The depth to the water table can change (rise or fall) depending on the time of year. During the late winter and spring when accumulated snow starts to melt and spring rainfall is plentiful, water on the surface 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 falls 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. If no wells are available, surface geophysical methods can sometimes be used, depending on surface accessibility for placing electric or acoustic probes.
Databases containing depth-to water measurements can also be helpful, though they don't always have current data:
- The USGS National Water Information System (NWIS) has depth-to-water measurements made in the present and the past. A convenient way to find data for your area is by using the NWIS Mapper and selecting "Groundwater Sites" in the menu on the left. Click on any red groundwater pin to access the data.
- The National Groundwater Monitoring Network is a compilation of groundwater monitoring wells from federal, state, and local groundwater networks across the nation. Use their Data Portal to zoom in to your area of interest and click on any site.
- Your state government probably maintains a database of drillers' logs that have water-levels recorded when a well was drilled, and hydrologic consultants often have reports that contain water-level data from shallow boreholes.
Consulting any or all of these sources is a good first step in finding out the depth to the water table.