The link between land and water prediction of sediment sources in a previously forested watershed in Lampung, Sumatra, Indonesia

Deforestation is often blamed for the loss of watershed functions. Little importance is given to what land use comes next. Resulting landscape mosaics with various degrees of tree cover are often perceived as not functional in guaranteeing these services. This was the root to often violent conflicts between guardians of forest and farmers opening the land. ICRAF and partner institutions study land use, its change and the hydrological impacts in and around Sumberjaya watershed, West-Lampung, Sumatra an area of about 730 km2. The area was transformed in the past three decades from a large forest cover to a mosaic of coffee farms with rice paddies in the valleys and has seen quite some conflict over the past 10 years. The (weak) knowledge base used for evaluating these issues for landscape mosaics covering the wide range between pure forests and purely cropped lands is now challenged by the development of different erosion equations and models over the past ten years. In an earlier erosion modelling exercise various scenarios for the USLE, WEPP and GUEST (Rose) equations were compared at different scales. Results between models were strikingly different, especially in the spatial location of point sources of sediment delivery. Turbidity measurements in the river in a subwatershed indicate the importance of phenomena in and close by the river itself. For example criteria based on slope require definition of map resolution, as steep slopes, but short slopes close to the river are ignored in coarse resolution maps and DEM's. A good characterisation of ‘filter’ phenomena is more important than a qualification of land use per se. Simulation of the impact of government regulations made clear that a blind application of 'simple' regulations issued at the country level bypasses the local variety of conditions.

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