According to the conclusions of the United Nations, ensuring freshwater resources is one of the key challenges facing humanity in the twenty-first century. Estimates of the Earth's potentially renewable freshwater resources range from 35,000 to 50,000 cubic kilometers per year, of which only one-third (12,500 cubic kilometers per year) can be utilized to satisfy human needs. At the same time, due to water pollution, the share of potentially available freshwater resources tends to decline over time, while global demand for water continues to grow in the opposite direction. It has been established that during the twentieth century global water consumption increased sevenfold, reaching 4,000 cubic kilometers annually, with this growth rate exceeding population growth by more than two times. According to forecasts, the world's population will reach 8.3 billion by 2025 and exceed 10 to 12 billion by 2050. Given current trends, it can be assumed that within the next fifty years problems related to water scarcity or water pollution will arise, to varying degrees, in every country of the world. The worsening global water crisis will have its greatest impact on food security and ecological safety in many regions and countries. The United Nations Agenda 21 (Rio de Janeiro, 1992) convincingly demonstrated that water is a key factor in sustainable development, that is, development that does not undermine the social, economic, and environmental conditions necessary for its continuation and is capable of reproducing its own foundations without depletion [1, 2]. At the United Nations World Summit on Sustainable Development held ten years later in Johannesburg (2002), Kazakhstan's President Nursultan Nazarbayev pointed out that despite significant scientific and technological progress, humanity had still failed to overcome the problems that had accompanied it throughout history: poverty, hunger, disease, and destructive wars. He also reminded participants that more than one billion people lived in poverty and more than one billion lacked access to safe drinking water. Nursultan Nazarbayev argued that preventing irreversible environmental damage must become the core of sustainable development in order to preserve the resources necessary for life for future generations. Sustainable development is a form of human survival. During the summit's plenary session, Nursultan Nazarbayev proposed that all countries reduce military expenditures by one percent and direct these funds toward the development of the world's poorest nations. Analysis of the global water-resource situation shows that a strategy based solely on water conservation measures cannot solve the problem of water scarcity. Another promising direction toward balanced water use is water-resource augmentation, that is, increasing the potential resources of natural waters through their redistribution in space and time. According to UNESCO experts, by 1990 the flow regimes of one-tenth of the world's rivers had already been regulated, resulting in a 22 percent increase in stable river runoff. Forecasts indicate that by around 2030, two-thirds of global river runoff will be regulated to varying degrees [3-8]. Altering the natural state of water systems may increase water availability and help resolve many hydraulic engineering problems, but it also causes numerous side effects and adverse changes in natural and economic conditions across vast territories, thereby triggering justified public concern. Consequently, the significance of research conducted in this field in many countries around the world can hardly be overstated. The severity of Kazakhstan's water-resource challenges stems from limited available water resources, their uneven distribution, substantial temporal variability, and significant pollution levels. Future prospects indicate that the situation may become even more severe due to reductions in river inflows originating from neighboring countries and decreases in local runoff caused by climatic factors. At the same time, the frequency of extreme hydrological events, including catastrophic floods and droughts, is expected to increase. The potential consequences of changes in river runoff constitute a genuine threat to the country's sustainable development. The fundamental economic reforms currently underway, including reforms within the water sector, place special demands on solving the problem of sustainable water-resource provision. The Concept for the Development of the Water Management Sector and Water Policy of the Republic of Kazakhstan through 2010, approved by Government Resolution No. 71 of January 21, 2002, defined the overall goals, primary tasks, and main directions of national water-resource policy, including: • Rational use of water resources; • Preservation and maintenance of a healthy ecological environment; • Provision of high-quality drinking water to the population; • State management of the water-management sector; • International cooperation in the use of transboundary water resources; • Development of the legal framework for water-resource management; • Scientific research, design, and education. The approval of this document by the Government confirmed both the necessity and timeliness of preparing the General Scheme for Water Resource Provision of the Republic of Kazakhstan, which serves as the basis for planning water-management measures. It is equally evident that an appropriate scientific foundation must be established to ensure a sufficiently high level of reliability for this scheme and its conclusions. Based on the analysis conducted, the following urgent scientific issues in water-resource provision and water use in the Republic can be identified: developing theoretical foundations for the assessment and forecasting of natural water resources, their condition and quality, taking into account anthropogenic and climatic changes; developing principles for integrated water-resource management, including issues of interstate water allocation within the framework of complete hydrological cycles of river basins; establishing scientific foundations for optimizing the use of nontraditional water resources, including the transition of population centers, industry, and part of agriculture (irrigated farming) to groundwater-based supply; developing scientific principles for water-conservation strategies aimed at reducing withdrawals from natural sources, decreasing pollutant discharges, and improving water-use efficiency in industry and agriculture; and conducting fundamental research on the assessment and prediction of water-related natural hazards and the responses of natural-economic systems to anthropogenic and climate-induced changes in water resources. To date, the work of both foreign and domestic researchers has created a certain theoretical basis for the rational use of natural resources and the introduction of active management elements within this framework. This progress was facilitated by the timely inclusion of experimental transformation tasks within geographical research, an area that received substantial development in works cited in [9-29]. Given the difficulty, or even impossibility in principle, of conducting physical experiments on large natural-economic systems, the systems approach has come to occupy a central position in studies of natural-geographical systems, particularly in addressing water-resource issues. Mathematical modeling has been widely applied in this field. Numerous researchers throughout the post-Soviet region have devoted their work to the application of mathematical methods in the analysis, forecasting, and optimization of water-management systems [30-38]. The heightened attention paid to environmental problems in recent decades has stimulated the active development of hydroecological research. This field is based on evaluating current and anticipated climate-induced and anthropogenic changes in water bodies and on developing theories and methods for understanding the effects of such changes on natural-economic systems and their components [39-80]. While the generally high level of development of theories concerning the rational use of natural resources, including water resources, is widely recognized, it should also be noted that the current methodological foundations for managing large-scale water systems exhibit certain shortcomings in ensuring objective management decisions. This is particularly related to the need to improve the theoretical and methodological basis of water-resource management within the framework of modern concepts of sustainable development, while taking into account the regional characteristics of water-resource challenges in natural-economic systems. In accordance with these issues, the present monograph is devoted to solving the following tasks: • Developing the concept of a water-resource provision system as the “support mechanism” for integrated water-resource management in natural-economic systems; • Conducting retrospective and predictive assessments of the stability of water-resource provision in the endorheic transboundary basins of the Aral Sea and Lake Balkhash; • Identifying contemporary patterns of water-salinity dynamics in river deltas on the basis of hydrological monitoring conducted in the Syr Darya Delta; • Developing a typical conceptual model for the rehabilitation of ecologically destabilized deltaic waters in order to curb environmental degradation and alleviate severe socio-economic conditions (poverty) in rural areas, using the Syr Darya as an example; • Investigating patterns of water-salt exchange in closed water bodies through field experiments and simulation of intrabasin hydrological processes in Lake Balkhash; • Substantiating the possibility of managing intrabasin hydrological processes as a means of compensating for anthropogenic and climatic changes in river inflows into closed water bodies, using Lake Balkhash as an example; • Developing methodological tools for assessing the stability of water-resource provision systems based on principles of multi-objective optimization and dynamic stochastic modeling, using the Aral Sea-Syr Darya Basin as an example. The principal source materials for this monograph consist of a synthesis of research results obtained by the authors over the past twenty-five years in the course of implementing national scientific and technological programs, contractual research projects, and international programs and projects.