Soil erosion characterization in an agricultural watershed in West Sumatra, Indonesia.

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Quantitative evaluation of soil erosion rates provides important baseline data to investigate, manage and improve land use systems. However, soil erosion analyses have not been sufficiently conducted in Indonesia. In the present study, we investigated the spatial distribution of soil erosion rates in relationship to land use patterns in the Sumani agricultural watershed, the primary rice-producing region in West Sumatra. The soil erosion rate was estimated applying the Universal Soil Loss Equation (USLE) using representative soil survey data obtained in the watershed. Sediment delivery ratio (SDR, i.e. sediment yield / soil erosion rate) was determined from sediment yield data, which was generated in a previous study. Soil erosion rate in the Sumani watershed was estimated as 43.13 Mg ha-1y-1 in 1992 and 58.91 Mg ha-1y-1 in 2002 (annual averages). These values are far greater than the tolerable soil erosion rates (TER) for Indonesia i.e. 14 Mg ha-1y-1. Natural factors, including heavy rainfall and local soil properties in a landscape susceptible to soil erosion were the fundamental factors responsible for the high soil erosion in the watershed. In addition to these causes, changes in land use accelerated soil erosion. From 1992 to 2002, the soil erosion rate showed a 37% increase due to forest conversion to agricultural fields. SDR in five sub-watersheds exhibited relatively small values ranging from 6% to 15%, indicating an accumulation of eroded soil particles in flat areas in the lower part of the watershed where the land is primarily allocated to sawah.