Irrig Sci DOI 10.1007/s00271-007-0060-5
WATER PRODUCTIVITY: SCIENCE AND PRACTICE
Beyond irrigation efficiency Marvin E. Jensen
Received: 27 November 2006 / Accepted: 4 January 2007 Springer-Verlag 2007
Abstract Parameters for accounting for water balance on irrigation projects have evolved over the past century. Development of the classic term irrigation efficiency is summarized along with recent modifications such as effective irrigation efficiency. The need for terms that describe measurable water balance components of irrigated agriculture is very important, as demands and competition for available renewable water supplies continue to increase with increasing populations. Examples of irrigation efficiency studies conducted during the past few decades are summarized along with related irrigation terminology. Traditional irrigation efficiency terminology has served a valid purpose for nearly a century in assisting engineers to design better irrigation systems and assisting specialists to develop improved irrigation management practices. It still has utility for engineers designing components of irrigation systems. However, newer irrigation-related terminology better describes the performance and productivity of irrigated agriculture. On a river-basin level, improved terminology is needed to adequately describe how well water resources are used within the basin. Brief suggestions for improving irrigation water management are presented.
Communicated by R. Evans. M. E. Jensen (&) 1207 Springwood Drive, Fort Collins, CO 80525-2850, USA e-mail:
[email protected]
Introduction Efficient management of water for irrigation requires a full understanding of water balance for the field, irrigation project, or river basin under consideration. Terms describing the components of water balance have evolved over the past century. The most familiar term is irrigation efficiency and its use today, or a replacement term, is even more important as demands and competition for available renewable water supplies continue to increase with increasing populations. This paper describes relevant components of water balance for an irrigated area. Basic terms and their development are described. The paper also reviews measurable water balance component terms for quantifying the performance of irrigation systems or projects. Related terms are also summarized along with several examples illustrating the misuse of these terms. Recent studies of irrigation efficiency are summarized, and recent publications on this subject are identified. Alternatives are suggested for planning, development and management of water resources for irrigated agriculture to facilitate better understanding of water balance in irrigation systems, projects and river basins. Finally, suggestions for improving irrigation water management are presented. Engineers and scientists need to carefully define the efficiency terms that they use in reporting irrigation studies to avoid misinterpretation by readers. More important, they need to consider using terminology based on the physics of the water resource system and conservation of mass to avoid misunderstandings by the general public. Authors also need to avoid making claims that are not valid or can be misleading.
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Water balance The main water balance components of a field, irrigation system, or project are: Wg ¼ E þ T þ DS þ Rs þ D
ð1Þ
where Wg = the gross volume of water delivered to a field or study area including effective precipitation, Pe, E = evaporation from the soil and plant surfaces, T = transpiration, DS = change in water stored in the root zone of the soil, Rs = surface runoff, and D = vertical drainage below the crop root zone that may be natural or enhanced with a subsurface drainage system. The first two terms of Eq. 1 represent liquid water that has been converted to vapor and transferred to the atmosphere. It is commonly referred to as water that has been consumed. For many studies, it is not practical to separate E and T components. Therefore, the combined E and T components are usually represented by the term evapotranspiration (ET). Surface runoff and vertical drainage often provides the water supply for downstream fields or projects within a hydrologic unit or river basin, or it returns to the underlying ground water from which it was pumped. Water that is incorporated within plant tissue or the harvested component of a crop is usually very small part of the water balance and is generally ignored in water balance and water management calculations. When evaluating the performance of an irrigated unit, it is common to express the water balance component as ratios relative to the gross water supply delivered to the study area. The main ratio is commonly referred to as irrigation efficiency (Ei). It represents the portion of irrigation water delivered to the target area that has been evaporated, or consumed. This fraction is usually expressed as a percentage. Ei ¼
ET i Wg P e
ð2Þ
where ETi is the component of irrigation water delivered that was consumed by E and T, Wg is the gross supply and Pe is the effective precipitation or precipitation that reduces the amount of irrigation water that is needed. The ratio of the water balance components that are not consumed, i.e., that remain in liquid form somewhere in the system or that are stored or drain from the system to irrigation water delivered is: ð1 Ei Þ ¼
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DS þ Rs þ D : Wg P e
ð3Þ
On an annual basis, the change in the soil root zone water content is often very small so that for evaluation purposes, the portion of the irrigation water supply that is not consumed becomes: ð1 Ei Þ ¼
Rs þ D : Wg Pe
ð4Þ
The sum of Rs and D may be reusable for irrigation or other uses if it does not flow to a sink where it cannot be recovered or it is degraded and becomes unsuitable for reuse for irrigation. The quality of Rs and D will always be degraded to the extent that the original soluble salt content that was in the ET component of irrigation water will remain in the component not consumed. If the natural drainage capacity is limited, drainage water may accumulate in the root zone causing the soil to be waterlogged. Molden et al. (2001a, b) presented graphical diagrams of various possible process and non-process depletions of the non-consumed component of gross water supply at the basin level.
Background In the USA, during the early days after the Reclamation Service was established in 1902 the two criteria for determining whether or not land could be irrigated were: (1) available water, and (2) getting the water to the land (Tekrony et al. 2004). Within a few years the soils in many irrigated areas became waterlogged and saline because the natural drainage capacity for removing the non-consumed component of water delivered to the projects was inadequate. The technology for drainage of irrigated land was not developed until the 1940s and 1950s. Technology also did not exist for accurately estimating the ET component or crop water requirement so as to manage the quantity of water delivered and to estimate the quantity of drainage required. Even today with improved technology for estimating the ET component, large-scale drainage and salinity problems still exist in some areas. One such example is the area along the Syr Dara River in the Aral Basin (Heaven et al. 2002). The development and management of irrigation projects must consider the water balance including the quantity and timing of water delivered to a project relative to crop ET and natural or supplemental drainage capacity of the area. The Doroodzan project in Iran (Javan et al. 2002) illustrates typical conveyance and distribution problems that have been encountered. These problems emphasize the need to rigorously consider all water
Irrig Sci
balance components in the planning, development, and operation of irrigation projects. Good operation requires timing water deliveries to meet varying crop water needs. Early definitions of irrigation efficiency O.W. Israelsen, a pioneer in irrigation technology, was concerned about the quantity of water being applied to irrigated land in the USA, and he developed several parameters for characterizing irrigation performance. The first parameter, that he called water application efficiency, related to the quantity of water that was added to soil (DS) during irrigation. The second parameter was the ratio of the quantity of irrigation water consumed in ET relative to the quantity applied that he called irrigation efficiency. Water-application efficiency Israelsen et al. (1944) defined water-application efficiency as ‘‘the ratio of the amount of water that is stored by the irrigator in the soil root zone and ultimately consumed (transpired or evaporated or both) to the amount of water delivered to the farm.’’ They indicated that Beckett et al. (1930), in reporting a study of water requirements of citrus and avocados, ‘‘made observations of ‘irrigation efficiency’—a term used by them with the same meaning as the term ‘water-application efficiency’ is herein used.’’ Israelsen et al. (1944) reported measurements of water applications on 23 farms in Utah and Salt Lake Counties in Utah using gravimetric soil sampling techniques and calculated water-application efficiencies. The objective of the study was to obtain information on how uniformly water was being distributed over farm fields and to provide information to farmers on how to improve water applications. Studies were conducted from 1937 to 1940 on Utah County farms and in 1941 on Salt Lake County farms. This was a period of time before modern land leveling equipment, such as laser-controlled land smoothing equipment and modern water control structures, were available. Likewise, it was long before climate data were being used to calculate daily crop water requirements for scheduling irrigations. Most of the measured field water-application efficiencies were in the range of 20 to 60%. The summary of results included detailed data from each set of some 145 measurements in Utah County and 28 measurements in Salt Lake County. The average farm water-application efficiency in Utah County was 40%.
Water-application efficiency determinations are still made in special studies. Such studies are expensive and today only a few are being conducted. The time period commonly involved for an irrigation event being evaluated is from the time of irrigation, or date soil water content is measured before irrigation, until 2– 3 days later when the rate of drainage of soil water, if any, from the root zone has greatly decreased. For a season, water-application efficiency is the ratio of the sum of water stored to the sum of water applied for all irrigations. Irrigation efficiency Israelsen (1950) stated ‘‘With a given quantity of water diverted from a river, the larger the proportion that is stored in the root-zone soil of the irrigated farms and held there until absorbed by plants and transpired by them, the larger will be the total crop yield.’’ He then defined irrigation efficiency as the ratio of the irrigation water consumed by the crops of an irrigation farm or project to the water diverted from a river or other natural water source into the farm project canal or canals. In equation form, he defined irrigation efficiency as Ei = Wc/Wr where Wc is irrigation water consumed by the crops and Wr is the water diverted from a river or other natural source (Israelsen 1950; Eq. 1). Clearly, his definition of irrigation efficiency was the ratio of water ‘‘consumed’’ to that diverted or applied. In 1993, because authors of numerous publications were inferring that new water for other uses could be created by increasing irrigation efficiency as defined I suggested that we need to consider changing the name of this ratio to a term such as a consumptive use coefficient, Ccu (Jensen 1993). The consumptive use coefficient would represent the fraction of water diverted, or applied to a field, farm, or project that is converted to vapor or consumed. The fraction that is not consumed would be Cncu = (1 – Ccu). In any water balance study of a project or river basin, or when estimating the impact of some intervention, the authors must consider both terms, Ccu and Cncu and not just Ccu. In 1967, I indicated that for sustained irrigated agriculture, the quantity of water effectively used to control soil salinity should be considered as beneficial use (Jensen 1967). Therefore, I defined irrigation efficiency as the ratio of ET of irrigation water plus the water ‘‘necessary’’ for leaching on a steady state basis to the volume of water diverted, stored, or pumped specifically for irrigation. It was intended to show that for sustained irrigated agriculture, soil salinity had to
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be controlled by the only practical method which is soil leaching. Therefore, in calculating the irrigation requirement per unit area with perfectly uniform application, in addition to water for crop ET not provided by precipitation, some water may be required for leaching. In retrospect, this may not have been good physics because the numerator of the resulting term contained a consumptive component and a small nonconsumptive component making water balance calculations more complex. The term has been misused in that calculations of the water required for leaching was added to the net ET requirement over the entire field or area. With surface irrigation and normal irrigation practices, the upper two-thirds of the field may already receive excess water that satisfies the leaching requirement and water for leaching does not need to be added for that portion of the field. Efficiency Webster’s Unabridged Dictionary (New World Dictionaries 1979) defined efficiency as (1) the ability to produce the desired effect with a minimum of effort, expense, or waste; and (2) the ratio of effective work done to the energy expended in producing it, as of a machine; output divided by input. Efficient was defined as producing the desired effect, or result, with a minimum of effort, expense, or waste. When applied to irrigation, the traditional term ‘‘irrigation efficiency’’ is partially applicable in that it considers the water consumed (crop ET) in producing the desired effect (crop production), but it is an inappropriate term if it considers the water that is not consumed to be wasted. If expense is the main criterion under consideration, then a properly managed surface, or gravity, irrigation system may be as efficient as more sophisticated and elaborate systems sprinkler or microirrigation system. The performance of an irrigation system or project may better be described in specific terms describing the physical or economic productivity of the system rather than using efficiency parameters. Irrigation efficiency and performance studies The US Bureau of Reclamation (USBR) conducted major farm irrigation efficiency studies in the 1960s and 1970s. Each of these studies involved several farms, measurement of water delivered to fields, surface runoff from fields, and periodic measurements of soil water contents over a period of 3–5 years. The studies were conducted in Idaho, Nebraska, Washington (the Columbia Basin), and Wyoming (USBR 1970, 1971, 1973). Data summarized involved mainly measure-
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ments of seepage in farm distribution systems, water applied, surface runoff, estimated deep percolation, estimated ET, and calculated seasonal field and farm irrigation efficiencies. The distribution of farm irrigation efficiencies was similar to average farm waterapplication efficiencies measured in Utah in the 1930s except that there were fewer low and fewer high values and the averages were slightly higher. The area in Idaho had an average annual precipitation of about 250 mm with about 100 mm occurring during the growing season. The average depth of annual water delivery was 0.95 m. Average farm irrigation efficiency was 43%. In Nebraska, the average annual precipitation was 530 mm, and the average seasonal precipitation ranged from 250 to 500 mm. The average depth of annual water deliveries ranged from 0.37 to 0.55 m. Average farm irrigation efficiency was 45%. In Wyoming, the average May–October precipitation was about 300 mm. The average depth of annual water deliveries was 0.55 m. Average farm irrigation efficiency was 44%. In Washington, the average April– October precipitation was very low ranging from about 45 to 85 mm. The average depth of annual water deliveries to surface-irrigated farms was 1.2 m. The soil at this location was a coarse sandy loam over coarse sand. Both surface and sprinkler irrigation were studied at this site. For surface-irrigated farms, the average farm irrigation efficiency was 35%. Oster et al. (1986) reported detailed measurements of water applied, drainage and runoff on nine fields in the Imperial Valley of California during the period 1977–1981. Eight of the fields had drainage systems that could be instrumented to measure the drainage volume. All fields had inlet structures for installing flow meters or rectangular weirs and they had sites to measure surface runoff. The soils were of mostly of silty clay loams and silty clays. Irrigation efficiencies exceeded 70% for most crops, but were lowest for lettuce and cantaloupe. Crop effects on irrigation efficiency were greater than field effects. Surface runoff for furrow-irrigated crops tended to be higher than for border-strip irrigated crops. Measuring water-application efficiency of individual events and fields is labor intensive and expensive. Most recent studies of irrigation practices over many fields and farms have involved measuring field soil water contents in the spring and fall, measuring water applied to fields and rainfall, measuring runoff from the fields, and then estimating evaporation and transpiration during the growing season. With improved technology for estimating crop ET and evaporation from the soil, future studies will likely involve estimates of crop ET and evaporation (Allen et al. 1998; EWRI 2005) and
Irrig Sci
calculated or measured water applications. With the development of detailed models of soil water, atmosphere, and plants and the availability of satellite remote sensing, studies of irrigation performance over large areas are now possible (Allen et al. 2002; Droogers and Bastiaansen 2002). Irrigation efficiency publications Many publications describe irrigation efficiency terminology and summarize the results of efficiency studies. Bos and Nugteren (1974, 1982) published the results of a joint effort of the International Commission on Irrigation and Drainage (ICID), the University of Agriculture, Wageningen, and the International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. The definitions of efficiency terms were refined in the 1982 edition. Distribution efficiency was defined as the ratio of the volume of water furnished to the fields to the volume of water delivered to the distribution system. Field application efficiency was defined as the ratio of the volume of irrigation water needed, and made available for ET by the crop to avoid undesirable water stress in the plants throughout the growing cycle to the volume of water furnished to the fields. Farm, or tertiary unit, efficiency is the combined efficiency of the distribution system and the application process. The results presented in these publications were based on questionnaires that had been sent to national committees of the ICID. In 1979, a US Interagency Task Force (TF) completed a report on Irrigation Water Use and Management (Interagency TF 1979). The TF did not make independent measurements of farm irrigation practices, but based the report on available literature and input from a number of specialists. Onfarm efficiency was defined as the ratio of the volume of water stored in the root zone and used by the crop to the volume of water delivered to the farm. The report contained chapters on water laws and institutions, causes of inefficiencies and their results, and measures, costs and impacts. Regarding irrigation efficiency, the Task Force stated ‘‘Therefore, any report dealing with irrigation efficiencies must first define ‘efficiency’ with a great deal of care. Many different and sometimes conflicting definitions have been published. It is frequently assumed that because irrigation efficiency is low, much irrigation water is wasted. This is not necessarily so.’’ (p. 22). The latter statement pertains to the reuse of return flows for irrigation that can occur within large projects and in river basins.
In 1990 and 1991, more than 100 surface irrigation events on fields in northern Colorado were analyzed as part of a water rights transfer case (Walter and Altenhofen 1995). Several different methods were used to calculate farm irrigation efficiency. The method of measuring inflow and runoff from each field and estimating deep percolation appeared to provide the best results. The resulting farm irrigation efficiencies typically were higher than field efficiencies because surface runoff from some fields was reapplied to the same or to other fields. The Colorado Water Court ruled that the applicant for a water transfer must replace return flows that have been recharging the shallow aquifer supplying wells in the area. Wolf et al. (1995) conducted field research in Jordan during 1993–94 comparing drip irrigation systems with surface systems. Daily water measurements of water delivery to 31 farms were made over a 1-year period. Three types of irrigation methods were studied: (1) conventional surface irrigation; (2) drip (primarily) or microjet systems; and (3) drip irrigation in greenhouses. The farms were small, ranging in size from 3 to 5 ha. During the winter, water generally could be obtained on demand and the Jordan Valley Authority encouraged farmers to take water for leaching purposes. In the summer, deliveries were on a strict rotation schedule. Water delivery to farms was via closed conduits with deliveries measured with water meters. Water deliveries were compared with estimated crop ET to calculate irrigation efficiency. The calculated average on-farm efficiencies for the year were 70% for surface systems and 56% for the drip systems. Some of the reasons for the lower efficiency with the drip systems were: the drip systems were about 20 years old and in need of replacement; in-line emitters originally used for closely-planted crops were being used on orchards with widely-spaced trees; some drip lines were moved and installed up and down slopes leading to pressure differences; water was being applied in excess of the infiltration rate resulting in ponded water at the base of trees; weed growth was excessive indicating non-beneficial ET; and few farmers had any basis for irrigation scheduling. Furthermore, the owners of the drip system may have had access to more water than farmers with surface systems. However, economic returns from crops grown under drip systems were higher than returns from crops grown under surface systems. The distribution of crop type grown under the drip and surface systems was not given. Wolters (1992) published a comprehensive discussion of the results obtained from a second ICID questionnaire. He found that interpreting the questionnaires was not always easy. He summarized various
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efficiency terms found in the literature in Chap. 2 beginning with those by Israelsen (1932). Appendix III summarized three groups of definitions. Group A, based on volumes, are those of Israelsen (1932), Hansen (1960), Jensen et al. (1967), Erie (1968), ICID (1978), Bos (1980), Hart et al. (1979), Greenland and Bhuiyan (1980), Jensen et al. (1980), Levine (1982), Keller (1986), Weller et al. (1988), and Schuurmans (1989). Group B, based on ‘‘measured’’ depths of application, were those of Christiansen (1942a, b), Hall (1960), Hansen (1960), and ASCE (1978). Wolters’ publication is a must reading for engineers and scientists contemplating a major study that will involve calculating irrigation efficiencies. More recently, Javan et al. (2002) evaluated the performance of three irrigation projects in Iran with a total area of about 211,000 ha. The Doroodzan project was designed using USBR standard methods. The Zayandeh Rud project was designed by the European Nertech Module System. The Moghan project is an older system based on mixed standards. Measured discharges at the intakes of the three projects varied from –26 to 30%. Studies by the International Water Management Institute (IWMI) during 1990–91 showed that managers rarely monitor the performance of the system under their supervision, researchers not managers collect performance data, and data collected rarely influence the management of projects. The overall project efficiency in wet and dry years was about 46%. Distribution and conveyance was unreliable in both wet and dry years mainly due to lack of conformity between the actual water requirement of crops and the water distribution schedule. Ali et al. (2004) evaluated the performance of the Muda Irrigation Project in Malaysia which covered 126,000 ha of which 96,000 ha was under double cultivation of rice. The performance was determined by the efficiency by which water was diverted, conveyed, applied, and by adequacy and uniformity of application in each field. Rainfall provided 51% of the irrigation requirements. Two dams contributed about 29% while uncontrolled river flow and recycling contributed about 15%. Conveyance efficiency for a 67-km canal from the Pedu dam to the barrage was estimated to be 67%, and from the dam to the end of central and northern canals, 59%. The overall project efficiencies for the main season and off season were found to be 18 and 32%, respectively. River basin studies Accounting for water use and productivity requires meaningful calculations of the consumption of water to
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achieve a river basin water balance (Molden 1997). Molden stressed the importance of distinguishing between water depletion and water diverted for a use or service because not all water diverted is depleted from the supply. With increasing competition for available water resources, there is a need to not only consider farm irrigation efficiency, but also the combined effects of several projects within a river basin on the volume of water depleted. Water planners and managers have been slow to recognize this need and to apply this concept. If we consider that the volume of water diverted from a river, or ground water system, for irrigation is either consumed or not consumed, i.e., Vw = Vc + Vnc, the volume depleted, Vdep, would be Vdep = Vc + (1 – fr)Vnc where fr is the fraction of water diverted for irrigation and not consumed that returns to the river, or ground water system and can be reused for irrigation. The resulting net or effective irrigation efficiency for the water resource, based on irrigation efficiency defined as Ei = Vc/Vw, would be Ee ¼ Ei þ fr ð1 Ei Þ
ð5Þ
where Ee is net or effective irrigation efficiency (Jensen 1977). Effective irrigation efficiency in essence states that if all of the water that is not consumed returns to the river or ground water system, the effective efficiency for the water resource would be 100%. In most projects, evaporation from open drains and transpiration from phreatophytes and vegetation along the drains and waterways and from wetland areas consume some of the water that is not consumed on irrigated fields. Therefore, the return flow factor is usually less than 1.0. In this example, the salinity of the water was not considered. Under steady state conditions, and assuming minimal soil mineralization, the increase in dissolved solids, or salt content, in the return flow will be inversely proportional to the fraction of irrigation water consumed. The increase in salt content in the river below the project will depend on the rate of flow in the river and the return flow rate. If none of the non-consumed water returns to the river (fr = 0), there will be no change in the salt content of the river below the project and Ee = Ei. If all of the non-consumed water returns to the river (fr = 1.0), the salt concentration below the project will be increased depending on salt content of water diverted, the flow rate in the river at the point of return flow, the volume of irrigation water consumed during the irrigation season on the irrigated land, and any incidental pickup of salt from saline geologic formations such as marine shale.
Irrig Sci
Keller and Keller (1995) expanded the concept of net irrigation efficiency. They stated, ‘‘Classical water use efficiency concepts are appropriate tools for irrigation design and management, but they are poorly suited for formulating water allocation and transfer policies.’’. . . ‘‘Classical efficiency concepts ignore the value of return flows—irrigation water runoff and seepage that re-enters the water supply.’’ Keller and Keller (1995) defined effective efficiency as the ratio of irrigation water consumed by crops to effective use, Ee = Uci/Ue where Uci is crop ET less effective precipitation and Ue is effective inflow less effective outflow. Using Israelsen’s definition of irrigation efficiency, Ie , Ie was defined as Ie = Uci/ VD = (CropET – Pe)/VD where CropET is evapotranspiration, ET, Pe is effective rainfall, and VD is the irrigation water delivered from a surface or ground water source to the canals or farm headgates. Then, the leaching requirement, LR, was added where LR = VLR/(Uci + VLR). VLR = the minimum amount of water that must pass through the root zone to maintain a favorable salt balance. They defined the classical concept of irrigation efficiency, Ei, as Ei = Ie/(1 – LR), or Ei ¼
ðCropET Pe Þ þ VLR Uci þ VLR ¼ : VD VD
ð6Þ
Keller and Keller (1995) defined effective irrigation efficiency, Eei, as the crop consumptive use of applied irrigation water, Uci, divided by the effective use, Ue. Eei ¼
Uci Uci CropET Pe ¼ ¼ Ue Vei Veo ð1 LRi ÞVi ð1 LRo ÞVo ð7Þ
where (Vei – Veo) is effective water supply, the subscripts i and o denote inflow to and outflow from the irrigated system. Eei is the efficiency of the system in terms of the quantity of water effectively consumed by the system. Equation 7 is basically the same as Eq. 5 except in Eq. 7 the volumes of water involved are discounted by their respective salt contents.
Keller and Keller (1995) presented three examples comparing classical and effective efficiency values: (1) the Grand Valley in the Upper Basin of the Colorado River where a program had been implemented to reduce salt loading that occurs when seepage moves through underlying or adjacent saline strata before returning to the river; (2) the Imperial Irrigation District (IID) where a set of conservation projects was implemented to create ‘‘real water savings’’ that could be diverted out of the Basin to serve the Metropolitan Water District of Southern California (MWD); and (3) a portion of the Nile Valley in Egypt. A summary of the resulting calculations for these three examples is presented in Table 1. The calculation procedures are in the Keller and Keller paper. The salinity of the water for the Grand Valley was 573 ppm for the diversion, and the return flow salinity was 2,268 ppm before the intervention and 1,563 ppm after the intervention. The salinity of the Colorado River water used in the IID was 629 ppm. The return flow salinity was 2,506 ppm; however, none of the return flow is usable for irrigation since it drains to the Salton Sea. The most significant differences were between the classical and effective efficiencies in the Grand Valley and in the Nile Valley. The main differences between Keller and Keller’s effective efficiency and Jensen’s net or effective efficiency are the inclusion of the estimated LR and resulting effective volume of water in the equation by Keller and Keller. The differences between Keller and Keller’s Eei and Jensen’s Ee would have been less in the IID if a more rigorous procedure such as that by Rhoades et al. (1992) had been used to estimate the LR. For example, instead of an LR of about 15% as was calculated for the IID, it would have been about 8– 9% using the 1992 Rhoades procedures. The large difference in Ee for the Grand Valley is because Keller and Keller discount the effective volume of return flow because of the large increase in salinity. Seckler (1996) and Seckler et al. (1998) presented more information on the background and purpose of Keller and Keller paper. Papers presented by Willardson et al. (1994), Allen et al. (1996, 1997), and Willardson and Allen (1998) suggested that the classi-
Table 1 Summary of the comparison of classical and effective irrigation efficiencies for the Grand Valley, Imperial Irrigation District in California, and the Nile Valley irrigation system in Egypt Efficiency
Grand Valley Pre-intervention (%)
Grand Valley Post-intervention (%)
Imperial ID Pre-intervention (%)
Imperial ID Post-intervention (%)
Nile Valley(%)
Classical, Ei Jensen’s net, En Effective, Ee
26.0 64.9 36.8
30.4 86.0 61.7
71.9 61.0 71.9
74.6 63.4 74.6
41.2 92.6 91.3
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cal efficiency term was outmoded. This series of papers recommended using ratios or fractions to define water use so as to better consider impacts of return flows. The consumed fraction, CF, is essentially the same as the term I called consumptive use coefficient, Ccu, (Jensen 1993), and the reusable fraction, RF, where RF = (1 – CF), is essentially the same as Cncu = (1 – Ccu). Burt et al. (1997) modified the consumptive use coefficient equation to include the change in stored irrigation water in the denominator. The concepts presented by Jensen (1977) and Keller and Keller (1995), Keller et al. (1996), Willardson et al. (1994), and that of Seckler (1993) provided the foundation used in the development of the new International Water Management Institute (IWMI) paradigm referred to as the ‘‘IWMI Paradigm’’ which analyzes water use in the context of the water balance of the river basin. Seckler (1999) pointed out that the classical irrigation efficiency definition leads to the belief that water is wholly ‘‘wasted’’ or ‘‘lost’’ because it treats outflows as though they vanish from river basins. The effective irrigation efficiency concept can also be applied to a river basin as the ratio of the total net irrigation requirement to the total or primary water supply. The concepts mentioned above also played a role in the development of world water demand and supply estimates from 1990 and projections to 2025 (Seckler et al. 1998). Molden (1997) developed procedures for accounting for water use, or water accounting based on a water balance approach. Water accounting is a procedure for analyzing the uses, depletion, and productivity of water. A key term is water depletion, which is the use or removal of water from a water basin such that it is permanently unavailable for further use. He described process and non-process depletions. Process depletion is where water is depleted to produce an intended good. In agriculture, process depletion is transpiration plus that incorporated into plant tissues—the product. Nonprocess depletion includes evaporation from soil and water surfaces and any non-evaporated component that does not return to the fresh water resource. The depleted fraction is that part of inflow that is depleted by both process and non-process uses of water. The process fraction is similar to the effective irrigation efficiency of Keller and Keller. The productivity of water is a performance parameter that can be related to the physical mass production or economic value per unit volume of water. Molden suggests that the productivity of water can be measured against gross or net inflow, depleted water, process-depleted water, or available water in contrast to the production per unit of water consumed in ET as defined by Viets (1962). Water accounting can be done at various levels such as the
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field, irrigation service, basin or sub-basin levels. Molden et al. (1998a, b) presented a detailed example of water accounting at the basin level using data from Egypt’s Nile River where some detailed information on water use and productivity was available. Molden and Sakthivadival (1999) presented another example for a district in Sri Lanka. Molden et al. (2001b) refined the basin method by dividing the area into six zones to enable defining, characterizing, and development strategies for areas with similar characteristics. The zones enable better understanding of complex water interactions within river basins and development of water management strategies tailored to different conditions within basins. For example, Renault et al. (2001) described how water consumption by non-crop perennial vegetation in a humid tropical watershed needs to be considered in a basin. In that case, some of the perennial vegetation consisted of fruit and coconut trees which were considered to be profitable while other trees provided shade and enhanced the environment. If non-crop perennial vegetation is considered to be the beneficial use of water, then the performance of the system being considered should be evaluated using effective irrigation efficiency terminology. Numerous conferences have been held resulting in papers concerning the quantification of ‘‘real water’’ that may be available for transfer or reallocation to higher value uses. Irrigation efficiency concepts enter in when discussing possible real water amounts that can be saved and made available for transfer by making changes on irrigated lands. Roos (2001) indicated that unless depletion changes as a result of interventions, there is no real water savings available for transfer.
Terminology Basic terms Basic irrigation efficiency terms were defined in the previous section. These involve water-application efficiency, irrigation efficiency (classical), and effective irrigation efficiency. A comparison of these terms is presented in Table 2. Several other terms that are related to irrigation efficiency components and terminology are described subsequently Consumptive use The total quantity of water taken up by vegetation for transpiration or building of plant tissue, plus the
Irrig Sci Table 2 Irrigation efficiency and performance parameters Term
Equation
Water-application efficiency
Ea = Vs/Va
Classical irrigation efficiency
Net or effective irrigation efficiency Effective irrigation efficiency
Application
Comment
Measure of the volume of water stored in the Requires measuring soil water content soil relative to the volume applied during a before and after each irrigation. single irrigation or for multiple irrigations. Requires accurate definition of equation Ei = ETi /(Wg – Pe) Measure of how well irrigation water delivered (or diverted from a river or other components. Represents the fraction of source) was used for its intended purpose. applied water consumed in the intended area. Requires accurate definition of equation Ee = Ei + fr(1 – Ei) Measure of how well water delivered was used for both its intended and noncomponents, and the non-intended intended, but beneficial purposes. beneficial purposes. Salt content is not considered. Eic = Uci/(Vei – Veo) Measure of crop consumptive use relative to Effective use discounts the volumes of effective use taking into account salt water involved by their respective salt content. contents.
unavoidable evaporation of soil water, snow, and intercepted precipitation associated with vegetal growth. The US Geological Survey (Solley et al. 1997) definition is: ‘‘That part of water withdrawn that is evaporated, transpired, incorporated into products or crops, consumed by humans or livestock, or otherwise removed from the immediate water environment. Also referred to as water consumed.’’ Evapotranspiration The combined processes by which water is transferred from the earth’s surface to the atmosphere (evaporation plus transpiration). Return flow That portion of the water diverted for irrigation that returns to a fresh-water ground water or stream system for potential rediversion or instream use (Interagency TF 1979). Net depletion Includes all crop consumptive uses and irrecoverable water losses. Does not include return flows to estuaries or direct to oceans (Interagency TF 1979).
Efficiency terms often misinterpreted Improving farm irrigation efficiency The most common misinterpretation is that improving farm irrigation efficiency will create new water that can be used for other purposes or applied to other lands without affecting the water supply for other water uses that depend on return flow. Many comments can be found in publications stating that if irrigation efficiency could be increased but a few percentage points; other water demands could easily be met. Likewise, increasing irrigation efficiency will result in saved ‘‘wet water’’ or real water. In 1993, I listed a number of comments made about irrigated agriculture that are partially correct and partially incorrect due to misinterpretation of irrigation efficiency (Jensen 1993, pp. 10–12). Improving irrigation efficiency will reduce soil salinity This is partially true in that improving the uniformity of water application can reduce soil salinity in those areas that typically have been under-irrigated. However, under steady state conditions, the salinity of the soil solution depends on evaporation (and transpiration) that removes pure water leaving the salts in the soil that were in the irrigation water. The only practical way to control soil salinity, where leaching by precipitation is insignificant, is to apply about 5–10% more irrigation water than that consumed in ET to move the salts below the root zone and to natural or constructed drains.
Productivity of water The physical mass of production or the economic value of production measured against gross inflow, net inflow, depleted water, process depleted water, or available water (Molden 1997).
Irrigation efficiency can be improved without additional costs to the farmers Better timing of irrigations and controlling amounts applied (scheduling) can improve irrigation efficiency
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and productivity of water with little additional cost. Improving the farm irrigation system, such as installing a drip system (or trickle system) usually requires substantial investment in new equipment; higher annual operating costs, and may require higher skills. Installing a drip system does not always result in higher irrigation efficiencies. As reported by Wolf et al. (1995), unless a drip system is properly maintained and operated, the irrigation efficiency achieved may be no better than that achieved with a traditional surface system. Irrigation systems do not consume water. Crops consume water by evapotranspiration. Related terminology and technologies Water saved Many publications often refer to or claim ‘‘water saved’’ as a result of some intervention. The Interagency TF defined water saved (salvaged) as ‘‘That water gained in a river basin through reduction of incidental uses and irrecoverable ground-water.’’ Keller and Keller (1995) indicated that decisions intended to raise water use efficiency based on classical irrigation efficiency calculations often do not result in real water savings. In the USA, to avoid this problem, many states now work with depletion of water. The quantity of water that can be transferred from one use to another, or out of the basin, without damage to downstream users is the quantity that is being depleted by consumptive use and not the quantity that is diverted for a use such as irrigation. Performance indicators Use of ‘‘efficiency’’ in irrigation-related terms is often intended to be an indicator of performance. Molden et al. (1998a, b) defined nine performance indicators for irrigated agricultural systems. Four main indicators relate to output or production, three relate to water supply and system capacity, and two are financial indicators. Benchmarking A more recent procedure for assessing the performance of irrigation and drainage systems is called benchmarking. Malano and Burton (2001) defined benchmarking as: A systematic process for securing continual improvement through comparison with relevant and achievable internal or external norms and standards. The World Bank has extended the bench-
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marking initiative in water supply and sanitation sector to a wider audience by establishing a web site (World Bank 2003). Malano et al. (2004) indicate that benchmarking in the irrigation sector is a tool for change. Data envelopment analysis (DEA) The DEA is a linear programming technique to determine the relative efficiencies of a company when the inputs and outputs of production units within a company are known, but the productive process itself is not (Rodrı´guez et al. 2004). The DEA analysis based on inputs and outputs of an irrigation system enables the authors to determine the relative efficiency of an organization or productive function within an organization. Transpiration efficiency (TE) The net photosynthesis of a leaf defined by Bierhuizen and Slatyer (1965) and later discussed by Tanner and Sinclair (1983) has become known as transpiration efficiency. TE is the amount of biomass accumulated per unit of water transpired and is commonly expressed as TE = k/De where k is approximately a constant for a crop species and De is the vapor pressure deficit. In a glasshouse study, Mortlock and Hammer (1999) examined the variation in TE for a range of sorghum genotypes grown under well watered (WW) and water limited (WL) conditions. The purpose of the study was to seek selection indices for this trait by measuring a range of associated physiological and morphological attributes. Keller (2005) showed that a near linear relationship existed between maize grain yield and transpiration plus evaporation from many locations when normalized for estimated daytime saturated vapor pressure deficit. Improving irrigation water management Adequate knowledge and methodologies exist for calculating water balance components of irrigated agriculture and for estimating daily crop water requirements. The remaining challenge is improving systems and implementing practices to achieve improved irrigation efficiencies and crop water productivity. Surface or gravity irrigation is widely used. If crop production is below expectations and the irrigator has some control of timing irrigations, improving water management begins at the field or farm level. The first step in improving irrigation water is to improve the
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timing of irrigations to match soil water depletions. Where climate conditions are not greatly affected by rainfall, estimated schedules of near optimal irrigations can be calculated or derived from irrigation studies conducted in the area. For example, Mishra et al. (2001) showed that maximum yields of winter maize in India could be achieved by irrigating at tasseling, flowering, silking, and grain-filling stages. Providing computerized irrigation scheduling services to individual growers for several years is a very effective educational tool. Typically, growers using irrigation scheduling information on the same crops and fields soon learn how climate affects evapotranspiration rates and how these rates affect soil water depletion on their fields. At a minimum, broadcasting calculated daily reference ET or estimated crop ET over a period of years is an economical method of improving the growers’ understanding of the ET-soil water depletion process. This methodology can be the beginning of further refinement in disseminating needed daily water management data to growers. The next step is considering improvements in the farm physical irrigation system to enable growers to apply the quantity of water needed at each irrigation. Such improvements typically begin with land leveling or smoothing followed by improvements in the farm water distribution systems. After implementing these on-farm improvements, the deficiencies in the project system for storing and distributing irrigation water soon become apparent along with the need for improvement in the systems and their management. Replacing surface or gravity irrigation systems with drip or sprinkler systems can facilitate achieving more accurate irrigation scheduling, especially when the system is automated and controlled by soil water sensors. However, even with improved systems, such as center-pivot sprinklers, rational irrigation scheduling can be effective in managing the quantity of water applied. For example, an irrigation scheduling program based on the FAO–56 Penman-Monteith ET estimation and neutron probe measurement was in operation on a 28,000-ha center-pivot irrigation project in northern New Mexico, USA for the period 2000–2004 (Bliesener et al. 2005). The program resulted in a 20% reduction in applied water compared with the average of the previous 10 years, and a reduction in nitrate concentration in the groundwater. Reduced ground water recharge reduced phreatophyte growth and consumptive use. Although yield/consumptive use has improved with scheduling, it was not possible to separate the effects of scheduling and the general
improved farming practices that occurred during the study period. From a river basin viewpoint, the use of hydronomic zones, a term proposed by Molden et al. (2001b), would facilitate defining, characterizing, and developing management strategies for areas with similar characteristics. Such a tool would help better understand complex water interactions within river basins. It would isolate similar areas to develop better water management strategies that are tailored to specific conditions with basins.
Summary and conclusions The traditional water-application efficiency term has been used for nearly a century. Should this term be called efficiency? Efficiency implies that the water applied and not stored may be ‘‘wasted.’’ Thus, the use of ‘‘efficiency’’ may not be correct, or appropriate for the ratio of ET to water applied. The term can be misleading, since the only water lost to the system during an irrigation event is that portion that evaporated from the time of application until the soil water content was measured 2 or 3 days later. Most of the remaining water not stored and that percolated beyond the root zone to the ground water, or that flowed to an open drain, is still somewhere in the system. The traditional irrigation efficiency term has served a valid purpose for nearly a century in assisting engineers to design better irrigation systems and assisting specialists to develop improved irrigation management practices. It still has utility for engineers designing irrigation systems. However, the traditional irrigation efficiency term should not be called efficiency since it incorrectly implies that the non-consumed component of the water balance equation is lost. The term having the same meaning would be more accurate if it were called something like the consumptive use fraction, or consumptive use coefficient. When planning and managing water resource systems within river or other closed basins, or even within large projects that capture and reuse return flows, the portion not consumed must be taken into account. As competition for available water supplies within river basins increase and water resource planning and management decisions are made, traditional irrigation efficiency terminology should be replaced with more exact and descriptive terminology that is based on the physics of the hydrologic system and conservation of mass. Vague, obsolete terms should be avoided.
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