Our water supply

The water supply at any place depends on several factors including

• the hydrologic cycle
• precipitation
• evaporation
• stream flow
• subsurface flow

The water budget is the sum of the inputs, outputs, and storage of water in a system. The amount of water vapor in the air is equivalent to 40,000 billion gallons of liquid water. Of this about 10% falls as rain. Out of this roughly two thirds evaporates.

The remaining water enters the

• surface or ground water storage systems
• flows to the oceans or across the nation's boundaries
• is used by consumption
• evaporates from reservoirs

Our ground water supply

Nearly half the population of the United States uses ground water as the primary source of drinking water. Fortunately, ground water availability is enormous. Within the U.S. the amount of ground water within 0.8km of the land surface is between 125,000 and 224,000 km.

To put into perspective, the lower estimate is about the total discharge of the Mississippi River in the last 200 years! Unfortunately, owing to the cost of pumping and exploration, much less than the total quantity of ground water is available.

Protecting ground water is of crucial importance. It takes thousands of years to replenish aquifers. Once they are damaged by pollutants they are difficult or impossible to reclaim for drinking.

Aquifers are also important because 30% of the stream flow in the U.S. is supplied by ground water that emerges as springs or seepage along stream channels. This is counts for the dry season flow of most perennial streams.