Assessment of the best management practices under a semi-arid basin using SWAT model (case of M’dez watershed, Morocco)

In Morocco, several major development projects have been planned to address the worrying concern for soil and water resources. They aimed to reduce the soil degradation and loss of soil fertility caused mainly by water erosion, deforestation, and agricultural practices. The objective of this work is the quantification of the water balance and soil erosion and its spatial distribution, using the hydro-agricultural SWAT model, followed by the evaluation of the best management practices (BMPs) at the M’dez basin (3350 km2), such as contour tillage, bench terraces, and stone line. The coefficient of determination (R2) and Nash–Sutcliffe values were both 0.65 during the calibration step (1993–2002) and 0.56 and 0.61 (2003–2013), respectively, during the validation step, indicating satisfactory performance of the SWAT simulation. The water balance established at the M’dez watershed level shows that the average annual rainfall was around 382 mm, of which 79.9% was lost by evapotranspiration (300.05 mm). Surface runoff was about 13.83 mm, corresponding to 3.6% of precipitation. The results of the implementation of BMPs in agricultural areas showed that the contour tillage scenario was the most efficient option, with a contribution of baseflow to the surface flow of about 7.8%, and an infiltration into the groundwater of approximately 48.2%, which means better preservation of surface water resources. For soil losses, the reduction was significant at 64.90%. At watershed level, the tillage contour scenario coupled with a forestation strategy of the most degraded sub-basins (Slope < 25%) reduced the annual average specific degradation from 3.95 to 1.57 t/ha/year, i.e. (60.25%), and decrease the mean annual sediment input by 60.25% compared to the baseline scenario. The siltation rate of the projected M’dez retention dam was estimated at 0.8 Mm3/year for the baseline scenario, and was reduced to 0.78 Mm3/year and 0.34 Mm3/year, respectively, for the contour tillage scenario and its coupling with the forestation strategy. This work has shown that hydrological modeling using SWAT coupled with BMPs will help planners to manage water and soil resources in an integrated manner at the watershed scale. It will also provide useful information and effectively target the best water and soil conservation practices to select the most appropriate practice for agricultural watersheds.