Soil-sediment nutrient transport dynamics in a Himalayan watershed

Nutrient dynamics in the Bela watershed, Nepal were examined using a range of GIS budget modelling and field analysis techniques. The fertility status analysis showed that soil acidity, phosphorus availability, and lack of base cations were the key soil fertility issues. Nutrient budget calculations revealed significant annual phosphorus deficits for maize rota­ tions, but only minor deficits in rice rotations. The effect of biophysical conditions and management inputs were analysed and the link between poor soil fertility, annual deficits, soil erosion and degradation was made. The erosion and sediment transport rates from degraded sites (rilled and gullied, with <25% vegetation cover), were found to be twice those from nonirrigated agricultural sites. The nutrient content in these sediments was very low and of little benefit to farmers who re-capture sediments in their irrigated fields. In contrast, re-deposition of sediments from non-irrigated agriculture was found to enrich irrigated fields significantly. Rehabilitation of degraded areas is needed to prevent negative effects downstream.