Modelling the impact of soil and water conservation structures at various scales in Tunisian semi-arid region

Abstract

In central-northern Tunisia, a combination of dryness, erratic rainfalls, and the undulating terrain create an erosion-prone environment. Soil and water conservation (SWC) measures have been widely applied to reduce the losses of water and top-soil from agricultural fields and thereby increase crop-production and the rural communities’ livelihoods. However, the impacts of various SWC interventions are interrelated and difficult to predict. The objective of this study is to evaluate the impacts of Bench Terraces (BTs), in combination with small scale reservoirs (Hill Lakes (HLs)), on runoff and erosion using the Soil and Water Assessment Tool (SWAT) across semi-arid environments of Tunisia. Development and testing of the BT parameter set was performed using monthly surface runoff and multiple bathymetric sediment measurements obtained from the HLs of small BT-treated catchments (< 20 km2). The established Bench Terraces (BTs) parameter set was applied to the Rmel watershed (675 km2) to eventually test the parameters’ performance at the integrated landscape level. SWAT simulation using the defined BT parameter set and ‘pond’ approaches for the scattered HLs across Rmel produced a ‘good’ modelling performance indicated by NSE values of 0.89 and 0.91 during calibration and validation. A combination of BTs and HLs overall reduced runoff and sediment yield by 33% and 17% respectively. The Rmel case study demonstrates the robustness of the BT parameter-set obtained from small and quasi-homogeneous training catchments. The enhanced knowledge about SWC impacts, and its consideration in modelling (across scales), will eventually support planning and management in Tunisia’s dry and degradation-prone landscapes.