Relating Agricultural Land Uses to In-Stream Stormwater Quality

Sixteen largely agricultural watersheds in the upper portion of the North Bosque River of central Texas were instrumented to collect storm event samples for nutrient analysis. Flow-weighted storm-event mean concentrations were averaged across storm events to characterize the water quality at each site for storms sampled between November 1992 and August 1995. Nutrient concentrations were related to land uses above sampling sites using correlation and regression analysis to indicate major sources of nutrient nonpoint source pollution to the upper North Bosque River. Consistently, N and P concentrations increased as the proportion of land area used for dairy waste application fields (or milking cow densities) increased in the drainage basins above sampling sites. The proportion of total P (TP) in runoff represented by soluble reactive P (SRP) also increased as the percent of dairy waste application fields above a sampling site increased; likely as a response to the common practice of surface application of manure to permanent pasture. This increase in SRP is of particular importance because SRP is readily bioavailable in aquatic systems, thus increasing the potential for accelerated eutrophication in receiving waterbodies. The results of these analyses indicate a strong association between in-stream nutrient concentrations during storm events and the percent of dairy waste application fields comprising a drainage area. This indicates a need to manage the movement of nutrients, particularly soluble P, from manure application fields in areas where in-stream nutrient levels are considered a nonpoint source pollution problem.