Estimating sediment and nutrient loads of Texas coastal watersheds with SWAT

Texas Water Development Board (TWDB) has been studied and estimated the total fresh water inflow to bays in Texas using TxRR (Texas Rainfall Runoff) model. Recently, TWDB requested to develop and apply the SWAT model using up to date technologies for estimating inflow, sediment and nutrient loads to the bays. In 2011, SWAT (Soil and Water Assessment Tool) model was developed and applied for estimating inflow to the bays over TxRR by Lee et al. In this study, SWAT model was used to estimate sediment and nutrient loading from a coastal watershed based on the developed model in 2011. A rural watershed, Matagorda, was selected as pilot study. Model calibration and validation was conducted using an average annual and monthly sediment and nutrient loads from USGS water quality samples and literatures (gaged) and the same parameter settings were applied to the rest of the watersheds (ungaged). The total sediment and nutrient loads from the watershed by SWAT was compared to the estimation by available reports and literatures. The monthly sediment calibration showed a good coefficient of determination (R²) 0.76 with Nash Sutcliffe coefficient (NS) 0.70. The correlation for validation was 0.61 for R² and 0.33 for NS. The correlation for total nitrogen and total phosphorus calibration and validation ranged from 0.61 to 0.87 and 0.45 to 0.47 with NS 0.38 to 0.80 and 0.26 to 0.38, respectively. The comparison of SWAT and available literature estimation showed a good agreement in monthly total sediment and nutrient load to the bay.