LOSS OF NITRATE-NITROGEN BY RUNOFF AND LEACHING FOR AGRICULTURAL WATERSHEDS.

The loss of nutrients in runoff and leaching water from agricultural land is a major cause of poor water quality in the United States. Scientists (NRCS) developed a technique to estimate the impact of agricultural watersheds on natural water resources. The objectives were to apply the technique on Wagon Train (WT) watershed in Nebraska to predict: (i) loss of water by surface runoff and subsurface leaching, (ii) loss of nitrate-N from soils by runoff and leaching, and (iii) nitrate-N loading for WT reservoir. The annual loss of water was estimated at 4.32 million m^3 for runoff and 0.98 million m^3 for leaching. The observed annual inflow for WT reservoir was 4.25 million m^3. The predicted annual nitrate-N loss by runoff was about 7.0 Mg and could be considered the annual loading for the reservoir. The predicted nitrate-N loss by leaching was 7.73 Mg, however, the fate was not clear. The estimated average nitrate-N concentration in runoff and leaching water at field sites was 1.63 and 7.88 mg/L, respectively. The observed nitrate-N concentration in water samples taken from 12 major streams ranged between 0.37 and 1.56 mg/L with an average of 0.90 mg/L. Nitrogen uptake by algae, weeds, and aquatic plants and emission of gaseous nitrogen oxides from fresh water under reducing conditions might explain the lower nitrate-N concentration. No attempt was made to monitor the nitrate-N concentration in soil leachate or groundwater. When factors affecting N concentration in streams are considered, the technique could provide a reasonable estimation of N concentration in stream water. We concluded that the technique could be applied to estimate the loss of nitrate-N by runoff and leaching from soils and the impact on surface waters.