Estimation of Sediment and Nutrient Loads to Bays from Gauged and Ungauged Watersheds

Abstract. Due to the lack of water quality data in the lower watersheds of the Texas coastal region, a Soil and Water Assessment Tool (SWAT) version 2009 model was used to estimate terrestrial sediment and nutrient loads from non-tidal fresh water inflows to Texas bays in this study. The Texas Water Development Board (TWDB) has used the TxRR (Texas Rainfall-Runoff) model to estimate the total fresh water inflow and annual sediment, nitrogen, and phosphorus loads from measured and simulated stream flows to the bays in Texas. Applying newer methods, such as distributed hydrologic models, e.g., SWAT, enables the temporal and spatial distribution of freshwater, sediment, and nutrient loads to also be predicted. To develop the SWAT model for the estimation of sediment and nutrient loads from two watersheds, the Matagorda as a rural watershed and Galveston as an urbanized watershed, the average of the calibrated model parameter values were applied to the ungauged lower sub-watersheds in consideration of land use homogeneity. The monthly sediment calibration showed good agreement compared with the observed total suspended sediment, with Nash Sutcliffe Model Efficiency (NSE) values ranging from 0.55 to 0.75 and Percent Bias (PBIAS) values ranging from +10.3% to +51.0%. The predicted monthly total nitrogen (TN) and total phosphorus (TP) at gauge stations exhibited poor correlation with the observed values in the urbanized sub-watersheds but good-to-acceptable correlation in the other watersheds, with NSE values ranging from -0.15 to 0.85 and PBIAS values ranging from+13.3% to +84.0%. The comparison of the SWAT and available estimations from previous studies showed good agreement for the mean annual total suspended sediment and nutrient loads from ungauged sub-watersheds to the bays. Using parameter settings from gauged watersheds for ungauged watersheds, SWAT can be applied to other Texas coastal watersheds to estimate flow and sediment and nutrient loads. However, extending the global average values of the adjusted parameters to the highly developed ungauged sub-watersheds may lead to poor results, and limited regionalization in parameter allocation to these areas should be considered.