Analisis debit sungai akibat alih guna lahan dan aplikasi model genriver pada das way besai, sumberjaya

Forest conversion into coffee gardens in Sumberjaya over the last three decades has led to concerns over the hydrological functions of the upper watershed. Forests generally are associated with positive watershed functions and all land use change is expected to negatively affect the quantity and quality of river flow from the perspective of people living downstream. A recently developed set of criteria for watershed functions aims at a focus on the impacts of land use change, given the climate and inherent properties of a site. We analyzed data for the Sumberjaya benchmark area to derive a set of quantitative indicators for the criteria ‘transmit water’, ‘buffer peak rain events’ and ‘release of water gradually’. The conversion of forest to coffee gardens with declining forest area from 60% to 12% from 1970 to 2000. Based on the 23 years (from 1975 to 1998) of hydrological data. Annual river flow as a fraction of rainfall has increased. The data show a decline in the ‘buffering indicator’ that relates peak flows to peak rainfall events, but this decline does not exceed the increase in average water yield. There was no negative impact on dry season flows. This implies that the distribution of daily flows is shifted upwards, but without specific breakdown of buffering for peak rainfall events. A simulation model (GenRiver) was used to explore our understanding of historical changes in river flow due to land use change and use it as a basis for exploring plausible future scenarios. GenRiver is a distributed process-based model that extends a plot-level water balance to subcatchment level. Our GenRiver application for Sumberjaya compares three different land use changes scenarios (all forest, current land use and degraded land). An acceptable agreement was obtained between the measured and simulated values of the watershed function indicators for the current land use mosaic. Watershed function indicators derived by the model can thus be used to explore the ‘degradation’ scenarios where the positive impact of forest conversion on total water yield would become associated with negative impacts such as flooding risk and declines in river flow during the dry season. In the model the key factor for such change is the condition of the soil, and current evidence suggests that coffee-based agroforestry does not pose a threat to watershed functions in Sumberjaya.

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