Topography and its Effect on Groundwater

 

(Data Source:  Portage County Planning and Zoning Department)

 

Topography represents the contour or the arrangement of the land surface including its relief and the position of its natural and man-made features.  The topography of the County includes five moraines running from north to south.  These moraines provide the subtle relief or contour of the land, as well as divide the County into two watersheds or groundwater basins

The topography helps form the abundant hydrologic features of Portage County.  Wetlands cover 19,000 acres in the northwestern and central parts of the County.  15,000 acres are covered by 104 lakes (of which 95 are in the Tomorrow River Watershed) and 110 miles of streams. 

To view more topography maps, click here.

Besides surface water, topography of the land surface also determines the general direction of groundwater flow, and it influences groundwater recharge and discharge.  A recharge area is where water moves downward from a topographical high area into the zone of saturation.  In other words, recharge areas replenish groundwater.  A discharge area is where groundwater moves towards the surface to escape into a spring, lake, wetland, or a stream.   Groundwater discharge also occurs from a pumping well.  Groundwater flow from the recharge area to a discharge area is called a flow system.
 

In an unconfined aquifer like the one covering most of Portage County, recharge areas usually occur in high topographic areas.  In Portage County, a groundwater divide is formed by glacial moraines that runs from north to south through the center of the County. 4, 7  

As the map shows, groundwater flowing west of this divide empties or discharges into the Wisconsin River system and groundwater flowing east of the divide empties into the Tomorrow River system. 4, 7  

This groundwater contour map represents the County's regional groundwater flow system with the recharge area at the basin or watershed divide and the discharge area into the Wisconsin River. 

(Data Source:  Central Wisconsin Groundwater Center)

 

(Modified from M.K. Hubbert, Journal of Geology, University of Chicago Press, 37)

 

Now, let's look at this from another perspective.  This groundwater map shows a cross-sectional view of the regional flow system.   The dashed lines represent the groundwater contours (points of equal hydraulic head) and the solid lines with arrows represent groundwater flow lines.  Flow lines cross contour lines at right angles; therefore, flow lines are perpendicular to contour lines.  Also, flow lines tend to move away from each other in recharge areas and come back together in discharge areas.

 

Within the regional flow system are smaller local groundwater flow systems.  The recharge area is still at a topographical high spot, but the discharge area is at a nearby topographical low spot.  For example, when precipitation falls on a hill and recharges groundwater, and then discharges into the nearby lake.   Many streams and lakes in the County that are fed by groundwater exist within a local flow system.

 

(Modified from J.A. Toth,  38
Copyright 1963 by the American Geophysical Union.)

 

Even though recharge often occurs at topographic high areas, direct runoff may also occur.  During high intensity storms, water falling in steep areas is more likely to become surface or direct runoff, rather than replenish the groundwater.  Conversely, flat areas are more likely to absorb the water.  

In addition to topography, soil type, water levels, and underlying bedrock can also affect recharge.  "In the drift-crystalline rock province (northwest part of Portage County), extensive marshes, shallow water levels, clay till, and crystalline rocks near the surface all have combined to decrease recharge and increase surface runoff. 4"


(Italicized words defined in the glossary.)

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