Hydrological modeling in a data-poor Mediterranean catchment (Merguellil, Tunisia). Assessing scenarios of land management and climate change

In the Mediterranean regions, hydrologic processes are quite specific due to the temporal variability of precipitation characterized by a succession of drought and flash-flood periods. \nThese processes may also have changed due to a range of human activities such as land use \nchanges, dams building, soil and water conservations works. The Merguellil catchment (Central Tunisia) is a typical Mediterranean semi-arid basin which suffers regular water shortage aggravated by current drought. It extends on an area of about 1200 km² upstream of the El Houareb dam which presents its outlet. This semi-arid zone is exposed to a high variability of rainfall in time and space. Annual means vary between 300 mm in the plain and 500 mm in the highest parts. During the recent decades the continuous construction of small and large dams and Soil and Water Conservation Works (SWCW) (ie. Counter ridges) has taken place within the watershed. These practices, that currently cover nearly of the basin \nsurface, are classified in two categories: the practices on basin slopes constituted essentially by contour ridges (200 Km²) and the practices on the hydrographical network by implantation of small hilly dams draining about 170 Km. These water harvesting systems may intercept runoff at the upstream part of the catchment, thus depriving potential downstream users of their share of the resources. However, little is known about the effect of these water harvesting systems on the water balance components of arid watersheds. In such a vulnerable situation of water resources availability, it can be expected that the impact of climate change will further worsen the situation. \nThe work presented here attempts to simulate the actual water and nutrient balance using the integrated hydrological model “Soil and Water Assessment Tool” (SWAT 2005). The \nsimulation results revealed that vapotranspiration is the major component of the hydrological balance. Hydrological Calibration (1992-1994) and validation (1998) have been carried out referring to daily flow data at the Hafouz and Skhira flow-gauges. The model performance \nwas satisfactory and the Nash-Sutcliffe Efficiency coefficient ranges from 0.6 to 0.7. \nThe model was rather successful in reproducing water flow. However the low sampling frequency and the lack of detailed water quality measurement data did not allow an in-depth evaluation of the SWAT performance in predicting nutrient and sediment. Some scenarios were further generated. The first one regards the removal of contour ridges to assess their impact to water and sediment load. The results show that the contour ridges contribute to the \nretention of high quantity of sediment. These regulations reduce the surface runoff by 32 %. \nPlanting the olive trees between contours could improve its yield. The second scenario \nconsists in the reduction of the applied fertilizers. By reducing 20% in the applied fertilizers no change was detected in the olive yield, while a small change was noted for durum wheat yield’s (-2%). Whereas a net decrease in nutrient load was observed at the outlet. This reduction ranges, from 3 to 10% for nitrates, from 2.5 to 8% for total nitrogen and from 13 to 16.5 % for total phosphorus. Finally, the SWAT model was used to study the impact of future climate on water resources of this Mediterranean catchment. Future climate scenarios for periods of 2010-2039 and 2070-\n2099 were generated from the Canadian Global Coupled model (CGCM 3.1) for scenarios \nA1B, B1, and A2. These CGCMs data were then statistically downscaled to generate future \npossible local meteorological data of precipitation and temperature in the study area. SWAT model was run first under current climate (1986-2005) and then for the future climate period to analyze the potential impact of climate change on flow, evapotranspiration, and soil moisture across this catchment. Finally, Richter et al.’s Indicators of Hydrologic Alteration \n(IHA) were used to analyze the flow regime alterations under changing climate. The main \nresults indicate that this catchment would suffer a combination of increased temperature and reduced rainfall that will reduce water resources in this area. Consequently, summer droughts would be intensified. Different spatial responses to climate change were observed in the \ncatchment for near future simulations. Higher altitude regions would experience an increase of the total water yield, while a reduction is foreseen for lower parts. For far future, a noticeable decrease would affect water resources in all part of the catchment.

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