Basin-Scale Modeling as Tool for Water-Resource Planning

A basin-scale model was developed to simulate long-term average losses of water, sediment, and nutrients from large rural watersheds. In addition to simulating diffuse sources using the Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) model, the model incorporates point sources, nitrogen leaching from septic tanks, riparian and stream channel attenuation processes, and routing algorithms. When applied to a 73-km2 basin in New Zealand, the model predicted a 14-year annual average water yield of 740 mm, very close to the observed value of 738 mm. Predictions of long-term average sediment and nutrient losses were generally within 30% of those estimated from data gathered for the basin outlet. Importantly, the model was able to predict well the direction and magnitude of changes in pollutant loss brought about by implementation of a riparian pasture retirement scheme, a popular best management practice in New Zealand. Model predictions of soluble nutrient losses were sensitive to variation in stream attenuation coefficients, and we conclude that further development of basin-scale models will require a better understanding of riparian and stream-channel processes.