USGS - science for a changing world

Drought & Groundwater

Groundwater, which is found in aquifers below the surface of the earth, is one of the nation's most important natural resources. Groundwater provides drinking water for a large portion of the nation's population, supplies business and industries, and is used extensively for irrigation.

Groundwater can also contribute to surface water supplies. Some groundwater seeps into rivers and lakes, and can flow to the surface as springs.

The water level in an aquifer that supplies water to a well does not always remain the same. Droughts, seasonal variations in precipitation, and pumping affect the height of underground water levels. If a water is pumped at a faster rate than an aquifer is recharged by precipitation or other sources of recharge, water levels can drop. This can happen during drought, due to the extreme deficit of rain.

Long-term water-level data are fundamental to the resolution of many of the most complex problems dealing with groundwater availability and sustainability. Significant periods of time - years to decades - typically are required to collect water-level data needed to assess the effects of climate variability, to monitor the effects of regional aquifer development, or to obtain data sufficient for analysis of water-level trends.

Select California Groundwater Wells


Several well sites are shown on the map for historical comparison of groundwater levels affected by the drought. Well sites were selected based upon public and water management interest, and for their water level fluctuations during historical drought periods.

2015 and 2016 water year data are provisional and subject to change.

Click or Touch a Streamgage Icon On the Map to Display Drought Related Water Data Graphs for Select Water-Years. Click the home button () to reset map extent and/or reset the Explanation pane.


An extensometer measures the one-dimensional (1D) change in thickness of a specified depth interval. In other words, it measures the compaction and expansion of the aquifer system to a specific depth. More than two dozen extensometers in the Central Valley were constructed in the 1950s, 1960s, and 1970s by the U.S. Geological Survey in cooperation with the California Department of Water Resources (Ireland and others, 1984), the early group of which represent the first extensometers ever built in the United States. Additionally, several extensometers have been constructed in the San Joaquin Valley more recently.

Continuous GPS Station

A CGPS station continuously measures the three-dimensional (3D) position of a point on, or more specifically, near the earth's surface. There are more than 1,000 Continuous Global Positioning System Stations operating in Western North America, and hundreds of them in California alone; many of them are managed by the Plate Boundary Observatory/UNAVCO and by Scripps Orbit and Permanent Arrary Center (SOPAC), but other groups such as Caltrans, also operate some of them as part of their Central Valley Spatial Reference Network. They generally have been constructed to monitor motions caused by plate tectonics, but are widely used for other applications including subsidence monitoring. These GPS stations generally collect position information every 15 seconds which are then processed to produce a daily position. These daily positions are then concatenated to produce a daily time series, which allow us to track the 3D position of the station.