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 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.