Groundwater is one of Earth’s most abundant natural resources. It is also one of the most abused natural resources, largely because a majority of people fail to recognize that groundwater is not infinitely renewable. Renewability of any resource requires a balance between the rate at which it is produced versus the rate at which it is used. For groundwater then, the rates of recharge versus discharge determine whether the volume of water held in the aquifer shrinks, grows, or is maintained over time. When groundwater is pumped out faster than it can seep in, the aquifer becomes less saturated, the water table drops, and eventually the groundwater supply becomes depleted. When water is pumped out of an aquifer at rates much higher than the recharge rate, the aquifer is effectively being mined.

Many groundwater reservoirs are "fossil" aquifers, that is the water in them is hundreds to thousands of years old. Modern recharge rates in this case are so low that the reservoir is actually not renewable in a practical sense at all. Pumping water from fossil aquifers depletes the supply in the same way that extraction from an oil well depletes fossil fuels. Overuse of groundwater is now ubiquitous in many parts of the world, including China, India, Mexico, Thailand, North Africa, the Middle East, and the western United States. The major use of this water is irrigation water for agriculture. Worldwide, irrigation efficiency is estimated at 40%. This means more than half the water diverted for irrigation never benefits a crop. Although some of the "lost" water is returned to rivers or aquifers where it can be used again, the water quality is often degraded from pesticides, salts, and toxic chemicals it picks up off the land. Thus, not only is water becoming scarcer, the existing supplies are becoming unusable.

The largest aquifer in the world is the Ogallala Aquifer which extends across Nebraska and parts of Kansas, Colorado, Oklahoma, New Mexico, Wyoming, South Dakota, and Texas (see Figure). The Ogallala Aquifer supplies about 30% of the water used in the United States for irrigation. Recharge rates in the Ogallala are very low and withdrawal for irrigation is so high that the water table is dropping an average of 2 meters per year. At this rate of discharge, this groundwater resource will be depleted in less than fifty years; over half the total volume of the Ogallala will be gone by the year 2020. The farms, industries, and people that depend on the Ogallala for their primary water supply will soon have to face the problem of what to do when their wells run dry.