Water being drawn from a well was once precipitation that fell onto Earth's surface. It seeped into the ground and, over time, occupied the porous space in some subsurface material.
Water levels in wells are constantly changing both in the short term and over the long term. Some wells even have a seasonal change. In the short term, water levels can be lowered just by pumping water out of the well for use.
The U.S. Geological Survey WaterAlert service sends e-mail or text messages when certain parameters measured by a USGS data-collection station exceed user-definable thresholds.