The role of the USGS has grown along with the changing needs of the country, and the USGS has diversified to include not only a Geologic Division and a National Mapping Division, but also a Water Resources Division and a Biologic Resources Division. Of course there is some overlap among the divisions, which is why we all work together and are collectively known as the U.S. Geological Survey. Here in the Water Resources Division, the maps we produce are specific to the hydrologic projects that we are working on. Just like you, we depend upon the National Mapping Division for their most updated maps, which we use as base maps for our hydrologic information. The main body of subsurface water is found in the saturated zone of aquifers.
Aquifers can be only a few feet below the surface or more than a thousand feet deep.
This zone is called the unsaturated zone. The unsaturated zone includes a layer directly above the aquifer called the capillary fringe where water in the saturated zone is drawn upward by capillary action. During the late winter and spring when accumulated snow starts to melt and spring rainfall is plentiful, water on the surface of the earth infiltrates into the ground and the water table rises. When water-loving plants start to grow again in the spring and precipitation gives way to hot, dry summers, the water table will fall because of evapotranspiration. The most reliable method of obtaining the depth to the water table at any given time is to measure the water level in a shallow well with a tape.