Natural Resources, School of


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A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Natural Resource Sciences, Under the Supervision of Professor John D. Lenters. Lincoln, Nebraska: May 2011

Copyright 2011 Mehmet Evren Soylu


Evapotranspiration (ET) is an important component of the water and energy balance, yet it is also one of the most challenging components to estimate. There has been great effort to understand the nature of controlling mechanisms and interactions between ET and other earth system processes. The controlling factors of ET can be grouped into two broad categories – namely moisture availability and energy availability (e.g., solar radiation). Soil moisture is a key factor that most of the land surface hydrologic processes are dependent on. While plant water use is mainly controlled by radiation, temperature is another key factor for ET in terms of controlling the atmosphere’s moisture demand. In this study, the overall goals are to: 1) quantify the impact of groundwater and climate on ET and other components of the surface water and energy balance, 2) assess the observed and modeled interactions among ET and groundwater when the water table is close to the surface, and 3) determine the interdependencies among interannually varying climatic variables and their combined effect on the surface energy and water balance.

First, we investigated the role of different numerical model parameterizations in quantifying the impact of groundwater on root zone soil moisture and ET – as well as model sensitivity to soil texture and water table depth – by comparing land surface ET models with varying complexity in a shallow water table environment (i.e., a riparian wetland in south central Nebraska, USA). Then, the impact of ET on groundwater was examined by analyzing diurnal water table fluctuations at multiple observation wells at the wetland field site. In addition, we proposed a new method to estimate ET more effectively than existing methods by using Fourier series to represent diurnal variations in water level hydrographs. Finally, we used a high-resolution, distributed land surface hydrologic model (the Integrated Biosphere Simulator) to evaluate the impact of interannual climate variation, vegetation type, and groundwater depth on variations in ET across the central U.S.

Adviser: John D. Lenters