The hydrochemical impacts of terraced agriculture, Nepal

The contribution of different water sources to streamflow in catchments of the Middle Hills, Nepal is quantified. Mixing relationships for a conservative component, alkalinity, were used to enable hydrograph separation into two end-members, rainfall and water stored within a catchment. Alkalinity is estimated from continuous conductivity measurements using calibrations from spot sampled weekly data. The results indicate that stormflow in cultivated catchments is dominated by low-alkalinity rainwater which dilutes the higher-alkalinity stored waters, often causing a decrease in stream pH. The drains and irrigation canals associated with terraced agriculture provide quick flowpaths causing rainfall to bypass the soil horizons. In a forested catchment high infiltration rates and the absence of these pathways maintains stored water flows and reduces the rainfall contribution to stormflow. Water quality implications are examined given the predicted future increase in atmospheric pollution in Asia.