CALIBRATION AND VALIDATION OF THE SWAT MODEL FOR PREDICTING DAILY ET OVER IRRIGATED CROPS IN THE TEXAS HIGH PLAINS USING LYSIMETRIC DATA

摘要

The Soil Water Assessment Tool (SWAT) is a widely used watershed model for simulating stream flow, overland flow, and sediment, pesticide, and bacterial loading in response to management practices. All SWAT processes are directly dependent on accurate representation of hydrology. Evapotranspiration (ET) is commonly the most significant portion of the hydrologic cycle, especially for irrigated lands in semiarid environments when ET demands are met or exceeded. However, no studies using long-term data to evaluate the SWAT model's capacity to estimate daily, monthly, and seasonal ET have been performed. In this study, daily and monthly ET values were simulated using ArcSWAT 2012 for an irrigated lysimeter field at the USDA-ARS Conservation and Production Research Laboratory (CPRL) at Bushland, Texas, and compared to measured ET values from 2001-2010. Crops grown during the study period included cotton, soybean, grain and silage sorghum, sunflower, and corn silage. A one-year warm-up (2000) and equal division of the remaining years were used for the calibration (2001-2005) and validation (2006-2010) periods. SWAT achieved a Nash-Sutcliffe efficiency (NSE) of 0.67 for daily ET during the calibration period, resulting in a "good" performance rating. An NSE of 0.78 resulted in a "very good" rating for the validation period. NSE values for simulated average monthly ET were improved, at 0.77 and 0.85 for the calibration and validation periods, respectively. Analysis of simulated versus measured ET values both during and outside of the growing season revealed better agreement during the former than the latter. The SWAT model generally underestimated ET at both daily and monthly levels but overestimated annual cotton ET due to overestimation of leaf area index during the senescing stage. Overall, SWAT was able to simulate daily and average monthly ET reasonably well for major summer crops grown in the semiarid Texas High Plains. These results should reinforce confidence in the SWAT model's capacity to accurately simulate ET in fully irrigated watersheds. However, limitations in accuracy appear to exist for certain crops, such as cotton and sunflower, and particularly under limited irrigation conditions. These deficiencies may be related to issues with the embedded Leaf Area Index (LAI) crop growth model and default plant parameter values in the crop database in SWAT.