Mitigating the Impact of Irrigation With Effluent Water: Mixing With Freshwater and/or Adjusting Irrigation Management and Design

The use of treated wastewater (WW) for irrigation is steadily increasing worldwide. However, irrigation with WW requires special attention as it may alter soil hydraulic properties and, eventually, affect crop yields. The main goal of this study was to investigate two approaches that could mitigate this adverse impact: (i) mixing WW with local freshwater (FW) and (ii) adjusting the irrigation management and design to the deteriorated soil hydraulic properties. Four water qualities resulting from mixing FW and WW at different ratios for irrigation of an avocado orchard planted on a clayey soil were considered: FW, M-1/3(2/3 FW - 1/3 WW), M-2/3(1/3 FW - 2/3 WW), and WW. Increasing the WW component in the irrigation water reduces the soil's saturated hydraulic conductivity and infiltrability, and this reduction is depth dependent. It also reduces the transpiration of trees and their trunk growth. This is supported by numerical simulations that showed that irrigation with WW produced the reduction in plant transpiration relative to irrigation with FW. Numerical simulations indicated that irrigation management and design could be adjusted to improve plant water uptake and reduce the differences due to water quality. The results indicate that decreasing the irrigation rate and increasing the wetted area per tree could increase the water uptake in WW-irrigated soils to almost equal that in FW-irrigated soils. This can be achieved by using drippers with lower discharge and by using more drippers per dripline, more driplines per tree, and more drippers per tree (concentric loops of driplines around each trunk).