Dissertations / Theses on the topic 'Evapotranspiration'

Subsurface drainage (SSD) has been used to remove excess water from fields in the United States upper Midwest for more than a century, but only since the last decade in the Red River Basin of the North in North Dakota (ND). The water leaving from a SSD system can affect both the quality and quantity of water that flows to a surface water system. Therefore, determination of the water balance components is the first step to study the impact of SSD on water quantity, while evapotranspiration (ET), one of the most important components in the water balance, needs to be accurately measured for SSD field. A field experiment was conducted to study the water balance in SSD and undrained (having no artificial drainage system) fields in southeast ND. The field had three different water management systems: 22 ha undrained (UD), 11 ha subsurface drained, and the remaining 11 ha subsurface drained and subsurface irrigated. The ET rates were measured directly using an eddy covariance (EC) system for the SSD and UD fields. The changes in water table were monitored in 8 wells installed in both fields. Rainfall, SSD drainage volume, and soil moisture at six different depths at two locations were measured in both fields. The measurements were conducted in the growing seasons of 2009 and 2010. The ET rates were calculated for two different field crops: Com (Zea Mays) in 2009 and soybean (Glycine Max) in 2010. Crop coefficient (Kc) value was also developed using the ET measured by the EC system and the reference ET (ETref) estimated using the American Society of Civil Engineers Environmental and Water Resources Institute (ASCE-EWRI, alfalfa) method. The ETref was also estimated using the ASCE-EWRI grass and the Jensen Haise (JH) methods. The results indicated that the water table in the SSD field was lower during spring and fall than that in the UD field. The shallow water table and high soil moisture content in the spring and fall have resulted in higher ET rates in the UD field. In the summer, SSD field has favorable soil moisture at the root zone depth; the ET in the SSD field was 30% and 13% higher than that in UD field in summer 2009 and 2010, respectively. For the entire growing season, the ET in the SSD field was 15% higher compared to UD field and the difference was minimal in 2010. Though there were differences in the ET values, they were not statistically different. However, difference in magnitude of ET during summer 2009 yielded a statistical difference. During the peak growing season in July and August, the Kc values were greater in the SSD field due to healthy crops.
USDA (Grants CSREES NRI 2008-35102-19253)
USDA NRCS
North Dakota Agricultural Experiment Station
North Dakota State Water Commission
North Dakota Water Resource Research Institute
North Dakota Department of Health

12 pp.
This publication describes the procedure that is now recommended as a US standard for computing reference evapotranspiration. Included in the publication are: the rationale for developing the new procedure, the equations utilized in the new procedure, a discussion of how the new procedure differs from the established AZMET procedure, and tables to facilitate conversion between procedures (new and AZMET).

A new evapotranspiration package for the U.S. Geological Survey's groundwater -flow model, MODFLOW, is documented. The Riparian Evapotranspiration Package (RIP-ET), provides flexibility in simulating riparian and wetland evapotranspiration (ET) not provided by the MODFLOW-96 Evapotranspiration (EVT) Package, nor by the MODFLOW-2000 Segmented Function Evapotranspiration (ETS1) Package. This report describes how the package was conceptualized and provides input instructions, listings and explanations of the source code, and an example simulation. Traditional approaches to modeling ET processes assume a piecewise linear relationship between ET flux rate and hydraulic head. The Riparian ET Package replaces this traditional relationship with a segmented, nonlinear dimensionless curve that reflects the eco-physiology of riparian and wetland ecosystems. Evapotranspiration losses from these ecosystems are dependent not only on hydraulic head but on the plant types present. User-defined plant functional groups (PFGs) are used to elucidate the interactive processes of plant ET with groundwater conditions. Five generalized plant functional groups based on transpiration rates, plant rooting depth, and drought tolerance are presented: obligate wetland, shallow-rooted riparian, deep-rooted riparian, transitional riparian and bare ground/open water. Plant functional groups can be further divided into subgroups (PFSG) based on plant size and/or density. The Riparian ET Package allows for partial habitat coverage and mixtures of plant functional subgroups to be present in a single model cell. This requires a determination of fractional coverage for each of the plant functional subgroups present in a cell to simulate the mixture of coverage types and resulting ET. The fractional cover within a cell has three components: 1) fraction of active habitat, 2) fraction of plant functional subgroup in a cell, and 3) fraction of plant canopy area. The Riparian ET package determines the ET rate for each plant functional group in a cell, the total ET in the cell, and the total ET rate over the region of simulation.

Rapid population growth in arid regions of the western US is placing increased demand on water resources. Variability in precipitation and common occurrence of drought have promoted scrutiny of water use in urban lawns and gardens. However, few reliable measurements of water use of these landscapes exist. Quantifying the amount of water used vs. required by landscapes such as turfgrass would allow significant water conservation. Evapotranspiration (ET) is affected by biophysical factors such as: available energy, turbulent mixing, saturation deficit, soil water, and stomatal conductance. In order to simulate the water use by turfgrass, the relative importance of these processes must be determined for this environment. This study measures ET rates for Kentucky bluegrass using eddy covariance techniques, to quantify water use under various conditions. The results are combined with a coupled form of the Penman-Monteith Equation to determine which biophysical factors affect the ET rate under various atmospheric conditions, especially the advection of heat and saturation deficit from the regional atmosphere. In addition, changes in ET and other properties of the vegetation were monitored during a period of reduced irrigation or dry-down. These results will help determine the amount of water such landscapes actually need.

Thesis (B.S.)--California Polytechnic State University, 2010.
Project advisor: James Harris. Title from PDF title page; viewed on Apr. 19, 2010. Includes bibliographical references. Also available on microfiche.

Evapotranspiration is an essential component of the energy and water bud­get, but its estimation depends on available data sources and the environment of an area. Remote sensing techniques, combined with routine meteorological data, are used to estimate evapotranspiration over central Australia through the development and application of a number of models, ranging from physically based instantaneous models to a daily simulation model. The proposed models are evaluated using aircraft observations over two dis­tinct vegetation regimes in south-western Australia. Among the three physically based instantaneous models, single-source models using an excess resistance term empirically determined performed better than a two-source model which does not require such a parameterization. The mean absolute difference between measured and estimated values of the sensible heat flux is below 17 wm-2 in comparison to approximately 40 Wm-2 for evapotranspiration. Estimates of evapotranspiration depend on the closure of the surface energy balance and incorporate all residual errors in this closure. All models perform better over the agricultural vegetation than over the native vegetation. As these physically based models only provide instantaneous estimates of evapotranspiration at satellite overpass, a coupled one dimensional soil-canopy­atmosphere model and a simple budget water balance model have been used to simulate the daily evapotranspiration. Comparison of these results with the air-craft observations shows that the coupled model provides a good estimate of sur­face heat fluxes over the agricultural area with mean absolute differences between measured and estimate values being approximately 25 wm-2 for both sensible heat flux and evapotranspiration. Over the native vegetation, the mean absolute difference between measured and observed fluxes increased to 49 and 47 wm-2, respectively, for the sensible heat and evapotranspiration. This increase results from the inability of a simple water balance model to incorporate the effects of the underlying aquifer on deep rooted native vegetation, particularly during the dry summer season. It also highlights the sensitivity of the one dimensional soil-canopy-atmosphere model to the specification of soil moisture. Since the model simulation of surface temperature is also very sensitive to the soil moisture, a comparison between model simulation of surface temperature and satellite derived surface temperature was used to adjust parameters of a water balance model resulting in better estimates of soil moisture and consequently improved predictions of evapotranspiration. These models have been applied to estimating evapotranspiration in central Australia, using limited routine meteorological data and the NOAA-14 AVHRR overpass. Minimizing the difference between model predicted surface temperature and satellite derived temperature to adjust the estimated soil moisture, both the instantaneous physically based model and the simulation yielded consistent re­sults for 8 representative clear sky days during 1996-1997. These results highlight the sensitivity of surface temperature to soil moisture and suggest that radiomet­ric surface temperature can be used to adjust simple water balance estimates of soil moisture providing a simple and effective means of estimating large scale evapotranspiration in remote arid regions.

Revised. (Originally published: 2000).
4 pp.
Introduction: Local information on evapotranspiration (ET) is now readily available from on-site weather stations and/or public weather networks to assist turfgrass professionals with irrigation management decisions. Proper utilization of ET information can provide accurate estimates of daily water use and thus can assist irrigation managers with the all important decisions of when to apply water and how much water to apply. The concept of ET can be confusing and often is presented in a highly technical manner. The objective of this and subsequent bulletins in the Turf Irrigation Management Series is to simplify the subject of ET and thereby increase the effective utilization of ET in irrigation management. This bulletin provides some basic background on the related subjects of evaporation and evapotranspiration.

4 pp.
The objective of this and subsequent bulletins in the Turf Irrigation Management Series is to simplify the subject of ET and thereby increase the effective utilization of ET in irrigation management. This bulletin provides some basic background on the related subjects of evaporation and evapotranspiration.

The thesis first explored and evaluated some of the most used models that were developed to account for the effect of CO2 on evapotranspiration. This review depicts the complexity of the modeling procedure and underlines the advantages and shortcomings of each model. Then, the projected climate change in the near future (2021-2050) in different locations in Emilia-Romagna (Italy) was studied, with an emphasis on the opposite effect of an increase in both air temperature and CO2 levels on ETo. The case study used reanalysis data as a surrogate to historical weather stations measurements and an ensemble of regional climate models (RCMs) for the future projections. Results show that higher CO2 levels moderated the increase in ETo that accompanies an increase in air temperature, taking in consideration the change in other weather variables i.e. solar radiation, wind speed and dew point temperature. The outcomes of this study show that considering the CO2 fertilization effect when calculating reference evapotranspiration might give a more realistic estimation of water use efficiency and irrigation requirements in Emilia-Romagna and a better analysis of the future availability and distribution of water resources in the region. Finally, data from a model forecasting reference evapotranspiration (FRET) and the different variables involved in its calculation for the state of California (USA) were compared with similar data from the regional weather station network (CIMIS) to evaluate their accuracy and reliability. The evaluation was done in locations with different microclimates and included also sample irrigation schedules developed using FRET ETo. The obtained results demonstrate that FRET ETo forecasts are a viable alternative to traditional ETo measurements with some differences depending on the climatic condition of the location considered in this study. This implies that FRET could be replicated in other areas with similar climate settings.

Fresh water provides a range of essential services and is often used for irrigation purposes. Decreasing precipitation and increasing temperatures caused by climate change together with increased usage by humans has put these resources under stress, especially in relatively dry areas. This project takes a closer look at the irrigation of agricultural areas in the Guadalquivir river basin in southern Spain. An indication of irrigation intensity is attained by estimating the evapotranspiration using the S-SEBI method. This method is based on the surface energy balance and uses Landsat satellite images as its main input. Secondarily, a random forest classifier is trained to differentiate between irrigated and non-irrigated agricultural areas. Evaluation of these implementations produced a Root Mean Squared Difference of 0.8mm/day for daily actual evapotranspiration and an overall accuracy close to 80% for classification of irrigated areas. The results indicate that both the level of evapotranspiration and the irrigated agricultural surface area were stable over the period 2000-2020. This should not be taken to indicate that current freshwater management is therefore sustainable. This project shows the value ofcloud-computing services such as Google Earth Engine for remote sensing research. With this tool evapotranspiration estimation and irrigation classification was performed on an unprecedented temporal and spatial scale.

5 pp. / Originally published 2000; revised 2014.
Introduction: Accurate estimates of turf water use are required to effectively manage a turf irrigation system. In Volume I of this series entitled “Basics of Evaporation and Evapotranspiration (ET),” we indicated that actual turf water use (ETt) is rarely measured in the real world. Instead, we use meteorological data and a mathematical model known as the Penman-Monteith Equation to estimate reference evapotranspiration (ETos) — the ET from a tall, cool-season grass that is supplied with adequate water. In the lower elevations of Arizona the ETos value would seem of limited value since we rarely grow turf that is equivalent to the reference surface. However, we get around this problem by adjusting the ETos value to account for differences in turf type, quality and stage of development. This document describes the procedures used to adjust ETos for use on managed turf surfaces in Arizona.

This study applied a simple empirical evaporation model (SWET) proposed by Osmolski (1985) to estimate evapotranspiration over alfalfa using only solar radiation and wind data. SVVET was used to inventory evapotranspiration from irrigated alfalfa in lowland deserts in the southern half of Arizona. Meteorological data were obtained from the Arizona Meteorological Network (AZMET) for 21 sites in key agricultural regions. The number of years at each site ranged from 10 to 3 making possible for the first time the expression of long term means and standard deviations of alfalfa evapotranspiration. Results from SWET are compared to values of Penman reference evapotranspiration produced by AZMET and to estimates derived from the Blaney-Criddle and Thornthwaite methods. Temporal variability of daily estimates is examined as the scale lengthens from days to years. The spatial variability of predicted evapotranspiration is small across the width of southern Arizona, as illustrated by contour plots of evapotranspiration.

Grassland is the dominant vegetation cover of many of the 19 Water Catchment Areas within South Africa. The inappropriate management of some of these grassland catchments by the communities that depend on them for their livelihoods, often results in overgrazed lands with low biomass or invasive alien species. The short grass maintained by grazing policies of many communities results in high storm flows that have an adverse effect on the quantity and quality of runoff and recharge. Catchment-scale water balances depend on accurate estimates of run-off, recharge and evapotranspiration (ET). This study focuses on the ET component of the catchment scale water balance and explores the effect of two different grazing strategies on ET. To achieve this, two contrasting but adjacent quaternary catchments namely: P10A (a high biomass site) and Q91C (a low biomass site) were selected within the Bushman’s River Primary catchment as primary study sites. Within each catchment, a relatively homogenous pixel of 1 km was selected, representing contrasting example of high and low intensity grazing. From an eleven year MODIS leaf area index (LAI) data stack (March 2000 – 2010), 8-day LAI values was extracted for each pixel in each catchment. Using the Penman- Monteith equation, potential evapotranspiration (ET0) was calculated using data from a nearly automatic weather station. Actual evapotranspiration was estimated by adjusting ET0 using the values extracted from the MODIS LAI product. The MODIS LAI ET (ETMODIS) obtained for the eleven year period for both 1 km pixels decreased consistently, reflecting a general trend in declining LAI throughout the Eastern Cape. The highest ETMODIS obtained from P10A was 610.3 mm (2001) and the lowest was 333.1 mm (2009). Then from Q91C the highest ET obtained was 534.7 mm (2006) and the lowest was 266.2 mm (2009). The ETMODIS results were validated for each catchment using the Open Top Chamber (OTC) which sums the water lost from vegetation and soil within the chamber. This validation was conducted during the growing season of 2010–11. Wind speed; relative humidity and temperature were measured both at the inlet and the outlet of the chamber on five clear sunny days for each 1 km pixel. ETa for the same period was compared to the OTC ET (ETOTC) using the regression analysis and a good relationship was observed with the r2 of 0.7065. The relationship observed confirmed that ETOTC closely approximates ETMODIS and that the OTC can be used as a tool to validate MODIS LAI ET on clear, low winds and sunny days. In order to demonstrate proof-of-concept for the use of this modeling of ETMODIS within a Payment for Ecosystem Services framework, the approach was applied to two other quaternary catchments under communal tenure. Within each catchment, three land use scenarios were created for each catchment to reflect potential changes in the standing aboveground biomass. For Scenario 1, the status quo was maintained; for Scenario 2, MODIS pixels representing 28 km in each catchment were selected and the LAI of these pixels was doubled; and for scenario 3, LAI was halved. ETMODIS was calculated for each scenario by adjusting the ET0 data from a nearby automatic weather station with the MODIS LAI product. The results showed that the estimated annual ETMODIS obtained from the high biomass catchment was 111 mm greater than that obtained from the low biomass catchment. When comparing between the scenarios, the annual ETMODIS obtained from scenario 2 was the highest of the 3 scenarios for both sites. These results confirm that increased leaf area results in higher annual ETMODIS. This has a positive long term impact on stream flow, as high grass biomass allows the rainfall to infiltrate the soil and be gradually released to the dams with reduced magnitude of storm flows. This approach has the potential to quantify the benefits to down-stream water users of improving above-ground biomass in catchments.

Coastal environments can be highly susceptible to environmental changes caused by anthropogenic pressures and natural events. Both anthropogenic and natural perturbations may directly affect the amount and the quality of water flowing through the ecosystem, both in the surface and subsurface and can subsequently, alter ecological communities and functions. The Florida Everglades and the Sian Ka’an Biosphere Reserve (Mexico) are two large ecosystems with an extensive coastal mangrove ecotone that represent a historically altered and pristine environment, respectively. Rising sea levels, climate change, increased water demand, and salt water intrusion are growing concerns in these regions and underlies the need for a better understanding of the present conditions. The goal of my research was to better understand various ecohydrological, environmental, and hydrogeochemical interactions and relationships in carbonate mangrove wetlands. A combination of aqueous geochemical analyses and visible and near-infrared reflectance data were employed to explore relationships between surface and subsurface water chemistry and spectral biophysical stress in mangroves. Optical satellite imagery and field collected meteorological data were used to estimate surface energy and evapotranspiration and measure variability associated with hurricanes and restoration efforts. Furthermore, major ionic and nutrient concentrations, and stable isotopes of hydrogen and oxygen were used to distinguish water sources and infer coastal groundwater discharge by applying the data to a combined principal component analysis-end member mixing model. Spectral reflectance measured at the field and satellite scales were successfully used to estimate surface and subsurface water chemistry and model chloride concentrations along the southern Everglades. Satellite imagery indicated that mangrove sites that have less tidal flushing and hydrogeomorphic heterogeneity tend to have more variable evapotranspiration and soil heat flux in response to storms and restoration. Lastly, water chemistry and multivariate analyses indicated two distinct fresh groundwater sources that discharge to the phosphorus-limited estuaries and bays of the Sian Ka’an Biopshere Reserve; and that coastal groundwater discharge was an important source for phosphorus. The results of the study give us a better understanding of the ecohydrological and hydrogeological processes in carbonate mangrove environments that can be then be extrapolated to similar coastal ecosystems in the Caribbean.

Inner Mongolian grasslands are part of the vast Eurasian steppe belt and were used for nomadic pastoralism for thousands of years. As a result of political and economic changes in China in the last century, this mobile grazing management has been replaced by a sedentary and intensified livestock production. Stocking rates have increased substantially, overshooting the carrying capacity of the grasslands. These land use changes have induced severe grassland degradation. The impact and causes of grassland degradation have been investigated by the Sino-German joint research group MAGIM (Matter fluxes in grasslands of Inner Mongolia as influenced by stocking rate) in the Xilin River catchment of Inner Mongolia since 2004. This work is part of subproject P6, which amongst others pursues the goal of quantifying water balance exchange by micrometeorology and remote sensing. The dominating process of water balance losses in Inner Mongolian grasslands is evapotranspiration (ET), whereby water vapour is released into the lower atmosphere. ET is highly variable in both time and space in this semi-arid environment, as it is coupled with the typically fluctuating amount of precipitation (P). However, despite ET being the key output process of the hydrological cycle of Inner Mongolian grasslands and despite its important role as an indicator for ecosystem functioning, little is known about its spatiotemporal distribution and variability in this remote area. Recent studies on ET have demonstrated variations due to phenology, soil moisture and land use, but these studies have been limited to short periods and have been conducted on a few field sites in close proximity with debatable representativeness for the 2600 km² of grasslands in the Xilin River catchment. The development of a number of remote sensing methods in the last decades has introduced various approaches to determining spatial ET from space, but the application of remotely sensed ET in regional long-term studies is still problematic. Nevertheless, a variety of surface parameters are provided by the sensor MODIS (moderate resolution imaging spectroradiometer) at a resolution of approx. 1km. The aim of this work was (1) to close the gap between the limitations of available local ET measurements and the need for long-term studies on spatial ET in Inner Mongolian grasslands and (2) to analyse the spatiotemporal variability of ET and its implications on livestock management in this area. Therefore, micrometeorological data, remote sensing products and hydrological modelling with BROOK90 were integrated to model spatial ET for the grasslands of the Xilin River catchment over 10 years. The hydrological model BROOK90 calculates ET based on a modified Penman-Monteith approach including the separation of energy into transpiration and soil evaporation. The spatial application of the model was based on a land use classification restricted to the land use unit typical steppe. BROOK90 was parameterised from eddy covariance measurements, soil characteristics and MODIS leaf area index (LAI). Location and canopy parameters were provided individually, as well as the essential daily model input, including P and air temperatures for each pixel. Minimum and maximum air temperatures were calculated based on a relationship between measured air temperatures and MODIS surface temperatures (R²=0.92 and R²=0.87, n=81). Spatial P was estimated from a relationship found between the measured cumulative P of six rain gauges within the grasslands and the increase of MODIS LAI around these measurements (R²=0.80, n=270). Modelled ET is plausible and fits in the range of published results. ET was demonstrated to be highly variable in both time and space: the high spatiotemporal variability of eight-day ET is reflected by the coefficients of variation, which varied between 25% and 40% for the whole study area and were up to 75% for individual pixels. Soil evaporation reacts considerably more sensitively to precipitation pulses than transpiration. Modelled annual ET sums approached or exceeded precipitation sums in general; however, P exceeded ET in 2003, when exceptionally high precipitation occurred. The strong dynamics and the high spatiotemporal variability of ET clearly demonstrate that the current static livestock management is not adapted to the conditions of Inner Mongolian grasslands. New concepts for a sustainable livestock management could be developed in consideration of the intrinsic long-term patterns of spatial ET distribution and spatiotemporal variability identified in this work. Moreover, as this method for modelling spatial ET is not restricted to the grasslands of the Xilin River catchment, livestock management in other semi-arid grasslands could benefit from it as well
Die Grasländer der Inneren Mongolei sind Teil des riesigen eurasischen Steppengürtels und wurden seit Tausenden von Jahren für die nomadische Weidewirtschaft genutzt. Als Folge der politischen und wirtschaftlichen Veränderungen in China im letzten Jahrhundert ist diese mobile Weidewirtschaft durch eine ortsgebundene und intensivierte Tierhaltung ersetzt worden. Besatzdichten wurden erheblich erhöht und die Tragfähigkeit der Grasländer wurde deutlich überschritten. Diese Landnutzungsänderungen haben schwerwiegende Degradationserscheinungen der Grasländer induziert. Die Ursachen und Auswirkungen der Degradation sind von der Deutsch-Chinesischen-Forschungsgruppe MAGIM (Matter fluxes in grasslands of Inner Mongolia as influenced by stocking rate) im Einzugsgebiet des Xilin-Flusses in der Inneren Mongolei seit 2004 untersucht worden. Diese Arbeit wurde im Rahmen des Teilprojektes P6 erstellt, welches unter anderem das Ziel verfolgt, Wasserhaushaltsprozesse mit Mikrometeorologie und Fernerkundung zu quantifizieren. Der dominierende Prozess der Wasserbilanz-Verluste in den Grasländern der Inneren Mongolei ist die Verdunstung (ET), wobei Wasserdampf in die untere Atmosphäre freigesetzt wird. ET ist in diesem semi-ariden Ökosystem in Zeit und Raum sehr variabel, da an die in der Regel schwankenden Niederschläge (P) gekoppelt. Trotz der Schlüsselrolle, die ET im Wasserkreislauf der Inneren Mongolei einnimmt, und der wichtigen Rolle als Indikator für die Funktionsweise des Ökosystems, ist wenig über die raum-zeitliche Verteilung und Variabilität von ET in dieser abgelegenen Region bekannt. Neuere Studien haben ET-Schwankungen aufgrund von Phänologie, Bodenfeuchte und Bodennutzung dargestellt, aber diese Studien sind auf kurze Zeiträume beschränkt und wurden auf nur wenigen Standorten, die sich in unmittelbarer Nähe befinden, durchgeführt. Dies stellt ihre Repräsentativität für die 2600 km² an Grasland im Xilin-Einzugsgebiet in Frage. Die Entwicklung von Fernerkundungsmethoden in den letzten Jahrzehnten hat verschiedene Ansätze zur Bestimmung der räumlichen ET hervorgebracht, jedoch ist die Anwendung von ET aus Fernerkundungsdaten in regionalen Langzeitstudien immer noch problematisch. Dennoch werden eine Vielzahl von Oberflächenparametern durch den Sensor MODIS (Moderate Resolution Imaging Spectroradiometer) bei einer Auflösung von ca. 1km zur Verfügung gestellt. Das Ziel dieser Arbeit war (1) die Lücke zwischen den verfügbaren lokalen ET-Messungen und dem Bedarf an langfristigen Untersuchungen zu räumlicher ET im Grasland der Inneren Mongolei zu schließen und (2) die räumlich-zeitliche Variabilität von ET vor dem Hintergrund des Beweidungsmanagements zu analysieren. Daher wurden mikrometeorologische Daten, Fernerkundungsprodukte und hydrologische Modellierungen mit BROOK90 integriert, um die räumliche ET für die Grasländer des Xilin-Einzugsgebietes über 10 Jahre zu modellieren. Das hydrologische Modell BROOK90 berechnet ET auf Basis eines modifizierten Penman-Monteith-Ansatzes einschließlich der Aufteilung in Transpiration und Bodenverdunstung. Die räumliche Anwendung des Standortmodells basiert auf einer Landnutzungsklassifikation und wurde für die Landnutzungsklasse typical steppe durchgeführt. Eddy-Kovarianz-Messungen, Bodeneigenschaften und MODIS-Blattflächenindex (LAI) wurden zur Parametrisierung von BROOK90 verwendet. Sowohl Lage- und Pflanzenparameter, als auch die notwendigen Modelleingangsdaten (Tageswerte von P und Lufttemperaturen), wurden für jeden Pixel individuell zur Verfügung gestellt. Minimum- und Maximum-Lufttemperaturen wurden mittels einer Beziehung zwischen gemessenen Lufttemperaturen und MODIS-Oberflächentemperaturen berechnet (R²=0.92 und R²=0.87, n=81). Räumliche P wurden aus einem Zusammenhang zwischen gemessenen kumulierten P von sechs Niederschlagsmessern im Untersuchungsgebiet und der Erhöhung des MODIS-LAI im Bereich dieser Messungen abgeleitet (R²=0.80, n=270). Die modellierte räumliche ET ist plausibel und liegt im Wertebereich der publizierten Ergebnisse. Es wurde gezeigt, das ET sehr variabel in Raum und Zeit ist: die raum-zeitlichen Schwankungen der achttägigen ET wurden durch den Variationskoeffizienten dargestellt, welcher zwischen 25% und 40% für das gesamte Untersuchungsgebiet variiert und für einzelne Pixel bis auf 75% ansteigt. Die Bodenverdunstung reagiert wesentlich empfindlicher auf Niederschlagsereignisse als die Transpiration. Modellierte Jahres-ET-Summen erreichen oder überschritten die Niederschlagssummen in der Regel, jedoch übertraf P die ET im Jahre 2003, als außergewöhnlich hohe Niederschläge aufgetreten sind. Die starke Dynamik und die hohe raum-zeitliche Variabilität der ET zeigen deutlich, dass die aktuelle statische Tierhaltung nicht an die Bedingungen in den Innermongolischen Grasländern angepasst ist. Neue Konzepte für eine nachhaltige Viehwirtschaft könnten unter Berücksichtigung der inhärenten langfristigen Muster der räumlichen Verteilung von ET und ihrer raum-zeitlichen Variabilität, die in dieser Arbeit identifiziert wurden, entwickelt werden. Außerdem ist die Anwendung der entwickelten Methode für die Modellierung räumlicher ET nicht auf die Grasländer des Xilin-Einzugsgebietes beschränkt; die Weidewirtschaft in anderen semi-ariden Grasländern könnte ebenfalls davon profitieren

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.)--University of North Carolina at Chapel Hill, 2006.
Title from electronic title page (viewed Oct. 10, 2007). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Geography." Discipline: Geography; Department/School: Geography.

L’évapotranspiration (ET) joue un rôle important en hydrologie, météorologie et en agriculture et l’estimation précise de l’ET régionale à différentes échelles temporelles et spatiales est essentielle pour ces disciplines. La télédétection est identifiée comme le seul moyen rationnel de cartographier l’ET de la surface terrestre à l’échelle régionale de façon globale, cohérente et économiquement raisonnable. Cette thèse porte donc sur l’élaboration et la mise au point de méthodes permettant de déterminer l’ET régionale des surfaces terrestres à partir des données de l’instrument MODIS à bord des satellites polaires Terra et Aqua. Le travail de cette thèse a permis de montrer les atouts de la méthode de triangle Ts-VI par rapport aux autres méthodes traditionnellement utilisées pour la détermination de l’ET régionale et de proposer des méthodes permettant de calculer la température de surface et l’émissivité de surface à partir des luminances mesurées par les satellites. Cette thèse a montré également qu’il était possible d’estimer l’ET sur des régions arides et semi-arides à partir uniquement de données satellitaires et ce avec une précision acceptable. Elle ouvre des perspectives intéressantes. Dans la restitution de l’ET régionale, l'exactitude de cette restitution dépend principalement de l'exactitude de la détermination quantitative des limites sèche et humide dans le triangle Ts-VI et de la performance du modèle d'interpolation impliqué dans l'évaluation de la fraction évaporative. Les performances du modèle et du nouvel algorithme développé dans cette étude devront donc être évaluées de façon précise et avec attention
Evapotranspiration (ET) plays an important role in hydrology, meteorology, and agriculture. Accurate estimates of regional ET at different temporal and spatial scales are essential in these disciplines. Remote sensing technology is recognized as the only viable means to map regional- and meso-scale patterns of ET at the earth’s surface in a globally consistent and economically feasible manner and surface temperature helps to establish the direct link between surface radiances and the components of surface energy balance. This thesis thus concerns the methodological development permitting to determine the regional land surface ET from the MODIS data onboard the polar satellites Terra and Aqua. The work of this thesis showed the advantage of the Ts-VI triangle method compared to the other methods traditionally employed for the determination of the regional ET and proposed methods to calculate land surface temperature and emissivity from the radiances measured by the satellites. This thesis showed also that it was possible to estimate ET in arid and semi-arid areas only from the satellite data with an acceptable precision. This work opens interesting prospects. In the restitution of regional ET, the exactitude of this restitution depends mainly on the exactitude of the dry and wet edges determination in the Ts-VI triangle and on the performance of the interpolation model involved in the evaluation of the evaporative fraction in the ET estimation model. The performances of the model and the new algorithm developed in this study have to be evaluated in the future in a precise and attentive way

Since 2011, California has been under drought conditions. These conditions have not only affected water availability for farmers, but also production. California’s second most valuable crop, almonds, has been affected by drought conditions. This study used three models (Model 1-3) to describe almond yield variability from year to year and almond yield variability within a year in Kern County, CA. The study evaluated 185 almond farms that were classified in three locations (east side, west side and north west side). The years of the study were 2011 (wet year) and 2013-2015 (drought condition years). Model 1 determined a functional regression between almond yield and annual evapotranspiration during the 4 years of the study. The R2was 7.9%, meaning low association between both variables and high unexplained variability (92.1%). Model 2 evaluated year to year variation. A regression function between almond yield and annual evapotranspiration after adjusting for location, precipitation, chilling hours and year was made. The R2of this model 62.6%, and all the variables used had a p2was higher than Model 1; however, there was high unexplained variability (47.4%). Model 3 evaluated within-year variation. A regression function between almond yield and annual evapotranspiration after adjusting for tree age and location (east, west and northwest side) was made for each year (2011 and 2013 -2015). Coefficient of variation of evapotranspiration and soil available water storage were analyzed as additional variables in Model 3; however, they were not introduced in Model 3 due to the low increase in R2 in each year (2 of Model 3 for each year were, 60.4%, 49.7%, 53.8% and 53.2% for the years 2011, 2013-2015, respectively. Model 3 also had high unexplained almond yield variability in each year (39.6%-50.3%). This high unexplained variability leads to introduce additional variables to the functional regression model for further studies. Identifying these additional variables and having a functional regression model with high R2 would lead to understand howlow evapotranspiration could potentially lead to a positive response on yield in drought conditions; thus, making farmers improve water use efficiency and hence, lowering production cost. However, the high unexplained variability clearly indicates that evapotranspiration is only one of many factors that influence yield. If improved yield is an important outcome, future studies must examine large- scale almond-producing farms with multiple agricultural system variables.

Land Use Land Cover Change (LULCC) is a major component of global environmental change, which could result in huge impacts on biodiversity, water yield and quality, climate, soil condition, food security and human welfare. Of all the LULCC types, urbanization is considered to be the most impactful one. Monitoring past and current urbanization processes could provide valuable information for ecosystem services evaluation and policy-making. The National Land Cover Database (NLCD) provides land use land cover data covering the entire United States, and it is widely used as land use land cover data input in numerous environmental models. One major drawback of NLCD is that it is updated every five years, which makes it unsatisfactory for some models requiring land use land cover data with a higher temporal resolution. This dissertation integrated a rich time series of Landsat imagery and NLCD to achieve annual urban change mapping in the Washington D.C. metropolitan area by using time series data change point detection methods. Three different time series change point detection methods were tested and compared to find out the optimal one. One major limitation of using the above time series change point detection method for annual urban mapping is that it relies heavily on NLCD, thus the method is not applicable to near-real time monitoring of urban change. To achieve the near real-time urban change identification, this research applied machine learning-based classification models, including random forest and Artificial Neural Networks (ANN), to automatically detect urban changes by using a rich time series of Landsat imagery as inputs. Urban growth could result in a higher probability of flooding by reducing infiltration and evapotranspiration (ET). ET plays an important role in stormwater mitigation and flood reduction, thus assessing the changes of ET under different urban growth scenarios could yield valuable information for urban planners and policy makers. In this study, spatial-explicit annual ET data at 30-m resolution was generated for Virginia Beach by integrating daily ET data derived from METRIC model and Landsat imagery. Annual ET rates across different major land cover types were compared, and the results indicated that converting forests to urban could result in a huge deduction in ET, thus increasing flood probability. Furthermore, we developed statistical models to explain spatial ET variation using high resolution (1m) land cover data. The results showed that annual ET will increase with the increase of the canopy cover, and it would decrease with the increase of impervious cover and water table depth.
Doctor of Philosophy
Motoring past and current urbanization processes is of importance in terms of ecosystem services evaluation and policy-making because urban growth has huge impacts on the environment. First, this dissertation designed and compared three different methods for annual urban change mapping in Washington D.C. metropolitan area by using a rich time series of Landsat imagery and National Land Cover Database (NLCD). Then, machine-learning based classification models were implemented to achieve near real-time urban change identification. Finally, spatially-explicit evapotranspiration (ET) data for Virginia Beach, a case study location, were generated and annual ET rates for major land cover types were compared to assess the urbanization's impacts on ET.

Two sets of Bowen ratio and lysimeter measurements of evapotranspiration (ET) were compared for a field of winter wheat in Phoenix, Arizona. Daytime data for ten days of clear skies were examined. Daily lysimeter ET (LET) generally exceeded Bowen ratio ET (BRET). Advective cases were compared with lapse cases. For one set of lysimeter and Bowen ratio measurements, average LET was 10% more than BRET during advective conditions, while average LET and BRET were equal during lapse conditions. Results for the other pair of measurements were less conclusive due to unresolved lysimeter problems, with average LET 13% more than BRET during advection, and average LET 13% less than BRET during lapse conditions. These results suggest that the assumption of equal eddy diffusivities for heat and vapor caused BRET to underestimate evapotranspiration during advection. The Bowen ratio, wind speed, and wind direction were identified as possible variables for correcting BRET underestimation.

Vineyards are often grown in semi-arid climates, such as California’s central valley, where water resources can be limited. Summer weather conditions result in high water use by these plants. For wine grapes, a high-value commodity, there are known benefits to fruit quality in irrigating grapevines with slightly below optimum for the plant. Growers would like to be able to precisely irrigate without overusing water or overstressing the vines. This calls for improving ways to monitor vineyard water use by estimating the combined soil evaporation and plant transpiration known as evapotranspiration (ET). A computer model developed by the USDA called the Two-Source Energy Balance model (TSEB) can estimate ET through satellite or aircraft measurements of land surface temperature. The model has been successful for simple, uniform vegetation such as maize, soybeans, and grasslands. The ability of TSEB to estimate vineyard ET has been tested through a field experiment called the Grape Remote sensing, Atmospheric Profile,& Evapotranspiration eXperiment or GRAPEX. Water is primarily transported away from the ground and plants by turbulent swirls in the wind. Models such as TSEB assume these swirls occur in a consistent manner over a few minutes to hours. Yet, interactions between the wind, the complex vineyard canopy, and heating near the ground can cause them to be episodic or intermittent. There are questions of if and how intermittent water vapor transport might happen in vineyards, and whether the TSEB model will still estimate ET well in such cases. In this study wind, humidity, air temperature, and surface temperatures are used to examine when intermittent behavior occurs, how it affects ET from the vineyard canopy, and how TSEB performs for intermittent versus more steady conditions. Results show that intermittent turbulence significantly alters the microclimate in the vineyard canopy compared to more ideal, steady behavior. The TSEB model was successful but showed reduced ability to estimate ET during times of intermittent behavior. The knowledge gained is an important step toward using TSEB as a powerful tool for sustainable water management, not only in vineyards, but other cash crops with complicated canopies such as orchards, as well as natural ecosystems.

Stodmarsh National Nature Reserve includes the largest reedbed in Southern England and is an important habitat for breeding waders and several rare bird species, including Bitterns. A succession of drought years in the 1990s brought the issue of the hydrology and water requirements of the wetland to the attention of managers and there is concern about future water supplies to the reserve. This study aims to calculate the amount of water required by the site in order to maintain optimum habitat conditions. The greatest area of uncertainty in the water balance is the evapotranspiration rate of the reedbeds and therefore a secondary aim is to increase understanding of this flux. Detailed hydrological measurements were carried out over two years to establish the water balance of the site. Evapotranspiration was measured using the Bowen ratio technique, accompanied by additional physiological and meteorological measurements. Results showed that evapotranspiration from reeds was generally less than reference evapotranspiration and that stornatal resistance was the most important factor controlling evapotranspiration rates. The hydrology of the site was modelled using a thirty year historical data series to quantify the return periods of flood and drought conditions of different severity. These were used to predict water resource requirements and availability and confidence limits were attached to the results. In 70% of years, summer deficits in the rainfall-evapotranspiration balance require the addition of water from the Lampen Stream. In 10% of these years, the entire surmner discharge of the Lampen Stream would be insufficient to meet site water requirements and an additional source of water is required. Competition with other water users and limits on abstraction will increase the number of years an additional water source is required. In addition future climate change is likely to increase summer water requirements whilst decreasing resource availability.

A change in surface condition of a watershed, which is usually caused by development, can have measured effects on the naturally occurring hydrologic cycle and nitrogen cycle. This could result in environmental problems, such as reduced springflow and eutrophication. In an effort to address these issues, a combination of best management practices (BMPs) can be adhered to. The practice of using excess stormwater as a source for irrigation is proposed as a BMP for the minimization of impacts by development to the hydrologic and nitrogen cycles. To study the proposed BMP, a field experiment was installed in an outdoor location on the UCF main campus in Orlando, Florida. The experiment consists of three soil chambers, (2x2x4 ft, L:W:H), filled with compacted soil and covered with St. Augustine grass to simulate a suburban lawn. The grass was irrigated up to twice a week with detained stormwater with a nitrate nitrogen concentration of up to 2 mg/L. A mass balance and a total nitrogen balance were performed to determine evapotranspiration (ET) and impacts on groundwater nitrogen content. It was determined that the groundwater characteristics are largely dependent on the characteristics of the soil. The input nitrogen (precipitation and irrigation) was mostly in the form of nitrate and the output nitrogen (groundwater) was mostly in the form of ammonia. A total nitrogen mass balance indicated the mass output of nitrogen was significantly larger than mass input of nitrogen, which was due to ammonia leaching from the soil. Only small concentrations of nitrate were detected in the groundwater, resulting in an estimated nitrate removal (conversion to ammonia) of 97 percent at a depth of four feet when the input nitrate concentration was 2 mg/L. The average ET of the three chambers was compared to the estimated ET from the modified Blaney-Criddle equation on a monthly basis and a yearly basis. The modified Blaney-Criddle equation was proven to be accurate for estimating the actual ET for this application: irrigated St. Augustine grass in the Central Florida climate. In conclusion, using the available literature and the data collected from the field experiment, it was shown through an example design problem that the proposed BMP of using excess stormwater as a source for irrigation can help achieve a pre- versus postdevelopment volume balance and can help control post-development nitrate emissions.
M.S.Env.E.
Department of Civil and Environmental Engineering
Engineering and Computer Science
Environmental Engineering

A new version of an evapotranspiration package for the U.S. Geological Survey's groundwater -flow model, MODFLOW, is documented. The Riparian Evapotranspiration Package (RIP-ET) provides flexibility in simulating riparian and wetland evapotranspiration (ET) not provided by the MODFLOW -2000 and MODFLOW 2005 traditional Evapotranspiration (EVT) Package, nor by the MODFLOW-2000 Segmented Function Evapotranspiration (ETS1) Package. This report describes how the package was conceptualized and provides input instructions, listings and explanations of the source code, and an example simulation. Traditional approaches to modeling ET processes assume a piecewise linear relationship between ET flux rate and hydraulic head. The RIP-ET replaces this traditional relationship with a segmented, nonlinear dimensionless curve that reflects the eco-physiology of riparian and wetland ecosystems. Evapotranspiration losses from these ecosystems are dependent not only on hydraulic head but on the plant types present. User -defined plant functional groups (PFGs) are used to elucidate the interactive processes of plant ET with groundwater conditions. Five generalized plant functional groups based on transpiration rates, plant rooting depth, and water tolerance ranges are presented: obligate wetland, shallow-rooted riparian, deep- rooted riparian, transitional riparian and bare ground /open water. Plant functional groups can be further divided into subgroups (PFSG) based on plant size, density or other user defined field. The RIP -ET allows for partial habitat coverage and mixtures of plant functional subgroups to be present in a single model cell. Habitat areas are designated by polygons. A polygon can contain a mixture of PFSGs and bare ground, and is assigned a calculated land surface elevation. This process requires a determination of fractional coverage for each of the plant functional subgroups present in a polygon to simulate the mixture of coverage types and resulting ET. The fractional cover within a cell has two components: 1) the polygonal fraction of active habitat in a cell, and 2) fraction of plant flux area in a polygon. The RIP -ET determines the ET rate for each plant functional group in a cell, the total ET in the cell, and the total ET rate over the region of simulation.

To close the water use budget of irrigated agricultural fields in floodplains with substantial riparian corridors, it is necessary to understand groundwater usage by dominant phreatophytic vegetation, particularly when the primary source of water for irrigation comes from groundwater abstraction. We report here results of estimated evapotranspiration (ET) of a riparian forest, which were based on measurements of sap flow in phreatophytic vegetation within a riparian corridor. The riparian corridor was within a study area 75 to 140 meters wide in the lower portion of the Scotts Creek watershed, which is bounded to the west by the Pacific Ocean in Santa Cruz County, California. Canopy coverage in the study area often approaches 100% during the growing season, with dominant trees being red alder (Alnus rubra Bong.), arroyo willow (Salix lasiolepis Benth.), and pacific willow (Salix lasiandra Benth. var. lasiandra). Other trees include boxelder (Acer negundo L.), bigleaf maple (Acer macrophyllum Pursh.), California bay laurel (Umbellularia californica (Hook. & Arn.) Nutt.), and coastal redwoods (Sequoia sempervirens (D. Don) Endl.). Common understory vegetation includes California blackberry (Rubus ursinus Cham. and Schlecht.), stinging nettle (Urtica dioica subsp. gracilis L.), poison hemlock (Conium maculatum L.), Cape ivy (Delairea odorata Lem.), Italian thistle (Carduus pycnocephalus L. subsp. pycnocephalus), and western poison oak (Toxicodendron diversilobum (Torr. & A. Gray) Greene). We hypothesized that the ET of a riparian forest could be estimated by measuring the sap flow of riparian phreatophytic trees. For the study reported here, only the two most dominant phreatophytic species, namely red alders and arroyo willows, were instrumented with thermal dissipation probes. In addition to diurnal fluctuations, sap flow data collected hitherto also showed expected seasonal variation with summer maxima and winter minima, with transition fall and spring periods. Sap flow measurements from the study area were used to estimate riparian forest ET by projecting them across the canopy areal extent of the riparian forest using sampled tree sapwood areas from six sample plots. The sap flow-based ET results were then compared to ET results reported by two other methods. Additional research, including increased number of trees with thermal dissipation probes, further analysis of sap flow behavior, and continued long-term measurement of sap flow, is needed to further improve the method of using long-term sap flow measurements to estimate the ET of a riparian forest.

There is on-going need for reliable estimates of evapotranspiration (ET) at catchment scales to support objective decision-making for managing water supplies, and enhancing understanding of processes and modelling. Without reliable estimates of ET, water supply and catchment management agencies are exposed to significant economic, social and even environmental risks. This thesis focuses on identifying possible methodologies for estimating ET in a catchment dominated by eucalypt forest and woodland. More specifically, this thesis tests the hypothesis that different methods of deriving daily, catchment ET for a headwater in Australia meet underlying assumptions and yield similar results. The hypothesis was tested by using three approaches to estimate catchment ET: soil moisture changes (point scale), satellite imagery of leaf area index (MODIS, hillslope scale), and discharge (streamflow) and the storage-discharge relationship (catchment scale). Data from Corin Catchment, an unregulated catchment vital to the Australian Capital Territory and the surrounding region, is the basis of this study. After the General Introduction (Chapter 1), methods for estimating ET in eucalypt communities throughout Australia at various temporal and spatial scales are systematically reviewed (Chapter 2). Of the 1614 original research papers investigated, 52 were included for further investigation. A clear outcome is that transpiration by the overstorey, measured using sap flow, is the most frequently measured component of ET, and that physiological studies dominate estimates of ET. Very few studies were conducted at the catchment scale. Further, scaling ET from tree to catchment scales was rarely attempted, and the effect of scaling for water resource management is mostly unquantified and requires attention. The first method used to calculate catchment ET is based on up-scaling of soil moisture changes on the basis of a digital soil map (Chapter 4). The data presented here rejects the hypothesis that ET (derived from soil moisture) and overstorey transpiration (derived from sap flow) are well correlated. Instead, the data suggest that soil moisture-derived ET and overstorey transpiration obtained water from different sources. The key findings of this chapter are that this approach is not suitable for estimating ET at catchment scales because it is restricted to drier periods, and because trees did not solely rely on the defined root-zone for water supply. The second method to calculate catchment scale ET (Chapter 5) tests if hillslope-scale satellite imagery (MODIS leaf area index) can be up-scaled to estimate catchment ET. An outcome of this work is that caution is needed when using MODIS leaf area index for water resource planning in evergreen forests across the globe, particularly for forests with significant understorey and a relatively open overstorey canopy at some periods of the year. This method is deemed not suitable for estimating ET over the study area. The third method to calculate catchment scale ET (Chapter 6) is based on integrating discharge using a single non-linear equation to characterise the study area. This method yielded catchment ET far greater (18 times larger) than the largest observed measure of potential ET. As with the method based on soil moisture changes, it was restricted to drier periods. This method was clearly unsuitable for estimating ET over the study area due to relatively quick recession, large range in hourly discharge and significant scattered recession at low discharge. Overall, this thesis rejects the hypothesis that different methods of deriving daily, catchment ET for a headwater in Australia meet underlying assumptions and yield similar results. An important limitation identified through this research is the ability to determine a ‘correct’ estimate of catchment ET. Further research should focus on enhancing understanding of scaling ET within and beyond Australia, generating more daily catchment ET from up-scaled soil moisture changes, further evaluating ET from up-scaled satellite imagery, and identifying catchment characteristics to allow ET to be derived from discharge. Water resource managers must be diligent when selecting and applying a method to estimate catchment ET.

Evapotranspiration as a major component of the water balance has been identified as a key factor in hydrological modelling. Water management can be improved by means of increased use of reliable methods for estimating evapotranspiration. The limited availability of measured climate and discharge data sets, particularly in the developing world, restricts the reliability of hydrological models in these regions. Furthermore, rapid changes in hydrological systems with increasing development mean uncertainties in water resource estimation are growing. These changes are related to the modification of catchment hydrological processes with increasing human activity. Dealing with data uncertainty and quantifying the impacts of catchment activities are significant challenges that scientists in the field of hydrology face today. Uncertainties in hydrometeorological data are associated with poor observation networks that provide data at point scales which are not adequately representative of the inherent heterogeneity within catchment processes. Using uncertain data in model applications reduces the predictive power of hydrological models as well as the ability to validate the model outcomes. This study examines the potential of using remote sensing-based evapotranspiration data to reduce uncertainty in the climatic forcing data and constraining the output of a rainfall-runoff hydrological model. It is common to use fixed seasonally variable potential evapotranspiration (PET) instead of temporally varying PET data as inputs to standard rainfall-runoff models. Part of the reason is that there are relatively few stations available to measure a variety of meteorological input data needed to compute PET, as well as the apparent lack of sensitivity of rainfall-runoff models to different types of PET inputs. As hydrometeorological data become more readily available through the use of earth observation systems, it is important to determine whether rainfall-runoff models are sensitive to time-varying PET derived from these earth observations systems. Further potential includes the use of actual evapotranspiration (ETa) from this type of data to constrain model outputs and improve model realism. It is assumed that a better representation of evapotranspiration demands could improve the efficiency of models, and this study explores some of these issues. The study used evapotranspiration estimates (PET and ETa) from the MOD16 global product with one of the most widely used hydrological models in South Africa. The investigation included applying the Pitman model in a number of case study catchments located in different climatic regions of the country. The main objectives of the study included (i) the establishment of behavioural model parameter sets that generate acceptable hydrological response under both naturalised and present-day conditions, (ii) the use of time-varying PET estimates derived from MOD16 data to force the model, and (iii) the use of MOD16 ETa estimates to constrain model-simulated ETa. Before examining the use of different PET forcing data in the model, a two-step modelling approached was employed both a single-run and an uncertainty version of the Pitman model. During the first step (using a single-run version), available information on catchment physical properties and regionalised groundwater recharge together with model calibration principles were used to develop model functionality understanding and establish initial parameter sets. The outcomes from the first step were used to define uncertain parameter ranges for the use in the uncertainty version of the Pitman model (second step). Further, catchment water uses were quantified to ensure comparability with present-day flow conditions represented by the stream flow records. The effects of forcing the Pitman model with MOD16-based time-varying PET data inputs were evaluated using static and dynamic sensitivity analysis approaches. In the static approach, parameter sets calibrated using fixed seasonal distributions of PET data remain unchanged when forcing the model with other forms of PET, whereas in the dynamic method, the model is recalibrated with changing PET inputs. In both approaches, model sensitivity was assessed by comparing objective function statistics of reference flow simulations with those simulations incorporating changing PET data inputs. The use of the MOD16 ETa data to constrain model- simulated evapotranspiration losses was conducted by calibrating the parameters such that the simulated-ETa matched the evapotranspiration loss estimated from the MOD16 data. Despite issues around model equifinality and significant uncertainty within water use information, the Pitman model simulations were generally satisfactory and compared with observed stream flow data where available. The use of time-varying PET data does not improve the efficiency of the model when both static and dynamic sensitivity approaches are used. This was highly expected with the static approach where fixed model parameter sets do not account for the changes in evapotranspiration demands. However, with the dynamic approach, it was difficult to conclude why the model efficiency did not improve given the flexibility of the model to achieve appropriate parameter sets to different forms of PET. The study noted that the insensitivity of the model to changes in PET demands could be due to uncertainties in the model structure and MOD16 data. Attempts to constrain the model-simulated actual evapotranspiration with MOD16 ETa estimates were hampered by large errors in the MOD16 data and resulted in the non-closure of the catchment annual water balance, even when likely errors in the other components of the water balance were accounted for. There is still a great deal of work that needs to be done to reduce uncertainties associated with the use of earth observation data in hydrological modelling. This study has identified some of the specific gaps within the application of evapotranspiration data from earth observation information. While the MOD16 data applied with the Pitman model did not achieve improved simulations, the study has demonstrated the enormous potential of the data product in the future should the identified uncertainties be resolved. Lastly, the investigation highlighted some of the possible model structural uncertainties specifically associated with the simplified soil-moisture accounting routines within the model. It is possible that amending the model structure through investigating the dynamics of the relationship between soil moisture and evapotranspiration losses would assist in the improved utilisation of earth observation products related to the MOD16 ET data.

Irrigation uniformity, efficiency, leaching fraction, salt and water ages, and evapotranspiration rate were estimated from subsoil salinity data for three cotton fields in Arizona. The estimation of these parameters was based on the assumption of steady-state water and salt flow through the crop root zone. The levels of salt concentration in the irrigation water were 21.3, 11.5, and 11.6 meq/L for Fields 1, 2, and 3, respectively. Two of these fields were furrow irrigated, and the third was subsurface drip irrigated. Each field was sampled for salt concentrations to a depth of 1.5 m at 10-15 sites. A total of 514 soil samples were collected. Significantly lower salt concentrations were observed in the soil profiles in Fields 1 and 2 compared to Field 3, but lower variations in the salt concentrations were observed in Field 3 compared with Fields 1 and 2. These variations in salt concentration could be due to restricted water movement within the soil profile caused by stratified soil. Since a soil-water extract model indicated little or no chemical precipitation of salt within the soil profile, there was no need to correct the data for chemical effects. The calculated irrigation uniformity was highest in Field 3 and lowest in Field 1. This may be related to more accurate land leveling in field 2 than Field 1. The irrigation efficiencies were 83.0%, 89.0%, and 80.0% for Fields 1, 2, and 3, respectively. The correlation coefficient between the ages of salt and water was 0.98, 0.99, and 0.97 for Fields 1, 2, and 3, respectively. Leaching fraction was highest in Field 3 and lowest in Field 2. Mean actual ET calculated from the Blaney-Criddle method were 372, 314, and 308 mm for Fields 1, 2, and 3, respectively. Mean ET calculated from the salinity data were 1,250, 1,590, and 1,140 mm for Fields 1, 2, and 3, respectively. Statistically significant correlation coefficients were, however, found between both methods of estimating ET. These values were 0.97, 0.86, and 0.93 for Fields 1, 2, and 3, respectively.

Land surface actual evapotranspiration (ET) is an important process in terrestrial water balance and reliable estimates of ET are necessary to improve water resources management. In this regard, there is a growing body of literature that recognizes the importance of an accurate ET model. Among them, the complementary relationship between ET and potential ET (ETP) has been the subject of many studies because it uses only meteorological data as inputs. However, there is an increasing concern that some complementary relationship models perform poorly under dry conditions. To overcome this limitation, this dissertation was designed to extend the latest complementary relationship model, Modified GG, using both meteorological data and vegetation information, NDVI, which is readily available from remote sensing data. The proposed model, Adjusted GG-NDVI, was validated by comparing to other ET models and measured ET data. With Adjusted GG-NDVI, this dissertation addressed the applicability of using ET as a proxy for drought monitoring. As a result, the drought patterns from the proposed drought index, EWDI, were consistent with commonly used USDM in the United States. More importantly, this study described drought conditions by comprehensively considering both precipitation and vegetation conditions. Taken together, these findings have significant implications for the understanding of how ET can assist in water resources management.

Water is undeniably among the most important natural resources and the most critical in semi-arid regions like the Intermountain West of the United States. Such regions are characterized by low precipitation, the majority of which is transferred to the atmosphere from the soil and vegetation as evapotranspiration (ET). Quantification of ET is thus crucial for understanding the balance of water within the region, which is important for efficiently planning the available water resources. This study was motivated towards advancing the estimation of actual ET (ETA) in mountain ecosystems, where the variation in different types of vegetation and non-uniformity of soil including considerable stone content creates challenges for estimating water use as ET. With the aim of addressing the effect of stone content in controlling soil moisture and ET, this study examined the influence of stone content on bulk soil hydraulic properties. An averaging model referred to as a binary mixing model was used to describe the way in which water is held and released in stony soil. This approach was based on the individual hydraulic behavior of the background soil and of the stones within the soil. The effect of soil stone content on ETA was evaluated by accounting for the water retention properties of stones in the soil using a numerical simulation model (HYDRUS-1D). The results revealed overestimation of simulated ETA when effects of stone content were not accounted for in comparison to ETA measured by the state-of-the-art “eddy covariance” measurement method for ETA. An even larger-scale model was evaluated, named the Noah-Multiphysics (Noah-MP) land surface model. The land surface model was run using different arrangements of complexity to determine the importance of stone content information on simulation results. The version of the model with information about stone content along with detailed soil properties was able to provide the best Noah-MP prediction of ET. The study suggests that improvement in representation of soil properties including stone content information, can substantially advance the ability of numerical and land surface models to more accurately simulate soil water flow and ETA.

Evapotranspiration (ET) of center pivot irrigated alfalfa was studied in the windy, arid, Curlew Valley, Northern Box Elder County, Utah, during the summers of 2009 and 2010. ET was estimated using eddy covariance (EC) and surface renewal (SR) techniques. ET estimates from the EC and SR analyses were compared with estimates using ASCE Standardized Reference ET Equation, with both dual and mean crop coefficients. EC energy balance closure was 0.80, on average, in 2009 and 0.76 in 2010. The SR weighting parameter (α) was calculated through linear regression of EC and SR sensible heat flux estimates. Alpha was found to be 0.70 if EC energy balance closure was forced and 0.55 if closure was not forced. ET from SR analysis with α = 0.70 (ETSRα=0.70) was 409 mm in 2009 and 331 mm in 2010. ET from EC analysis with forced closure (ETECforced) was 390 mm in 2009 and 326 mm in 2010. In contrast, ETSRα=0.55 was 408 and 333 mm in 2009 and 2010, respectively, while ETECunforced was 315 and 251 mm in 2009 and 2010, respectively. Combined ETECforced and ETSRforced were compared with estimated crop ET from the ASCE Std. Eq. with both dual and mean crop coefficients (ETcDual and ETcm, respectively). ETcDual was 689 mm in 2009, as compared to ETcm and ETEC-SRforced, which were 677 and 617 mm, respectively. In 2010 ETcDual was 674 mm, with ETcm and ETEC-SRforced being 629 and 576 mm, respectively. The Kcm approach more closely approximated the estimated wet soil evaporation determined from the ETEC-SRforced for the measurement conditions and stated assumptions. ETEC-SR estimates were compared with irrigation application information to approximate field scale water balances. Effective precipitation plus net irrigation application (less wind drift and evaporation) were nearly equal to ETEC-SRforced for 2nd and 3rd crops of alfalfa in 2009 and 2010. No deep percolation was calculated using ETEC-SRforced; however, soil moisture measurements were not sufficient to verify that this was true. The water balances suggested that the fields were being underirrigated which may have caused salt accumulation in the soil, as evidenced by the low reported yields.

The aim of this thesis was to investigate the processes behind an unexpected runoff behaviour after a clear-cut in a boreal forest in northern Sweden (Balsjö). The risks of increased flooding, erosion, nutrient leakage and changes in the local ecosystems are some reasons why it is important to fully understand the effect of clear-cuts on the water balance. In northern boreal forests the snow is of great importance as it results in the main hydrological event of the year, the spring flood. In general, open areas accumulate more snow, have a lower evapotranspiration and therefore maintain a higher runoff than a forest. In a recent paired catchment study at Balsjö the expected pattern after a clear-cut was only shown in three out of five years (2007-2011). The expected increase in runoff did not occur in 2010 and 2011. Two hypothesized alternatives were year-to-year variation of ET or changes in soil water storage. In order to investigate this further the rainfall-runoff model HBV was used. First, the model was calibrated for the forest catchment (Ref) and the clear-cut catchment (CC), using observed data from Balsjö. To account for parameter uncertainty the calibration was performed using parameter optimization, resulting in 100 different parameter sets. Model results were evaluated using observed snow data from Balsjö and ET from Flakaliden, a nearby forest. Both the simulated snow and ET were quite consistent with the observed values. Finally the annual and the spring water balance were studied, using the simulated data. The simulated results did not detect the unexpected runoff behavior for the two years as clearly as the observations. The reason for this was that the model was calibrated for all five years, which meant that annual variations were not taken into account. The hypothesis, that higher ET could be the reason for the unexpected runoff behavior, could neither be dismissed nor confirmed by this thesis. This was because there were no observed data for the clear-cut area and limitations within the HBV model, which meant that sublimation and interception processes could not be analyzed separately. The model results indicated that the change in soil water storage was a more likely explanation for the unexpected runoff behavior. The simulation result showed that the meltwater was stored in the soil water storage. However, this theory does not seem likely since a clear-cut is normally wetter than a forest. The results of this thesis are consistent with other studies as they indicate that clear-cut effects should be studied seasonally as well as annually. The special feature of this thesis was the opportunity to study observed ET and investigate its influence on the water balance.