From the site: "The USGS maintains a network of wells to monitor the effects of droughts and other climate variability on groundwater levels. The network consists of a national network of about 130 wells monitored as part of the Groundwater Resources Program, supplemented by wells in some States monitored as part of the Cooperative Water Program. The water-level changes in the Climate Response Network should primarily reflect climatic variability and not human influences. The climate variations of interest are those that affect recharge on monthly and longer time scales; not barometric or tidal influences. Observation wells should satisfy the following criteria: -Open to a single, known hydrogeologic unit -Known well construction that allows good water-level measurements -Located in unconfined aquifers or near-surface confined aquifers that respond to climatic fluctuations -Minimally affected by pumpage and likely to remain so -Essentially unaffected by irrigation, canals, and other potential sources of artificial recharge -Long-term accessibility -Well has never gone dry (not susceptible to going dry)"