Thesis (Ph. D. - Renewable Natural Resources)--University of Arizona, 1985. / Includes bibliographical references (leaves 132-136).

Seasonal droughts are becoming more prevalent in recent years as a result of climate change and associated fluctuations in precipitation events. Forested ecosystems react very differently to changes in the hydrological cycle in different regions. This study investigates how atmospheric water fluxes react to changes in meteorological variables and subsurface hydrological conditions in a 90-year old temperate deciduous forest in southern Ontario, Canada, using eddy covariance flux data over five years (2012-2016). Results show that although drought occurrences in 2012, 2015 and 2016 put stress on the forest ecosystem, with significantly varying climate and soil conditions, no substantial impact on forest evapotranspiration appeared to occur. Annual eddy-covariance mean evapotranspiration of the study was 382 ± 46 with a growing season average of 353 ± 36. Highest annual evapotranspiration (422 mm) occurred in 2012 with 389 mm occurring during the growing season while the lowest annual evapotranspiration (341mm) occurred in 2014 with 316mm occurring in the growing season. Air temperature and vapour pressure deficit had the dominant control on evapotranspiration as expected, with the highest sensitivity occurring during drought years. Soil water potential (SWP), as the soil moisture stress proxy, reached maximum values during drought years, but was not found to have a significant control with evapotranspiration. SWP peaked when evapotranspiration values reach maximums combined with warm air temperatures, progressively increasing when precipitation input was minimal. Our results indicate that evapotranspiration rates in this forest ecosystem were predominantly determined by atmospheric controls when accessibility of deep soil water was possible. These findings suggest that temperate deciduous forests were able to sustain ET during low to moderate intensity droughts and observed over the study period due to their ability to access deep soil moisture stores during periods of stress. However, these forests may experience significant declines in ET if severe or multi-year drought conditions arise, similar to those that occurred in the last two years of this study. These conclusions will help to elucidate how ET in deciduous forests will respond to future climate regimes in this region. / Thesis / Master of Science (MSc) / With seasonal droughts occurring more frequently from climate change, water required by tree species may reach limitations to sustain atmospheric demand. In particular, temperate deciduous forests, that are common in the northern hemisphere, may be affected due to predicted future climate scenarios. It is important that we understand how they will respond to future climate conditions. Understanding how forest evapotranspiration is affected by various meteorological and hydrological parameters will help gauge how forests will cope with limiting water resources in the future. An assessment of a temperate deciduous forest in southern Ontario, which is part of the Turkey Point Flux Station, was conducted in this study over 5 years (2012-2016). Over this period the forest experienced varying drought intensities. Results show that air temperature and vapour pressure deficit were the dominant controls on forest ET during times of water limitations. The forest was able to withstand low-moderate droughts by accessing deep soil water stores to keep up with the extreme demand. This study suggests that temperate deciduous forests in the region are well adapted to drought stress and they may be able to cope with similar intensity droughts in the future by maintaining water flow.

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 70-75). Also available on the World Wide Web.

Existing empirical evapotranspiration formulas were evaluated using climatic data for two consecutive years (1980 and 1981) from the Lower Colorado River Valley (Blythe, California). It was demonstrated that existing simple empirical models are inapplicable in arid zones where climatic ranges are wide. Extensive measurements of evapotranspiration rates using the Bowen ratio energy budget approach and selected climatological variables were made over well irrigated alfalfa sites in Avra Valley near Tucson, Arizona, for the season beginning in early May 1982 and ending in mid-October 1982. Forty-three days of 12-minute data gathered during the study were used as a basis for developing new models and for calibrating the Penman combination model. Several empirical models estimating evapotranspiration from various numbers of climatic variables were derived using a multiple linear regression technique. Their performance over the wide range of climatic conditions during the study prove the utility of certain empirical models for estimating evapotranspiration. The most promising is a simple solar radiation and wind model PET = a + bK + CU which uses only two climatic variables, incoming solar radiation (I%) and wind (U) which are widely available from routine weather measurements. The Penman combination model, with improvements suggested in literature, underestimated evapotranspiration as measured over irrigated alfalfa. A new arid zone wind function was developed using 12-minute data throughout the entire measurement period. The Penman model with the arid zone wind function performed very well throughout the season.

A method of calculating surface evapotranspiration by separately including the effects of vegetation and atmospheric evaporative demand under the condition of nonlimiting soil moisture is presented. A literature survey is conducted to determine the effects of plants on evapotranspiration. To represent the atmospheric evaporative demand, the original potential evaporation equation of Penman (1948) is utilized and then modified to include the effect of atmospheric stability using turbulent exchange coefficients formulated by Louis et al. (1982). The original and modified Penman expressions are compared for different asymptotic cases. Using boundary layer data from the Wangara experiment (Clarke et al., 1971), the diurnal variations of the original and modified Penman equations are compared. The daily total potential evaporation using linearized and integrated forms of the original and modified expressions are also compared. Finally, the nonlinear effects of averaging both the original and modified expressions are examined. It is found that including the diurnal variations of stability in the modified expression causes large hourly differences with the original expression under non-neutral conditions, while daily averages of the two compared fairly well. The diurnal variation of the surface moisture flux appears to be much larger than predicted by the original Penman expression. However, the original Penman expression remains a reasonable estimate of the 24-hour total potential evaporation. / Graduation date: 1983

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009. / Title from title screen (site viewed February 25, 2010). PDF text: xvii, 164 p. : ill. (chiefly col.) ; 7 Mb. UMI publication number: AAT 3386611. Includes bibliographical references. Also available in microfilm and microfiche formats.

Recent invasions of woody plants into semiarid grasslands are a world-wide phenomena with potential ramifications for global-scale carbon cycling. An understanding of how biological and non-biological processes within ecosystems influence water loss to the atmosphere is important to evaluating the consequences of woody plant encroachment on carbon and water cycling in semiarid lands. Accordingly, evapotranspiration in a Chihuahuan Desert grassland was partitioned into its component fluxes for the 2005 summer growing season using a combination of microlysimeters, to quantify soil evaporation, and eddy covariance, to quantify evapotranspiration and net ecosystem exchange of CO2 (NEE). While some of the results of this study (e.g., the ratio of T to ET) are expected to be highly dependent on the particular characteristics of the 2005 summer rainy season, many of them reveal a more general picture about the timing and magnitude of the biological and non-biological water and carbon cycling responses for a warm-season semiarid grassland. This will be important for trying to understand what happens to the carbon and water cycling processes as grasslands are invaded by shrubs.

Thesis (Ph. D. - Hydrology and Water Resources)--University of Arizona, 1971. / Includes bibliography.

Thesis (M. S. - Renewable Natural Resources)--University of Arizona, 1997. / Includes bibliographical references (leaves 106-109).

Thesis (M. S. - Soil, Water and Environmental Sciences)--University of Arizona, 2004. / Includes bibliographical references (leaves 50-54).