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The objective of this study was to evaluate the effect of check dam infrastructure on soil and water conservation at the catchment scale using the Soil and Water Assessment Tool (SWAT). This paired watershed study includes a watershed treated with over 2000 check dams and a Control watershed which has none, in the West Turkey Creek watershed, Southeast Arizona, USA. SWAT was calibrated for streamflow using discharge documented during the summer of 2013 at the Control site. Model results depict the necessity to eliminate lateral flow from SWAT models of aridland environments, the urgency to standardize geospatial soils data, and the care for which modelers must document altering parameters when presenting findings. Performance was assessed using the percent bias (PBIAS), with values of +/-2.34%. The calibrated model was then used to examine the impacts of check dams at the Treated watershed. Approximately 630 tons of sediment was stored behind check dams in the Treated watershed over the 3-year simulation, which increases the water quality to an acceptable level for fish habitat there. A minimum precipitation event of 15mm was found necessary to instigate the detachment of soil, sediments, or rock from the study area, which occurred 2% of the time. The resulting watershed model is useful as a predictive framework and decision-support tool to consider long-term impacts of restoration and potential for future restoration.

In most watershed-modeling studies, flow is calibrated at one monitoring site, usually at the watershed outlet. Like many arid and semi-arid watersheds, the main reach of the Santa Cruz watershed, located on the Arizona-Mexico border, is discontinuous for most of the year except during large flood events, and therefore the flow characteristics at the outlet do not represent the entire watershed. Calibration is required at multiple locations along the Santa Cruz River to improve model reliability. The objective of this study was to best portray surface water flow in this semi-arid watershed and evaluate the effect of multi-gauge calibration on flow predictions. In this study, the Soil and Water Assessment Tool (SWAT) was calibrated at seven monitoring stations, which improved model performance and increased the reliability of flow predictions, in the Santa Cruz watershed. The most sensitive parameters to affect flow were found to be curve number (CN2), soil evaporation and compensation coefficient (ESCO), threshold water depth in shallow aquifer for return flow to occur (GWQMN), base flow alpha factor (ALphA_Bf), and effective hydraulic conductivity of the soil layer (Ch_K2). In comparison, when the model was established with a single calibration at the watershed outlet, flow predictions at other monitoring gauges were inaccurate. This study emphasizes the importance of multi-gauge calibration to develop a reliable watershed model in arid and semi-arid environments. The developed model, with further calibration of water quality parameters will be an integral part of the Santa Cruz Watershed Ecosystem portfolio Model (SCWEpM), an online decision support tool, to assess the impacts of climate change and urban growth in the Santa Cruz watershed.

In many watershed modeling studies, due to limited data, model parameters for flow, sediment, and nutrients are calibrated and validated against observed data only at the watershed outlet. Model parameters are adjusted systematically for the entire watershed to obtain the closest match between the model-simulated and observed data at the watershed outlet (lumped calibration). It is hypothesized that the relative loadings of pollutants and/or sediments contributed by each computational unit are not affected by this calibration procedure. In other words, areas generating relatively higher pollutant loads with an uncalibrated model will still generate relatively higher loads after calibration. This study explored the effect of lumped calibration of the Soil and Water Assessment Tool (SWAT) on locations of sediment and nutrient critical source areas (CSAs). Two watersheds in Alabama with differing size, topography, hydrology, and land use/cover characteristics were used to study the variations in locations of sediment, total phosphorus (TP), and total nitrogen (TN) CSAs identified by calibrated and uncalibrated SWAT models. Identified CSAs for sediment, TP, and TN were mostly the same with and without the calibration of the model in both watersheds. This study thus concluded that lumped calibration of the SWAT model using data at the watershed outlet has little effect on the locations of CSAs. Based on the results from these two watersheds, it was further concluded that SWAT can be used without calibration for identification of CSAs in watersheds that lack sufficient data for model calibration, but not for all other modeling purposes. More studies are encouraged to support these findings.