Controlled drainage in the Nile River delta of Egypt: a promising approach for decreasing drainage off-site effects and enhancing yield and water use efficiency of wheat

North Africa is one of the most regions impacted by water shortage. The implementation of controlled drainage (CD) in the northern Nile River delta of Egypt is one strategy to decrease irrigation, thus alleviating the negative impact of water shortage. This study investigated the impacts of CD at different levels on drainage outflow, water table level, nitrate loss, grain yield, and water use efficiency (WUE) of various wheat cultivars. Two levels of CD, i.e., 0.4 m below the soil surface (CD-0.4) and 0.8 m below the soil surface (CD-0.8), were compared with subsurface free drainage (SFD) at 1.2 m below the soil surface (SFD-1.2). Under each drainage treatment, four wheat cultivars were grown for two growing seasons (November 2018–April 2019 and November 2019–April 2020). Compared with SFD-1.2, CD-0.4 and CD-0.8 decreased irrigation water by 42.0% and 19.9%, drainage outflow by 40.3% and 27.3%, and nitrate loss by 35.3% and 20.8%, respectively. Under CD treatments, plants absorbed a significant portion of their evapotranspiration from shallow groundwater (22.0% and 8.0% for CD-0.4 and CD-0.8, respectively). All wheat cultivars positively responded to CD treatments, and the highest grain yield and straw yield were obtained under CD-0.4 treatment. Using the initial soil salinity as a reference, the soil salinity under CD-0.4 treatment increased two-fold by the end of the second growing season without negative impacts on wheat yield. Modifying the drainage system by raising the outlet elevation and considering shallow groundwater contribution to crop evapotranspiration promoted water-saving and WUE. Different responses could be obtained based on the different plant tolerance to salinity and water stress, crop characteristics, and growth stage. Site-specific soil salinity management practices will be required to avoid soil salinization due to the adoption of long-term shallow groundwater in Egypt and other similar agroecosystems.