Rainfall, runoff and sediment transport from a forested Lesser Himalayan watershed in Himachal Pradesh

Sediment transfer from continents to oceans via rivers is one of the most important processes. The amount of sediment transported from a watershed system is an indicator of the rate of erosion or the soil loss. In the present study, the long-term annual, seasonal, monthly and diurnal variations in sediment transport have been studied at the outlet (concentration point) of Tirthan watershed using the monitored data during 1981-2004. The sediment transport at the outlet of watershed is a function of rainfall and runoff occurring over the watershed area. A temporal and spatial variation in the amount of rainfall and runoff clearly exhibits annual, seasonal, monthly and diurnal variation in the sediment generation response behaviour. The higher concentration of rainfall depths exceeding 5 mm or more with significant variability could result in generation of more sediment fluxes. Both sediment concentration and load have been found to be the highest in the month of July followed by August and nearly 67% of the total sediment load has been transported in these two months. The peak of sediment load in July and August indicates that materials weathered during the dry season get mobilized during this period. Sediment load for the entire study period is computed as 454 t km(-2) yr(-1). Sediment yield from the watershed was lower as compared to the average sedimentation rate of the north Indian reservoirs. Availability of good forest cover (56%) within the watershed system may be attributed to low sediment transport rates from this watershed. The lower sedimentation rate from the watershed will enhance the operational efficiency of hydro-power projects to be built or already constructed in the Tirthan watershed system.