"Groundwater stored underground in the continental United States on November 28, 2011, as compared to the long-term average from 1948 to 2011."
Groundwater is fed mostly by rain. Without 'normal' amounts of rain, shallow well water levels fall and well water may become more saline. Rivers and lakes once maintained by groundwater discharges dry up. People may be forced to drill even deeper wells or to seek out new surface water sources to produce water for industrial, farming, and domestic consumption.
Deeper drilling and piping water from farther away, aside from being more costly, pose long term risks: essentially you are mining very old water with deeper wells and distant reservoirs, too, may be depleted should drought continue. Texas in particular, is moving toward that position. NASA has mapped one indicator of that risk (see graphic). Below is a NASA provided description of the data and how it may be properly viewed (probabilistic).
The map above depicts the amount of groundwater stored underground in the continental United States on November 28, 2011, as compared to the long-term average from 1948 to 2011. Deep reds reveal the most depletion, with deep blues representing aquifers and soils that are nearly full. The maroon shading over eastern Texas, for example, shows that the ground has been this dry less than two percent of the time between 1948 and the present.