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Understanding the resource
Natasha Kassulke and Laura Chern
Water might be called our most recycled resource. The water you drink today contains the same water molecules that flowed in the Nile during the building of the Egyptian pyramids and froze in glaciers when mastodons roamed the earth. Distribution of the earth's total water supply changes in time and space, but the quantity remains constant.
Wisconsin receives an average 30 to 32 inches of precipitation per year. Seventy-five percent evaporates or transpires through plants and never reaches surface water or groundwater. The six to 10 inches that do not evaporate immediately or get used by plants run off into surface waters or soak into the ground, depending on local topography, soil, land use and vegetation. For every inch of water that runs off the land to a stream or lake in gently rolling Dane County, three inches seep to the water table. In the sandy plains of Portage County, nine inches seep into the ground for each inch running off the land.
Water distribution is governed by a phenomenon known as the hydrologic, or water cycle, which is kept in motion by solar energy and gravity. Start with a spring shower. As the rain falls to earth, some flows downhill as runoff into a stream, lake or ocean. Some evaporates; some is taken up by plants. The rest trickles down through surface soil and rock. This water eventually reaches the water table – the top of a saturated zone of soil or rock, which is called an aquifer. The water contained in the aquifer is groundwater. Groundwater is discharged to wetlands, lakes and streams – the low places where the water table meets the land surface. The sun causes evaporation from these surface waters, and, as water vapor accumulates in the atmosphere and clouds begin to form, the water cycle begins anew.
On the move
Geology controls the rate of groundwater movement. The size of the cracks in rocks, the size of the pores between soil and rock particles, and whether the pores are connected determine the rate at which water moves into, through and out of the aquifer.
Water generally moves quickly in coarse sand, sometimes as much as several feet per day. Openings between the grains are large and interconnected, resulting in high permeability. Very fine-grained material like clay has many pores where water can be stored, but the pores are so small that moving water through or out is difficult. Clay formations are relatively impermeable – water may move only a few inches a year. Permeability in limestone, on the other hand, primarily depends not on pore spaces, but on the size, frequency and distribution of fractures and cracks.
Groundwater is always moving toward a surface outlet or discharge area, following the slope of the water table. In Wisconsin, the natural movement is from upland recharge areas (places where rain or melt water infiltrates the ground and reaches the water table) to lowland discharge areas. Most precipitation seeping into the soil moves only a few miles to the point where it is discharged; in the vast majority of cases, it stays within the same watershed.
Perhaps you've wondered why some streams continue to flow during dry periods and in winter, when there's no rainfall. Winter stream flow is largely groundwater discharge (called baseflow) that remains at a relatively constant temperature year-round – about 50° F. During the winter, groundwater from surrounding uplands constantly replenishes streams, and most lakes and wetlands. That same 50° F groundwater baseflow is the reason streams stay icy cold in the summer.