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The Lessons of Water Scarcity Benchmarks The benchmarks of water stress and water scarcity used in this report are not intended to describe Malthusian limits to growth or strict natural thresholds governing population-environment interactions with consistent and unalterable effects. Rather, they serve as indicators of the likelihood of adverse consequences related to water shortage. As such, these benchmarks can help predict the future urgency of problems related to freshwater availability. Equally important, they can provide insight into how true natural thresholds related to population-environment interactions might operate. In the real world some countries with less than 1,000 cubic meters of renewable fresh water per person per year manage to develop economically, while many countries with abundant water still experience severe problems in supplying water to farms, factories and homes. Despite these apparent inconsistencies, these benchmarks are recognized and used by many hydrologists and by the World Bank and help illustrate important population-water relationships. To understand different responses to water availability, it is important to explain a few of the principles and limitations of the terms and analyses presented in this report. First, the figures for per capita water availability presented here refer only to renewable fresh water. This is defined as salt free water that is fully replaced in any given year through rain and snow that falls on continents and islands and flows through rivers and streams to the oceans. The figures do not include water that evaporates through the heat of the sun or transpires through plants to the atmosphere, processes known collectively as evapotranspiration. Excluding the amount of water lost to evapotranspiration helps standardize the water availability figures for countries with dry climates and those with wetter climates by counting only that amount of water that is available for human uses—except to the extent that water availability varies by season. Second, the water availability figures do not include supplies of groundwater that are not replenished by precipitation on human time scales—also called nonrenewable or fossil water. Many countries supplement their renewable water supplies by drawing down their groundwater aquifers. Relying on nonrenewable supplies of groundwater is one way that countries with less than 1,700 cubic meters of renewable fresh water per person per year can avoid, at least temporarily, feeling the constraints of limited supplies of renewable fresh water. However, for most countries this situation cannot be sustained 10
for long, especially as their populations continue to grow, their pumping costs begin to escalate and their development requires greater quantities of water. In addition, the water availability figures take no account of the timing or seasonality of this availability. Fresh water is often more abundant in some seasons than in others. Throughout the tropics, for example, rainy seasons often produce deluges of renewable fresh water that cause damaging floods and can scarcely be captured for later use. Months later the dry season sets in, drying rivers and streams to a trickle and causing severe, though temporary, freshwater shortages. Under these circumstances, a watershed or country that in theory has sufficient water for its inhabitants can face shortages not accounted for by the annual availability of renewable fresh water. Much more data on seasonal inventories of freshwater availability will be needed before the concept of timing can play a significant role in analyses of population and water relationships. As previously discussed, the ability of individuals and institutions to adapt to different challenges can obviously vary widely among nations. For this reason, some can naturally manage better than others as the per capita availability of renewable fresh water declines. It would be inappropriate, therefore, to propose any precise levels as absolute thresholds of water scarcity, or insist that they apply equally to all countries. Instead we use the term benchmark to convey that these figures represent approximate levels of freshwater availability— averaged over different climates, soil conditions and economic development levels—below which concern about water shortage tends to rise significantly. By applying these approximate benchmarks, we can establish a framework that helps explain how population dynamics interact with finite resources such as renewable fresh water. Finally, the benchmarks described here are not meant to imply that countries having more than 1,700 cubic meters of renewable fresh water available to each inhabitant are automatically considered water abundant. This term proved to be misleading when used in Sustaining Water: Population and the Future of Renewable Water Supplies because in many countries—from India to Iran to the United States—the per capita water availability is higher than the 1,700 cubic meter stress benchmark, yet the drier regions of these countries experience significant water shortages at times. In addition, although countries may appear to have a plentiful supply of renewable water, not all of these sources can be exploited at an acceptable cost, given their location relative to population centers. Thus the supply of economically available fresh water is often much lower than the estimates provided here. In this update of Sustaining Water, the term relative water sufficiency conveys the fact that countries with a per capita water availability in excess of the stress and scarcity benchmarks are not guaranteed an abundant freshwater supply in all times and in all places. Even the term “sufficiency” often overstates the reality of freshwater availability in these countries—in the United States the examples of southern California, the Florida Everglades and south Texas come to mind. Nonetheless, it seems the most apt term to describe the condition of renewable water resources in countries with a per capita renewable freshwater supply that exceeds the benchmark levels for water stress or scarcity described in this update. 11
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