WinHSPF model simulations of nitrogen and pH for a low-alkalinity stream impacted from atmospheric acid deposition

The Hydrologic Simulation Program-Fortran (WinHSPF) was used to simulate the stream flow, ammonium, nitrate and pH for the No land Divide watershed from 1999 to mid-2006, to evaluate the model applicability to watersheds impacted by acidic deposition. Noland Divide watershed is located in the Great Smoky Mountains National Park, Tennessee. Model calibration for hydrology was satisfactory showing total error of 2.1% for stream discharge during simulation period. The model simulated low ammonium concentration (0.04 mgN L^-1) in the stream, which is likely due to the high nitrification rate on the land converting most deposited ammonium to nitrate. The deviation between the simulated and measured nitrate concentration was 0.03 mgN L^-1 while the mean observed nitrate concentration was 0.57 mgN L^-1. Although the model output for pH (from 5.7 to 6.0) were similar to observed stream pH measurements (from 5.5 to 6.5), model calculations of pH did not consider stream acidification due to mineral anion acids. The WinHSPF model computes pH simply based on carbonate system and alkalinity equilibrium, and therefore is only applicable to waterbodies governed by the carbonate-dominated chemistry. In low alkalinity waters like those found in the Noland Divide watershed, WinHSPF can not predict stream chemistry response from impacts of acidic deposition.