Simulation on area threshold of urban building land based on water environmental response in watersheds.

: Urban sprawl has impacted increasingly on water environment quality in watersheds. Based on water environmental response, the simulation and prediction of expanding threshold of urban building land could provide an alternative reference for urban construction planning. Taking three watersheds (i.e., Yundang Lake at complete urbanization phase, Maluan Bay at peri-urbanization phase and Xinglin Bay at early urbanization phase) with 2009-2012 observation data as example, we calculated the upper limit of TN and TP capacity in three watersheds and identified the threshold value of urban building land in watersheds using the regional nutrient management (ReNuMa) model, and also predicted the water environmental effects associated with the changes of urban landscape pattern. Results indicated that the upper limit value of TN was 12900, 42800 and 43120 kg, while that of TP was 340, 420 and 450 kg for Yundang, Maluan and Xinglin watershed, respectively. In reality, the environment capacity of pollutants in Yundang Lake was not yet satura-ted, and annual pollutant loads in Maluan Bay and Xinglin Bay were close to the upper limit. How-ever, an obvious upward trend of annual TN and TP loads was observed in Xinglin Bay. The annual pollutant load was not beyond the annual upper limit in three watersheds under Scenario 1, while performed oppositely under Scenario 3. Under Scenario 2, the annual pollutant load in Yundang Lake was under-saturation, and the TN and TP in Maluan Bay were over their limits. The area thresholds of urban building land were 1320, 5600 and 4750 hm2 in Yundang Lake, Maluan Bay and Xinglin Bay, respectively. This study could benefit the regulation on urban landscape planning.