The Water Hydrologic Cycle

All life depends on water and in its absence life ceases. The kind of vegetation present at a site depends upon the amount of free water available and a principal factor in terrestrial net primary production is the amount of precipitation a site receives. The movement of water on the face of the Earth affects the rate of weathering, the amount of materials carried to the seas and lost to the bottom sediments, the erosion of soils, as well as the global heat balance and rainfall patterns from pole to pole.

Water is the carrier of the elements, and all of the biogeochemical cycles include portions of time in the hydrosphere. Water itself cycles and would do so in the absence of organisms (unlike the other major elements, which require organisms in their cycles). Oceans, which cover over three‐quarters of the Earth's surface, are the reservoir for water. Evaporation from their surfaces amounts to 425,000 km 3 per year. About 90 percent of the water returns to the oceans as precipitation, and the remaining 10 percent falls as precipitation on the land. Transpiration (the loss of water from plants) and evaporation from the soil, together called evapotranspiration, add 71,000 km 3 to the atmosphere yearly. The amount of water in the atmospheric reservoir is small, however, and the terrestrial water reserves are in the groundwater, the lakes and streams, and the soil water. It is estimated that 96 percent of the freshwater in the United States is held in the groundwater reservoir. Groundwater doesn't enter the global biosphere directly and is unavailable to plants except through human activities that remove water in wells drilled into the supply. Recharge of the reservoir is from seepage through the soil and is a slow process.

Of particular significance is the unevenness of the water cycle across the Earth. Some regions receive quantities of rain, others none. Nor is evaporation from the oceans uniform. Near the equator evaporation may average 4 mm/day and in polar latitudes <1 mm/day. Because solar energy powers evaporation and is not uniformly received due to the tilt of the Earth toward the sun, the heat balance of the Earth depends on the transfers of heat through evaporation and precipitation of water.