A map of the piezometric surface can be made by comparing the levels to which the water rises in wells, reflecting the pressure the water is under in that portion of the aquifer. Groundwater is made up of rainfall or snowmelt that percolates down through the soil until it reaches the saturated zone of an aquifer. This process is known as aquifer recharge. Recharge may take place throughout the aquifer, but occurs principally in aquifer recharge areas. These are usually located in the uplands.
Lowlands tend to be discharge areas where groundwater is discharged to surface waters.
Surface watersheds can usually be used to approximate shallow groundwater hydrologic boundaries when specific information is not available. Surface watershed boundaries and groundwater divides do not, however, always coincide. Specific site investigations require the use of detailed hydrologic maps or field data to properly identify groundwater flow patterns. Regional groundwater flow in Wisconsin is similar to surface topography. Groundwater usually enters the aquifer in upland areas and flows toward low points in a drainage basin.