Simulating and monitoring water flow, salinity distribution and yield production under buried diffuser irrigation for date palm tree in Saharan Jemna oasis (North Africa)

Abstract

Irrigation in traditional oases in the Mena region was possible in the past via traditional techniques (foggaras, tabia) and a relative availability of water resources. To promote the development of sustainable oases, in the context of global changes and water resources scarcity and degradation, an assessment of an innovative subsurface irrigation system (Chahbani) was conducted in Saharan Jemna oasis (South of Tunisia). Irrigation water salinity (ECw) was classified brackish (ECw ranged between 3.2 dSm-1 and 4.5 dSm-1). Field investigations were conducted during two crop years (2018, 2019) for palm trees. Capacitance sensors (Decagon 5TE) were installed at six soil depths (0.15 m, 0.40 m, 0.60 m, 0.80, 1 m and 1.20 m) for real time monitoring of soil water moisture (θ) and pore electrical conductivity (ECp). A numerical model (HYDRUS (2D/3D)) was calibrated then applied to investigate the effect of different filed irrigation amounts ((T1F (73% crop evapotranspiration (ETc); T2F (120% ETc)) and different buried diffuser optimization (opt) positions from the trunk of the palm tree (X) and soil depths (Z): (T1opt (X = 1.5 m and Z = 0.3 m); T2Opt (X = 0.5 m and Z = 0.3 m); T3opt (X = 1.5 m and Z = 0.6 m); T4opt (X = 0.5 m and Z = 0.6 m)) on root water uptake, yield production and soil salinity distribution. Simulation results for both field treatments T1F and T2F showed that the amount of root water uptake was always lower than 100%(ETc) with observed yield equal to 45% and 50% for T1F and T2F, respectively. According to buried diffuser position scenarios, installation of the buried diffuser at the canopy of the date palm T1opt (X = 1.5 m) as used in Jemna oasis, showed the least calculated relative yield (37%). However, for lateral distance, X = 0.5 m, root water uptake was equal to 649 mm for T2Opt (Z = 0.3 m) and 855 mm for T4Opt (Z = 0.6 m) with relative yield equal to 54% and 71%, respectively. According to simulation results, all position scenarios showed salt accumulation on the top soil layers and leaching requirement fraction (LR) application failed to decrease salinity for 0–0.4 m soil depth. Supplemental leaching using surface irrigation is thus recommended. Further studies for irrigation with buried diffuser under arid climate and brackish water need to focus on its impact on oasis sustainability especially when used for traditional oases as irrigation system.