MAC Educational Materials/ 2: Hydrologic Cycle
The Hydrologic Cycle
Groundwater is part of the hydrologic cycle (Figure 1). The hydrologic cycle describes the global circulation of water and helps us understand groundwater flow. On a global scale, water is found as water vapor in clouds, as liquid water in the oceans, rivers, lakes, and aquifers, and as ice in polar ice caps and glaciers. About 97.2% of the Earth’s water is salt water in oceans and seas. Polar ice caps and glaciers store 2.2% of the Earth’s water. The remaining water is stored as groundwater (0.61%), surface water (0.017%), soil moisture (0.005%), and in the atmosphere (0.001%). These figures illustrate the relative abundance of groundwater compared to surface water and water in the atmosphere.
Figure 1. Arrows show the movement of water
through a complete hydrologic cycle that includes precipitation, surface runoff
into surface water, seepage into the ground, groundwater flow from recharge
into discharge, evaporation and transpiration, and condensation. The triangle
indicates the position of the water table.
Water is constantly moving along the hydrologic cycle. Water can move rapidly through the atmosphere and cover long distances from the oceans to the continents. After precipitation falls, it may run off to a stream or seep into the soil. Surface runoff can carry water through the watershed back to the ocean. Along the way, the water may evaporate or be transpired by plants.
Water that seeps into the ground may also evaporate or be transpired by plants. This seepage may also move deeper into the subsurface to recharge an aquifer. Once in the subsurface, groundwater may flow along a local or regional flow path (Figure 2). The local flow path may end at the local stream or lake, while the regional flow path may end at a major river such as the Illinois River or Sangamon River. Groundwater flow is much slower than surface water flow. Groundwater flow is generally on the order of feet per day, so it can take years or decades to move along some of the longer flow paths in Figure 2. Surface water flow is much faster, on the order of feet per second.
The connection between surface water and groundwater is also shown in Figure 2. Groundwater discharges to surface water at various scales, from the local to regional scale. This groundwater flow sustains surface flows during periods with no or little precipitation such as annual dry periods or even during droughts.
Figure 2. A cross-section showing local and regional groundwater flow systems. Water enters the groundwater flow system at the recharge area and moves to a local or regional discharge area. Groundwater in the local flow system will remain in the subsurface much less time than groundwater in the regional flow system.
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