GENERALIZED WATERSHED LOADING FUNCTIONS FOR STREAM FLOW NUTRIENTS¹

Abstract

ABSTRACT: 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 km 2 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.