HYDROLOGICAL REGIMES FOR FIVE SUB-CATCHMENTS IN THE HILKOT WATERSHED, DISTRICT MANSEHRA

Abstract

Stream discharge, sediment concentration and nutrient loadings are significantly influenced by land use and management practices in addition to climatic characteristics of Himalayan watersheds. A preliminary study was conducted to relate flows and sediment yields with land management from four sub-catchments and main catchment in Hilkot watershed. Climatic and stream flow data revealed that the bulk of the rainfall occurred during the monsoon i.e. July to September as discharge from all the streams was high. High flows occurred shortly after heavy storms because of the steep gradients and small catchment areas of the streams. High sediment concentrations were confined to critical periods of intense rains and during tillage/planting times when the soil is least protected and most disturbed. The objectives of study were to develop stage-discharge relationship to investigate sediment losses at different discharge levels at five hydro-stations representing different land uses. In the Hilkot watershed annual maximum rainfall ranged from 996 mm to 1458 mm while maximum monthly rainfall recorded was 319 mm. Resulting R2 values showed a strong relationship between stage and discharge. The relationship between stage and discharge was 99%, 97%, 79%, 89% and 95% at hydro-station 1, 2, 3, 4 and 5 respectively. There was a very good and clear relationship between sediment concentration and discharge on all hydro stations. In most of the cases per liter sediment concentration was below 10 grams. Only some events were most destructive when per liter concentration was above 12 mg/I. Hydrograph showed positive skewnes, with recessing time greater than the rising time. Flood reaches from base to peak flow within 3-5 hour and fell very rapidly in the sub-catchments, while it took more time at the outlet of the watershed. Normally base flow in the main stream is 20 to 50 litre/second.