Linking Montane Soil Moisture Measurements to Evapotranspiration Using Inverse Numerical Modeling


Ling Lv

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Utah State University


The mountainous areas in the Intermountain West (IMW) of the North America are considered as the major water reservoir for the Western US. Summer evapotranspiration (ET) and soil moisture are key factors affecting the annual water yield in the montane region of the IMW. This research estimated ET of four common vegetation types (aspen, conifer, grass, and sage) and areal soil moisture in an advanced instrumentation site located at the T.W. Daniel Experimental Forest (TWDEF). Among instrumented forest research sites worldwide, TWDEF is one of a few with triplicate measures of meteorological parameters, radiation, and soil moisture within four common vegetation types in the IMW. This unique dataset enables study and understanding of the ecological and hydrological responses to climate change in Utah and the IMW region. In a second phase of this study, summer water uses from the four common vegetation types were simulated using a numerical simulation model, Hydrus-1D. The simulation was informed by soil moisture measurements at three depths (0.1 m, 0.25 m, and 0.5 m) and by ET measured from an eddy covariance tower. The results confirmed the value of numerical simulations as a viable alternate method to estimated ET where no direct ET measurements are available. It also provided comparison of water use by these vegetation species including both high and low water years. In the third phase of this study, a comparison was made between the intermediate-scale areal soil moisture measured by a Cosmic-ray neutron probe (CRNP) and the in situ TDT soil moisture network at the TWDEF site. Improved correlations were obtained, especially after shallow rainfall events, by including numerically simulated soil moisture above 0.1 m where no measurements were available. The original CRNP calibration exhibited a dry bias during spring/early summer, leading to the need for a site-specific enhanced calibration, which improved the accuracy of the CRNP soil moisture estimate at the TWDEF site.