Comparing Runoff, Soil and Nutrient Losses from Three Small Watersheds in Indonesia

The effects of land cover at the watershed scale on soil and nutrient losses are important criteria for improved soil conservation management. The objective of this study was to assess runoff, soil and nutrient losses from 3 neighboring small agricultural watersheds in Java, Indonesia, in relation to the prevailing land use. In the area, Inceptisols and Alfisols are dominant. The three watersheds are characterized by distinctly different land uses, i.e., annual cropping (in the Tegalan watershed), perennial cropping (in Rambutan) and mixed cropping (in Kalisidi). Field measurements were carried out over a two-year period (2000 and 2001). The mean annual rainfall in the area was about 3180 mm. The annual sediment yields (soil losses) from the watersheds were 9.6 t ha-1 (in 2000) and 15.4 t ha-1 (in 2001) for Tegalan; 1.0 t ha-1 (in 2000) and 0.2 t ha-1 (in 2001) for Rambutan; and 0.5 t ha-1 (in 2000) and 0.8 t ha-1 (in 2001) for Kalisidi. The annual runoff from the watersheds in 2000 and 2001 were 1,980 m3ha-1 and 1,190 m3ha-1 (Tegalan), 230 m3ha-1 and 80 m3ha-1 (Rambutan), and 6,090 m3ha-1 and 6,200 m3ha-1 (Kalisidi). Enrichment ratios for P ranged between 1.2 and 2.3, for Mg between 1.1 and 2.4, and for K between 1.2 and 1.3. No significant overall correlation was observed between enrichment ratio (ER) and soil loss, but the differences between the watersheds were significant. Soil losses did not systematically increase with runoff. Overall, land cover has shown to be the most likely determinant of soil loss, and hence, nutrient losses in the study area. Nutrient losses due to erosion appear to be a major cause of soil fertility depletion in the area. Soil management practices to improve the levels of P, K and organic matter would be required to compensate losses of plant nutrient and to improve soil-quality in general.