Summary

How does water move through the hydrologic cycle? The water movements of the hydrologic cycle maintain a balance among the major reservoirs of water on Earth. Evaporation from the oceans, evaporation and transpiration from the continents, and sublimation from glaciers transfer water to the atmosphere. Precipitation returns water from the atmosphere to the oceans and the land surface. Runoff returns part of the precipitation that falls on land to the ocean. The remainder infiltrates the ground and forms groundwater. Differences in climate produce local variations in the balance among evaporation, precipitation, runoff, and infiltration.

How does water move below the ground? Groundwater forms as precipitation infiltrates the ground and travels through porous and permeable formations. The groundwater table is the boundary between the unsaturated and saturated zones. Groundwater moves downhill under the influence of gravity, eventually emerging at springs where the water table intersects the ground surface. Groundwater may flow through unconfined aquifers in formations of uniform permeability or in confined aquifers, which are bounded by aquicludes. Confined aquifers produce artesian flows and spontaneously flowing artesian wells. Darcy’s law describes the rate of groundwater flow in relation to the hydraulic gradient and the permeability of the aquifer.

What factors govern human use of groundwater resources? As the human population grows, the demand for groundwater increases greatly, particularly where irrigation is widespread. As discharge exceeds recharge, many aquifers, such as those of the Great Plains of North America, are being depleted, and there is no prospect of their renewal for many years. Artificial recharge may help to renew some aquifers. The contamination of groundwater by industrial wastes, radioactive wastes, and sewage further reduces supplies of potable groundwater.

What geologic processes are affected by groundwater? Erosion by groundwater in limestone terrains produces karst topography, characterized by caves, sinkholes, and disappearing streams. At great depths in the crust, rocks contain extremely small quantities of water because their porosities are very low. The heating of these waters forms hydrothermal waters, which may return to the surface as geysers and hot springs.