LOSS OF PHOSPHORUS BY RUNOFF FOR AGRICULTURAL WATERSHEDS.

The loss of nutrients in runoff from agricultural land is a major cause of poor surface water quality in the United State. Scientists (NRCS) developed a technique to estimate the impact of agricultural watersheds on natural water resources. The objectives of this study were to apply this technique on the Wagon Train (WT) watershed to predict (1) loss of water by surface runoff, (2) loss of phosphorus (P) from soils by runoff and P loading for WT reservoir. The annual loss of water by runoff was estimated at 4.32 million m^3. The USGS data for a 50-year period (1951 to 2000) indicated that the average annual inflow for WT reservoir was 4.25 million m^3. The predicted annual P loss by runoff was 844 kg and could be considered as the annual loading for WT reservoir. The predicted P concentration in the runoff water at field sites was 196 (mu)g/L. Phosphorus concentration observed in major streams at the beginning of spring (March) ranged from 99 (mu)g/L to 240 (mu)g/L with an average of 162 (mu)g/L (S.D.=40 (mu)g/L), and the average P concentration in water samples taken from different locations in the reservoir was 140(mu)g/L. Phosphorus uptake by algae, weeds and aquatic plants, as well as high pH in the reservoir and streams might explain the slight drop of P concentration in waters. Further, the average P concentration observed in the main stream samples for the entire rainy season (March through October), ranged between 157 and 346(mu)g/L with an average of 267 (mu)g/L (S.D.=65 (mu)g/L). Application of P fertilizers (April/May) for summer crops might explain the increase in P concentration. When factors affecting P concentration in streams are considered, the technique could provide a reasonable estimation of P concentration in stream water.