The Hydrologic Cycle Revisited
Water is essential to life. Nearly all organisms require constant access to water or at least a water-rich environment for survival. Humans are no exception. We need a constant supply of fresh water from precipitation over the lands. Some of this water is stored in soils, regolith, and pores in bedrock. And a small amount of water flows as fresh water in streams and rivers. In this chapter, we will focus on water at the land surface and water lying within the ground.
Fresh water on the continents in surface and subsurface water makes up only about 3 percent of the hydrosphere's total water. This fresh water is mostly locked into ice sheets and mountain glaciers. Ground water can be found at almost every location on land that receives rainfall, but only accounts for a little more than half of 1 percent of global water. This small fraction is still many times larger than the amount of fresh water in lakes, streams, and rivers, which only amount to about three-hundredths of 1 percent of the total water. Fresh surface water varies widely across the globe, and many arid regions do not have permanent streams or rivers.
PATHS OF PRECIPITATION
Figure 14.4 shows the flow paths of water between ocean, land, and atmosphere. The study of these flows is part of the science of hydrology, which is the study of water as a complex but unified system on the Earth. In this chapter, we will trace the part of the hydrologic cycle that includes both the surface and subsurface pathways of water flow. This part arises from precipitation over land.
As shown in Figure 14.5, there are three pathways for land precipitation. First, water can sink into the soil in the process of infiltration, remaining in the soil-water belt or passing through into the zone of ground water below. Second, the water can return to the atmosphere through evapotranspiration. Third, water can form surface runoff that flows directly downslope and into streams. This overland flow moves surface particles from hills to valleys, helping to shape landforms. By supplying water to streams and rivers, runoff also allows rivers to carve out canyons and gorges and to carry sediment to the ocean.