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 m3 for runoff and 0.98 million m3 for leaching. The observed annual inflow for WT reservoir was 4.25 million m3 . 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.
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