Loading functions are proposed as a general model for estimating monthly nitrogen and phosphorus fluxes in stream flow. The functions have a simple mathematical structure, describe a wide range of rural and urban nonpoint sources, and couple surface runoff and ground water discharge. Rural runoff loads are computed from daily runoff and erosion and monthly sediment yield calculations. Urban runoff loads are based on daily nutrient accumulation rates and exponential wash off functions. Ground water discharge is determined by lumped parameter unsaturated and saturated zone soil moisture balances. Default values for model chemical parameters were estimated from literature values. Validation studies over a three‐year period for an 850 km2 watershed showed that the loading functions explained at least 90 percent of the observed monthly variation in dissolved and total nitrogen and phosphorus fluxes in stream flow. Errors in model predictions of mean monthly fluxes were: dissolved phosphorus ‐ 4 percent; total phosphorus ‐ 2 percent; dissolved nitrogen ‐ 18 percent; and total nitrogen ‐ 28 percent. These results were obtained without model calibration.

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A general procedure was developed for calibrating multiparameter probability distributions of daily precipitation to single-parameter distributions. The approach uses monthly precipitation summaries and data from U.S. Weather Bureau Technical Paper 57 (Miller and Frederick, 1966). The threeparameter beta-P model of daily precipitation amount was calibrated for 33 sites east of the Rocky Mountains. The resulting single-parameter Weibull distribution and two other single-parameter precipitation distributions were compared with respect to their fit to Paper 57 summaries and historical daily precipitation records. The Weibull model was shown to yield significant improvement over the other models in reproducing precipitation probability distributions.