Groundwater and surface water are basically one resource, physically connected by the hydrologic cycle. Under natural conditions, rainfall percolates into the ground and recharges the groundwater system, and then water flows very slowly through the groundwater system, eventually discharging to streams. Changes in groundwater supplies are mainly influenced by climate and human consumption.
In the arid Southwest, groundwater plays a critical role in sustaining the flow of streams and rivers and maintaining riparian and wetland habitats. Groundwater also provides drinking water to urban and rural communities and supports agriculture and industry. Rapid population growth over the last century has led to increasing groundwater withdrawals that are outpacing the rate at which groundwater is naturally replenished.