How is ground water mapped?
USGS conducts regular survey in over 20000 wells across the United States. The frequency of data collection depends on the site with around 7000 providing daily data. For some of those wells (currently, 3777 groundwater sites) daily statistics are provided. An increasing number of wells are providing real time (updated every 15 or 30 min) data.  

Source: USGS

You can visualize the daily changes using this animation.

Go to https://waterdata.usgs.gov/nwis/gw for  information as well as the ability to get the data directly from the source.