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. 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.