On-farm Irrigation Efficiency Research Articles

Soil moisture-based irrigation interval and irrigation performance evaluation: In the case of lower kulfo catchment, Ethiopia

The lack of soil moisture-based irrigation intervals, poor distribution of irrigation water among users, and the time-based and spatial variability of water supply have been challenges for the productivity of irrigation schemes in the Lower Kulfo catchment, Southern Ethiopia. This study was conducted to develop soil moisture-based irrigation intervals and to evaluate irrigation water delivery and field level irrigation efficiencies. Soil water content, and flow along the canal and in the field were measured directly, and irrigation duty was estimated by using CropWat 8 model. To minimize water stress or excess problems, irrigation needs to be applied when soil water content drops to 35.7 % for onion and pepper, 34.4 %% for watermelon, and 32.5 % for wheat and maize from field capacity. However, irrigation was applied at 36.2 % for onion, 35.4 % for watermelon, 36.4 % for pepper, 36.2 % for maize, and 35 % for wheat in the existing irrigation scheme that increased irrigation amount in the field. The average percentage of soil moisture depletion (p) at time of irrigation was 27.4 %, which was below the recommended value. The average adopted irrigation and design irrigation interval were 4 & 6 days for onion and pepper, 5 & 7 days for watermelon and wheat, and 6 & 7 days for maize, respectively. The mean irrigation adequacy and dependability of the irrigation scheme in the lower Kulfo catchment were 1 & 0 for Arba Minch irrigation scheme, 0.5 & 0.2 for Arba Minch University farmland, 0.4 & 0.25 for private farmland and 0.1 & 0.43 for Kollashara farmland, respectively. The value of irrigation equity was 0.7 in January, 0.6 in February, and 0.8 in March which indicates the highly temporary variation of irrigation adequacy. The mean value of canal conveyance was 82.7 % and the average on-farm irrigation efficiency also was 56.6 %. The average value crop yield in the present study were 0.5ton/ha for wheat, 4.9ton/ha for onion, 6.2ton/ha for pepper, 0.6ton/ha for watermelon, 4.2ton/ha for maize that was very low compared with other control irrigation in the study area. Inadequate soil moisture-based intervals, inequitable water distribution, and variable supply hinder irrigation in the Lower Kulfo catchment; adopting optimized practices and robust management can enhance efficiency, equity, and crop productivity.

Using UAV-thermal imaging to calculate crop water use and irrigation efficiency in a flood-irrigated pecan orchard

Pecans are a major crop in New Mexico’s Lower Rio Grande Valley (LRGV) where about 30 % of pecans are produced in USA. Due to low amount of precipitation in the LRGV, pecans are highly depended on irrigation from surface and ground water. The area has gone through two decades of drought which has significantly reduced the availability of surface water. There is an urgent need for proper estimation of crop water use and irrigation scheduling to improve water use efficiency and productivity. The objective of this study is to extend a recently developed model for calculating real time pecan water use (ETa) from UAV-thermal imaging when the canopy cover is variable. The extended methodology was used to calculate ETa and seasonal irrigation efficiency. The calculated seasonal irrigation efficiency was compared with irrigation efficiency estimated by chloride tracing method. The results showed the proposed methodology can be used to calculate ETa and on-farm irrigation efficiency in the area when pecan canopy cover is changed.

The impact of water-pricing policies on water productivity: Evidence of agriculture sector in Iran

Water pricing and reducing the costs of the implementation and maintenance of irrigation systems have been a challenging topic for a long time. Water pricing issues combine two complex dimensions. The first dimension is the economy of the farm and its association with the broader economic system and farming practices. The second dimension is the hydrology and interconnection of the plot with the irrigation system, the river basin, and the underlying water policy framework. Accordingly, this study aimed to investigate how the institutionalization of water applied would affect water productivity through the implementation of a water-pricing policy. To this end, this study formulated a Positive Mathematical Programming (PMP) model with a gross margin maximizing objective function to assess the data collected from 213 farms in Hamadan-Bahar Plain, Iran. The results revealed that a water-pricing policy can change the cropping pattern and also irrigation system within the limitations of the case study. It can also be a driver to motivate farmers to use a modern and more efficient irrigation system. The potential environmental benefits from the improved on-farm irrigation efficiency depend on the government's ability to prevent the extension of the irrigated area. In absence of such controls, the aquifer depletion will be accelerated. Therefore, an appropriate water-pricing policy could improve water productivity and also reduce water applied.

Improving groundwater recharge estimates in alfalfa fields of New Mexico with actual evapotranspiration measurements

Quantifying groundwater recharge from irrigation practices of arid regions is necessary for efficient and sustainable groundwater resources management. Recharge is difficult to measure directly, many studies have quantified recharge from agricultural fields as a residual of the soil water balance method (WBM) or by other mathematical models estimating evapotranspiration (ET) and measuring soil moisture, rainfall, and irrigation. ET is often estimated using weather parameters from climate stations that are distant from crops. Flooding of fields is the most common form of irrigation used by farmers in the Mesilla Valley, New Mexico to grow alfalfa, one of the state’s major crops. The objectives of this study were to quantify recharge in flood irrigated alfalfa fields in the Mesilla Valley, New Mexico using the WBM and actual ET measurements during the 2017 growing season. The study fields were different sizes, crop ages, and soil textures. Actual ET of alfalfa was measured on the largest field using the eddy covariance − energy budget method and applied to the two other nearby fields as estimated ET. All other WBM parameters were measured at each study site (i.e. soil moisture, rainfall, and applied irrigation). Recharge using ET referenced to grass (ETsz) was then calculated using weather parameters and compared with measured ET. Recharge ranged from 618 to 716 mm with no significant statistical differences between the three alfalfa fields (f-ratio of 0.8876 and a p-value of 0.42 using ANOVA). Recharge to irrigation ratio ranged from 37% to 45%. On-farm irrigation efficiency ranged from 54.86% to 59.94%. When compared to measured ET, ETsz underestimated recharge of alfalfa by 11% in the WBM and overestimated recharge by 6% when a reduction in ETsz during harvest periods are considered. The results show that the measurement of actual ET improved recharge estimates in the alfalfa fields.

Factors that Influence the Rate and Intensity of Adoption of Improved Irrigation Technologies in Alberta, Canada

Despite the importance of adopting improved irrigation technologies to increase on-farm irrigation efficiency, our understanding of what determines farmers’ adoption decisions in southern Alberta remains relatively poor. The overall goals of this study are to examine the extent of adoption (proportion of all irrigators that have started the adoption process), how far along they are in the adoption process, and the intensity of adoption (percentage of irrigated land on which the technology is adopted) of improved irrigation technologies in southern Alberta, and to assess the major factors that influenced farmers’ adoption decisions. The data were collected in a farm-household survey conducted in the 12 largest irrigation districts (IDs) as well as among private irrigators in southern Alberta. Results show that adoption of improved irrigation technologies is widespread at various levels of intensity. By 2011, 81.3% of farmers had started the adoption process, are now using some kind of improved technology to apply water to their crops, and used it on 76.8% of all irrigated land. The most commonly used irrigation technology is a low pressure center pivot system. Receiving support services following the adoption decision played an important role in increasing the intensity of adoption. Obtaining information on irrigation technologies from individual farmers or farmers’ associations, and extension agencies significantly influenced farmers’ decisions to adopt. Farmers who increased their social capital through attending meetings related to agricultural production practices were more likely to adopt while farmers who participated in recreational or social organizations were less likely to adopt. Finally, the extent and intensity of adoption are higher for those with corporate farm structure, larger families, more generations of ownership and higher education.

Water budget for agricultural and aquatic ecosystems in the delta of the Colorado River, Mexico: Implications for obtaining water for the environment

In arid lands, wetland loss is the result not only of the scarcity of water itself, but also of the management of water to maximize off-stream uses. In the Colorado River delta, Mexico, no in-stream flows are allocated for aqutic ecocystems, yet agricultural return flows and canal operational releases support 36,377ha of valuable wetland and riparian habitat. We evaluated the relationships between water use in the Mexicali Irrigation District (DR014) and the water supply for the Colorado River delta wetlands. Mexicali farmers applied on average 1024mmyr−1 to fields in 2008, less than half of what is applied to irrigated fields in a typical irrigation district in the U.S. portion of the river. Farm-level agricultural efficiency is high, as 90% of water applied to fields can be accounted for in evapotranspiration (ET), as estimated by a remote sensing technique and by a district water budget approach. By contrast, in a comparison U.S. irrigation district, ET accounts for only 60% of applied water and the rest emerges as irrigation return flows. Despite the apparent high on-farm irrigation efficiency in DR014, sufficient water is discharged to support valuable ecosystems. Seepage losses from canals are higher than in the comparison district in the U.S., and this water contributes to formation of a high, non-saline aquifer that supported riparian trees along the river. On the other hand, surface drainage water supports brackish wetland habitats south of the irrigation district. The goal of obtaining water for environmental uses by increasing agricultural efficiency in the irrigation districts would not by itself be an effective strategy for the delta aquatic habitats as the delta wetlands and riparian areas are currently dependent on canal seepage, return flows and waste spills from both the U.S. and Mexico.

Evaluating on-farm irrigation efficiency across the watershed: A case study of New Mexico's Lower Rio Grande Basin

Irrigation efficiency is a critical factor in irrigation water management. Irrigation efficiency is used in economic analysis when selecting an irrigation system design, and in irrigation management. It is also used in water rights adjudication and administration. On-farm irrigation efficiency is spatially and temporally variable and measuring irrigation efficiency is time consuming and costly. This paper describes a process to evaluate on-farm irrigation efficiency across the watershed using a combination of remote sensing and ground level measurements. On-farm irrigation efficiency was evaluated for three major crops in New Mexico's Lower Rio Grande Basin (LRG). The results of on-farm irrigation efficiency evaluation of 152 alfalfa fields, 189 pecan fields and 38 cotton fields showed that the average on-farm irrigation efficiency was 64%. However, on-farm irrigation efficiency values ranged from 11% to 95%. Accounting for delivery efficiency of 54%, the overall district efficiency was calculated as 35%. The study shows significant potential for improving irrigation efficiency in the LRG watershed; however, the inefficiency of the system is a major factor in recharging and sustaining the local aquifer. Significant improvement in on-farm and delivery efficiency can potentially change the hydrologic balance and result in depletion of the historically stable groundwater of the Mesilla Bolson.

Impacts of Combined Technical and Economic Measures on Water Saving in Agriculture under Water Availability Uncertainty

In Emilia Romagna region (northern Italy), the Water Protection Plan (WPP) — a tool designed to reach the objectives of the Water Framework Directive — established different measures to achieve a good quantitative status of water bodies. The objective of this study is to evaluate the combined economic, agronomic and environmental impacts of four measures of the WPP on the “Renana” reclamation and irrigation scheme, and to quantify the water conservation in agriculture under the uncertainty of water availability. To this purpose, a mathematical stochastic model able to represent uncertainty in water availability was designed and implemented. Some data were collected from farms, and local sources were used as well to test the model. Each analyzed measure was simulated in a single scenario, and subsequently combined with other measures. The performed simulations are the following: application of volumetric water pricing (Scenario WFD1), awarding incentives for farmers to improve on-farm irrigation efficiency (Scenario WFD2a), combination of WFD1 and WFD2a (Scenario WFD2b), reduction of water abstraction in case of water shortage (Scenario WFD3a), combination of WFD1 and WFD3a (Scenario WFD3b), increasing distribution efficiency in the channel system (Scenario WFD4a) and combination of WFD1 and WFD4a (Scenario WFD4b). Results have shown that scenarios WFD2a and WFD4a generate a higher income for farmers and produce a positive ecological impact (water use and soil cover) but tend to increase total water use. Scenarios WFD1 and WFD3a have the opposite effects since they reduce farmers’ income but decrease water use as well. Only WFD1 could be favorable for the Board governing the scheme (RIB) in terms of economic returns. Finally, results have shown that combined scenarios WFD2b and WFD4b, respectively, could modify the trade-off between economic and environmental objectives and could be more effective since they significantly reduce water consumption and minimize losses in farmers’ income.

Irrigation evaluation based on performance analysis and water accounting at the Bear River Irrigation Project (U.S.A.)

Abstract The purpose of this work is to contribute to the development of a combined approach to evaluate irrigated areas based on: (1) irrigation performance analysis intended to assess the productive impacts of irrigation practices and infrastructures, and (2) water accounting focused on the hydrological impacts of water use. Ador-Simulation, a combined model that simulates irrigation, water delivery, and crop growth and production was applied in a surface irrigated area (1213 ha) located in the Bear River Irrigation Project, Utah, U.S.A.. A soil survey, a campaign of on-farm irrigation evaluations and an analysis of the database from the Bear River Canal Company and other resources were performed in order to obtain the data required to simulate the water flows of the study area in 2008. Net land productivity (581 US$ ha −1 ) was 20% lower than the potential value, whereas on-farm irrigation efficiency (IE) averaged only 60%. According to the water accounting, water use amounted to 14.24 Mm 3 , 86% of which was consumed through evapotranspiration or otherwise non-recoverable. Gross water productivity over depleted water reached 0.132 US$ m −3 . In addition, two strategies for increasing farm productivity were analyzed. These strategies intended to improve water management and infrastructures raised on-farm IE to 90% reducing the gap between current and potential productivities by about 50%. Water diverted to the project was reduced by 2.64 Mm 3 . An analysis based on IE could lead to think that this volume would be saved. However, the water accounting showed that actually only 0.91 Mm 3 would be available for alternative uses. These results provide insights to support the decision-making processes of farmers, water user associations, river basin authorities and policy makers. Water accounting overcomes the limitations and hydrological misunderstandings of traditional analysis based on irrigation efficiency to assess irrigated areas in the context of water scarcity and competitive agricultural markets.

SURFACE IRRIGATION PERFORMANCE IN THE UPPER TUNUYÁN RIVER BASIN MENDOZA, ARGENTINA

The irrigated area in the Upper Tunuyan River Basin comprises 41,601 ha with surface irrigation rights. It is, therefore, essential to develop indicators to determine irrigation water use efficiency -which will, in turn, help to detect the main problems and find solutions for the sustainable development of the basin. The overall objective of this study is to obtain information on irrigation water use within the agricultural properties in the command area of the Upper Tunuyan River and to determine the potential efficiencies to be achieved considering possible operational changes and salt balance. Surface irrigation methods, with and without tailwater runoff, prevail in the basin. Walker and Skogerboe's methodology (1987) and the ASAE standards (1999) were used. Soil and irrigation characteristics include: furrow irrigation with runoff; sandy-loam soil textures; 0.87% slopes; soil salinity from 0 to 100 cm: 1.12 dSm -1 ; irrigation water salinity: 0.70 dS m ; managed and unit flows of 53 L s -1 and 1.24 L s -1 m -1 respectively; 123 m long irrigation plots; 7-h application periods; and 5.3 mm h -1 basic soil infiltration. The following results were obtained: on-farm irrigation efficiency; and conveyance, application, storage and distribution efficiencies: 91, 43, 99 and 91%, respectively. Potential efficiency may rise to 68% if plot irrigation management is improved; it may well be higher than 68% if the salt balance is taken into account.

Protecting water resources in biofuels production

The Energy Independence and Security Act of 2007 mandates a five-fold increase in current biofuels production. This will substantially increase the current production of corn-based ethanol despite widespread concerns over its negative impact on the availability and quality of already stressed water resources. The National Academy of Sciences' (NAS) recently-released report on water use in biofuels production (principally corn-based ethanol) proposes improved on-farm irrigation efficiency and water recycling within biorefineries to reduce the impact on water availability. In a classic case of unintended consequences, we demonstrate that these measures may decrease the sustainability of biofuels production by increasingly stressing water supplies. Instead, these measures may be more effective in reducing the water quality impact of biofuels production—a role that the NAS report does not emphasize.

A Systems Engineering Approach to Viticulture On-Farm Irrigation

Water resources management presents an important research topic, because our planet is facing a serious water crisis. About 70% of all fresh water usage goes towards agriculture. Moreover, low water application efficiency are well reported in the literature. Improving on-farm irrigation efficiency can make a substantial contribution to a more sustainable utilization of the world's fresh water resources. It is argued that systems engineering principles can assist to realize the goal of improving water efficiency or produce quality in on-farm irrigation whilst maintaining productivity and quality of service. In approaching this resource management problem, wireless sensor network technologies and automation ideas are combined to improve economic productivity in dairy, horticulture and viticulture industries in such a way as to support continued growth in these major food industries in the face of a competitive water market. This paper reports the early progress of the project on smart irrigation system for viticulture and initial attempt in modeling the viticulture soil-water dynamics is briefly discussed. The results obtained are encouraging indicating that water automation is a promising technology.

Agricultural and environmental changes after irrigation management transfer in the Develi Basin, Turkey

Develi Basin is a semi-arid basin in central Turkey where water sustains both irrigated agriculture and an internationally important wetland, the Sultan Marshes. Agricultural and environmental changes in the Develi Basin have occurred since irrigation management was transferred in 1994 from a state authority (DSI) to irrigation associations (Kovali and Agcaasar IAs). In this paper we evaluate the practices of the IAs using extensive data from interviews with farmers and IA officials, as well as data from reports prepared by DSI and the IAs, using comparisons with case studies reported in the scientific literature. Irrigated areas and surface water use in the Develi Basin showed significant fluctuations from 1995 to 2003. The area allocated to high water-consuming plants increased. Maintenance activities became dependent on fee collection rates. Quality of the irrigation water did not changed significantly. Ground-water levels, flow rates from springs, and water levels in the Sultan Marshes all dropped. Overall analyses indicate that the water requirements of the Sultan Marshes have not been met, while water use for irrigation has been effective but not efficient. To reconcile agricultural and wetland water requirements, a basin-wide approach in water planning is recommended. Amounts of water to be allocated to the IAs and wetlands need to be clearly defined. DSI has to monitor canal maintenance by the IAs more closely, and IAs need to be given more responsibilities for future rehabilitation of the canals. Realistic water pricing, increased reliability of irrigation scheduling, higher on-farm irrigation efficiency, and in the long-term, modernization of the irrigation system need to be considered.

Measuring On-Farm Irrigation Efficiency with Chloride Tracing under Deficit Irrigation

Water is a limited resource in agricultural production in arid climates. Under such conditions, high irrigation efficiency can be obtained either through implementation of efficient irrigation systems such as drip or sprinkler systems or through the age-old practice of deficit irrigation with gravity systems. The method used to increase irrigation efficiency is often dictated by economic and/or social factors. In either case, the effectiveness of water management at the farm level needs to be evaluated by measuring irrigation efficiency. The objective of this study was to evaluate the irrigation efficiencies for three crops in Southern New Mexico using the chloride technique. The chloride technique is a simple method in which the natural chloride in the irrigation water is used as a tracer to estimate the leaching fraction and the irrigation efficiency at the farm level. Soil samples were collected from various fields in 15 cm increments to a depth of 180 cm at the end of the irrigation season. The samples were analyzed for moisture and chloride content. In addition to the chloride technique, on-farm irrigation efficiencies were measured using applied water, yield, and water production functions. Water production functions and yields were used to estimate total evapotranspiration while flow measurements were used to calculate the amount of applied water. The results showed that high irrigation efficiency can be accomplished using deficit irrigation. Irrigation efficiency values ranged from 83 to 98%. Irrigation efficiencies using the chloride technique were compared with efficiencies estimated from direct flow measurements. The differences between the two methods ranged from 2 to 11.4%. The results showed that even though the chloride technique is subject to sampling errors and simplified theoretical assumptions, it can be used to estimate on-farm irrigation efficiency with considerable accuracy.

Farm size, irrigation practices, and on-farm irrigation efficiency

Relationships between farm size, irrigation practices, and on-farm irrigation efficiency in the Elephant Butte Irrigation District, New Mexico, USA are explored using 2001 water delivery data supplied by the irrigation district. The study area is experiencing rapid population growth, development, and competition for existing water supplies. It is conventionally assumed that in the future water will be transferred from agriculture to other uses. Analysis of pecan orchard water delivery data, fieldwork, and interviews with irrigators found extremely long irrigation durations, inefficient irrigation practices, inadequate on-farm infrastructure, and lack of interest in making improvements to the current irrigation system or methods on the smallest farms. These findings are attributed to the nature of residential/lifestyle or retirement agriculture. Irrigation practices on large, commercial orchards are notably different from the smallest farms: irrigation event durations are shorter, less water is applied, and the producers are commercially oriented. With respect to future increases in the efficiency of irrigation water usage, large, commercially oriented producers already have a high level of physical efficiency. Small producers appear to view irrigation as a consumptive, recreational, social, or lifestyle activity, rather than an income-generating pursuit, thus the cost of inducing changes in their practices may be extremely high. Copyright © 2005 John Wiley & Sons, Ltd.

Water balance and irrigation performance analysis: La Violada irrigation district (Spain) as a case study

Diagnosis of water management at the irrigation district level is required for the rational modernisation of the irrigation schemes and the subsequent increase in the efficiency of water allocation and application. Our objectives were to: (i) evaluate the global irrigation performance in the 5282 ha La Violada surface-irrigated district (Ebro River Basin, northeast Spain), and (ii) estimate the water that could potentially be conserved under two scenarios of modernisation and three increased irrigation efficiencies. The main district’s water inputs and outputs were measured (irrigation, precipitation, and outflow surface drainage) or estimated (canal releases, lateral surface runoff, municipal wastewaters, and actual evapotranspiration of crops) during the 1995–1998 hydrological years. The annual average water outputs were 23% higher than the corresponding water inputs, presumably due to canal seepage and lateral groundwater inflows from the 14 355 ha dry-land watershed. The district-level irrigation performance was poor (mean 1995–1998 seasonal irrigation consumptive use coefficient (ICUC)=48%), due to the low distribution (DE) and on-farm (ICUCf) efficiencies (i.e., mean estimates of 83% (DE) and 61% (ICUCf) for the 1995–1996 irrigation seasons). Thus, despite the high volume of applied irrigation water, the actual district ET was 16% lower than the maximum achievable ET, indicating that the water-stressed crops yielded below their maximums. Potential reductions in water allocation were estimated for three ICUC values (65, 75 and 85%) and two scenarios of modernisation (I and II). In scenario I, where the aim was to achieve maximum ET and crop yields, water allocation could be reduced from 8 to 30% of the current allocation. In scenario II, where the aim was to achieve the maximum conservation of water under the actual ET and crop yields, reductions in water allocation would be much higher (from 26 to 43% of current allocation). Thus, significant volumes of water could be conserved in the rehabilitation of this 50-year-old district by increasing the distribution efficiency and, in particular, the on-farm irrigation efficiency.

A Theoretical Analysis of Economic Incentive Policies Encouraging Agricultural Water Conservation

A conceptual model of a representative irrigated farm is formulated to study farm responses to two economic policies commonly suggested to encourage agricultural water conservation, and to characterize the hydrological and economic circumstances in which these responses provide the desired conservation. The economic policies studied are to increase the irrigator's cost of applied water and to subsidize the irrigator's cost of investing in improved on-farm irrigation efficiency. Comparative statics results demonstrate that increasing the cost of applied water may be a more effectual water conservation policy than subsidizing the cost of improved on-farm irrigation efficiency.

The allocative efficiency and conservation potential of water laws encouraging investments in on-farm irrigation technology

Agricultural water conservation statutes are emerging in the West encouraging private irrigators to improve on-farm irrigation efficiency as a basinwide conservation measure. We investigate whether private improvements promote the economic efficiency and conservation of water use basinwide under a wide variety of hydroeconomic circumstances. The standard of efficiency is how an irrigation district manager should optimally invest in improving the irrigation efficiencies of individual farms located along a stream while internalizing intrabasin allocative externalities of these investments. The results indicate that the popular Oregon legislative model may be the least effective in conserving water and promoting economically efficient water allocation.

Minimizing salt in drain water by irrigation management — Leaching studies with alfalfa

An alfalfa experiment was conducted in the Wellton-Mohawk Irrigation and Drainage District of Southwest Arizona to determine the potential for minimizing the salt load in irrigation return flow by decreased leaching. Three leaching treatments of 5, 10, and 20%, replicated five times, were imposed on a 2-ha field. The crop was irrigated with Colorado River water (electrical conductivity of 1.3 dS m −1) through a lateral-move, spray-type irrigation system. Results were compared with those of an adjacent area irrigated with level basin flooding. The average annual evapotranspiration during the 4-year study was 1930 mm. Several indirect measures of the leaching fractions attained gave average values of 6.4, 9.3, and 13.1% for the three leaching treatments. Mean annual yields were 21.5 and 22.9 Mg ha −1 for 1977 and 1978 in the experimental plots, with no significant differences among leaching treatments, and 25.7 and 20.8 Mg ha −1 in the adjacent flooded check. The results suggest that full yields could be attained with as little as 5% leaching. Estimates based on average on-farm irrigation efficiency for alfalfa in the District in 1979, indicate that 5% leaching, if attainable, would reduce the salt load in the irrigation return flow by 39 000 Mg year −1 on 8 000 ha of alfalfa.