Calibration of NPS Model Loading Factors

NPS continuous simulation model was calibrated to 11 single‐land use watersheds in Chesapeake Bay drainage area to derive nonpoint pollution loading factors (total N and total P) for rural, agricultural, and urban land use categories. Flow, rainfall, and nonpoint pollution monitoring records for each test watershed cover approximately 1–1.5 years. Parameter adjustments for calibration of both the hydrologic and nonpoint pollution submodels relied upon NPS model executions with continuous 15‐minute rainfall record covering the entire monitoring period. Parametric and nonparametric statistics were used to assess goodness‐of‐fit for nonpoint pollution loading calibration. In light of the higher power than normal statistics for datasets characterized by small sample sizes and skewed distributions, nonparametric statistics are recommended for goodness‐of‐fit assessments for nonpoint pollution loading models. Calibrated NPS loading factors for total N and total P are presented for the following land uses: (1) Forestland; (2) pastureland; (3) high and low tillage cropland; (4) single family residential; and (5) commercial. Simulated annual unit area loads for each land use are compared for years of normal and above‐average rainfall.