Soil and Water Degradation Following Forest Conversion in the Humid Tropics (Indonesia)

Indonesia’s annual deforestation rate of –1.2% is dramatic compared for example to Brazil’s 0.4% loss of forest cover. Sulawesi region of Indonesia still has a forest cover of 48%, but as population has been growing by 66% over the past 2 decades, massive land cover transformations are going on that have changed the land cover pattern and consequently soil and water resources of the region are altering as well. Since 2001, we investigate the impact of forest conversion on the water balance, nutrient losses and soil erosion of a small mesoscalic tropical catchment, which is integrated into the long-term interdisciplinary collaborative project STORMA (http://www.storma.de). The study was conducted in a small mountainous catchment, which is located at the north-eastern border of the Lore Lindu National Park Central of Sulawesi. The Nopu catchment (51S 01757231E, 9867683 N) covers an area of approximately 2.6 km2. Since 2001 the catchment is monitored with three weirs (automatic stage recorder), one climate station and six rain gauges. Traditional slash & burn cultivation is predominant and heavy machinery is not used. At two sub-catchments (rainforest weir 3, slash & burn weir 2) changes in river discharge and nutrient outputs with time and differences were analysed since 2002. As indicators for land degradation we studied the changes of infiltration rate, water flow path (increasing interflow and surface flow) and soil nutrient output with river discharge. In the Nopu catchment slash & burn activities increased mainly since 2003/2004 with forest reduction until 2007 in the sub-catchment weir 2 from 87% (2001) to 26% forest cover (2007). Due to forest conversion, river discharge increased from 9 to 17% for the period 2002/2003–2005/2006 mainly driven by increase in overland flow and quick interflow. Three scenarios (forest/cacao/slash & burn (corn with cassava)) were simulated with the application of the water balance model WASIM-ETH. Simulation results supported the experimental results showing an increase from 8% to 17% annual discharge for cacao and slash & burn scenario and give insight into the changing discharge components. The increase in overland flow leads to an increase in soil erosion with doubling of suspended sediment output from 2003 to 2005. Also, higher soil nutrient leaching and increase in quick interflow has caused an increase by ratios of 1.5–1.9 of the main cations (Ca, Mg, K) and nitrogen (TNb) in the sub-catchment partially deforested compared to the natural forest sub-catchment. After 2005/2006, land degradation indicators of suspended sediment output and soluted nutrient output indicate a “stabilizing effect” with decrease due to land use change to cacao agroforestry in the mid slopes of the catchment.