Balancing water scarcity and quality for ... - Wiley Online Library

PUBLICATIONS Water Resources Research RESEARCH ARTICLE 10.1002/2015WR017071

Special Section: The 50th Anniversary of Water Resources Research Key Points: Irrigation with treated effluent has adverse impacts on soil properties Irrigation with desalinated water improves yields and save water Quantitative models are used to delineate trends

Correspondence to: S. Assouline, [email protected]

Citation: Assouline, S., D. Russo, A. Silber, and D. Or (2015), Balancing water scarcity and quality for sustainable irrigated agriculture, Water Resour. Res., 51, 3419–3436, doi:10.1002/ 2015WR017071. Received 10 FEB 2015 Accepted 17 APR 2015 Accepted article online 21 APR 2015 Published online 8 MAY 2015

Balancing water scarcity and quality for sustainable irrigated agriculture Shmuel Assouline1, David Russo1, Avner Silber2, and Dani Or3 1 Department of Environmental Physics, Institute of Soil, Water, and Environmental Sciences, A.R.O.—Volcani Center, Bet Dagan, Israel, 2Northern R&D, Rosh Pina, Israel, 3Department of Environmental Systems Science (D-USYS), Swiss Federal Institute of Technology (ETH), Zurich, Switzerland

Abstract The challenge of meeting the projected doubling of global demand for food by 2050 is monumental. It is further exacerbated by the limited prospects for land expansion and rapidly dwindling water resources. A promising strategy for increasing crop yields per unit land requires the expansion of irrigated agriculture and the harnessing of water sources previously considered ‘‘marginal’’ (saline, treated effluent, and desalinated water). Such an expansion, however, must carefully consider potential long-term risks on soil hydroecological functioning. The study provides critical analyses of use of marginal water and management approaches to map out potential risks. Long-term application of treated effluent (TE) for irrigation has shown adverse impacts on soil transport properties, and introduces certain health risks due to the persistent exposure of soil biota to anthropogenic compounds (e.g., promoting antibiotic resistance). The availability of desalinated water (DS) for irrigation expands management options and improves yields while reducing irrigation amounts and salt loading into the soil. Quantitative models are used to delineate trends associated with long-term use of TE and DS considering agricultural, hydrological, and environmental aspects. The primary challenges to the sustainability of agroecosystems lies with the hazards of saline and sodic conditions, and the unintended consequences on soil hydroecological functioning. Multidisciplinary approaches that combine new scientific knowhow with legislative, economic, and societal tools are required to ensure safe and sustainable use of water resources of different qualities. The new scientific knowhow should provide quantitative models for integrating key biophysical processes with ecological interactions at appropriate spatial and temporal scales.

1. Introduction The human population is estimated to exceed 9 billion by 2050, representing a 30% increase of the current global population [Roberts, 2011; Tilman et al., 2011; Foley et al., 2011]. The increase in population and changes in income and diets are expected to increase the demand for food by 70–100% from current levels [Tilman et al., 2011; World Bank, 2008; Evans, 2009; Kearney, 2010, Gregory and George, 2011]. At the same time, the prospects for substantial agricultural land expansion to meet the increased demand for food by a larger and more affluent population are limited. While the extent of convertible land area to agricultural use €m et al., 2009; Running, 2012], the expansion of agriremains a subject of debate [Smith et al., 2010; Rockstro cultural land would contribute only 20% of the required increase in crop production [Foley et al. 2011; Smith et al., 2010; Fritz et al., 2013]. The primary contribution to meet the expected rise in demand for food must come from increasing crop yields and food distribution efficiency rather than from land expansion [Barnosky et al., 2012; Godfray et al., 2010]. Considering that most of the agriculture in the world is rainfed [Rost et al., 2008], an important strategy is to enhance the role of efficient irrigated agriculture, thereby increasing crop yield per unit land. A transition from rainfed to irrigated agriculture under water scarce conditions could increase crop yields by a factor of 3 on average [Howell, 2001].

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Limited water resources in regions where increased crop production is most needed presents a major constraint to the expansion of irrigated agriculture. Irrigation presently amounts to nearly 70% of fresh water (FW) withdrawals (rivers, lakes, aquifers), yet accounts for only 10% of global agricultural water use (the balance is ‘‘green water,’’ the amount of water used by rainfed agriculture). The pressure on the global freshwater resources is steadily increasing, and will be amplified in countries chronically short of water where the population is projected to increase from half to four billion people by 2050 [Evans, 2009; Taikan and

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Kanae, 2006]. Hence, despite the typically high efficiency of irrigation (supporting 30% of crop production with 10% of total water used for agriculture), the real potential of irrigation in closing the food production gap would vary significantly among geographical regions (most significantly in arid and semiarid regions). Even in scenarios where irrigation expansion is possible, the growing competition for FW resources for €m, 2006]. Consequently, alternative water domestic use must be considered [Falkenmark and Rockstro resources for irrigation must be developed and supported by advanced and environmentally sustainable irrigation water management schemes [Gleick, 2000; Tal, 2006; Grant et al., 2012]. In addition to issues related to water quantity, the quality of irrigation water plays an important role in the sustainability of irrigated lands, especially in the context of salinity buildup that could adversely impact agricultural crop productivity [Maas and Hoffman, 1977; Bresler et al., 1982; La€ uchli and Epstein, 1990; Pitman and La€ uchli, 2002]. By some estimates, about 20%–50% of the global irrigated land is salt-affected to some extent [Ghassemi et al., 1995; Flowers, 1999; Tanji, 2002; Pitman and La€ uchli, 2002]. Application of saline water containing high concentrations of sodium affects soil hydraulic properties and reduces soil permeability [Bresler et al., 1982; Shainberg and Letey, 1984; Russo, 2005]. Presently, the adverse impacts of salinity on agricultural land degradation and loss of productivity are estimated at $12 billion per year, with more land expected to be affected due to deteriorating water quality [Ghassemi et al., 1995]. The projected intensification and expansion of irrigated agriculture would invariably enhance the risk of salinization due to the growing reliance on marginal sources of water available in the arid regions with high population growth and where irrigation expansion is most needed. Adverse effects of irrigation with low water quality could be enhanced by modern practices that incorporate copious amounts of fertilizers with irrigation water. Traditional salinity management schemes rely on root zone leaching (the concept of leaching fraction) that, in turn, may discharge salts to groundwater and surface water resources [Ayers and Westcot, 1985; Ghassemi et al., 1995] leading to a vicious cycle with the gradual increase in water demand for irrigation. Irrigation-induced salinity buildup and ultimate collapse of agricultural production are among the earliest man-made ecological disasters responsible for the demise of the civilizations of Mesopotamia and the Indus valley [Hillel, 1991; van Schilfgaarde, 1994; Ghassemi et al., 1995]. A rapidly expanding alternative source for water irrigation in regions with limited freshwater resources is treated effluents (TE) [Hamilton et al., 2007; Qadir et al., 2007; Pedrero et al., 2010]. The volumes of available TE are proportional to the steadily increasing demand for freshwater (FW) for domestic use worldwide, with 80% of the urban ‘‘blue water’’ becoming TE in the developed world, at a rate of 100 m3/yr per household. The ecological footprint of untreated effluent is unsustainable even in regions where water is plentiful (e.g., South East Asia) due to alternation of nutrient loads in rivers and coastal regions and health hazards. At the other extreme, the increased reliance on treated effluent for irrigation in arid regions is often practiced with little consideration of long-term impact on soil, hydrology, and ecology of the producing area. The benefits of TE for irrigation, and conservation of local natural water resources [Murray and Ray, 2010] are not without certain drawbacks. Recent studies have shown that long-term effects of TE irrigation resulted in a significant degradation of soil structure and hydraulic properties due to increased exchangeable sodium percentage (ESP) [Coppola et al., 2004; Lado et al., 2005; Levy and Assouline, 2011; Assouline and Narkis, 2011; 2013]. Evidence from other studies have shown other negative effects related to chemical aspects [Xiong et al., 2001; Wallach et al., 2005; Lado et al., 2012], and human health and other ecological risks associated with introduction of pathogenic microorganisms, heavy metals, and toxic organic compounds into the soil and crop [Aiello et al., 2007; Toze, 2006; Pedrero et al., 2010; Scheierling et al., 2010; del Mar Alguacil et al., 2012]. Hence, the sustainability of a coupled agro-urban hydrological cycle where TE is used for irrigation hinges on proper management to mitigate adverse impacts of long-term TE application to avoid potential collapse of soil ecological functions. Along with the expansion of TE for irrigation, the use of desalinated sea and brackish water at large-scale for irrigation [Grant et al., 2012] is rapidly becoming feasible with advances in desalination techniques and dramatic reduction in desalination costs [Beltran et al., 2006; Tal, 2006; Elimelech and Phillip, 2011]. Desalinated water (DS) is becoming a competitive source of water for irrigation, especially for high cash crops. Spain ranks first in the world in terms of use of DS in agriculture, where a significant part (25%) of the total production of DS is allocated to irrigation [Medina, 2006; Veza, 2006]. In Israel, due to fluctuations in domestic water demand, DS is sporadically allocated to agriculture, revealing the need to adapt special fertilization protocols to this mineral-free water [Yermiyahu et al., 2007; Ben Gal et al., 2009]. Beside the evident, positive

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impact of water desalination on water resources and environment, such as increasing good-quality water availability and recycling poor-quality water, it presents also several negative impacts for the environment, mainly: brine disposal from desalination process, chemical additives used for antifouling, and anticorrosivity; and high consumption of energy that may increase emission of greenhouse gases. The projected intensification of agriculture with expansion of irrigated areas and application of marginal water will undoubtedly affect an already fragile environment and could threaten the sustainability and functionality of such agroecosystems. The challenge is thus to devise strategies for increasing food production while preserving soil ecological functionality, minimizing human health risks, and ensuring sustainable land and water resources for agricultural use. Among the scientific tools necessary for efficient guidance of future management scenarios are advanced models capable of simulating the complex interactions between physical, chemical, and biological processes taking place in the soil that would enable hypothesis and scenario testing to provide reliable predictions of outcomes. Such tools should contribute to improved understanding of conditions associated with salinization and contamination hazard, and help design of efficient ways to control and minimize damage to agricultural production and environment quality. The development of such tools that bridge basic research, management, policy, and societal needs would require a monumental interdisciplinary effort by experts from many fields (it is certainly beyond the scope of this or any single study). We thus limit our study to identification of the water-related risks of irrigated agriculture in a changing world, and some of the most critical knowledge gaps, that must be addressed for sustainable and environmentally responsible intensive (irrigated) agriculture. These gaps are of several types: (1) knowledge gaps that represent practices that enhance direct risks to public health (antibiotic resistance induced by wastewater use), or to the ecological functioning of the soil system; (2) areas that are poorly studied and not well understood (interactions of marginal water with biological and ecological components); (3) unknown impacts of future forcing such as climate extremes on agroecosystem sustainability. Naturally, the emphasis of certain knowledge or technological gaps over others, reflect our best present understanding on the areas with most risk or most promise. The main objective of this study is to provide a systematic evaluation of the scope and potential risks associated with the projected expansion of irrigated agriculture in arid and semiarid regions, focusing on challenges associated with increased use of marginal water (saline water, treated effluent, and desalinized water) with respect to sustainability and soil function. The study is organized as follows: following this introduction, we review the strategy associated with the use of saline water for irrigation. We then discuss the issue of allocating TE for irrigation, and present experimental evidences for adverse impacts resulting from long-term exposure to TE. We introduce DS irrigation and describe its benefits based on field data from a recent experiment is Israel. We finally conclude with an outlook on expansion of irrigation with marginal water sources, considering several issues affecting sustainability, such as the role of irrigation methods, extreme climatic events, soil ecology, and the accelerated agro-urban water cycle.

2. Agricultural, Hydrological, and Environmental Impacts of Irrigation With Marginal Water 2.1. The Use of Saline Water for Irrigation 2.1.1. Overview of the Impact of Water Salinity in Agriculture Globally, about 33% of the potentially arable land area is salt-affected, with 950 million ha in arid and semiarid regions. About 20% of the global irrigated land (450,000 km2) is salt-affected and every day 2000 ha of farm land is lost to salt-induced degradation [Nellemann et al., 2009; Qadir et al., 2014]. Some of the most severely salt-degraded regions include the Aral Sea Basin, Central Asia; Indo-Gangetic Basin, India; Indus Basin, Pakistan; Yellow River Basin, China; Euphrates Basin, Syria and Iraq; Murray-Darling Basin in Australia, and the San Joaquin Valley in California. The inflation-adjusted cost of salt-induced land degradation in 2013 was estimated at $440 per hectare, yielding an estimate of global economic losses in excess of $27 billion per year [Qadir et al., 2014]. Crop response to the spatial and the temporal distributions of soil water content and soil salinity is complex and not fully understood. Soil water content and soil salinity interact and the partitioning of their respective effects on crop response is difficult to separate. As water evaporates from the soil or transpires through the

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plant, salts are left in the soil and gradually increase osmotic effects that directly affect plant ability to take up water. The effects of osmotic and capillary components of the soil solution pressure head on plant transpiration could be considered either additive [Childs and Hanks, 1975; Bresler and Hoffman, 1986; Bresler, 1987; Bras and Seo, 1987; van der Zee et al., 2014] or multiplicative [van Genuchten, 1987; Simunek et al, 1999; Hanson et al., 2008]. Under daily drip irrigation where the most active soil volume remains wet, there was practically no difference between the two approaches to accounting for these two components of the water potential [Russo et al., 2009]. Regardless, the combined effect of the osmotic and capillary components of the soil solution water potential limits plant water uptake and transpiration, hence, reduces crop yields. Data on salinity sensitivity of many crops provide specific thresholds for concentration of solutes in the soil solution, cT, that could be tolerated by each crop without impacting yields significantly, and values for the expected decrease in crop yield when the soil salinity exceeds cT [Maas and Hoffman, 1977; Ayers and Wetscot, 1985; Maas, 1990]. Some of the strategies allowing the use of saline water (SW) for irrigation while limiting yield losses include mixing water of different qualities, selection of salt-tolerant crops, and avoidance of overly sensitive soils. Another strategy (under debate) advocates compensating for high salinity water by increasing the irrigation dosage above plant transpiration demand [Russo and Bakker, 1987; Shani and Dudley, 2001; Shani et al., 2007; Dudley et al., 2008]. Technically, this means that an additional amount of water beyond estimated plant needs is allocated to leach the salinity buildup in the root zone, this additional amount being larger as irrigation water salinity increases. This approach where the drainage fraction, or leaching fraction LF, is adjusted with irrigation water salinity has considerable implications in terms of environmental quality, since the solutes leached below the root zone may increase substantially the salt load toward groundwater resources. Consequently, this approach may intensify salinization problems and contribute to reduce available freshwater resources at the regional scale [Assouline and Shavit, 2004; Schoups et al., 2005; Shani et al., 2005]. The appropriate irrigation management implementing the concept of water quantity compensating for water quality is based on a simple solute mass balance approach in the root zone (applied typically for an irrigation season): cI I 1 cP P 5 cD D 1cSW DW

(1)

where I, P, and D are the rates of irrigation, rainfall, and drainage, respectively; cI, cP, and cD, their respective solute concentrations; and DW, the change in the water content in the root zone with the corresponding concentration of solutes in the soil solution, cSW. The basic idea is to apply irrigation in an amount that exceeds the water needs of the crop (or the estimated potential evapotranspiration) such that a fraction of the water will flow downward past the root zone and carry with it excess salts. Otherwise, salts accumulate in the root zone in direct proportion to the crop water uptake and their concentration in the soil solution. The leaching fraction (LF) for steady state conditions is calculated therefore as: LF 5

D cI ECI 5 ECD I cD

(2)

where ECI and ECD are the electrical conductivity of the irrigation (I) and drainage (D) waters. It states that if, for example, the maximum allowable electrical conductivity in drainage water is five times that of the irrigation water, then 1/5 of the irrigation water must drain below the root zone. For a linear plant root water uptake with depth (z), the steady state salinity concentration profile in soil water c(z) for a root zone with depth L is simply: c ðz Þ 5

cI 1 1 ð1 2 LF Þ z=L

(3)

The leaching fraction LF could be defined as the ratio between the solute concentration in the irrigation water, cI, and the solute concentration at the bottom of the root zone, cL, [Skaggs et al., 2014a]: LF5cI =cL ECI =ECL

(4)

where ECL is the electrical conductivity of the soil solution at the bottom of the root zone. The leaching requirement, LR, is the minimum LF that maintains a salinity threshold, ECT, tolerated at the bottom of the

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root zone by the crop with only a minor reduction in yield. Based on experimental data [Rhoades and Loveday, 1990; Marcar et al., 2011], the empirical relationship for the case of conventional irrigation is: LR530:86 ðECI =ECT Þ1:7

(5)

where LR is in percent. This relationship for ECT51.5 dS/m is illustrated in Figure 1. The LR for a specific crop, therefore, results from the combination of the crop sensitivity to salinity and the irrigation water salinity, but also from a priori knowledge of the thickness of the root zone and the solute conFigure 1. Leaching requirement, LR, as a function of the ratio between the electrical conductivity of the irrigation water, ECI, and a prescribed threshold centration of the soil solution in the root electrical conductivity characterizing the crop, ECT51.5 dS/m for convenzone, which in turn both depend upon the tional irrigation following Rhoades and Loveday [1990] [Marcar et al., 2011]. specific climatic (rainfall and evapotranspiThe colored dots illustrate the LR for the representative salinity of the different water qualities (TE, FW, and DS). ration) and irrigation conditions (irrigation type, rate, and frequency). Most steady state models assume a constant value of transpiration irrespective of controllable variables, i.e., irrigation water salinity, ECI, and quantity, I, [van Schilfgaarde et al., 1974; Ayers and Westcot, 1985], although such assumption is not valid in the case where salinity limits plant transpiration [Suarez, 2012]. Although true steady state conditions are rarely attained in irrigation systems, the basic steady state approach presented above, with some variations, may provide in some cases reasonable estimates when compared to experimental data [Letey et al., 1985; Shani et al., 2007; Skaggs et al., 2014a]. Skaggs et al. [2014a], for example, abandon the ‘‘leaching requirement’’ concept and instead present irrigation decision making in terms of the ‘‘irrigation requirement’’ needed to obtain a targeted yield. Another approach applies transient numerical models [Russo, 1988a, 1988b; Suarez and Simunek, 1997; Pang and Letey, 1998; Russo et al., 2004; van Dam et al., 2008; Russo et al., 2009; Simunek et al., 2013; Russo, 2013]. Such models are much more detailed and can account for soil, water, crop, and climatic variables. The latter approach should theoretically provide more reliable quantitative estimates leading to better irrigation management [Letey et al., 2011; Oster et al., 2012]. However, these models are highly parameterized and their calibration and validation against limited appropriate field data is often challenging [Rhoades, 1999; Skaggs et al., 2014b]. The LR concept described above focuses on aspects of salt balance often ignoring the complexity of soilplant-water-salt interactions or the resulting temporal and spatial distribution of the salt concentrations within the root zone [Meiri et al., 1977; Corwin et al., 2007; Russo et al., 2009]. For example, it ignores possible root-zone internal compensation mechanisms allowing plants to control their uptake and by-pass areas in the root zone with unfavorable conditions [Simunek and Hopmans, 2009; Kuhlmann et al., 2012]. Consequently, this management approach may fail to predict actual yield reductions for a given LR [Bresler and Hoffman, 1986, Ben-Gal et al., 2008] or recommend more irrigation than is necessary [Hoffman, 1985; Letey et al., 1985]. A crucial aspect linked with salinity hazard is soil sodicity. Variations in soil sodicity are related to the accumulation of monovalent sodium cations (Na1) in the soil solution, that gradually dominate the soil Cation Exchange Capacity (CEC) by displacing bivalent cations such as calcium (Ca21) and magnesium (Mg21). The level of soil sodicity is expressed by the Exchangeable Sodium Percentage (ESP): ESP5100½Naex =CEC

(6)

where [Naex] represent the exchangeable sodium and is expressed like the CEC in (meq/100 g) [Bresler et al., 1982]. In clayey soils with large values of ESP irrigated with water of low salinity or by rainfall (low ECI), severe soil structural degradation may ensue due to dispersion of clay particles and soil swelling. Such processes may significantly reduce the soil saturated hydraulic conductivity, eliminate large pores, and shift the water retention curve toward smaller pores [Russo and Bresler, 1977a, 1977b; Bresler et al., 1982; Shainberg

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and Letey, 1984; Russo, 2005]. All these effects on porosity and hydraulic functions should be accounted simultaneously. Consequently, aeration, plant water uptake, and soil leaching capabilities are reduced, thus impacting the spatial and temporal distribution of the solutes in the root zone and altering strategies based on the LR concept. The effect of sodicity is clay-type dependent and will be stronger in montmorillonitic soils than in kaolinitic ones, more common under tropical climate. van der Zee et al. [2014] have shown that for infiltration of rainfall water (low salinity) into sodic soil, the impact on soil hydraulic properties depends on the temporal structure of the wetting events. The impact is negligible for rainfall regimes inducing low variation in soil wetness and therefore low variation in soil salinity, whereas the impact is more significant for seasonal rainfall patterns. Russo et al. [2004] and Russo et al. (Salinity control in a clay soil beneath an orchard irrigated with treated waste water in the presence of a shallow water table: A numerical study, submitted to Journal of Hydrology, 2015) analyzed flow and transport in 3-D spatially heterogeneous soils whose clay fraction is dominated by montmorillonite, under typical Mediterranean climate with a long dry season requiring irrigation and a distinct rainy period during the winter. They analyzed long-term effects of interactions between the soil solution and the soil matrix on water and solute movement, demonstrating a minor decrease in the hydraulic conductivity function K(w) during the irrigation season (due to the relatively concentrated soil solution), and a more significant decrease in K(w) during the rainfall seasons associated with diluted soil solution. The system exhibits a reversible, cyclic behavior that persists while the soil ESP remain below a threshold value (e.g., ESPc 5 15%) [Shainberg and Kaiserman, 1969], above which a breakdown of the clay particle domains, and the subsequent relocation of the clay particles, may lead to a continuous decrease in the soil hydraulic conductivity, and, eventually, to an unrecoverable situation in which most of the soil upper layer is essentially sealed. 2.1.2. The Agricultural and Environmental Consequences of Excess Irrigation to Compensate for Poor Water Quality: A Case Study Recently, Russo et al. [2009] have provided a comprehensive analysis of the management strategy based on increasing Qr (irrigation water quantity I relative to the potential evapotranspiration ETp; Qr5I/ETp) to compensate for the adverse effects of high irrigation water salinity (expressed as total soluble salt concentration, cI). The analyses focused on drip irrigation (considering a two-dimensional flow in a spatially heterogeneous domain). The model accounts for the coupling between water flow and salt transport, plant water uptake, soil evaporation, and interactions between adjacent drip line laterals. The study considered sandy and clayey soils to illustrate the impact of soil hydraulic properties, for a wide range of cI and Qr values to represent various irrigation management conditions. The simulated results illustrate that soil volume per unit drip irrigated length, Vs, from which most (90%) of the plant water is taken up by roots decreases with increasing cI for both Qr values (Figure 2a). The sensitivity of root uptake zone to water salinity (dVs/dcI) was higher for the clayey soil than in sandy soil. Variations in the volume of soil explored by plants with irrigation water salinity and irrigation method [Mmolawa and Or, 2000; Stevens and Douglas, 2004; Dudley et al., 2008] play an important role in soil and irrigation management, and crop selection. For both soils, the relative amount of solutes leached beyond the root zone increases with Qr, suggesting an increase in salt load that could affect groundwater resources (Figure 2b). In summary, the study of Russo et al. [2009] has shown that the excess irrigation to compensate for poor water quality in drip irrigation: (i) exhibits diminishing efficiency as Qr increases; (ii) is more efficient in sandy than in clayey soils in controlling the solute concentrations in the root zone; (iii) is more efficient in clayey soils (than in sand) for controlling transpiration of the plants (and possibly crop yield), and the amount of salt leached below the root zone; and (iv) could adversely impact salinization of groundwater resources. 2.2. Treated Effluent (Wastewater) for Irrigation 2.2.1. Overview of the Allocation of Treated Effluent for Irrigation Effluent reuse in agriculture has a long tradition [Shuval et al., 1986], where most irrigated lands are located near urban areas where effluents are generated. Global estimates of effluent reuse indicate that about 20 million hectares of agricultural land are irrigated with wastewater [Jimenez and Asano, 2008]. The past few decades mark an increase in wastewater use in agriculture in developing countries and in semiarid and arid areas of industrialized countries. The ten countries with the largest volume of wastewater used for irrigation are shown in Figure 3 [Jimenez and Asano, 2008]. Irrigation with wastewater may pose a risk to public health due to exposure to microbial pathogens (viruses, bacteria, and protozoa) or chemical compounds (heavy

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metals, toxic organics, and anthropogenic compounds), thus requiring appropriate regulations [Aiello et al., 2007; Toze, 2006; Scheierling et al., 2010; Qadir et al., 2010; Shuval, 2011]. Wastewater use in agriculture could also induce various environmental risks to soil ecology and function and might lead to groundwater pollution. The primary risks associated with TE irrigation involve high concentrations of salts, especially sodium (Na1), and of organic compounds [Feigin et al., 1991; Balks et al., 1998; Hamilton et al., 2007; Pedrero et al., 2010]. The combination of these elements increase the exchangeable sodium percentage (ESP) of the irrigated soils [Shainberg and Letey, 1984; Halliwell et al., 2001] and affect their wettability [Wallach et al., 2005]. The consequence is a significant deterioration of soil physical and chemical properties [Lado et al., 2005; 2012; Aiello et al., 2007; Levy and Assouline, 2011]. A detailed quantitative description of the changes in the soil physical and hydraulic properties of a clayey soil following long-term irrigation with TE can be Figure 2. (a) Soil volume per unit length, Vs, from which most (90%) of the volume of water is extracted by the plant roots versus irrigation water concentrafound in Assouline and Narkis [2011]. An tion cI for two given Qr values (1.4 and 2.0) and two soil types (clay in black and interesting finding was that the level of sand in gray); (b) Relative cumulative mass of chloride leached below the root soil deterioration was depth-dependent. zone versus the irrigation water salinity cI for given Qr values and two soil types (clay in black and sand in gray). The decrease in the Ks values induced by TE irrigation was maximal at the upper soil layer and decreased gradually with depth. However, the amplitude of the impact on the water retention curve and the hydraulic conductivity function was different at each depth, indicating that the long-term use of TE for irrigation will affect different zones in the soil profile differently, in response to soil properties, water quality, irrigation management parameters, plant uptake characteristics, and climatic conditions (rainfall and evapotranspiration). The resulting changes in soil properties reflect on the fluxes of the main flow processes (infiltration, drainage, and evaporation) in the soil, and consequently, affect water and nutrients availability to plants in the root zone.

Figure 3. The ten countries with the largest volume of wastewater used for irrigation (* indicates that only the use in California and Florida are accounted for). Data based on Jimenez and Asano [2008].

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Oxygen at sufficient concentrations in the root zone is crucial for proper plant development [Armstrong, 1979; Glinski and Stepniewski, 1985]. Assouline and Narkis [2013] have shown that the changes in the hydraulic properties of the TE-irrigated soil have a great impact not only on the water regime but also on the aeration of the root zone. In addition, TE irrigation has been shown to

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Yield (TE) / yield (FW)

affect soil microbial activity [Elifantz et al., 2011; del Mar Alguacil et al., 2012] and bacterial community composition [Frenk et al., 2013]. The increase in input of organic substrates with TE irrigation with concurrent changes in water retention properties and enhanced microbial activity could be responsible for lowering oxygen concentrations and diminishing soil aeration rates.

2.2.2. Evidence for Adverse Impacts of Long-Term Irrigation With Treated Effluent Figure 4. The ratio between yields from TE-irrigated and FW-irrigated avocado and The arbitrarily short duration of most citrus trees versus duration of TE irrigation (data for citrus were provided by Asher funded research projects (rarely Aizenkot and for avocado, by Myriam Silberstein, Anat Lowengart, Ami Keinan, and Udi Gafni). exceeding 3 years) reflects on the body of knowledge with respect to long-term impacts of irrigation with treated effluent. Most past studies have concluded that there are no significant statistical differences between TE and local FW irrigation in terms of crop yields, with the exception of specific toxicity issues such are related to high boron concentrations [Pedrero et al., 2010]. Recent long-term studies in Israel have shown a systematic decrease in yields of orchards planted on clayey soils (50% clay) and drip-irrigated with TE (Figure 4). Following more than 10 years of consecutive TE irrigation, avocado and citrus yields have dropped by approximately 20–30% in comparison with yields from parts of the orchard receiving local FW irrigation under similar agrotechnical management. The mechanisms for such loss of productivity remain unclear and reflect a complex interplay of chemical, physical, and biological soil attributes affecting plant function. Years of TE- irrigation

Evidence for potential effects of long-term TE application on soil properties emerges from monitoring the spatial distribution of bulk soil electrical conductivity. An electromagnetic induction (DUALEM 1-S) survey was conducted in the middle of the irrigation season in an avocado orchard near Acre (Northern Israel) planted on a clayey soil, where several tree rows were drip-irrigated with FW next to rows drip-irrigated with TE. Result clearly show that TE-irrigated rows exhibit higher electrical conductivity than FW-irrigated rows, suggesting wetter and more saline root zones for TE-irrigated trees (Figure 5). Due to the relatively high sodium adsorption ration (SAR) of TE, the higher soil salinity is accompanied by a higher sodicity, inducing higher ESP values in the TE-irrigated soils that increase with depth [Assouline and Narkis, 2011; 2013]. Additional effects of TE application were demonstrated in the cumulative infiltration versus time in laboratory experiments on disturbed samples from clayey soils from two orchards (Acre and Rosh Pina) (Figure 6). Long-term irrigation with TE resulted in a drastic reduction of the soil infiltrability compared to the FWirrigated soil. The decrease in the infiltrability of TE-irrigated soils expresses Figure 5. Distribution in space and with depth of the soil bulk electrical conductivity in the upper soil layer of two adjacent rows of avocado tree in the experimental changes in the soil hydraulic properfarm of Acre, one irrigated with freshwater (FW) and the other with treated effluent ties [Coppola et al., 2004; Aiello et al., (TE). Measurements were carried out using DUALEM-1S (Courtesy of Scott Jones— 2007; Assouline and Narkis, 2011] that Utah State Univ.).

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may also affect soil aeration regime [Assouline and Narkis, 2013], thus impacting soil biological activity [Elifantz et al., 2011; del Mar Alguacil et al., 2012]. The oxygen concentrations in the soil gas phase measured within the avocado root zone in the Acre orchard (20 cm below the soil surface near tree trunks) in the TE-drip irrigated rows were consistently about 20% lower than in the FW-irrigated ones (Figure 7). A clear aeration response to irrigation events is also depicted, revealing a Figure 6. Cumulative infiltration versus time for disturbed samples from the 20– drop in oxygen content following 40 cm layer in the experimental orchards of Acre (dashed curves) and Rosh Pina water application that is more accentu(solid curves). The blue curves correspond to FW-irrigated soil samples, and the ated in TE treatments. Interestingly, the red-ones to TE-irrigated soil samples. oxygen concentrations measured at mid distance between trees have shown no particular impact of the irrigation water quality, indicating the complex interactions between the chemical (saline and sodic conditions), physical (hydraulic properties; plant water uptake), and biological (root and microbial activity) aspects involved with TE irrigation.

O2 conc. (%)

Water amount (mm)

2.3. Large-Scale Use of Desalinized Water in Irrigated Agriculture Recent technological advances make desalinated water (DS) a competitive source of water for irrigation in certain regions and for high cash crops [Beltran and Koo-Oshima, 2006]. A comprehensive study regarding the impact of DS-irrigation on plant growth, yield, and water use efficiency, relying on data from a field experiments conducted in a banana plantation located in the Jordan Valley, Northern Israel, was published recently [Silber et al., 2015]. Relative to irrigation with FW from the Sea of Galilee (with ECFW 51.5 dS m21), irrigation with DS (ECDS 50.3 dS m21) resulted in higher yields for all seasonal irrigation amounts investigated (Figure 8). Greater impact was observed for low irrigation treatments (deficit irrigation), and the difference decreases with increasing seasonal irrigation amount. In practical terms, banana yields (75 tons/ha) that under FW irrigation require 2100 mm/yr, could be achieved with 30% less water under DS-irrigation (1500 mm/yr) (Figure 8). A similar trend was observed for irrigated bell pepper [Ben Gal et al., 2009]. The environmental benefit of DS irrigation in arid regions is a reduced leaching of salts into groundwater resources [Silber et al., 2015]. The use of DS for irrigation, and for domestic use as well, present a few challenges of restoration of mineral balance for plant and human consumption [Avni et al., 2013; Ben Gal et al., 2009], which could be relatively easily met by remineralization through mixing with seawater or other FW sources. In general, DS irrigation emerges as a viable alternative for water saving, crop yield increase, and reduction of pollution hazards in certain regions and for certain crops and economic scenarios, thereby broadening the range of options for increased food production and sustainable irrigated agriculture.

DOY

Figure 7. Measured oxygen concentrations by means of sensors (Figaro, Japan) installed 15 cm aside from the dripper and 20 cm below the soil surface close to avocado trunks and at mid distance between trees (trees are planted at 6 m intervals and are irrigated by two dripping lines with drippers every 50 cm) in the FW (blue) and TE (red) drip-irrigated plots at the experimental farm of Acre.

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For climatic conditions where annual ETp is high and annual rainfall limits salt leaching (arid regions), the use of poor irrigation water quality requires prescribed excess amounts of water for salt leaching from the root zone (Figure 1). The salinity level of the applied water relative to the salinity tolerance of the specific crop under interest is a key factor in the seasonal amount of irrigation water

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to be allocated to attain target (high) yields. Consider for example, three different water sources with salinity expressed by ECDS 5 0.3 dS/m for DS, ECFW 51.2 dS/ m for FW, and ECTE 52.5 dS/m for TE, and a crop characterized by ECT5 1.5 dS m21 [threshold value for full potential yield following Ayers and Westcot, 1985]. As the salinity of the irrigation water decreases, the LR is lower, and the water saving potential is greater (Figure 1). Considering salinity aspects alone and for the assumed irrigation water salinity values, a shift from using FW irrigation to TE irrigation will require an increase in Figure 8. Banana yield for the 2011 season for the different irrigation amounts of freshwater (FW; blue) and desalinated water (DS; dark blue). The dotted line indithe allotted irrigation water amounts of cates the level of the commercial yield of banana in the region under FW irriga50% to achieve similar yields. Alternation (75 T/ha). The values indicated below this line indicate the irrigation rates tively, considering DS irrigation will genthat correspond to the intersection of the curve with the dashed line, i.e., the irrigation rates required to achieve the commercial yield for each water quality. erate 20% water saving, thereby increasing the potential of available water for irrigation. In addition, the lower LR (with DS) will significantly reduce salt leaching to groundwater resources. Naturally, the use of DS would be costly, requiring economic analysis and proper infrastructure. However, preliminary estimates indicate that a 15% increase of banana or avocado yields should be sufficient to cover the related additional expenses. The analysis above, based on Figure 1, considers salinity aspects solely. To consider sodicity aspects, we applied a coupled numerical model of flow and transport that accounts for the dependence of the hydraulic properties on soil solution concentrations [Russo, 2013] to simulate the impact of long-term irrigation (10 years) with TE and DS on agricultural and environmental metrics (Figures 9a and 9b). Two annual irrigation water amounts were applied in relation to the irrigation water quality (accounting for the different corresponding LR): 2400 mm/yr for the more saline TE water and 1800 mm/yr for the DS water. The agricultural metric presented is the plant transpiration rate relative to the potential transpiration, T/Tp (Figure 9a), considered to be a good proxy for the relative yield [Skaggs et al., 2014a]. During the first 2 years, the simulated curves for DS irrigation and TE irrigation are similar and higher water application rate allowing salt leaching does compensate for water quality, in agreement with short-term studies [Pedrero et al., 2010]. However, from that point, due to the buildup of salinity in the root zone and changes in soil hydraulic properties resulting from sodicity effects, the curves diverge and a drop of 20% in the yield is predicted. The emerging time scale for stabilization of the low yield in case of long-term TE irrigation seems to be at the order of a decade. The positive impact of DS irrigation providing higher yields while saving 25% of irrigation water is also simulated (Figure 9a). An environmental impact metric for gauging the relative effect of different water sources for irrigation is represented by the mass of chloride leached below the root zone (a reference horizontal plane 1.50 m below the soil surface) relative to the applied chloride mass in each of the cases (Figure 9b). For the TE irrigation, the mass of salts leached toward the groundwater is practically equal to the applied mass of salts via irrigation because of the high LR. On a relative basis, only half of it is leached in the case of DS. However, in terms of absolute values, the salt load leached under DS-irrigation conditions is only 10% of the load leached under TE irrigation because of the lower LR and water salinity. The results in Figures 9a and 9b could have a tremendous impact when applied on a global scale to available water resources and crop productivity: (i) more freshwater will be available due to the saving of most of the excess water needed for salt leaching and the resulting drastic reduction of the salinization rate of groundwater, thus allowing the increase of the relative part of the irrigated land; (ii) higher yields resulting from the joint effect of the increase of the total area of irrigated land and of the yield per unit area of irrigated land due to the improvement in water quality.

3. A Global Outlook on the Expansion of Irrigation Based on Marginal Water Sources The range of strategies required for meeting the global challenge of producing more food with practically the same land footprint but less available FW have not yet been developed. One of the few options is the

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expansion of irrigation, yet, limited FW resources where irrigation is needed most requires expansion of (presently) marginal alternative water resources (SW, TE, and DS). For such an expansion to become a viable option, it must offer agroecological sustainability and economic feasibility. The global map of where irrigation expansion is feasible and sustainable varies dramatically across geographic and climatic regions and is shaped by local land and water resources and socioeconomic development. It is thus important to analyze the risks and challenges for expansion of marginal water-based irrigated agriculture in arid regions. 3.1. Long-Term Effects on Soil Ecology and Emerging Risks The long-term effects of irrigating a complex ecological system such as soil with marginal water (SW and TE) are largely unknown. The functionality and Figure 9. (a) and (b) Simulated agricultural and environmental metrics during 10 balance among the highly diverse and years of consecutive irrigation with desalinated water at a rate of IDS51800 mm/ interacting soil fauna and flora and yr (DS; dark blue), treated effluent at a rate of ITE52400 mm/yr (TE; red), and alternating between TE and DS (ALT_TE_DS; green) at a constant rate of 2400 mm/ especially the microbial communities yr—Figure 9a shows ratio between plant actual transpiration, T, to potential tranare critically dependent on the physical spiration, Tp; Figure 9b shows mass of chloride leached below a reference plane and chemical environment they inhabit 1.5 m below soil surface relative to the applied mass of chloride. [Wieland et al., 2001; Mills, 2003; Young and Crawford, 2004; Or et al., 2007]. The transition to more saline and sodic soil water alters not only the composition of the aqueous phase, but affects also dispersion and swelling processes that reduce pore spaces and change hydraulic connectivity and diffusion processes essential to microbial life and to critical interactions with plant root rhizosphere [Vessey, 2003; Berg, 2009; Barea et al., 2005]. An important aspect of the success and sustainability of expanding irrigation would be the systematic consideration of long-term exposure of soil biological function to marginal water. The potential impacts on soil ecology and microbial community composition [del Mar Alguacil et al., 2012; Frenk et al., 2013; Garcıa-Orenes et al., 2015] are not only important for the primary objective of increasing crop yields, but also to address concerns that have been raised with respect to risk to human health [Pruden, 2014; Becerra-Castro et al., 2015]. The risk is not limited to potential exposure to pathogens in food webs, but also to unknown consequences of developing antibiotic resistance (soil bacteria and genes) as a result of extended exposure of unknown microbial populations to persistent amounts of residual antibiotics in TE irrigation water [Pruden, 2014; Negreanu et al., 2012]. Some of the risk could be reduced by changing behavior in producing the effluents, or developing new technologies to target and remove such unwanted compounds. However, the main challenge to the sustainable use of marginal water sources remains the unintended consequences on the ecology, hydrology, and productivity of irrigated soils. Hence, comprehensive monitoring strategies for soil health, that could provide timely and actionable indications of substantial shifts in key variables that affect soil biota, hydraulic and transport properties, biochemical and biophysical conditions would have to be developed and integrated in future intensive irrigated agriculture. 3.2. Effluent Reuse and A Sustainable Agro-Urban Water Cycle Considering the vast amounts of effluent produced in urban areas and growing costs of treatment and disposal, it is not surprising that national and international initiatives (e.g., ReNUWIt) are seeking a more comprehensive solution to this rapidly growing challenge [Pruden, 2014; Grant et al., 2012]. The transformation of effluent into a dependable and renewable resource for irrigation require new approaches toward its

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generation and treatment to meet health and composition guidelines [Pruden, 2014; Becerra-Castro et al., 2015], and to ensure a balance between irrigation water availability, public health, and long-term sustainability of the lands that are exposed to this renewed resource. Depending on the scale and level of the effluent treatment, modern methods are now available, which offer an array of solutions at different scales ranging from a household to megacities that balance operational simplicity, costs, and regulatory constraints [van Loosdrecht and Brdjanovic, 2014, Grant et al., 2012]. The volumes and quality of effluent depend on public awareness and participation influencing how effluent is generated, and acceptance of various forms of reuse, while, at the same time, changing how freshwater is sourced, used, managed, and priced [Grant et al., 2012]. Localized agricultural reuse of effluent seems the logical and preferable option to long distance or interbasin transfers. Hence, an integral element in planning sustainable future cities in arid and semiarid regions is the incorporation of agricultural land that would benefit from the agro-urban water cycle either by providing a fraction of the food production locally, or by using this renewed water resource within other economically feasible agricultural schemes. The sociohydrological time bomb due to the accelerating effluent cycle in the underdeveloped world, and the growing reliance on TE as a dependable resource in the developed world, are two sides of this important hydro-urban challenge. At the same time, the conditions in the temperate climate of central Europe with established regulation and advanced treatment are significantly different from those in semiarid regions requiring different approaches, treatment strategies, and applications. Therefore, appropriate approaches that account for the specific regional conditions and constraints have to be developed. 3.3. Effects of Extreme Climatic Events on the Sustainability of Irrigated Agriculture Extreme events such as prolonged droughts or large rainfall events impart significant ecological, economic, and societal influences [Diffenbaugh et al., 2005], especially on the trajectories of water and salt balances in soil and on surface and groundwater resources. Prolonged droughts contribute to increase the rates of soil salinization, especially in agricultural systems irrigated with marginal water. In contrast, the episodic occurrence of extremely wet events offers opportunities for massive salt leaching that may help reset salinization trends that cause slow drifting away from sustainability. A class of stochastic models for salinity buildup and flushing under random rainfall events have been proposed [Suweis et al. 2010], which provide a formal framework for considering such reset events in the long-term management scheme despite their random nature. Some of the models do not yet consider aspects such as the possible development of sodic conditions in soils, which could change the nature of the impact of extreme rainfall events, reducing the efficiency of salt leaching and increasing flooding and erosion damages [van der Zee et al., 2014]. Additionally, the ecological component of the soil that undoubtedly would be affected by extreme drought or rainfall events is largely absent in such modeling schemes. Many climate models predict an increase in the frequency of such extreme events [Meelh and Tebaldi, 2004; Diffenbaugh et al., 2005], a factor that could be exploited and integrated in future irrigation management schemes with marginal water. Naturally, the full pathways analysis of the fate of salt removed or accumulated during such extreme events should be considered as part of the full salt balance in these systems. 3.4. The Role of Irrigation Methods The expansion of irrigated land and the increased reliance on marginal water sources are aimed at increasing yields and enhancing water use efficiency in a sustainable manner. An important tool for the success of such strategy is the adoption of appropriate irrigation methods for the soil and climatic conditions, and development of irrigation and fertigation management protocols suited to the particular irrigation method. A range of pressurized irrigation methods (sprinkler, drip) offer alternatives to the traditional surface irrigation (furrow, basin) in terms of efficiency, environmental impact, health risks, and more. These advantages come at a cost in terms of infrastructure, knowledge, maintenance, and potential vulnerability to crop failure (limited root zones and small wetted soil volumes by drip irrigation), or soil degradation [Phene and Sanders, 1976; Schneider et al., 2001; Assouline and Ben-Hur, 2003; Assouline et al., 2006]. The local climatic conditions, soil and crop characteristics, water quality and availability, water pricing, and technological infrastructure play an important role in the selection and success of an irrigation method and its related management. In contrast with the large body of knowledge related to the performances of irrigation methods with respect to efficiency and crop response, very little is known about the long-term effects of different

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irrigation methods using marginal water on soil health and ecological function. Evidence suggests that the extrapolation of knowledge gained from ‘‘blue water’’ irrigation with the various methods to their long-term performance with marginal water is unwarranted (see examples with TE and DS above) and require far more monitoring of below ground soil ecological and hydrologic responses to avoid potential of ecological collapse of the soil system at a time it is needed most. Considering salinity management in irrigated agriculture, the vast number of studies in the 1960–1990 [Maas and Hoffman, 1977; Bresler et al., 1982; Ayers and Wetscott, 1985; Rhoades, 1999] provides a wealth of knowledge concerning how to improve salt leaching efficiency while reducing deleterious effects of salinity on crop production. There is a large gap in the impacts of various irrigation methods on long term salt loading rates and residence times in the subsurface [Schoups et al., 2005]. Sustainability of a selected irrigation method require a broader picture (beyond crop yields) including ground water recharge patterns, depth to water table and long-term drought or rainfall record, and consideration of all relevant temporal and spatial scales involved [Passioura, 2005]. Arid regions with episodic rainfall may support drip irrigation with highly localized salt loads that would not be possible without episodic flushing events (or availability of DS water for the same purpose). Similarly, surface irrigation is arid region underlain by shallow water table are typically unsustainable (see the demise of ancient agricultural civilizations) [Hillel, 1991]. In general, for a certain irrigation water quality and crops, the impact of an irrigation method on the rate of (the inevitable) salinization will depend on several variables including vertical and spatial distribution of soil properties, topography, irrigation and cultural practices, climatic conditions, and regional hydrological conditions (depth and water quality of local water table). Techniques for improving the quality of available irrigation water by mixing water sources of different qualities have been considered and could be adapted to the irrigation method [Ben Gal et al., 2009] (Russo et al., submitted manuscript, 2015). Multiple water sources could help the selection of appropriate ECI/ECT target values, allowing the adaptation of specific values to different stages of the crop. Certain combination may be used to reduce the LR (Figure 1) while increasing crop yields (Figure 9a) and reducing groundwater salinization (Figure 9b). The mixing ratio corresponding to the required high ECI/ECT values becomes an operational state variable depending on the specific soil properties, climate conditions, and crop characteristics of the system under interest. In systems where the two-end members of available water quality for irrigation are DS and TE, one scenario could be for example shifting from TE irrigation to DS irrigation according to some prescribed, user-controlled, salinity criterion, ccr (Russo et al., submitted manuscript, 2015). Considering the chloride concentration, c[zcu(t)], at the vertical position of the centroid of the soil volume active in water uptake, zcu(t), irrigation water quality is alternated between TE and DS in the course of the irrigations, according to the rule ‘‘replace TE by DS if c[zcu(t),t]>ccr’’ and vice versa. The results, based on numerical simulations using the same model and parameterization as above, for I5 2400 mm/yr and for ccr 515meq/‘, are illustrated in Figures 9a and 9b (green curve). Notice that the resultant mixing ratio, Rm5QDS/QTS, where QDS and QTS are the cumulative volumes of DS and TS applied during the irrigations, respectively, depends on the selected value of the critical chloride concentration, ccr, and on the interrelationships between flow, transport, and water uptake by the plant roots; hence, Rm is time-dependent, i.e., Rm5Rm(t; ccr). Naturally, ability to implement such refined management strategy within the crop root zone requires control over the water supply that would be critically dependent on the irrigation method. Alternating between TE and DS according to ccr led to higher yields than if DS is used solely (Figure 9a), reflecting the higher irrigation rate applied in the alternating irrigation scenario compared to the rate of DS irrigation (2400 mm/yr versus 1800 mm/yr). It led also to intermediate salt leaching between TE and DS irrigation (Figure 9b). In terms of absolute values, the salt load leached under alternating irrigation is 59% of the load leached under TE irrigation. For the time period of 10 years, the resultant time-averaged mixing ratio for this specific simulation, , was 1.0. Figures 9a and 9b suggests, therefore, that alternating irrigation water quality between TE and DS allows higher yields and lower groundwater salinization rate while using less of the more expensive DS water. Human health concerns related to irrigation with TE water may dictate (by regulation or incentives) selection of irrigation methods that do not wet parts of the crop consumed by humans. Buried drip irrigation method offers isolation of TE water application from above ground consumable plant parts. In this context, we need to improve our understanding of rhizospheric process including water and nutrients uptake at soil-root interfaces. Measurements have shown that the physical, chemical, and biological characteristics of

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the rhizosphere differ significantly from those of the bulk soil [Hinsinger et al., 2009]. Exudation of mucilage and other compounds by roots and the biophysical modifications and stimulation of biological activity, all contribute to the formation of very different environments (hot spots) at the vicinity of plant roots [Carminati and Vetterlein, 2013]. Integration of these modern insights into the macroscopic root zone models is critical for sustainable irrigation where salts (and especially sodium) may preferentially accumulate at the soil-root interface at concentrations well above predictions by diffuse macroscopic approach [Hamza and Aylmore, 1992; Hopmans and Bristow, 2002].

4. Summary and Conclusions The projected expansion of irrigated agriculture with a growing reliance on (presently) marginal water resources are essential strategies for increasing food production while establishing a more sustainable agrourban water cycle. The adaptation of irrigation methods and the extent of their expansion and management are expected to vary widely across geographic regions, climate, and available water sources. The primary challenges to the sustainability of irrigated agroecosystems in arid and semiarid regions lies with the hazards of saline and sodic conditions due to salt accumulation in the soil profile, and the unintended consequences of using TE on soil hydroecological functioning. Salinization hazard is not new, and strategies have been devised to prolong the sustainability window of irrigated systems through judicious implementation of the concept of leaching fraction (irrigation in excess of crop water needs to leach salts). The blending of water sources of different qualities, the water application through appropriate irrigation methods, and the selection of salt tolerant crops, are management options that have been implemented in the past and require additional refinements for future use in a more intensive agriculture under possible global climate changes. An interesting angle linked to this climate changes is the predicted increase in extreme events (droughts or intense rainfall events) that could play a significant role in the long-term management of irrigated agroecosystems. The projected increase in TE use for irrigation presents unique challenges with largely unknown long-term influences on the soil hydrology and ecology. Concerns regarding the potential enhancement of antibiotic resistance of soil microbial communities may require not only rigorous and timely soil monitoring, but possibly introduction of treatment methods for reducing the loads of certain anthropogenic compounds in the TE. Mounting evidence suggests that long-term use of TE may affect various aspects of soil hydrology, due to increased load of salts, organic matter, surfactants, nutrients, and subsequent interactions with the soil minerals. Among these effects, a systematic reduction in soil infiltrability and in soil aeration has been observed in some field experiments. Although the mechanisms, frequency of occurrence and magnitudes of adverse impacts associated with long-term irrigation with TE are not yet fully known, the potential ramifications on soil function and productivity, and on public health, necessitate significant investment in research and monitoring of such irrigated systems to ensure their long-term sustainability. The development of DS as a viable water source for irrigation is strongly linked with local conditions, technological improvements, and the energy nexus. Recent expansion in Spain, the Middle East, Australia, and other semiarid regions was driven primarily by the need to meet demand of rapidly growing populations under climatic conditions that limited the capacity of available water resources. Naturally, for wetter periods where the capacity meets the local demand for potable water, the decision regarding the use of DS for irrigation hinges on the economics of the agricultural system and the alternatives in term of food distribution. The fluctuating energy prices and strategic considerations may also play a role in DS diversion to agriculture. In the long-term and for larger areas, the DS may be used to reduce the salt load (by mixing) under certain conditions (for example, during a drought or when salinization approaches critical levels). The beneficial effects of irrigation with DS have been shown in recent field experiments, both in terms of significant irrigation water savings or yield increase. The availability of DS water, subject to the technological and cost constraints, broadens the management options for sustainable irrigation in arid regions. Developing strategies for addressing the issues presented above require multidisciplinary approaches that combine new scientific knowhow with legislative, economic, and other societal tools to ensure safe and sustainable use of water resources of different qualities. The new scientific knowhow should provide quantitative models for integrating key biophysical processes with ecological interactions at appropriate spatial and temporal scales.

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Water Resources Research Acknowledgments The authors thank Scott Jones from Utah State University for providing the EMI data; Asher Aizenkot for the data on citrus yields; and Myriam Silberstein, Anat Lowengart, Ami Keinan, and Udi Gafni for the data on the avocado yields. They thank also Kfir Narkis and Rivka Gherabli for their technical help. The support of the Chief Scientist Fund of the Israeli Ministry of Agriculture is gratefully acknowledged (projects 301 0632 and 596-0526). The data in this study are available upon request.

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