Effects of Land Use Change on Nutrient Discharges from the Patuxent River Watershed

We explored the effects of land use and land use change on water and nutrient discharges from the Patuxent River watershed, a 2400 km² tributary basin of the Chesapeake Bay. We related measured discharges from 22 small (0.06-230 km²) watersheds to land cover and physiographic province. Nitrogen discharges increased with the proportion of cropland in a watershed, and the rate of increase with cropland proportion was stronger in the Piedmont than in the Coastal Plain physiographic province. Phosphorus discharges were less strongly related to geographic factors and more strongly driven by temporal variations in stream flow. Developed land had weaker but still significant effects on nitrogen and phosphorus discharges, and the effects again differed between physiographic provinces. We developed a set of statistical models incorporating the effects of geographic and temporal variability, and then we applied the models to predict the effects of land use change on nutrient discharges. Land use transitions involving the gain or loss of cropland had a much stronger effect on nitrogen, phosphorus, and silicate discharges than gains or losses of developed land. The effects of land use change and the differences among physiographic provinces were stronger in relatively wet year than in a dry year. Land cover data from different sources yield different estimates of land cover proportions, particularly the proportion of cropland. The differences can affect fitted statistical models and the resulting predictions of land use change effects. Therefore, improved land cover data are needed to better understand and quantify the effects of land use and land use change on watershed discharges.