APPLICATION OF SWAT FOR THE UPPER NORTH BOSQUE RIVER WATERSHED

Abstract

Agriculture, including livestock production, has been implicated as a major source of pollution to streams and lakes. This study was conducted to assess the effect of dairy production on water quality within Upper North Bosque River Watershed (UNBRW) of north central Texas. The UNBRW encompasses an area of approximately 93 250 ha and included 94 dairies at the time of the study. A river basin model (Soil and Water Assessment Tool, SWAT) was applied in two phases. During the first phase, SWAT was validated for the baseline condition within UNBRW. The baseline condition within UNBRW was simulated from 1988 through 1996; model output was compared to flow, sediment, and nutrient measurements for 11 stream sites within the watershed for the period of October 1993 to July 1995 for SWAT model validation. The Nash-Sutcliffe coefficient evaluating model efficiency of SWAT for predicting average monthly flow, sediment, and nutrient loading (organic-N, NO3-N, organic-P, and PO4-P) at 11 stream sites over the validation period ranged from 0.65 to 0.99, indicating reasonable predicted values. SWAT also adequately predicted monthly trends in average daily flow, sediment, and nutrient loading over the validation period with Nash-Sutcliffe coefficients ranging from 0.54 to 0.94 except for NO3-N which had a value of 0.27. Nutrient loadings were consistently highest in the subwatersheds with the most dairies operations. In phase two, to evaluate the effect of dairies, SWAT was executed by replacing the manure waste application fields with grassland. The results from this phase of study indicate that loading from the watershed could be reduced about 33% for total-N (organic-N plus NO3-N) and 79% for total-P (organic-P plus PO4-P) in the UNBRW if dairy waste application fields were replaced by grassland. Empirical equations based on the output with and without dairy loading were developed to illustrate the impact of cow density and percent area covered by dairy waste application fields on sediment and nutrient loading.