Hydrologic Cycle

The hydrologic cycle refers to the continuous cycling of water through the following processes: water evaporates from water bodies, or is transpires by plants into the atmosphere; it condenses and precipitates as rain and snow; it returns to the oceans via streams or seepage from groundwater aquifers. Precipitation may occur in minutes or hours, stream flow and infiltration to the water table take place in days, and groundwater may move to discharge areas in decades, centuries, or longer.

Retention of water in ice fields or in deep aquifers may extend the periods of local cycles through geologic ages. One must appreciate the hydrologic cycle to understand water resources and their best management.

The energy of the sun drives the hydrologic cycle by evaporating water and transporting water vapor in the earth's weather systems; the earth's gravity directs many hydrologic processes including stream flow. Only a small part of all the earth's water commonly occurs as fresh water; most occurs as salt water in the oceans, or as ice in ice caps and glaciers, or as hot mineralized brine deep in geologic formations. Humans purposefully interrupt natural cycling rates to supply water as needed rather than as delivered by the natural regime. For example, we construct storage reservoirs to supply municipal, industrial, and agricultural demands and return treated wastewater to our lakes and