Date of Award:

5-2014

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Scott B. Jones

Committee

Scott B. Jones

Committee

Roger K. Kjelgren

Committee

Lawrence E. Hipps

Committee

Shih-Yu Wang

Committee

R. Douglas Ramsey

Abstract

Evapotranspiration (ET) and soil moisture play important roles in annual water delivered from snowpack to reservoirs, lakes and streams. Indeed, ET and soil moisture are key factors dictating the performance of the regional climate models in the intermountain west (IMW) of the USA. Water resources management and climate modeling require accurate prediction of ET and areal soil moisture for reliable estimates of ongoing and future water needs. This research has examined ways to estimate ET from four common vegetation types in the IMW (aspen, conifer, grass and sage) using local soil moisture measurements from an advanced instrumentation network located in the T.W. Daniel Experimental Forest (TWDEF). The TWDEF is located within the Bear River Range of the Wasatch Cache National Forest in Northern Utah. Among instrumented forest research sites worldwide, TWDEF is unique, providing triplicate measures within a mixed forest system common to the IMW. Observations included continuous meteorological measurements such as air temperature, humidity, solar radiation, wind speed, soil moisture and others. In situ soil moisture values were measured at 0.10-, 0.25- and 0.50-m depths within each of the four vegetation types. In addition, areal soil moisture was measured using a Cosmic-ray neutron probe (CRNP) located in the middle of the site. This unique dataset enables study of the hydrological processes in Utah and the IMW region. Estimates of ET from aspen, conifer, grass and sage were simulated using a numerical model. Simulated ET values were compared with measured ET from an eddy covariance tower. Results suggest the numerical model is a viable method to estimated ET where no direct ET measurements are available. The simulations also enabled comparison of summer ET among vegetation species including both high and low water years. Finally, a comparison was made between the intermediate-scale areal soil moisture measured by the Cosmic-ray neutron probe (CRNP) and the in situ time domain transmissometry (TDT) soil moisture network at the TWDEF site. Improved correlations were obtained by including numerically simulated soil moisture above 0.1 m where no measurements were available. The original CRNP calibration showed a dry bias during spring/early summer, leading to the need for an additional site-specific calibration, which improved the accuracy of the CRNP soil moisture estimate at the TWDEF site.

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