Impacts of climate change and forestation on the hydrology of a tropical catchment in Indonesia

The aim of this study is to assess the impacts of climate change and forestation on the hydrology of a tropical catchment in Java, Indonesia. The study area is an exemplary case in Indonesia where a forestation programme has been implemented at the catchment scale. The hydrological effects of this forestation programme, in combination with climate change effects, have not been assessed. The individual and combined effects of climate change and forestation on the hydrology are assessed by employing a calibrated Soil and Water Assessment Tool (SWAT) model. Different water balance components are considered including streamflow, surface runoff, lateral flow, base flow and evapotranspiration. The results show that the forest area has increased from 2.7 to 12.8% of the catchment area from 2006 to 2019. In the same period the mean annual and seasonal temperature, rainfall and streamflow have increased as well. The SWAT simulations show that the effects of forestation on the annual and seasonal water balance components were relatively small, while the effects of climate change on these components were more pronounced. The combined effects of climate change and forestation show the dominant influence of climate change and both opposite effects of climate change and forestation on water balance components (e.g. for streamflow) and similar effects (e.g. for evapotranspiration). The outcomes of this study indicate that the forestation programme has small impacts on water balance components in the study catchment on an annual scale compared to the impacts of climate change. However, seasonally, forestation has resulted in a decrease in streamflow and surface runoff during the wet season which may reduce the flood risk. At the same time, the effects of forestation on severe drought events in the dry season should be carefully monitored. In conclusion, this study showed the relative importance of climate change and forestation impacts on water balance components and the potential of forestation programmes to flatten extreme events on a seasonal scale.