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.

Streams are classified into perennial, intermittent, and ephemeral streams based on flow durations. Perennial stream is the basic network, while intermittent or ephemeral stream is the expanded network. Connection between perennial stream and base flow at the mean annual scale exists since one of the hydrologic functions of perennial stream is to deliver runoff even in low flow seasons. The partitioning of precipitation into runoff and evaporation at the mean annual scale, on the first order, is captured by the ratio of potential evaporation to precipitation (Ep/P called climate aridity index) based on the Budyko hypothesis. The primary focus of this thesis is the relationship between base flow and perennial stream density (Dp) in the Budyko framework. In this thesis, perennial stream density is quantified from the high resolution National Hydrography Dataset for 185 watersheds; the climate control (represented by the climate aridity index) on perennial stream density and on base flow is quantified; and the correlation between base flow and perennial stream density is analyzed. Perennial stream density declines monotonically with the climate aridity index, and an inversely proportional function is proposed to model the relationship between Dp and Ep/P. This monotonic trend of perennial stream density reconciles with the Abrahams curve, and the perennial stream density is only a small portion of the total drainage density. The dependences of base flow ratio (Qb/P) and the normalized perennial stream density on the climate aridity index follow a similar complementary Budyko-type curve. The correlation coefficient between iv the ratio of base flow to precipitation and perennial stream density is found to be 0.74. The similarity between the base flow and perennial stream density reveals the co-evolution between water balance and perennial stream network.

Evapotranspiration ET is a dominant Earth System process that couples the water and energy cycles at the earth surface. The pressure of global environmental changes foster the broad scientific aim to understand impacts of climate and land-use on evapotranspiration under transient conditions. In this work, the spatial scale of river catchments is addressed through data analysis of hydrological and meteorological archives with ET classically derived through water balance closure. Through a synthesis of various catchments with different climatic forcings and hydrological conditions, the core objectives of this thesis are: - Did environmental changes in the past, such as climatic- or land-use and land cover (LULC) changes, result in detectable non-stationary changes in the hydro-climate time series? - How can the impacts of climatic- from LULC changes on the hydroclimatology of catchments be separated? - What are the factors that control the sensitivity of ET and streamflow to external changes? These research questions are addressed for the climatic scales of long-term annual averages and seasonal conditions which characterise the hydroclimatology of river catchments. Illustrated by a rich hydro-climatic archive condensed for 27 small to medium sized river catchments in Saxony, a method is proposed to analyse the seasonal features of river flow allowing to detect shifting seasons in snow affected river basins in the last 90 years. Observations of snow depth at these same times lead to the conclusion, that changes in the annual cycle of air temperature have a large influence on the timing of the freeze-thaw in late winter and early spring. This causes large changes in storage of water in the snow pack, which leads to profound changes of the river regime, particularly affecting the river flow in the following months. A model-based data analysis, based on the fundamental principles of water and energy conservation for long-term average conditions, is proposed for the prediction of ET and streamflow, as well as the separation of climate related impacts from impacts resulting from changes in basin conditions. The framework was tested on a large data set of river catchments in the continental US and is shown to be consistent with other methods proposed in the literature. The observed past changes highlight that (i) changes in climate, such as precipitation or evaporative demand, result in changes of the partitioning within the water and energy balance, (ii) the aridity of the climate and to a lesser degree basin conditions determine the sensitivity to external changes, (iii) these controlling factors influence the direction of LULC change impacts, which in some cases can be larger than climate impacts. This work provides evidence, that changes in climatic and land cover conditions can lead to transient hydrological behaviours and make stationary assumptions invalid. Hence, past changes present the opportunity for model testing and thereby deriving fundamental laws and concepts at the scale of interest, which are not affected by changes in the boundary conditions. / Die Verdunstung ist ein maßgeblicher Prozess innerhalb des Klimasystems der Erde, welche den Wasserkreislauf mit dem Energiehaushalt der Erde verbindet. Eine zentrale wissenschaftliche Herausforderung ist, zu verstehen, wie die regionale Wasserverfügbarkeit durch Änderungen des Klimas oder der physiographischen Eigenschaften der Landoberfläche beeinflusst wird. Mittels einer integrierten Datenanalyse von vorhandenen langjährigen Archiven hydroklimatischer Zeitreihen werden die folgenden wissenschaftlichen Fragestellungen dieser Dissertation diskutiert: - Haben beobachtete Änderungen der Landoberfläche und des Klimas zu nachweisbaren, instationären hydroklimatischen Änderungen geführt? - Lassen sich die hydroklimatischen Auswirkungen von Klimaänderungen und Änderungen der Landoberfläche voneinander unterscheiden? - Welche Faktoren beeinflussen die Sensitivität von Abfluss und Verdunstung auf Veränderungen der klimatischen und physiographischen Randbedingungen? Hierbei fokussiert sich die Arbeit auf Änderungen im langjährige Mittel und im Jahresgang von hydroklimatischen Variablen auf der räumlichen Skala von Flusseinzugsgebieten. Zur Untersuchung des hydrologischen Regimes wurde ein harmonischer Filter angewandt, der es erlaubt, die Eintrittszeit des Jahresgangs (Phase) zu quantifizieren. Diese klimatologische Kenngröße wurde für eine Vielzahl von Einzugsgebieten in Sachsen untersucht, wobei sich vor allem für die Gebiete in den Kammlagen des Erzgebirges signifikante Veränderungen ergaben. Es konnte gezeigt werden, dass die signifikante Phasenverschiebung der Temperatur seit Ende der 1980er Jahre zu einer verfrühten Schneeschmelze und dadurch zu einem Rückgang des Abflusses bis in die Sommermonate hinein geführt hat. Desweiteren wurde eine modellbasierte Datenanalyse entwickelt, welche auf Massen- und Energieerhalt von Einzugsgebieten im langjährigen Mittel beruht. Das entwickelte Konzept erlaubt es, Auswirkungen von Klimaänderungen von anderen Effekten, welche z.B. durch Landnutzungsänderungen bedingt sind, abzugrenzen und zu quantifizieren. Die Ergebnisse einer Sensitivitätsanalyse dieses Konzeptes sowie die Anwendung auf einen umfangreichen hydroklimatischen Datensatz der USA zeigen: (i) Veränderungen im Wasser- oder Energiedargebot beeinflussen auch die Aufteilung der Wasser- und Energieflüsse. (ii) Die Aridität des Klimas und nachgeordnet die physiographischen Faktoren bestimmen die Sensitivität von Verdunstung und Abfluss. (iii) Beide Faktoren beeinflussen die Stärke und Richtung der Auswirkungen von physiographischen Änderungen. (iv) Anthropogene Veränderungen der Landoberfläche führten zum Teil zu stärkeren Auswirkungen als klimatisch bedingte Änderungen. Zusammenfassend zeigt sich, dass Änderungen von Landnutzung und Klima zu Verschiebungen im Wasserhaushalt führen können und damit auch die Annahme von Stationarität verletzen. Hydroklimatische Veränderungen bieten aber auch eine Gelegenheit zum Testen von Theorien und Modellen, um somit die grundlegenden Zusammenhänge zu erkennen, welche nicht durch Änderungen der Randbedingungen hinfällig werden.

Study region Nyangores River watershed, headwater catchment of Mara River basin in Kenya. Study focus Climate variability and human activities are the main drivers of change of watershed hydrology. The contribution of climate variability and land use change to change in streamflow of Nyangores River, was investigated. Mann Kendall and sequential Mann Kendall tests were used to investigate the presence and breakpoint of a trend in discharge data (1965–2007) respectively. The Budyko framework was used to separate the respective contribution of drivers to change in discharge. Future response of the watershed to climate change was predicted using the runoff sensitivity equation developed. New hydrological insights for the region There was a significant increasing trend in the discharge with a breakpoint in 1977. Land use change was found to be the main driver of change in discharge accounting for 97.5% of the change. Climate variability only caused a net increase of the remaining 2.5% of the change; which was caused by counter impacts on discharge of increase in rainfall (increased discharge by 24%) and increase in potential evapotranspiration (decreased discharge by 21.5%). Climate change was predicted to cause a moderate 16% and 15% increase in streamflow in the next 20 and 50 years respectively. Change in discharge was specifically attributed to deforestation at the headwaters of the watershed.

Study region Nyangores River watershed, headwater catchment of Mara River basin in Kenya. Study focus Climate variability and human activities are the main drivers of change of watershed hydrology. The contribution of climate variability and land use change to change in streamflow of Nyangores River, was investigated. Mann Kendall and sequential Mann Kendall tests were used to investigate the presence and breakpoint of a trend in discharge data (1965–2007) respectively. The Budyko framework was used to separate the respective contribution of drivers to change in discharge. Future response of the watershed to climate change was predicted using the runoff sensitivity equation developed. New hydrological insights for the region There was a significant increasing trend in the discharge with a breakpoint in 1977. Land use change was found to be the main driver of change in discharge accounting for 97.5% of the change. Climate variability only caused a net increase of the remaining 2.5% of the change; which was caused by counter impacts on discharge of increase in rainfall (increased discharge by 24%) and increase in potential evapotranspiration (decreased discharge by 21.5%). Climate change was predicted to cause a moderate 16% and 15% increase in streamflow in the next 20 and 50 years respectively. Change in discharge was specifically attributed to deforestation at the headwaters of the watershed.

Evapotranspiration ET is a dominant Earth System process that couples the water and energy cycles at the earth surface. The pressure of global environmental changes foster the broad scientific aim to understand impacts of climate and land-use on evapotranspiration under transient conditions. In this work, the spatial scale of river catchments is addressed through data analysis of hydrological and meteorological archives with ET classically derived through water balance closure. Through a synthesis of various catchments with different climatic forcings and hydrological conditions, the core objectives of this thesis are: - Did environmental changes in the past, such as climatic- or land-use and land cover (LULC) changes, result in detectable non-stationary changes in the hydro-climate time series? - How can the impacts of climatic- from LULC changes on the hydroclimatology of catchments be separated? - What are the factors that control the sensitivity of ET and streamflow to external changes? These research questions are addressed for the climatic scales of long-term annual averages and seasonal conditions which characterise the hydroclimatology of river catchments. Illustrated by a rich hydro-climatic archive condensed for 27 small to medium sized river catchments in Saxony, a method is proposed to analyse the seasonal features of river flow allowing to detect shifting seasons in snow affected river basins in the last 90 years. Observations of snow depth at these same times lead to the conclusion, that changes in the annual cycle of air temperature have a large influence on the timing of the freeze-thaw in late winter and early spring. This causes large changes in storage of water in the snow pack, which leads to profound changes of the river regime, particularly affecting the river flow in the following months. A model-based data analysis, based on the fundamental principles of water and energy conservation for long-term average conditions, is proposed for the prediction of ET and streamflow, as well as the separation of climate related impacts from impacts resulting from changes in basin conditions. The framework was tested on a large data set of river catchments in the continental US and is shown to be consistent with other methods proposed in the literature. The observed past changes highlight that (i) changes in climate, such as precipitation or evaporative demand, result in changes of the partitioning within the water and energy balance, (ii) the aridity of the climate and to a lesser degree basin conditions determine the sensitivity to external changes, (iii) these controlling factors influence the direction of LULC change impacts, which in some cases can be larger than climate impacts. This work provides evidence, that changes in climatic and land cover conditions can lead to transient hydrological behaviours and make stationary assumptions invalid. Hence, past changes present the opportunity for model testing and thereby deriving fundamental laws and concepts at the scale of interest, which are not affected by changes in the boundary conditions.:Kurzfassung Abstract List of Manuscripts Symbols and abbreviations List of Symbols List of abbreviations 1 Introduction 1.1 Motivation and relevance 1.1.1 Scientific importance of evapotranspiration 1.1.2 Pressure of human driven changes 1.1.3 Practical importance of evapotranspiration 1.2 Scope 1.2.1 Focus on the catchment scale 1.2.2 Changes in the hydroclimatology of river catchments 1.2.3 Hydro-climate data analysis 1.3 Objectives and research questions 1.3.1 Shifting seasons in hydrology 1.3.2 Long-term annual average changes of evapotranspiration and streamflow 1.3.3 Methodological requirements 1.4 Structure of the thesis 2 Long term variability of the annual hydrological regime 2.1 Introduction 2.1.1 Motivation 2.1.2 Seasonal changes in hydrologic records 2.1.3 Regional climate in Saxony 2.1.4 Objective and structure 2.2 Methods 2.2.1 Annual periodic signal extraction 2.2.2 The runoff ratio and its annual phase 2.2.3 Descriptive circular statistics 2.2.4 Detection of nonstationarities, trends and change points 2.3 Data 2.4.1 Estimation and variability of the timing of the runoff ratio 2.4.2 Temporal variability of the timing 2.4.3 Does temperature explain trends in seasonality of runoff ratio? 2.4.4 Trend analysis in snow dominated basins 2.4.5 Uncertainty and significance of the results 2.5 Conclusions 2.A Preparation of basin input data 2.A.1 Precipitation 2.A.2 Temperature and snow depth data 3 Evaluation of water-energy balance frameworks 3.1 Introduction 3.2 Theory 3.2.1 Coupled water and energy balance 3.2.2 The ecohydrologic framework for change attribution 3.2.3 Applying the climate change hypothesis to predict changes in basin evapo transpiration and streamflow 3.2.4 Derivation of climatic sensitivity using the CCUW hypothesis 3.2.5 The Budyko hypothesis and derived sensitivities 3.3 Sensitivity analysis 3.3.1 Mapping of the Budyko functions into UW space 3.3.2 Mapping CCUW into Budyko space 3.3.3 Climatic sensitivity of basin evapotranspiration and streamflow 3.3.4 Climate-vegetation feedback effects 3.4 Application: three case studies 3.4.1 Mississippi River Basin (MRB) 3.4.2 Headwaters of the Yellow River Basin (HYRB) 3.4.3 Murray-Darling River Basin (MDB) 3.5 Conclusions 3.5.1 Potentials and limitations 3.5.2 Insights on the catchment parameter 3.5.3 Validation 3.5.4 Perspectives 3.A Derivation of the climate change direction 4 Climate sensitivity of streamflow over the continental United States 4.1 Introduction 4.1.1 Motivation 4.1.2 Hydro-climate of the continental US 4.1.3 Aims and research questions 4.2 Methods 4.2.1 Ecohydrological concept to separate impacts of climate and basin changes 4.2.2 Streamflow change prediction based on a coupled water-energy balance framework 4.2.3 Streamflow change prediction based on the Budyko hypothesis 4.2.4 Statistical classification of potential climate and basin change impacts 4.3 Data 4.4 Results and discussion 4.4.1 Hydro-climate conditions in the US 4.4.2 Climate sensitivity of streamflow 4.4.3 Assessment of observed and predicted changes in streamflow 4.4.4 Uncertainty discussion 4.5 Conclusions 4.A Mathematical derivations for the Mezentsev function 5 Summary and conclusions 5.1 Shifting seasons in hydrology 5.1.1 Major findings 5.1.2 Socio-economic and political relevance 5.1.3 Limitations and possible directions for further research 5.2 Long-term annual changes in ET and streamflow 5.2.1 Major findings 5.2.2 Socio-economic and political relevance 5.2.3 Limitations and further research 5.3 General conclusions and outlook 5.3.1 Regional and temporal limits and validity 5.3.2 Hydrological records carry signals of climate and land use change 5.3.3 Statistical significance of past changes 5.3.4 Improvements in assessing ET 5.3.5 Remote sensing 5.3.6 Learning from the past to predict the future? Bibliography Danksagung Erklärung / Die Verdunstung ist ein maßgeblicher Prozess innerhalb des Klimasystems der Erde, welche den Wasserkreislauf mit dem Energiehaushalt der Erde verbindet. Eine zentrale wissenschaftliche Herausforderung ist, zu verstehen, wie die regionale Wasserverfügbarkeit durch Änderungen des Klimas oder der physiographischen Eigenschaften der Landoberfläche beeinflusst wird. Mittels einer integrierten Datenanalyse von vorhandenen langjährigen Archiven hydroklimatischer Zeitreihen werden die folgenden wissenschaftlichen Fragestellungen dieser Dissertation diskutiert: - Haben beobachtete Änderungen der Landoberfläche und des Klimas zu nachweisbaren, instationären hydroklimatischen Änderungen geführt? - Lassen sich die hydroklimatischen Auswirkungen von Klimaänderungen und Änderungen der Landoberfläche voneinander unterscheiden? - Welche Faktoren beeinflussen die Sensitivität von Abfluss und Verdunstung auf Veränderungen der klimatischen und physiographischen Randbedingungen? Hierbei fokussiert sich die Arbeit auf Änderungen im langjährige Mittel und im Jahresgang von hydroklimatischen Variablen auf der räumlichen Skala von Flusseinzugsgebieten. Zur Untersuchung des hydrologischen Regimes wurde ein harmonischer Filter angewandt, der es erlaubt, die Eintrittszeit des Jahresgangs (Phase) zu quantifizieren. Diese klimatologische Kenngröße wurde für eine Vielzahl von Einzugsgebieten in Sachsen untersucht, wobei sich vor allem für die Gebiete in den Kammlagen des Erzgebirges signifikante Veränderungen ergaben. Es konnte gezeigt werden, dass die signifikante Phasenverschiebung der Temperatur seit Ende der 1980er Jahre zu einer verfrühten Schneeschmelze und dadurch zu einem Rückgang des Abflusses bis in die Sommermonate hinein geführt hat. Desweiteren wurde eine modellbasierte Datenanalyse entwickelt, welche auf Massen- und Energieerhalt von Einzugsgebieten im langjährigen Mittel beruht. Das entwickelte Konzept erlaubt es, Auswirkungen von Klimaänderungen von anderen Effekten, welche z.B. durch Landnutzungsänderungen bedingt sind, abzugrenzen und zu quantifizieren. Die Ergebnisse einer Sensitivitätsanalyse dieses Konzeptes sowie die Anwendung auf einen umfangreichen hydroklimatischen Datensatz der USA zeigen: (i) Veränderungen im Wasser- oder Energiedargebot beeinflussen auch die Aufteilung der Wasser- und Energieflüsse. (ii) Die Aridität des Klimas und nachgeordnet die physiographischen Faktoren bestimmen die Sensitivität von Verdunstung und Abfluss. (iii) Beide Faktoren beeinflussen die Stärke und Richtung der Auswirkungen von physiographischen Änderungen. (iv) Anthropogene Veränderungen der Landoberfläche führten zum Teil zu stärkeren Auswirkungen als klimatisch bedingte Änderungen. Zusammenfassend zeigt sich, dass Änderungen von Landnutzung und Klima zu Verschiebungen im Wasserhaushalt führen können und damit auch die Annahme von Stationarität verletzen. Hydroklimatische Veränderungen bieten aber auch eine Gelegenheit zum Testen von Theorien und Modellen, um somit die grundlegenden Zusammenhänge zu erkennen, welche nicht durch Änderungen der Randbedingungen hinfällig werden.:Kurzfassung Abstract List of Manuscripts Symbols and abbreviations List of Symbols List of abbreviations 1 Introduction 1.1 Motivation and relevance 1.1.1 Scientific importance of evapotranspiration 1.1.2 Pressure of human driven changes 1.1.3 Practical importance of evapotranspiration 1.2 Scope 1.2.1 Focus on the catchment scale 1.2.2 Changes in the hydroclimatology of river catchments 1.2.3 Hydro-climate data analysis 1.3 Objectives and research questions 1.3.1 Shifting seasons in hydrology 1.3.2 Long-term annual average changes of evapotranspiration and streamflow 1.3.3 Methodological requirements 1.4 Structure of the thesis 2 Long term variability of the annual hydrological regime 2.1 Introduction 2.1.1 Motivation 2.1.2 Seasonal changes in hydrologic records 2.1.3 Regional climate in Saxony 2.1.4 Objective and structure 2.2 Methods 2.2.1 Annual periodic signal extraction 2.2.2 The runoff ratio and its annual phase 2.2.3 Descriptive circular statistics 2.2.4 Detection of nonstationarities, trends and change points 2.3 Data 2.4.1 Estimation and variability of the timing of the runoff ratio 2.4.2 Temporal variability of the timing 2.4.3 Does temperature explain trends in seasonality of runoff ratio? 2.4.4 Trend analysis in snow dominated basins 2.4.5 Uncertainty and significance of the results 2.5 Conclusions 2.A Preparation of basin input data 2.A.1 Precipitation 2.A.2 Temperature and snow depth data 3 Evaluation of water-energy balance frameworks 3.1 Introduction 3.2 Theory 3.2.1 Coupled water and energy balance 3.2.2 The ecohydrologic framework for change attribution 3.2.3 Applying the climate change hypothesis to predict changes in basin evapo transpiration and streamflow 3.2.4 Derivation of climatic sensitivity using the CCUW hypothesis 3.2.5 The Budyko hypothesis and derived sensitivities 3.3 Sensitivity analysis 3.3.1 Mapping of the Budyko functions into UW space 3.3.2 Mapping CCUW into Budyko space 3.3.3 Climatic sensitivity of basin evapotranspiration and streamflow 3.3.4 Climate-vegetation feedback effects 3.4 Application: three case studies 3.4.1 Mississippi River Basin (MRB) 3.4.2 Headwaters of the Yellow River Basin (HYRB) 3.4.3 Murray-Darling River Basin (MDB) 3.5 Conclusions 3.5.1 Potentials and limitations 3.5.2 Insights on the catchment parameter 3.5.3 Validation 3.5.4 Perspectives 3.A Derivation of the climate change direction 4 Climate sensitivity of streamflow over the continental United States 4.1 Introduction 4.1.1 Motivation 4.1.2 Hydro-climate of the continental US 4.1.3 Aims and research questions 4.2 Methods 4.2.1 Ecohydrological concept to separate impacts of climate and basin changes 4.2.2 Streamflow change prediction based on a coupled water-energy balance framework 4.2.3 Streamflow change prediction based on the Budyko hypothesis 4.2.4 Statistical classification of potential climate and basin change impacts 4.3 Data 4.4 Results and discussion 4.4.1 Hydro-climate conditions in the US 4.4.2 Climate sensitivity of streamflow 4.4.3 Assessment of observed and predicted changes in streamflow 4.4.4 Uncertainty discussion 4.5 Conclusions 4.A Mathematical derivations for the Mezentsev function 5 Summary and conclusions 5.1 Shifting seasons in hydrology 5.1.1 Major findings 5.1.2 Socio-economic and political relevance 5.1.3 Limitations and possible directions for further research 5.2 Long-term annual changes in ET and streamflow 5.2.1 Major findings 5.2.2 Socio-economic and political relevance 5.2.3 Limitations and further research 5.3 General conclusions and outlook 5.3.1 Regional and temporal limits and validity 5.3.2 Hydrological records carry signals of climate and land use change 5.3.3 Statistical significance of past changes 5.3.4 Improvements in assessing ET 5.3.5 Remote sensing 5.3.6 Learning from the past to predict the future? Bibliography Danksagung Erklärung

<p> Reflection and heating coefficients were measured during the 3 month summer season at Simcoe, southern Ontario. Contrasting agricultural surfaces and atmospheric conditions were used to analyse their effect on the reflection and heating coefficients. The results confirm the postulate of Montieth (1959a) that <alpha> is close to 0.25 for many vegetated surfaces. <beta> values were positive and there was a tendency for <beta> to approximate 0.22 for many of the vegetated surfaces. Finally it was shown that the radiation balance equation for Simcoe can be generalized into a linear function of solar radiation using the same constants that are applicable to many areas in the world. </p> / Thesis / Master of Arts (MA)

Freshwater is a vital resource that circulates between the atmosphere, the land and the sea. Understanding and quantifying changes to the partitioning of precipitation into evapotranspiration, runoff and water storage change in the landscape are required for assessing changes to freshwater availability. However, the partitioning processes and their changes are complex due to multiple change drivers and effects. This thesis investigates and aims to identify and separate the effects of atmospheric climate change and various landscape drivers on long-term freshwater change. This is done based on hydroclimatic, land-use and water-use data from the beginning of the twentieth century up to present times and across different regions and scales, from catchment to global. The analyzed landscape drivers include historic developments of irrigated and non-irrigated agriculture and flow regulation. The thesis uses and develops further a data-motivated approach to interpret available hydroclimatic and landscape data for identification of water change drivers and effects, expanding the approach application from local to continental and global scales. Based on this approach development, the thesis identifies hydroclimatic change signals of landscape drivers against the background of multiple coexisting drivers influencing worldwide freshwater change, within and among hydrological basins. Globally, landscape drivers are needed to explain more than 70% of the historic hydroclimatic changes, of which a considerable proportion may be directly human-driven. These landscape- and human-driven water changes need to be considered and accounted for also in modeling and projection of changes to the freshwater system on land. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.</p> / VR, project 2009-3221

The main goal of this dissertation is to develop a seasonal water balance model for evaporation, runoff and water storage change based on observations from a large number of watersheds, and further to obtain a comprehensive understanding on the dominant physical controls on intra-annual water balance. Meanwhile, the method for estimating evaporation and water storage based on recession analysis is improved by quantifying the seasonal pattern of the partial contributing area and contributing storage to base flow during low flow seasons. A new method for quantifying seasonality is developed in this research. The difference between precipitation and soil water storage change, defined as effective precipitation, is considered as the available water. As an analog to climate aridity index, the ratio between monthly potential evaporation and effective precipitation is defined as a monthly aridity index. Water-limited or energy-limited months are defined based on the threshold of 1. Water-limited or energy-limited seasons are defined by aggregating water-limited or energy-limited months, respectively. Seasonal evaporation is modeled by extending the Budyko hypothesis, which is originally for mean annual water balance; while seasonal surface runoff and base flow are modeled by generalizing the proportionality hypothesis originating from the SCS curve number model for surface runoff at the event scale. The developed seasonal evaporation and runoff models are evaluated based on watersheds across the United States. For the extended Budyko model, 250 out of 277 study watersheds have a Nash-Sutcliff efficiency (NSE) higher than 0.5, and for the seasonal runoff model, 179 out of 203 study watersheds have a NSE higher than 0.5. Furthermore, the connection between the seasonal parameters of the developed model and a variety of physical factors in the study watersheds is investigated. For the extended Budyko model, vegetation is identified as an important physical factor that related to the seasonal model parameters. However, the relationship is only strong in water-limited seasons, due to the seasonality of the vegetation coverage. In the seasonal runoff model, the key controlling factors for wetting capacity and initial wetting are soil hydraulic conductivity and maximum rainfall intensity respectively. As for initial evaporation, vegetation is identified as the strongest controlling factor. Besides long-term climate, this research identifies the key controlling factors on seasonal water balance: the effects of soil water storage, vegetation, soil hydraulic conductivity, and storminess. The developed model is applied to the Chipola River watershed and the Apalachicola River basin in Florida for assessing potential climate change impact on the seasonal water balance. The developed model performance is compared with a physically-based distributed hydrologic model of the Soil Water Assessment Tool, showing a good performance for seasonal runoff, evaporation and storage change.