Nutrient loss is sensitive to land cover changes and slope gradients of agricultural hillsides: Evidence from four contrasting pond systems in a hilly catchment

•Long-term water quality dynamics in four contrasting pond systems were revealed.•N levels showed an increase by a factor of five once the land cover was changed.•N & P loss is sensitive to the slope gradients of the cultivated hillsides.•Forests and grasslands are crucial in reducing N & P loss from the hillsides.•An integrated management strategy is needed to reduce N & P loss in hilly regions.