James B. Callegary

Multi-gauge Calibration for modeling the Semi-Arid Santa Cruz Watershed in Arizona-Mexico Border Area Using SWAT

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.

Quantifying Ephemeral Streambed Infiltration from Downhole Temperature Measurements Collected Before and After Streamflow

A constant flux infiltration experiment was conducted to determine the feasibility of using downhole temperature measurements to estimate infiltration flux. Temperatures measured using a downhole thermistor within a 15.4-m-deep borehole compare well with temperatures measured with buried thermocouples in an adjacent borehole to 5 m depth. Numerical forward model simulations were conducted using VS2DI. A numerical sensitivity analysis showed that the temperature profile was most sensitive to the average temperature of the infiltrating water, the infiltration flux, and the specific heat capacity of dry soil. The high sensitivity of these variables allows for a simple sequential optimization to be used to estimate the average temperature of the infiltrating water, the water flux, and the specific heat capacity of dry soil from numerical inversion of temperature measurements. Downhole temperature measurements could be a useful complement to shallow streambed temperature methods, allowing for better quantification of the contribution of streambed infiltration to basin-scale recharge.