Water balance evaluations for monitored evapotranspirative cover systems at three sites in the semi-arid and arid Southwest U.S.



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The focus of this dissertation is the water balance of monitored evapotranspirative cover systems constructed at sites in Yucaipa, California, Albuquerque, New Mexico, and Sierra Blanca, Texas. The contributions of the research are: • a comprehensive assessment of the status of performance evaluations of evapotranspirative cover systems; • an evaluation of the short-term (2 to 5-year) performance of the three cover systems using data from field monitoring programs and numerical modeling, without calibration; • an evaluation of the suitability of using short-term performance assessments to predict intermediate-term (10 to 30-year) performance; and • a comparison of the long-term (100 years or more) reliability of two hypothetical evapotranspirative cover systems at the Albuquerque site, one designed with loosely placed soil to promote plant growth and the other designed with compacted soil having a low saturated hydraulic conductivity. For the short-term performance evaluation, the water balances of the cover systems were modeled using the HELP (version 3.07), LEACHM (version 4), and UNSAT-H (version 3.01) computer programs. The simulation results were compared to the results of the field monitoring programs. The comparison of simulated and measured drainage for the considered sites was not ideal because drainage was not monitored at the Yucaipa site and zero drainage was measured at the Albuquerque and Sierra Blanca sites. The suitability of using the short-term performance assessments to predict the intermediate-term performance of the cover systems was evaluated by simulating the water balances of the cover systems with historical weather data and comparing the results of these simulations to the results of the short-term performance assessments. The possible long-term performance of two hypothetical cover systems was evaluated using interval analysis that considers long-term soil density, vegetation, and precipitation. Simulation results suggest that, in semi-arid and arid climates that receive much of their precipitation in the summer, a cover system with looser soil and deeper roots may allow less drainage than a cover system with denser soil and shallower roots, but may be less reliable in the long term if precipitation patterns or vegetation changes. Recommendations for assessing performance of evapotranspirative cover systems and for increasing reliability of evapotranspirative cover systems are presented.