The application of poultry litter to agricultural land has become a topic of interest for policy makers due to public concern about its effects on water quality. The Soil and Water Assessment Tool (SWAT) version 2005 is designed to assess nonpoint and point sources of pollution. In this study, six subwatersheds in Texas (HUC‐8; 12070101) are used to evaluate the model's ability to simulate water quality at a small scale. Each of these subwatersheds randomly received poultry litter rates of 0.0 to 13.4 Mg ha-1. Monthly and daily data from 2002 were used for calibration purposes, while 2000, 2001, 2003, and 2004 were used for validation. The SCS runoff curve number for moisture condition II (CN2) and the soil evaporation compensation factor (ESCO) parameters were found to be more sensitive than the surface runoff lag time (SURLAG) and initial soil water content expressed as a fraction of field capacity (FFCB). The monthly and daily runoff model simulations for the six subwatersheds resulted in calibration Nash‐Sutcliffe efficiency (NSE) values of 0.59 and 0.53 and validation NSE values 0.82 and 0.80, respectively. The monthly and daily R2 runoff values for the six subwatersheds resulted in calibration values of at least 0.60 and 0.53 and validation R2 values of 0.86 and 0.81, respectively. The observed trends included SWAT's overestimation of runoff in the dry periods and underestimation in the wet periods. The monthly NSE and R2 values for sediment and nutrient losses were generally above 0.4 and 0.5, respectively. Paired t‐tests for the monthly manually adjusted parameter simulation of sediment, organic N and P, NO3-N, and soluble P for the 2000‐2004 period losses showed that their respective SWAT means were not significantly different from the measured values ( = 0.05), except for NO3-N losses for the Y10 subwatershed (p‐value 0.042). The control subwatershed's measured and simulated water quality results were significantly different ( = 0.05) from the treated subwatersheds, most likely due to the amount of inorganic N present. Almost all of the subwatersheds that had poultry litter applied resulted in higher sediment, organic N, organic P, and soluble P losses than the control subwatershed upon averaging the monthly validation values. High NO3-N losses may have been a function of poultry litter and commercial fertilizers being applied before a large rainfall event occurred. The subwatersheds that received smaller amounts of commercial fertilizer and/or poultry litter lost more sediment, organic N, and organic P than the subwatersheds that received the higher litter and/or fertilizer treatments. Overall, the SWAT simulated the hydrology and the water quality constituents at the subwatershed scale more adequately when all of the data were used to simulate the model, as evidenced by statistical measures.
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