Groundwater | The Unseen Part Of The Water Cycle

DANA FULTON PORTER AND JADA THUR

Groundwater is an often-overlooked component of the water cycle, but it plays a critical role in the health of our ecosystems and our communities. In fact, about 70% of Wisconsinites rely on groundwater for drinking water, more than any other state.

Pick a summer shower as the starting point of the water cycle. The precipitation falls to earth, with some moving or running downhill as surface runoff, some evaporating and some absorbed and transpired by plants. The combination of evaporation and plant usage is called evapotranspiration, a process that eventually places the water molecules back in the sky, where new clouds will form from condensation

In Wisconsin, about one-third of the precipitation slowly soaks down through the unsaturated zone of the earth; the soil and rock in this zone contain air as well as water in its pores. The amount of water that infiltrates the earth varies. In rolling hills, more of the precipitation may become runoff, while in sandy spots, more water can soak into the land. The water that does seep through will eventually reach the top of the saturated zone where all the pore spaces are filled with water and no air remains.

The top of the saturated zone is called the water table, and all the water contained below the water table is called groundwater. The water table can be very shallow, even at the land surface in some cases, or very deep, sometimes hundreds of feet below the land surface. Most wetlands, lakes and rivers in Wisconsin represent the water table. Infiltration is an important step to help the groundwater recharge or replenish. The seasonal and yearly changes in precipitation cause the water table to fluctuate, trending higher in the spring when there is often more precipitation combined with snowmelt. Though shorter drought periods do not usually impact the water table, extreme drought can cause the water table to lower.

In the saturated zone, the rock and soil that can be tapped to get water are called aquifers. The water in aquifers is moving all the time, groundwater flow, but not like rivers flowing below ground. It's more like water in a sponge. Gravity and pressure move water downward and sideways underground through spaces between rocks. The speed of the water flow through an aquifer will vary greatly due to both the makeup of the aquifer and the pressure the water is under. In general, coarse sand allows water to move more quickly. In contrast, clay is fine-grained, and water moves much more slowly. The water eventually emerges back to the land surface, into rivers and lakes to keep the water cycle going. These areas are also called groundwater discharge areas.