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The main goal of this study was to test the capability of irrigation water-based and soil-based approaches to control nitrate and chloride mass fluxes and concentrations below the root zone of agricultural fields irrigated with treated waste water (TWW). Using numerical simulations of flow and transport in relatively a fine-textured, unsaturated, spatially heterogeneous, flow domain, scenarios examined include: (i) irrigating with TWW only (REF); (ii) irrigation water is substituted between TWW and desalinized water (ADW); (iii) soil includes a capillary barrier (CB) and irrigating with TWW only (CB1TWW); and (iv) combination of (ii) and a CB (CB1ADW). Considering groundwater quality protection, plausible goals are: (i) to minimize solute discharges leaving the root zone, and, (ii) to maximize the probability that solute concentrations leaving the root zone will not exceed a prescribed, critical value. Results of the analyses suggest that in the case of a seasonal crop (a corn field) subject to irrigations only, with respect to the first goal, the CB1TWW and CB1ADW scenarios provide similar, excellent results, better than the ADW scenario; with respect to the second goal, however, the CB1ADW scenario gave substantially better results than the CB1TWW scenario. In the case a multiyear, perennial crop (a citrus orchard), subject to a sequence of irrigation and rainfall periods, for both solutes, and, particularly, nitrate, with respect to the two goals, both the ADW and CB1ADW scenarios perform better than the CB1TWW scenario. As compared with the REF and CB1TWW scenarios, the ADW and CB1ADW scenarios substantially reduce nitrogen mass fluxes to the groundwater and to the atmosphere, and, essentially, did not reduce nitrogen mass fluxes to the trees. Similar results, even better, were demonstrated for a relatively coarse-textured, spatially heterogeneous soil. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We examined the impact of two different approaches to managing irrigation water salinity: salt leaching from the field ('conventional' management) and water desalination before field application ('alternative' management). Freshwater commonly used for irrigation (FW) and desalinated water (DS) were applied to the high-water-demanding crop banana at four different rates. Both irrigation rate and water salinity significantly affected yield. DS application consistently produced higher yields than FW, independently of irrigation rate. The highest yield for FW-irrigation was achieved with the highest irrigation rate, whereas the same yield was obtained in the case of DS-irrigation with practically half the amount of water. Yield decreased with FW-irrigation, even when the water salinity, ECi, was lower than the limit considered safe for soil and crops. Irrigating with FW provided a massive amount of salt which accumulated in the rhizosphere, inducing increased osmotic potential of the soil solution and impairing plant water uptake. Furthermore, applying the 'conventional' management, a significant amount of salt is leached from the rhizosphere, accumulating in deeper soil layers, and eventually reaching groundwater reservoirs, thus contributing to the deterioration of both soil and water quality. Removal of salt excess from the water before it reaches the field by means of DS-irrigation may save significant amounts of irrigation water by reducing the salt leaching requirements while increasing yield and improving fruit quality, and decreasing salt load in the groundwater. [ABSTRACT FROM AUTHOR]

Abstract: A shortage of rain and water resources exits in semi-arid and arid zones, necessitating the use of marginal water such as treated sewage water (TSW) for irrigation. These waters may contain substantial amounts of mixed-sodium- calcium salts, which, in turn, interact with the soil matrix. The soil solution-soil matrix (SS-SM) interactions, particularly in the presence of smectite minerals (e.g., montmorillonite), may change the soil pore-size distribution, and, consequently, could affect the soil hydraulic properties. Since the magnitude of the SS-SM interactions depends on time-dependent flow-controlled attributes, i.e., water content, pressure head, and solute concentrations, the resultant hydraulic properties are also time-dependent. Numerical simulations of flow and transport in a three- dimensional (3-D), spatially heterogeneous, variably saturated soil, for the case in which the flow is coupled to the transport through the dependence of the soil hydraulic properties on solute concentrations, were employed in order to analyze flow and transport in salt-affected soil. The case under consideration was a citrus orchard planted on a structured clay soil in a region characterized by a distinct rainy period during the winter and a relatively long dry season, requiring irrigations. Results of the analyses suggest that enhanced SS-SM interactions, induced by increasing soil exchangeable sodium during the irrigation season and dilution of the soil solution during the rain season, may considerably reduce soil hydraulic conductivity during the rain season, particularly, close to the soil surface. Consequently, enhanced SS-SM interactions, may slow down the solute movement, may substantially increase the tailing of the mean solute breakthrough at a given horizontal control plane (CP), and may lead to mean and standard deviation solute profiles which exhibit bimodal distributions, characterized by a substantial, secondary peaks close to the soil surface. Practical implications of this study regarding the long-term use of TSW for irrigation are briefly discussed. [Copyright &y& Elsevier]

Detailed numerical simulations were used to analyze water flow and transport of nitrate, chloride, and a tracer solute in a 3-D, spatially heterogeneous, variably saturated soil, originating from a citrus orchard irrigated with treated sewage water (TSW) considering realistic features of the soil-water-plant-atmosphere system. Results of this study suggest that under long-term irrigation with TSW, because of nitrate uptake by the tree roots and nitrogen transformations, the vadose zone may provide more capacity for the attenuation of the nitrate load in the groundwater than for the chloride load in the groundwater. Results of the 3-D simulations were used to assess their counterparts based on a simplified, deterministic, 1-D vertical simulation and on limited soil monitoring. Results of the analyses suggest that the information that may be gained from a single sampling point (located close to the area active in water uptake by the tree roots) or from the results of the 1-D simulation is insufficient for a quantitative description of the response of the complicated, 3-D flow system. Both might considerably underestimate the movement and spreading of a pulse of a tracer solute and also the groundwater contamination hazard posed by nitrate and particularly by chloride moving through the vadose zone. This stems mainly from the rain that drove water through the flow system away from the rooted area and could not be represented by the 1-D model or by the single sampling point. It was shown, however, that an additional sampling point, located outside the area active in water uptake, may substantially improve the quantitative description of the response of the complicated, 3-D flow system. [ABSTRACT FROM AUTHOR]

Pollution of groundwater by nitrate originating from irrigated fields was considered for this study. We hypothesized that under cropped conditions, low-salinity irrigation water (e.g., desalinated water) could reduce nitrate leaching below the root zone, due to two possible mechanisms: (i) decreased vertical water fluxes and (ii) increased nitrogen uptake by plant roots due to chloride–nitrate competition. The main goal of this study was to investigate this hypothesis. Considering a citrus grove, the investigation relied on three-dimensional (3-D) simulations of flow and transport in a variably saturated and spatially heterogeneous flow domain performed for three successive years. Results of the analyses suggest that the main mechanism responsible for the reduction in the nitrate leached below the root zone under irrigation with low-salinity water is the effect of the latter on the spatial distribution of the rate of water uptake by the roots. The latter, in turn, significantly reduces water content, hydraulic conductivity, and vertical velocity, and, consequently, solute mass fluxes along the soil profile. On the other hand, chloride–nitrate interaction has only a relatively small effect on the nitrate mass fluxes at relatively deep soil depths, far below the root zone, particularly when the irrigation water salinity decreases. [ABSTRACT FROM AUTHOR]