Soil erosion under land use change from three catchments in Laos, Thailand and Vietnam

The systems often identified as "traditional" undergo rapid changes as a response to demographic, economic, political and cultural drivers. These transitional periods are often most critical for soil erosion. The on-site impacts of soil erosion reduce the soil chemical fertility through nutrient and organic depletion, and acid subsoil exposure. Erosion also damages the physical fertility by removing surface soil, reducing the soil depth and water holding capacity, and exposing gravel and rocks. These combined processes result in less productive soils, hence lower farm income. To obtain the initial crop yield prior to erosion, increased amounts of inputs are needed, which is most often beyond the economic capacity of the small holders. To study the impact of land use change upon erosion, concurrent case studies, as seen with a dynamic perspective, can compensate for long-term monitoring studies. This approach provides data, which can be used for prediction soil erosion based on global change scenarios. The main objective of this study was to assess the influence of the rapid change of cropping systems on water erosion from three small catchments in three countries of South-East Asia (Laos, Thailand, Vietnam), using a multidisciplinary approach. These three catchments were selected because of their similar biophysical components (very steep slopes on shales; Janeau et al., submitted) and their land use intensification gradient. This investigation was conducted under the auspices the Management of Soil Erosion Consortium (MSEC) started in 1998 (Amado et al., 2002). Water discharge and soil erosion were monitored during three years at the outlet of each catchment using weirs. These data were used to calibrate and validate the PCARES model (Predicting Catchment Runoff and Soil Erosion for Sustainability) in each cachment. This GIS- based model was developed in the Philippines for very steep slope conditions (Paningbatan, 2000). It is based on the concept developed by Rose & Freebairn (1985) and uses PCRaster, a GIS software designed to simulate overland flow and soil erosion for each erosive rainfall event. This model was run for the three study catchments using main scenarios for land-use change and climate. The first scenario for land-use was based on the most likely changes as based on the observed tendencies. The second scenario was based on the best bet options of land management. Climate change scenarios were essentially based on an increase of 20% and 40% of rainfall amount and mean intensity. Our results confirmed the very high sensibility of soil erosion to land use change as compared to climatic change. They also illustrated the dramatic hydrological changes (time-response and peak discharge).