In the Mediterranean Andes region '(MA; 30 degrees-37 degrees S), the main rivers are largely fed by melting snowpack and provide freshwater to around 10 million people on both sides of the Andes Mountains. Water resources in the MA are under pressure because of the extensive development of industrial agriculture and mining activities. This pressure is increasing as the region faces one of its worst recorded droughts. Previous studies have pointed to El Nioo-Southern Oscillation '(ENSO) as the main climatic force impacting the MA. However, the role of decadal and multidecadal climate variability, their spatial patterns, and the recurrence of long-term droughts remains poorly studied. In an attempt to better understand these factors, spatial and temporal patterns of hydroclimatic variability are analyzed using an extensive database of streamflow, precipitation, and snowpack covering the period between 1910 and 2011. These analyses are based on the combination of correlation, principal components, and kernel estimation techniques. Despite a general common pattern across the MA, the results presented here identify two hydroclimatic subregions, located north and south of 34 degrees S. While the interannual variability associated with ENSO is slightly stronger north of 34 degrees S, the variability associated with the Pacific decadal oscillation '(PDO) and/or the interdecadal Pacific oscillation '(IPO) index shows similar patterns in both regions. However, variations produced by the IPO forcing seem to be greater in the southern subregion since 1975. The estimations presented here on drought recurrence reveal a generalized increase in dry extremes since the 1950s. These findings suggest that the northern MA is more vulnerable to changes in hydrology and climate than the southern MA.

This study was conducted on two Mississippi watersheds. The SWAT model was applied to the Upper Pearl River Watershed (UPRW) to evaluate flow, sediment, nutrients, and fecal coliform bacteria (FCB) transport. The model was further applied to evaluate crop and sediment yields from three tillage systems (Conventional, Reduce 1, and Reduce 2) of the Big Sunflower River Watershed (BSRW). In the UPRW, flow and sediment simulations showed good to very good model performances (for flow R2 up to 0.76 and NSE up to 0.75; and for sediment R2 up to 0.72 and NSE up to 0.54). Both total nitrogen (TN) and total phosphorous (TP) simulations showed fair to good model performances (R2 up to 0.71 and NSE up to 0.63 for TN; R2 up to 0.70 and NSE up to 0.59 for TP). The FCB simulation showed good model performance (R2 up to 0.59 and NSE up to 0.58). In the BSRW, crop simulations showed good to very good model performances (for corn yield R2 up to 0.5 and NSE up to 0.9; and for soybean yield R2 and NSE up to 0.6). Furthermore, modeling outputs of the BSRW explained 64% of the water table fluctuations in the Mississippi alluvial aquifer. The future climates of the UPRW and the BSRW were evaluated for three emission scenarios (A1B, A2, and B1) from the Intergovernmental Panel on Climate Change (IPCC) with the help of the general circulation model, CCSM3. Simulations predict future sediment yields will increase as much as 25% in the UPRW. Both TN and TP yields will also be elevated as much as 7.3% and 14.3% respectively in future climates of the UPRW. Four best management practices (BMPs) were applied to the current and future climates in the UPRW and results showed that BMPs were able to reduce 51% of flow, 55% of sediment, 44% of TN, and 88% of TP in the baseline climate. Moreover, the effectiveness of TN removal will increase in future climates, while the effectiveness of TP removal will remain unchanged. The effects of climate variability on corn and soybean yield were insignificant in the BSRW.

Previous studies have shown significant warming through the 1990s in the West Antarctic Ice Sheet (WAIS); but the records used in those studies end in early 2000, preventing trend analysis into the latest decade. Fourteen new snowpits and firn cores were collected in 2010 and 2011, which have been combined with previous cores to extend the isotopic records over WAIS. Significance of these isotopic patterns across WAIS was determined and is used to re-evaluate the warming of the West Antarctic interior over recent decades. We find that isotopic records longer than 50 years are needed to assess climate trends due to decadal variability. When assessed over periods greater than 50 years, there is a statistically significant warming trend over central WAIS. However, the isotopes in the 2000s are anomalously low in the isotopic records, which challenge the recent suggestion that the warming trend is accelerating. We attribute the isotopic low over the most recent decade to the coupling effect of anomalously low temperatures over central WAIS and associated increase in sea ice in the adjacent seas. This work strongly indicates that decadal variability and likely climate trends are both driven, at least in part, by atmospheric variability in the tropics as well as at high latitudes.

The temporary slowdown in large-scale surface warming during the early 2000s has been attributed to both external and internal sources of climate variability. Using semiempirical estimates of the internal low-frequency variability component in Northern Hemisphere, Atlantic, and Pacific surface temperatures in concert with statistical hindcast experiments, we investigate whether the slowdown and its recent recovery were predictable. We conclude that the internal variability of the North Pacific, which played a critical role in the slowdown, does not appear to have been predictable using statistical forecast methods. An additional minor contribution from the North Atlantic, by contrast, appears to exhibit some predictability. While our analyses focus on combining semiempirical estimates of internal climatic variability with statistical hindcast experiments, possible implications for initialized model predictions are also discussed.

Large wildfires in California cause significant socioecological impacts, and half of the federal funds for fire suppression are spent each year in California. Future fire activity is projected to increase with climate change, but predictions are uncertain because humans can modulate or even override climatic effects on fire activity. Here we test the hypothesis that changes in socioecological systems from the Native American to the current period drove shifts in fire activity and modulated fire-climate relationships in the Sierra Nevada. We developed a 415-y record (1600-2015 CE) of fire activity by merging a treering-based record of Sierra Nevada fire history with a 20th-century record based on annual area burned. Large shifts in the fire record corresponded with socioecological change, and not climate change, and socioecological conditions amplified and buffered fire response to climate. Fire activity was highest and fire-climate relationships were strongest after Native American depopulation-following mission establishment (ca. 1775 CE)-reduced the self-limiting effect of Native American burns on fire spread. With the Gold Rush and EuroAmerican settlement (ca. 1865 CE), fire activity declined, and the strong multidecadal relationship between temperature and fire decayed and then disappeared after implementation of fire suppression (ca. 1904 CE). The amplification and buffering of fire-climate relationships by humans underscores the need for parameterizing thresholds of human-vs. climate-driven fire activity to improve the skill and value of fire-climate models for addressing the increasing fire risk in California.

Dans un contexte où l'irrigation est une pratique devenue essentielle aux équilibres alimentaires sur la planète, plus particulièrement dans le cadre d'une augmentation des aléas climatiques, les systèmes de tarification non linéaire peuvent jouer un rôle déterminant pour la répartition de la ressource en eau, tout en respectant différentes contraintes de gestion telles que la couverture des coûts. Récemment, en France, certaines compagnies de gestion de l'eau d'irrigation ont mis en place de tels systèmes originaux de tarification fondés sur une différenciation entre des volumes d'eau réservés et des volumes réellement consommés. Ce travail a un double objectif : il vise, d'une part, à évaluer, sur différents plans, ces nouveaux systèmes de tarification mis en place et d'autre part, à concevoir des méthodes de tarification non linéaires capables d'intégrer les risques liés à la variabilité de la disponibilité de l'eau. La thèse est organisée en deux parties. La première partie est composée de 2 chapitres qui fournissent des éléments de cadrage sur la gestion de l'eau d'irrigation et expliquent la problématique de la recherche. La deuxième partie est composée de 4 chapitres constituant chacun un article. Dans le chapitre 3, nous évaluons l'efficacité de systèmes de tarification non linéaire de l'eau d'irrigation mis en place par des compagnies d'eau en France en prenant en compte les aléas pluviométriques et nous proposons des pistes d'amélioration de ces systèmes. Le chapitre 4 se situe dans la continuité du précédent dans la mesure où une méthodologie similaire est utilisée. Il compare ces nouveaux systèmes de tarification à une tarification uniforme dans un contexte de rareté de l'eau d'irrigation et d'aléas climatiques. Les résultats montrent que ces systèmes sont d'autant plus efficaces que la tarification uniforme que la ressource est rare. Le chapitre 5 modélise une situation où les agriculteurs ont la possibilité de réserver de l'eau dans des stocks de niveaux de sécurité différents. Nous déterminons la tarification optimale sous trois hypothèses d'objectif de gestion : i) Le gestionnaire cherche à maximiser le bien-être social ; ii) Le gestionnaire cherche à maximiser le bien-être social mais en assurant l'équilibre budgétaire ; iii) Le gestionnaire cherche à maximiser son profit. Le chapitre 6 étudie la situation d'un gestionnaire qui a besoin d'informations sur la demande d'eau d'irrigation avant de transférer la ressource vers les périmètres irrigués. C'est le cas du système Neste où les réserves d'eau sont souvent très éloignées des périmètres agricoles. En plus, les agriculteurs sont confrontés à des aléas sur la pluviométrie qui peuvent altérer la précision de leur connaissance de leur demande d'eau future. Dans un tel contexte, nous développons un système de tarification avec des possibilités de réservations sur plusieurs périodes et nous déterminons la tarification optimale que doit appliquer le gestionnaire. La discussion des résultats de ces chapitres permet de proposer des recommandations par rapport à la conception de systèmes de tarification adaptés à la gestion d'une ressource et d'une demande en eau incertaines. / In a world where irrigation has become essential to balance global food production, especially in the context of increasingly variable climate conditions, nonlinear pricing systems can play a crucial role in the allocation of water resources, while respecting various management constraints such as cost recovery. Recently in France, some irrigation water management companies have implemented such innovative systems based on price differentiation between volumes of water reserved and volumes actually consumed. This work has two objectives: firstly it aims to assess, at different levels, the new pricing systems in place; secondly it seeks to devise methods of nonlinear pricing that can integrate the risks related to the variability of the availability of water. The thesis is organized into two parts. The first part (chapters 1 and 2) provides background information on irrigation water management and explains the research problem. In the second part (chapter 3 to 6) each chapter is an article. In chapter 3, we assess the effectiveness of nonlinear pricing systems implemented by water companies in France, taking into account the variability of rainfall. We analyze the advantages of these new pricing systems for managing climate variability. We suggest ways of improving these systems. Chapter 4 is in line with the previous chapter in that a similar methodology is used. It compares the new pricing systems to uniform pricing in a context of scarce irrigation water and variable climate conditions. The results show that nonlinear systems are even more efficient than uniform pricing when water scarcity is severe. Chapter 5 models a situation where farmers are offered the possibility of reserving water in stocks with different security levels. We determine optimal pricing for three management assumptions: i) The manager seeks to maximize social welfare, ii) The manager seeks to maximize social welfare while ensuring a balanced budget; iii) The manager seeks to maximize his/her profit. Chapter 6 addresses the situation of a manager who needs information on water demand before moving the resource to areas requiring irrigation. This is the case of the Neste system where water supplies are often far away from agricultural areas. In addition, farmers face uncertainty of rainfall that can alter the accuracy of their knowledge of their future water demand. In this context, we develop a pricing system with the possibility of reservations over several periods and determine the optimal pricing to be implemented. Our results provide guidelines for designing pricing schemes adapted to the management of an uncertain water resource and an uncertain demand.

The central United States is a region of the country plagued by frequent catastrophic flooding (e.g., flood events of 1993, 2008, 2011, 2013, and 2014). In the twentieth and twenty-first centuries, flooding has taken a devastating societal and economic toll on the central United States, contributing to dozens of fatalities and causing billions of dollars in damage. Moreover, previous studies have shown that flood damage has been increasing over the past century across this region, and seems to foreshadow a future increase in flood activity. Despite these large repercussions, the use of historical records to ascertain the changes over time in flooding has thus far proved inconclusive. It is therefore of paramount importance to examine whether the characters (i.e., magnitude and frequency) of recent flooding are different from the long-term averages over the central United States. The results of this thesis are based on long-term discharge records at 774 U.S. Geological Survey sites and show limited evidence suggesting increasing or decreasing trends in the magnitude of flood peaks over the study region. In contrast, there is much stronger evidence of increasing frequency of flood events. While the detection of changes in flood characteristics is essential, it is also of critical importance to start exploring what caused these changes. Therefore, in addition to the aforementioned investigation on the stream flow records, precipitation records were used to inspect whether possible changes in flood characteristics can be linked to the changes in heavy precipitation characteristics. The results indicate that there is a stronger signal of change in the frequency rather than in the magnitude of heavy precipitation events, similar to what found for the discharge records. Given that heavy precipitation is responsible for the observed changes in flooding, further analyses were performed to examine the climatic driving forces that are responsible for the observed changes in the frequency of precipitation, and consequently flooding at the seasonal scale; particular emphasis was paid to the role played by the Atlantic and Pacific Oceans. The results of this dissertation indicate that changes in the climate system play a significant role in explaining the variations in the frequency of heavy precipitation and flooding over the central United States at both the seasonal and sub-seasonal scales. The Pacific North American (PNA) teleconnection pattern was found to play a particularly prominent role. Therefore, these results suggest that recent observed changes in the frequency of flood events over the central United States can be largely attributed to changes in the frequency of heavy precipitation events, which were in turn driven by changes in the climate system.

The Wind-Evaporation-Sea Surface Temperature (WES) feedback is believedto play an important role in the tropics, where climate variability is governed byatmosphere-ocean coupled interactions. This dissertation reports on studies to distinctlyisolate the WES feedback mechanism over tropical oceans using a modiedversion of an NCAR-Community Climate Model (CCM3) thermodynamically coupledto a slab ocean model, where the WES feedback is deliberately suppressed inthe bulk aerodynamic formulation for surface heat uxes. A comparison of coupledintegrations using the modified WES-off CCM3 to those carried out using the standardCCM3 conclusively identifies the role of the WES feedback in enhancing theinter-annual variability over deep tropical oceans and the westward propagation ofthe equatorial annual cycle. An important role for near surface humidity in tropicalclimate variability in enhancing inter-annual variability and in sustaining the equatorialannual cycle is also suggested. Statistical analyses over the tropical Atlanticreveal that the free coupled meridional mode of the Atlantic Ocean is amplified in thepresence of the WES feedback. Similar analyses of coupled model integrations, whenforced with an articial El Ni~no Southern Oscillation (ENSO)-like SST cycle in tropicalPacific, reveal that only in the presence of the WES feedback is the meridionalmode the preferred mode of response of the Atlantic to ENSO forcings. It is also foundthat WES feedback reinforces the tendency of the ITCZ to stay north of the equator over the Atlantic during El-Nino events. Comparative studies between Last GlacialMaximum (LGM) equivalent imposed northern hemispheric sea-ice experiments withthe WES-off model and the standard model indicate a dominant role for the WESfeedback in the southward shift of the ITCZ as indicated by paleo-climate records.However, it is found not to be the sole thermodynamic mechanism responsible for thepropagation of high latitude cold SST anomalies to the tropics, suggesting significantroles for other mechanisms in the tropical response to high latitude changes.

In this study, a model is developed to assess risk to a municipal water supply system under the influence of population growth and climate change. To incorporate the uncertainly in water use, a model which combines time series Monte Carlo simulations and a deterministic artificial neural network (ANN) is developed to simulate the daily water demand under climate variability. The model is then expanded in two directions. One direction is to estimate the effects of demand management programs and system expansion on the reliability, resiliency, and vulnerability of the water supply system. Another direction is to capture the possible impacts of climate change on the risk of a water supply system. Twenty-six scenarios generated from different combinations of demand management programs, system expansions and Global Climate Model (GCM) scenarios were set to illustrate the risk indices: reliability, resiliency, and vulnerability. To illustrate the effects of a change of precipitation frequency and a higher population growth, twenty-five additional scenarios were evaluated.

In this study, a model is developed to assess risk to a municipal water supply system under the influence of population growth and climate change. To incorporate the uncertainly in water use, a model which combines time series Monte Carlo simulations and a deterministic artificial neural network (ANN) is developed to simulate the daily water demand under climate variability. The model is then expanded in two directions. One direction is to estimate the effects of demand management programs and system expansion on the reliability, resiliency, and vulnerability of the water supply system. Another direction is to capture the possible impacts of climate change on the risk of a water supply system. Twenty-six scenarios generated from different combinations of demand management programs, system expansions and Global Climate Model (GCM) scenarios were set to illustrate the risk indices: reliability, resiliency, and vulnerability. To illustrate the effects of a change of precipitation frequency and a higher population growth, twenty-five additional scenarios were evaluated.