Preliminary assessment of flows and sediment load in the river system of the Sumberjaya, Lampung, Sumatra: is land use change really the main culprit?

Land use change, and especially deforestation, is often blamed for the loss of watershed functions. Farmer-development landscape mosaics with various, degrees of tree cover are often perceived as not functional in providing these services. This perception is the root to often violent conflicts between guardians of (protection) forest (‘hutait lindung’) and farmers opening the land, e.g. for coffee gardens. 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 forest and coffee on ‘the ridges, coffee on the slopes and paddy rice in the valleys. Ambivalent government positions (attraction of migrants followed by eviction) has caused conflict with farmers over the past decades, Extrapolation of plot level research to the entire basin led to the conclusion that this was a ‘critical’ watershed, The (weak) knowledge base used for evaluating these landscape mosaics is now challenged. Turbidity and sediment concentration measurements in the Way Besai and its tributaries show large differences between catchments. Even under dense forest cover some pristine headwaters can turn quite turbid. It seems that land use per se is not the most determining factor, but other factors like geology, soils, roads and foot paths, ... play a significant role. In recent years an impressive amount of public money was spent on national reforestation campaigns, often intended to rehabilitate watershed functions, without specification of which functions are to be enhanced, where and how. Assessing key watershed functions in a more direct approach e.g. by monitoring the river discharge and its sediment load would allow a better diagnosis of the condition of catchments. As preliminary results differ significantly from a widely accepted belief, this could alter which catchments would need rehabilitation, and what might be appropriate measures, The above does not give ‘cane blanche’ for deforestation, but suggests that well established agroforestry gardens (‘kebun lindung’) can be an ecologically benign and economically more attractive alternative. Farmers also suffer from reduced watershed functions (paddy rice fields might be washed out in floods, lower dry season flows hampers the availability of household water). Rather than being traditional enemies in a top-down reforestation campaign, there is a potential to be partners in watershed assessment, river monitoring and eventual rehabilitation..

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