpiration. Water may also be evaporated from the soil surface and the combination of evaporation from leaf and soil surfaces is evapotranspiration (ET). Water transpired from the leaf surface is replaced by extracting water from the soil and transporting it through the root and stem system. The soil serves as a reservoir from which the plant extracts water to meet the demands of transpiration. The quantitative storage capacity depends on both the type of soil and the rooting depth of the crop plants. Soil texture and structure which result in large pore sizes, have lower storage capacity than soils with smaller pore sizes. More deeply rooted crops have a higher storage capacity as compared to shallow rooted crops. Irrigation is the practice of recharging the storage capacity that has been depleted by ET when natural precipitation is not adequate to meet the ET demands. The time to irrigate is usually before the soil is too dry for water to be extracted by the plant at a rate to meet the transpiration rate. If the rate of water movement to the leaf surface is less than the rate of transpiration, the plant responds by closing stomata to reduce the water loss. Carbon dioxide (CO2) used for plant photosynthesis enters the plant through these same stomata. Therefore, closure of the stomata reduces water loss but also reduces CO2 intake and therefore reduces the rate of photosynthesis. Thus the plant has a dual mechanism for protecting itself under a limited water supply. It reduces transpiration, thus attempting to maintain turgidity. At the same time it reduces photosynthesis, which would otherwise increase plant surface area causing greater interception of energy and more transpiration. Numerous studies have reported that total dry matter production in plants is linearly related to ET. This observed result is consistent with the mechanism that the plant uses to protect itself against inadequate water. However, the part of some crops that is sold is not linearly related to total dry matter production. For these crops the marketed product might be achieved or possibly increased by allowing the soil to dry to a level reducing transpiration at specific time periods. Dr. Goldhamer will present the application of this concept. The combination of a soil and crop rooting system that has a low storage capacity dictates the use of smaller quantity, more frequent irrigations; whereas a larger storage capacity allows less frequent though higher quantity irrigations. All irrigation waters contain some dissolved salts. Plants transpire pure water causing the soil solution to become concentrated with salts as transpiration proceeds. Because of this effect, occasionally irrigation must not only recharge the storage capacity, but additional water may be necessary to leach excessive salts from the root zone. The concentration of salts in the irrigation

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Agricultural Water Management: Proceedings of a Workshop in Tunisia water and the crop tolerance to salinity dictate the amount of leaching. Irrigation management entails consideration of salinity as well as water content in the root zone. Water Application Technology A canal, well, or pond may serve as the irrigation water supply. A means of transferring the water from the source to the root zone must be designed. Irrigation systems can be broadly characterized as being pressurized or nonpressurized. Pressurized systems are those in which water is delivered through a pipe under pressure and discharged by one of a variety of different outlet designs including sprinkler heads and drip emitters. Nonpressurized systems are those in which water is delivered and allowed to flow across the field. Various configurations for flow across the field such as furrows or basin borders are possible. All irrigation systems deliver water to the soil surface from which it must infiltrate into the soil to recharge the soil-root storage capacity. The infiltration rate therefore becomes an important factor in irrigation management. Nonpressurized irrigation systems deliver water in a manner that causes free-standing water on the soil surface. The following description of water infiltration into soils applies for the case of free-standing water on the soil surface and is therefore appropriate for nonpressurized irrigation systems. The infiltration rate decreases with time and approaches a constant steady state rate as depicted in Figure 1. The soil water content at the time of water application affects the initial infiltration rate. The initial infiltration rate is more rapid for a dry as compared to a wetter soil. The infiltration rate is largely controlled by the soil pore size with the most rapid infiltration occurring with soils with large pores. Therefore, different soils have different infiltration rates. Furthermore, the infiltration rate of a given soil can be modified by tillage operations. For example, a soil that is plowed and tilled to produce a seedbed at the beginning of the season has high infiltration rates. However, the process of wetting the soil causes the soil aggregates to disperse and become more compact and thus the infiltration rate for many soils decreases with successive irrigations after tillage. Modification of irrigation management during the growing season to accommodate changes in soil infiltration characteristics is very difficult to program. FIGURE 1 The relationship between infiltration rate (I.R.) and time for a soil and different initial degrees of wetness.

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Agricultural Water Management: Proceedings of a Workshop in Tunisia Water cannot infiltrate the soil at a rate greater than which it is applied. Pressurized irrigation systems can be designed to deliver water to the soil surface at a prescribed rate. A well-designed system would not deliver the water to the soil surface at a rate more rapid than it can infiltrate. Therefore, runoff from the field can be avoided with pressurized systems. Furthermore consideration can be given to the changes in infiltration rate during the season and designed not to surpass the infiltration rate at any time during the growing season. Pressurized systems have the advantage of having complete control on the amount of water applied and infiltrated by valves. Conversely, the amount of water that infiltrates the soil and potentially available for crop production for nonpressurized systems is partially dependent on soil characteristics. Although the irrigator has some design and control features for nonpressurized systems such as length of furrow, rate of water discharge, and time (duration) of water application; the quantitative control is limited. Runoff from the field is usually an unavoidable condition for nonpressurized systems. Water must be maintained on the soil surface at the lower end of the field to allow adequate infiltration. During that period of time, water is flowing off the field. Irrigation uniformity adds complexity to irrigation management. A uniform irrigation is one in which the same amount of water infiltrates the soil at all points in the field. Most frequently the amount of water that infiltrates into the soil is variable for different parts of the field. Nonuniform irrigation creates a dilemma. If irrigation is programmed to restore the storage capacity in the parts of the field that receive the most water, the other parts of the field will be under irrigated, causing yield reduction in those under-irrigated areas. Conversely, if irrigation is programmed to recharge the storage capacity zones with the lowest infiltration rate, the other parts of the field will be excessively irrigated, leading to unrecoverable water loss to deep percolation. Uniformity of irrigation therefore is one of the most critical factors affecting irrigation management. Properly designed and maintained pressurized systems can deliver water very uniformly across the field. However, if water is emitted into the air, such as through sprinkler systems, the wind currents can greatly disturb the distribution of the water. Therefore, although sprinkler systems can be designed to be very uniform, wind can cause a very nonuniform distribution. Pressurized systems that do not spray water into the air, such as drip systems, allow for very uniform irrigation. Nonpressurized irrigation systems have two sources of nonuniformity. First, water is on the soil for a longer period of time at the top than the lower end of the field. This provides the opportunity for more water to infiltrate at the top end as compared to the lower end of the field. This nonuniformity is referred to as opportunity time nonuniformity. Variability of soil infiltration rate across the field due to textural or structural differences also leads to nonuniform infiltration across the field. The total nonuniformity is a combination of the opportunity and soil variability. Accurate measurement of irrigation uniformity is important in developing the optimal management scheme for a given irrigation system. Unfortunately, measurement of uniformity is complex. Uniformity of sprinkler systems is measured by distributing containers throughout a

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Agricultural Water Management: Proceedings of a Workshop in Tunisia collection area and measuring the amount of water collected in each container. The data are then statistically analyzed for variability. The numerical result depends on the size of the container. Using large containers will result in a high uniformity value than using smaller containers under the same conditions. Thus, the value is already recognized as being somewhat subjective based on the measuring technique. Even a drip system of soil water is high at the location adjacent to the emitter and the amount of water between the emitters is very low. Nevertheless, depending on the type of plant, the plant root system can integrate different parts of the root zone and effectively even things out. This factor raises an additional point concerning uniformity. The plant root system can accommodate uneven water distribution and can extract more water where the soil water content is high. A tree with a large root system can accommodate considerable nonuniformity of the water application under the canopy. A shallow rooted vegetable crop would be more highly impacted by the same distribution. The nonuniformity for surface systems is determined by measuring the rate of advance of water down the furrow, and then inserting these numbers into equations developed to compute the nonuniformity associated with opportunity time. These measurements do not include the nonuniformity associated with soil variability, which can be considerable. Therefore, the numerical values for furrow systems are overestimates of the true uniformity. As long as the measurement procedure for a given irrigation system is consistently used, the comparative uniformity of different fields with that particular system can be determined. In other words, the uniformity from one sprinkler system can be compared to a different sprinkler system. However, it is not appropriate to compare a uniformity coefficient that has been measured for a furrow system to a sprinkler system. With all the factors considered, the drip system has better uniformity than a sprinkler system, which is better than a nonpressurized system. However, the costs for the systems are in the reverse order. The improved performance from a drip system may not justify the cost for the drip system. Furthermore, pressurized systems require an energy supply that may not always be present in the field. Water Use Efficiency and Water Conservation: Definitions Water use efficiency and water conservation are commonly used terms applied to irrigated agriculture. However, confusion and misunderstanding can arise because these terms have multiple definitions. Water use efficiency is calculated by a ratio of terms which is then multiplied times a hundred to report the efficiency as a percentage. Different measurements may be used in the ratio resulting in different numbers and yet all may be referred to as water use efficiency. For example, water use efficiency can be defined as a ratio of beneficial water use to the applied water (AW) to the field. However, beneficial use sometimes is defined as ET and at other times it is ET plus the amount of water required for leaching salts from the root zone. Some individuals include all of the water delivered to the field as AW, whereas others might only consider the infiltrated water that would be available for crop use. For example, in a surface irrigated system, the runoff may be considered as part of the AW, or it may be subtracted from the AW. Obviously different numbers result for different combinations of these terms. Furthermore the computations can be made on different area sizes. For example the ratio can be

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Agricultural Water Management: Proceedings of a Workshop in Tunisia calculated for a field, the total farm, or the total basin. Different numbers result depending on which is selected. Possibly the biggest problem however is the common belief that a higher efficiency number is always better than a lower number. Any number less than 100 percent is considered to have some degree of inefficiency. As will be discussed later in this paper, this is not usually the case. Water conservation, likewise, is subject to different definitions. One definition is to use less water. This can be accomplished by various means, each with a specific consequence. A farmer can use less water by not growing a crop. Or, a farmer can grow a crop and apply a small amount of water resulting in very low crop production. Farmers can also grow a crop for high production and either eliminate runoff or capture runoff and use it as part of the irrigation supply. The main point is that in using these terms, the definition must be clearly specified and the consequences of the action properly evaluated. A further complication is that all of the definitions are based on water quantity without reference to water quality and water quality cannot be ignored in a management scheme. A distinction between water use and water consumption is required to properly assess water conservation practices. Water consumption is water that is lost for future use. For example, ET is water consumption. It is water that is lost and not available until it is returned again in the form of precipitation, usually at a different location. For a nonpressurized irrigation system with runoff from the field, part of the water is consumed through ET and part is used but runs off and is available for other uses. Water that percolates beyond the root zone might be consumed or used depending upon the fate of that water. If the water migrates to groundwater or a stream, it is still available for use and therefore not consumed. Conversely, if it migrates to a location where it cannot be retrieved, it should be considered as consumed. Crop-Water Production Functions Agricultural production is a business operation and irrigation management can be evaluated in context of the business. The goal of any business is to maximize profits. Maximizing profits can include sustaining the business through a period when profits are not possible and generating growth in anticipation of future profits. Therefore, one definition of optimal irrigation management is that management which maximizes profits. Water is an input to the production system. The functional relationship between crop yield that is marketed and AW must be known for the economic analysis. Only water that infiltrates into the soil has an opportunity to contribute to crop production. The water running off the field cannot contribute to crop production so the crop-water production function can only be based on the amount of infiltrated water (IW). The runoff water has economic implications that must be accounted for separately. The uniformity of irrigation significantly affects the crop-water production function. The relationship between cotton lint yield and IW for various irrigation uniformities for climatic conditions of the San Joaquin Valley of California is presented in Figure 2. The uniformity is characterized by the Christensen’s Uniformity Coefficiency (CUC), where a value of 100

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Agricultural Water Management: Proceedings of a Workshop in Tunisia represents perfectly uniform irrigation and decreasing values of CUC represent increasing nonuniformity. Also depicted in Figure 2 is the amount of water percolating below the root zone for the various CUC and IW amounts. Considering the uniform irrigation first, crop yields increased with increasing amount of IW until a maximum yield is achieved and additional AW does not contribute to more production. When the irrigation is nonuniform, more average water must be infiltrated to get the highest yields. For a given amount of IW, the yield decreases as the irrigation uniformity decreases. FIGURE 2 The relationships between relative yield and deep percolation to the amount of infiltrated water for three levels of Christensen’s Uniformity Coefficient. Under uniform irrigation no deep percolation occurs until water has been applied to reach the maximum yield. In other words, all of the water is used by the crop. After maximum yield has been achieved, any additional AW goes directly to deep percolation. In contrast for nonuniform irrigation, some deep percolation occurs before a maximum yield is achieved. This is a consequence of some parts of the field having more water than necessary for maximum crop yield and other parts of the field having less water than required. In all cases increasing IW increases the amount of deep percolation. For a nonuniform irrigation there is a trade-off between irrigating for high crop yield and low deep water percolation. The salinity of irrigation water is another factor that affects the crop water production function. The salts in the irrigation water become concentrated through evapotranspiration and therefore some water must be applied to leach the excess salts from the root zone. The amount of water to be applied, however, depends on the salinity level of the irrigation water and the crop sensitivity to salinity.

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Agricultural Water Management: Proceedings of a Workshop in Tunisia FIGURE 3 The relative yield of corn and cotton as a function of applied water (AW) scaled by pan evaporation (Ep) for waters of different salinities. The number on each curve represents the irrigation water salinity in dS/m. Letey et al. (1985) developed a model to compute the relationship between relative crop yield and the seasonal applied water for crops with different tolerances to salinity. The crop-water

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Agricultural Water Management: Proceedings of a Workshop in Tunisia production functions are presented in Figure 3 for corn, which is a salt sensitive crop, and for cotton lint, which is a salt tolerant crop. The applied water (AW) assumes that all of the water infiltrates and EP is the pan evaporation. The ratio AW/EP may be used to facilitate comparison between different climatic zones with different EP values. The relationships for wheat are presented in Figure 4. FIGURE 4 The relative yield of wheat as a function of applied water (AW) scaled by pan evaporation (Ep) for waters of different salinities. The number on each curve represents the irrigation water salinity in dS/m. Note as the salinity of the irrigation water increases, more water must be applied to get the same yield. Irrigation water salinity may reach a level where maximum yield cannot be achieved regardless of the amount of water applied. Larger differences in yield for a given amount of AW or larger differences in AW for a given yield occur for the salt sensitive corn than the salt tolerant cotton. Indeed for cotton, irrigation water salinities up to 4 dS/m require relatively small amounts of additional water to achieve the maximum yields. Note that wheat is salt tolerant and the relationships are similar to those of cotton. All of the curves depicted in Figures 3 and 4 assume that the irrigation is uniform. Nonuniform irrigation would modify the results in a manner as depicted in Figure 2. In other words, increasing nonuniformity would cause each of the curves in Figure 3 to be lower for a given water application. Economic Irrigation Efficiency Because of deficiencies in the traditional water use efficiencies, as stated above, a different criterion must be used to characterize the optimal irrigation management. Since agriculture

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Agricultural Water Management: Proceedings of a Workshop in Tunisia production is a business operation, maximizing economic irrigation efficiency would be an appropriate goal. Economic irrigation efficiency is defined at the farm level as the irrigation management that maximizes profits. In a broader context economic irrigation efficiency could be defined as irrigation management that maximizes net social benefits. The difference between the two definitions is the result of externalities. An externality arises when some of the costs or benefits of irrigation agriculture accrue to society as a whole and the costs (as reflected in market prices) are not borne by the farmers or the consumers of their products. Externalities can be positive or negative. An example of a positive externality occurs when water purchased by a farmer runs off his farm and serves some beneficial societal use. However, if the water is polluted it can impose a cost to society and create a negative externality. The crop-water production functions as depicted in Figures 2, 3 and 4 can be converted to benefit curves by multiplying the yield by the market price for the crop. Such curves are depicted in Figure 5 as a hypothetical case representing two irrigation uniformities. The total benefit (TB) in $ ha−1 for a given IW is higher for the more uniform (TB1) than the less uniform irrigation (TB2). The total cost of water (TC) is also depicted in Figure 5 where the price of water for case 1 (TC1) is greater than for case 2 (TC2). FIGURE 5 The total benefit (TB) and average benefit (AB) as a function of infiltrated water (IW) for two levels of irrigation uniformity, when irrigation uniformity is more uniform for case 1 than case 2. Also presented is the total cost for water (TC) as a function of IW when the cost for water is higher for case 1 than case 2. The arrows represent IW that maximize profits. The highest profit is achieved where the differences between TB and TC is the greatest. These points are identified by arrows in Figure 5. Some general conclusions can be derived from the information depicted in Figure 5. The economically optimal (profit maximizing) level of IW depends on the shape of the crop-water production function and the price of the water. Improving the uniformity of irrigation results in a decrease in the value of IW that achieves economic

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Agricultural Water Management: Proceedings of a Workshop in Tunisia efficiency. Also raising the price of water lowers the value of IW that achieves economic efficiency. Raising the price of water has a greater effect on decreasing the economically efficient IW value under the nonuniform irrigation system as compared to the more uniform irrigation system. Indeed raising the price of water had relatively little effect on changing the economically efficient level of IW for the most uniform system. A shift in irrigation technology or management to achieve more uniform irrigation usually imposes a cost. The increased cost may not be offset by the increased benefits associated with improved irrigation uniformity to justify the investment. This factor must be evaluated on a case by cases basis. The average benefit (AB) as a function of IW is also depicted in Figure 5 for the two irrigation uniformities. AB is calculated as TB divided by IW and has the units of $ (ha cm)−1. AB is the average dollar return per hectare-centimeter of IW. Note that the maximum AB value occurs at a lower IW value than the economically optimal quantity. Even though the average return decreases, the cost for additional irrigation water is exceeded by additional return associated with an incremental increase in water. Economically optimal input is at the point where the marginal benefit equals the marginal costs. The marginal benefit is defined as the benefit associated with the next incremental increase in input and likewise the marginal cost is the increase in cost associated with the next incremental input. The marginal benefit is the slope of the TB curve and the marginal cost is the slope of the TC curve depicted in Figure 5. Note in Figure 5 that the slope of the TB curve equals the slope of the TC curve for the optimal irrigation as indicated by the arrows. The shape of the AB curves in Figure 5 is identical to the shape of the ratio of yield (Y) to infiltrated water (IW). This ratio (Y/IW) is a common definition of water use efficiency. Note that the IW that achieves maximum water use efficiency by this definition is not the IW that is economically efficient. Irrigating to achieve the maximum commonly defined water use efficiency results is a significant reduction in yield. Clearly maximizing water use efficiency by this traditional definition leads to results that are not the most economically beneficial. Therefore, as stated above, increasing the water use efficiency may not be a positive goal. A shift in production function from less uniform to more uniform irrigation does result in higher water use efficiency for a given value of IW. Therefore increasing the numerical value of water use efficiency by a change in management that entails a change in production function is positive. However, it is not necessarily economical. It is not obvious that the shift in production function from the less uniform to the more uniform irrigation is economically efficient. The main conclusion is that generalizations cannot be made, and each situation has to be thoroughly evaluated from a production and economic consideration. Irrigation Scheduling Irrigation scheduling refers to the time, duration, and quantity of an irrigation. Although crop water production functions as depicted in the figures provide the scientific and economic basis for optimizing irrigation, farmers do not have such complete detailed information available to guide their irrigation management. Nevertheless, the general principles still apply. Since the

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Agricultural Water Management: Proceedings of a Workshop in Tunisia purpose of irrigation is to replace the water lost from the storage zone between irrigations, knowing the amount of ET that has occurred since the last irrigation is important. Alternatively, the farmer could monitor the soil water content as a function of time to determine when the soil is sufficiently dry to warrant recharge. Therefore, a method of monitoring either ET or the change in soil water content is required for irrigation scheduling. Climatic conditions drive ET, therefore monitoring the potential ET by an evaporation pan or from other climatological data. In California several weather stations have been established throughout the state to form a California Irrigation Management Information System (CIMIS). Farmers with computer systems can get daily information from the weather station located nearest to their farms. The climatological data identify the potential ET. The crop ET is not always equal to the potential ET. For example, during the early part of the season for annual row crops, the plant is small, and the crop ET is much less than the potential ET. As the crop grows and the canopy cover increases, the crop ET approaches potential ET. Thus, to estimate crop ET the potential ET is multiplied by a crop coefficient (Kc), which must be empirically determined for each crop as a function of time. Studies have established the crop coefficients for several crops in California. Results from these studies can be used to guide irrigation management. Nevertheless, the study results are not absolute and the farmer must use judgment and make observations in the field to be sure that his irrigation is appropriate. Monitoring the soil water content as a function of time requires instrumentation. The neutron probe can be used to measure the water content in the soil profile, but this method is labor intensive. It requires the installation of neutron probe access tubes and then measurements on some predetermined schedule. Other instruments such as tensiometers can be installed at various depths and require reading on a timely basis. Some of the instruments have electrical signals that can be connected to a recorder for continuous monitoring with minimal labor input. Soil moisture monitoring requires capital investment and then some level of operational expense. Policy Issues Related to Irrigated Agriculture Water Pricing One belief, particularly in the United States, is that agricultural irrigation water is under priced and that if the cost for water was increased, it would induce the farmers to improve their irrigation management. A decrease in the optimal IW as a cost for water increases can be observed in Figure 5. However, based on the shape of the crop-water production function, cost of the water, and the sale price of the crop, the decrease in optimal IW may be relatively small. The effect of cotton and water prices on the optimal AW and the gross returns net of water cost to the farmer were calculated for a measured crop-water production function for cotton grown in the San Joaquin Valley of California. In this case it was assumed that all of the AW infiltrated and there was zero runoff. The results are summarized in Table 1. Note that increasing the water price or decreasing the cotton price decreased the optimal value of AW. However, the differences in optimal AW are very small and the main effect was in reducing the profitability of

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Agricultural Water Management: Proceedings of a Workshop in Tunisia growing the crop to the farmer. Indeed the computed differences in optimal AW are smaller than the degree of precision that the farmer has in controlling AW. TABLE 1 Effect of cotton sale price and water price on the economically optimal water application and gross monetary returns net water costs (GR). Cotton Prices $/kg Water Prices $/cubic meters Optimal AW centimeters GR $/ ha 0.32 59,207 84.1 0.031 0.32 118,414 80 0.025 0.23 59,207 82.6 0.021 0.32 250,398 71 0.001 The computations reported in Table 1 are based on no runoff from the field. If the farmer had runoff from the field, the increase in water price could provide an incentive for reducing or eliminating runoff from the field. But in this case, since the runoff from the field is useful for other societal benefits, very little is gained from a societal water availability point of view. The main effect of raising water prices is to decrease farmer profitability or more likely to induce a shift in the crop to be grown. Relatively little is to be achieved in a net societal water savings. Improving Irrigation Efficiency A common belief is that if agriculture increased the water use efficiency, more water would be available for societal use. Discussion of this matter is complicated by the several definitions of water use efficiency. However, some general statements can be made. First, for common irrigation practices, an efficiency value of 100 percent by any traditional definition is not the economically efficient practice. Therefore, increasing the efficiency number may or may not serve a positive economic effect. The pertinent scientific fact is that the purpose for AW is to meet the ET demands of the crop. Except in special cases, decreasing the crop ET also decreases the yield. Any AW in excess of ET is still potentially available for societal use. For example, water that runs off of a field or farm has some societal use depending upon the locale. The water that percolates below the root zone has to be evaluated on a case-by-case basis as to its availability for societal use. Reduction of AW or deep percolation in most cases will reduce the amount of water available for societal use. These statements are based on water quantity without consideration of water quality. Water quality must be considered as a significant factor but may have relatively little effect on total societal water availability. Water resource managers who are considering increasing irrigation efficiency as a major policy approach to increasing water supplies to meet future demands may be over estimating the potential increase in water supply. In some cases, the over estimates can be large.

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Agricultural Water Management: Proceedings of a Workshop in Tunisia Using Crop-Water Production Functions Traditionally, agricultural management concepts have been based on achieving maximum yields. Crop production research was directed towards determining the treatment that provided maximum yield rather than establishing crop-water production functions. The leaching requirement concept was established for irrigating with saline waters. The leaching requirement is defined as the leaching that results in maximum yield. A common approach to specify the amount of irrigation water to apply for nonuniform systems is as follows. The distribution of water application across the field is measured. The average of the lowest one-fourth of the numbers is divided by the average of all of the numbers. This results in a number less than 1 that is then divided into the average to determine the amount of water to be delivered to the field. This approach is to achieve maximum yield throughout the field. Focus on maximum yield can obscure the vision to other alternatives for irrigation management. Production functions reported in this paper will be used to illustrate this point. The effects of irrigation uniformity are depicted in Figure 2. Maximum yield for a uniform system can be achieved with 70 cm of IW. The distribution of numbers that resulted in the CUC of 86 were such that the lowest one fourth averaged 31.3 and the total average was 40. Dividing 31.3 by 40 and then dividing that number into 70 results in 90. Therefore using a common irrigation prescribing procedure, an IW of 90 cm would be prescribed. Note that the difference in yield for the CUC equal 86 curve between 70 and 90 cm of IW is very small compared to the increase in water application. For the case of CUC equal to 72, irrigating to achieve the maximum yield that can be achieved with a uniform irrigation is not practical. However, a very large water application would be prescribed using the standard procedure. Slightly less than 90 percent maximum yield is achieved with 70 cm of IW. However, the rate of increase in yield with increasing values of IW is not great. The second example will be related to saline irrigation water using the production functions depicted in Figures 3 and 4. Maximum yields of salt tolerant cotton and wheat can be achieved with an irrigation water of 4 dS/m by applying very little more water than used for nonsaline water. In this case the leaching requirement concept to achieve maximum yield is appropriate. However, what can be overlooked is the opportunity to save “fresh” water if a very saline water supply is available. For example, assume that a saline water of 8 dS/m was available. Irrigating either wheat or cotton with this water would be questionable. However, blending this water with nonsaline water to create a water of 4 dS/m is feasible and “saves” a large fraction of the nonsaline water that would otherwise be used to irrigate these crops. This conclusion would not be obvious if one only used the leaching requirement index. Like in all cases, more widespread implication of doing this must be evaluated. As a general rule, the opportunity to use very saline water to irrigate economic salt tolerant crops must be considered.

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Agricultural Water Management: Proceedings of a Workshop in Tunisia Using Water for its Highest Monetary Value Economists suggest that water should be used for its highest monetary value. For example, the analysis summarized in Table 1 indicates that raising the price of water induces a small amount of water application reduction, but a significantly large reduction in farmer profits. If the price of water is raised, the farmer’s option usually is to switch to a higher value crop. One might argue that this is an economically efficient management option for water. Water marketing is proposed as an option for stimulating transfer of water from agriculture to urban uses or from lower value to higher value uses. The urban dweller and industry can afford to pay more for water than many farming operations. This option is considered in light of an increasing human population that is living in urban areas. Moving water to irrigate higher valued crops has one serious potential consequence. The foods that feed the masses of population are the lower valued crops. If the trend for agricultural production throughout the world was towards the higher valued crops, this could have potentially serious consequences on the ability to feed the human population. Analyses of water necessary to meet increased urban human populations is usually done by calculating the water use by an urban dweller and multiplying that by the increased number of people. In one regard this may be an overestimate of the water required. Except for watering gardens and lawns where the water is consumed through ET, most of the urban water demand is using rather than consuming water. In other words, the water has been used and can be treated and reused. However, this analytical approach seriously underestimates the true urban water demand. The amount of water required to grow the crops that feed the people living in the city is almost always overlooked. An estimate of the water required to produce various foods in California was done. Using these data a daily total of 2680 liters were used to produce the food in a 2200-calorie menu suggested by the USDA Food Nutrition and Consumer Service. For comparison, a typical urban home in California uses about 473 liters per day. Of the total water therefore required for the livelihood of an urban dweller, 85 percent of the water was used to produce the food. Strictly by coincidence, in California approximately 85 percent of the developed water is devoted to irrigation agriculture and 15 percent to the cities. The main point is that a major water need for an urban dweller is that which is used to produce the food that sustains them. Based on this fact, there should never be a competition between urban and agricultural uses of water. Almost all of the water consumed by agriculture is to provide for the necessities of the city dweller. Therefore, one of the major long-range concerns related to water supply is that of supplying the food for the increased human population. Rather than managing water to maximize the monetary benefit per unit of water, it might be more appropriate to maximize the food calorie production per unit of water. This criterion may ultimately be more valid in achieving a goal of feeding the peoples of the world.

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Agricultural Water Management: Proceedings of a Workshop in Tunisia Reference Letey, J. , A. Dinar, and K.C. Knapp. 1985. Crop-water production function models for saline irrigation waters. Soil Sci. Soc. Am. J. 49: 1005-1009.