The SWAT model to assess hydrological processes in arid environment (SE Tunisia), in the frame of WADIS-MAR project

Arid regions of Maghreb (North Africa) suffer scarce water conditions and the erratic behaviour of rainfall events over brief intervals often produce short and intense floods events which converge into ephemeral wadi beds. The watershed of wadi Oum Zessar, in South-East Tunisia, is characterized by a rainfall rate of about 200 mm/year, overexploitation of groundwater resources and is highly exposed to climate change risk and desertification processes (Ghiglieri et al., 2014). This region represents one of the two study areas in the framework of WADIS-MAR demonstration project (www.wadismar.eu), funded by the European Commission under the Regional Programme SWIM (www.swim-sm.eu). WADIS-MAR Project aims: (i) to improve the traditional water harvesting systems (i.e. jessour and tabias) by applying “soft” modern rehabilitation interventions; (ii) to increase groundwater availability through managed aquifer recharge (MAR) systems (i.e. gabions, recharge wells, recharge trenches). Jessour andtabias capture surface runoff coming from degraded and rocky rangelands for crop production (fruit trees, mainly olives, and cereals) in upstream subbasins. The objective of this study was to assess the main hydrological processes and the sediment loads in this arid environment, by using the Soil and Water Assessment Tool (SWAT) model. Sediment load is an important parameter to be considered in the MAR systems because it may causes the clogging of the infiltrating surface, resulting in the reduction of water infiltration rates. The watershed delineation used the newly 2014 released SRTM 1 Arc-Second digital elevation model, available at www.earthexplorer.usgs.gov. A land cover classification obtained by visual interpretation of Landsat data was used for the modelling. Conversion to land use was supported by ancillary and detailed ground truth data. The classic version of SWAT was adjusted to adapt the model to this dry Mediterranean environment (Ouessar et al., 2009). In SWAT, subbasin may be divided into Hydrologic Response Units (HRUs) which possess unique landuse/management/soil attribute. Normally, the runoff from all HRUs is added directly to the outlet of the subbasin. In this study runoff is routed between HRUs within the subbasin. The main adjustment consists of allowing the simulation of the runoff collection behind the water harvesting structures by bringing the surface runoff and lateral flow generated by different HRUs within the same subbasin. Moreover, existing gabions and recharge wells were considered in the model for a more accurate simulation of the hydrological processes. Possible coupling with MODFLOW will be considered. After calibration and validation, the model will be also used for evaluating the efficiency of the MAR systems which are being realized within the WADIS-MAR project.