Effects of „ENSO-events” and rainforest conversion on river discharge in Central Sulawesi (Indonesia) – problems and solutions with coarse spatial parameter distribution for water balance simulation

Effects of „ENSO-events” and rainforest conversion on river discharge in Central Sulawesi (Indonesia) – problems and solutions with coarse spatial parameter distribution for water balance simulation

Natural interannual climate fluctuation as ENSO-events and forest conversion play an important role for the water balance of tropical catchments. Distributed hydrological modelling that relate land cover changes and climate changes with river discharge changes for humid tropical catchment areas at the mesoscale level are rare. This article applies a hydrological modelling approach to analyse the impact of land cover changes and ENSO-events on the water resources of a mesoscale humid tropical catchment.Because of a lack of detailed spatial soil information, a topographic based nested scale approach to parameterise soil physical properties was used for the hydrological model WASIM-ETH (Schulla and Jasper, 1999). The PHA (potential homogeneous soil texture areas) approach produces reasonable results for the water balance components for the whole catchment, but is too coarse for flow component differentiation within the catchment.Thereafter the distributed hydrological model WASIM-ETH (“Water balance Simulation Model”) was calibrated and validated for the current land use (2001/2002, Landsat/ETM+ scene). Model results were generally consistent with the observed discharge data and reproduced the seasonal discharge dynamics well. The implications of possible future climate and land use conditions on the water balance of the Gumbasa River sample catchment were assessed by a scenario analysis. The results of the scenario simulations clearly demonstrate a strong relationship between deforestation rates and increasing discharge variability. Especially a significant increase of high water discharges was simulated for the applied land use scenarios. The main results of the scenario analysis are:(1) that ENSO anomalies of precipitation (monthly decrease for an average ENSO -21 until -80% from June to October) lead to an increase of discharge variability.(2) that strong ENSO-events (El Nino) lead to 30% reduction of the water yield and annual crop scenarios up to 1,200 m a.s.l. showed a 42% and the perennial crop scenario (cacao) a 23% increase in river discharge with high increase in overland flow and flooding risks.(3) that ENSO-events (El Nino) decrease the potential (flooding) area of paddy rice cultivation to one third in the second half of the year.With regard to the high deforestation rates of the research catchment and increase of El Nino frequency with climate change in the future, one can expect further negative changes for the water resources in Central Sulawesi.