Water might be called our most recycled resource. The water you showered with this morning, for example, may have contained the same water molecules that caused a dinosaur's hide to glisten in the prehistoric sun or carried the Nina, Pinta and Santa Maria across the Atlantic. The distribution of the earth's total supply of water changes in time and space, but the quantity has remained constant.
Uneven water distribution is governed by a phenomenon known as the hydrologic cycle, which is kept in motion by solar energy and gravity. A rain storm is the start of the cycle. 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 formations, traveling through pore spaces and open cracks. This water eventually reaches the top of a water-saturated layer of soil or rock called the water table is called groundwater.
Groundwater seeps from upland to lowland areas and is released, or discharged, in lakes, streams, and wetland-low places where the water table meets the land surface. The sum release energy, causing evaporation from surface waters. The process that returns water to the atmosphere from water and land surfaces and by the activity of living plants called evapotranspiration. When water vapor accumulates in the atmosphere and clouds begin to form, the hydrologic cycle begins.
Wisconsin receives an average of 30 to 32 inches of precipitation per year. Seventy-five percent of that precipitation evaporates or transpires through plants and never reaches surface 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. All groundwater moves continually toward an area of discharge, but the rates of movement vary greatly.
The reason for this variability is a matter of geology. The size of the cracks in rocks, the size of the pores between soil and rock particles and weather the pores are connected contribute to the rate of movement to, through and out the saturated zone.
Water generally moves more quickly into, through and out of 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 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 pores spaces but on the size, frequency and distribution of fractures and cracks.
GROUNDWATER ON THE MOVE
As groundwater moves through the water cycle, it follows the slope of the water table. In Wisconsin, the natural movement is from upland recharge areas to a lowland discharge areas.