Renewable fresh water comprises a tiny fraction of the global water pool but is the foundation for life in terrestrial and freshwater ecosystems. The benefits to humans of renewable fresh water include water for drinking, irrigation, and industrial uses, for production of fish and waterfowl, and for such instream uses as recreation, transportation, and waste disposal. In the coming century, climate change and a growing imbalance among freshwater supply, consumption, and population will alter the water cycle dramatically. Many regions of the world are already limited by the amount and quality of available water. In the next 30 yr alone, accessible runoff is unlikely to increase more than 10%, but the earth's population is projected to rise by approximately one-third. Unless the efficiency of water use rises, this imbalance will reduce freshwater ecosystem services, increase the number of aquatic species facing extinction, and further fragment wetlands, rivers, deltas, and estuaries. Based on the scientific evidence currently available, we conclude that: (1) over half of accessible freshwater runoff globally is already appropriated for human use; (2) more than 1 x 109 people currently lack access to clean drinking water and almost 3 x 109 people lack basic sanitation services; (3) because the human population will grow faster than increases in the amount of accessible fresh water, per capita availability of fresh water will decrease in the coming century; (4) climate change will cause a general intensification of the earth's hydrological cycle in the next 100 yr, with generally increased precipitation, evapotranspiration, and occurrence of storms, and significant changes in biogeochemical processes influencing water quality; (5) at least 90% of total water discharge from U.S. rivers is strongly affected by channel fragmentation from dams, reservoirs, interbasin diversions, and irrigation; and (6) globally, 20% of freshwater fish species are threatened or extinct, and freshwater species make up 47% of all animals federally endangered in the United States. The growing demands on freshwater resources create an urgent need to link research with improved water management. Better monitoring, assessment, and forecasting of water resources will help to allocate water more efficiently among competing needs. Currently in the United States, at least six federal departments and 20 agencies share responsibilities for various aspects of the hydrologic cycle. Coordination by a single panel with members drawn from each department, or by a central agency, would acknowledge the diverse pressures on freshwater systems and could lead to the development of a well-coordinated national plan.
This article summarizes the theory of climate change and the relationship of climate-change forcing to hydrologic and aquifer processes. It focuses on regional aquifer systems and on the methods to link large-scale climate-change processes to ground-water recharge and to simulate ground-water flow and solute transport in a warmer, 2× CO2 climate. The article reviews methods currently available to generate climate-change forcing and to simulate regional aquifer systems under ensuing hydrologic conditions. In addition, it outlines the development of a methodology to quantify the effects of climate change and of changes in ground-water use by population growth on hydrologic response. An example illustrates a specific procedure and our current capabilities and limitations to assess the potential impacts of a warming climate and population growth on regional-scale aquifer systems. The results indicate that aquifer exploitation strategies must take into account climatic variability and climate-change patterns. During protracted drought, the competition between human and ecological water uses is sharply accentuated. Changes in ground-water use may affect aquifer response more profoundly than climate change associated with modern global warming.
Although linkages between water scarcity and conflict have received a great deal of attention, both in qualitative case studies as well as quantitative studies, the relationship remains unclear since the literature has generally not considered the effectiveness of governance. We distinguish between direct effects and indirect effects linking water resource scarcity and conflict by systematically examining how intervening factors, such as political institutions, might influence the impact of water scarcity on the probability of conflict. We find support for our hypotheses postulating both direct and indirect relationships between water scarcity, governance, and conflict.
Los ecosistemas agrícolas proveen servicios reconocidos como generadores de beneficios ambientales y sociales mediante la conservación de zonas boscosas y cambios en las prácticas de producción agrícola que ellos promueven. Las cuencas productoras de agua más importantes del Estado Táchira han sido afectadas debido al manejo de los ecosistemas, incidiendo particularmente en la producción, almacenamiento y regulación del caudal. Para valorar los beneficios derivados de la protección del recurso hídrico en las cuencas que surten al Acueducto Regional del Táchira, se aplicó el método de valoración contingente a través de una encuesta con una muestra de tamaño 806 para una población de 90.806 suscriptores. Las preguntas referentes a la disposición a pagar utilizaron formato tipo referéndum y abierto. La mediana resultante según tipo de pregunta y técnica de estimación varió entre 1000 a 1725,1 Bs mes-1 suscriptor-1. Los resultados muestran que la implementación del pago de servicios ambientales podría generar recursos para conservación que varían entre 100.000 a 172.458 Bs ha-1año-1, monto muy superior al obtenido actualmente por Decreto y al utilizado en otras experiencias en América Latina. Esta investigación constituye un primer paso para una gestión integral de cuencas, aun cuando deben esclarecerse otros aspectos básicos para establecer el pago final por servicios ambientales.