Impact of subsurface drainage system on waterlogged and saline soils in a Saharan palm grove

Abstract

Salinity and waterlogging associated with saline and shallow groundwater are major problems for the sustainability of irrigated agriculture in many Saharan oases located in southern Tunisia. The aim of this paper is to assess the performance of a subsurface drainage system installed in the modern Sagdoud oasis to control the perched water table depth and reclaim the saline waterlogged soils. Water and salt balances at the oasis scale and daily basis were evaluated during the hydrologic year 1995. Moreover, the water table depth, the mineralisation in the perched groundwater and the bare soil salinity were measured at 40 parcels of farmland in November 1993, October 1998 and June 2021. The results showed that the total depth of irrigation water was three times the potential evapotranspiration depth in 1995. Evaporation process and subsurface drainage system allowed the discharge of 81% and 42%, respectively, of the total volume of irrigation water. From a salt input of 4,865 tons/year by irrigation, 4,566 tons/year (94%) were exported by subsurface drainage system and about 300 tons/year were estimated to have been stored in the soil and perched groundwater. The spatial distributions of the perched groundwater depth and mineralisation, and bare soil salinity were influenced mainly by the topography. The mean water table depth increased by 87%, while the mean salinity of perched groundwater increased by 19% during November 1993-October 1998. The mean bare soil salinity in the 20–40, 40–60 and 80–100 cm depths decreased by 75%, 50% and 42%, respectively, during October 1998-June 2021. Overall, the study revealed that the subsurface drainage system allowed the control of the water and salt balances at the oasis scale, the decrease in the perched water table levels and the reclamation of saline soils. However, the provision of this system does not permit the control of the groundwater mineralisation.