Modelling the Impact of Land Use Changes on Hydrology using HBV-Light
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During the last years, problems of flows risks have become a very interesting issue in Tunisia due to many factors such as changes in the land use. Deforestation together with agriculture expansion, due to the anthropogenic activities, has an effect on the land use/cover change influencing the hydrology of a sub-basin by changing the magnitude of stream flow. In this work, the likely land use/cover change impacts on hydrology of the Mellegue river sub-basin in the Upper Medjerda River Basin have been evaluated using the semi-distributed HBV-Light hydrological model and remote sensing for two different periods. ArcGIS was used to generate the land use-cover maps from Landsat TM (1984-1997) for the year 1988 and second land use/ land cover map on 2003 was taken from the Regional Commissariat of Agricultural Development (CRDA) of Kef region. The land use-cover map of 1988 were generated using the Maximum Likelihood Algorithm of Supervised Classification. The accuracy of the classified maps was assessed using contingency matrix, it gives an overall accuracy of 89.6%. The result of this analysis showed that the forest area has decreased by 5.16% from 1988 to 2003 due to anthropic activities, as well, there is an expansion of irrigated land by 4% due to the converting toward more advanced agriculture technologies and the encouragement of the government by making more projects focusing in agriculture enhancing production. The model calibration was processed using observed hydro-meteorological data (precipitation, temperature, potential evapotranspiration, discharge) for the period ranging from 1980 to 1986. The validation of the model was run for a period between 1995 to 1999 using the same data input as calibration. The efficiency of the model was obtained directly from HBV model using the Reff method. The result was acceptable for the both calibration and validation. An efficiency of 0.38 for calibration and 0.1 for validation for the land use/cover of 1988. An efficiency of 0.33 for calibration and 0.1 for validation for the land use of 2003. By comparing the outputs of the two calibration and validation, the hydrograph shows a variation of the streamflow along the years by either increasing or decreasing, this therefore could be interpreted due to the occurrence of changes in land use/cover (deforestation and expansion of irrigated land).