The physical change of water from solid to liquid to gas is the basis of the hydrologic cycle (see Figure 6). The cycling of the Earth’s water is governed by evaporation, transpiration, precipitation, and surface runoff. Each process involves not only the change of water’s physical state, but also its transport and temporary storage.
The hydrologic cycle and its governing processes are essential for purifying water and distributing it across the Earth. The hydrological cycle begins when liquid water is converted into gaseous water vapor through one of two processes. The first process is evaporation, whereby heat from the sun and wind energy convert liquid water into its less dense, gaseous state. The second is transpiration, or the loss of water vapor directly from plants. When taken together, the two processes are called evapotranspiration (see Figure 7). When saltwater evaporates, only pure, freshwater molecules become vapor, leaving the mineral salt behind.
Solar radiation warms the surface of the Earth causing evaporation of moisture into the air. As the warm, moist, less dense air rises into the atmosphere, it cools and then condenses this water vapor into small, liquid water droplets or ice crystals we visualize as clouds. A cloud remains in the sky until the pull of gravity exceeds the ability of the atmosphere to hold it up. Precipitation can be liquid (rain), solid (snow, ice) or a mix of the two states, depending on the air temperature and the pressure.
Many religious traditions have used the hydrologic cycle of rains, floods, and droughts to portray--in mythic stories--the spiritual experience of moving from opportunity, through suffering and sacrifice, to abundance.
The highest levels of evaporation occur over the Earth’s oceans, especially near the equator, where solar heating is most intense, while the greatest amounts of precipitation occur over landmasses. In the oceans, more water is lost through evaporation than is returned via precipitation. On land, more water is gained through precipitation than is lost via evaporation. Surface flow and temporary storage of rainwater in aquifers and surface water bodies keep the water levels in the ocean and on land relatively constant over time.
Water has 'intrinsic value'. This is a value for "its own sake." You can already see the high value of water’s unique properties and its role in the hydrologic cycle.
The rate at which water moves through the hydrological cycle depends on its physical state and place at any given moment. Water moves most rapidly as water vapor in the atmosphere. Water does not stay in the atmosphere very long; the average retention time of atmospheric water vapor is nine days. The retention time of surface water stored in lakes is much longer, six to seven years on average. In some large, deep lakes the retention time is greater than 10,000 years. Groundwater stored underground in aquifers is slow moving and also has a very long retention time. The water in many groundwater aquifers can take thousands of years to return to the ocean. Most fresh water is stored in glaciers and ice caps, which are formed by long-term climate patterns. Our warming global climate is melting and releasing this water at a higher rate than usual, changing natural hydrological patterns. We have already encountered this problem in the Ganges case study that opened this chapter. The phenomenon of climate change will be thoroughly explored in Chapter 6.
Learn more about the water cycle and its connection to sustainability by watching Sustainability: Water, a video sponsored by the National Science Foundation and NBC Learn.
Some fluctuation in precipitation is a common part of the hydrologic cycle, and can cause periodic drought or floods. During times of drought, water stored in bodies with long retention times (e.g. snowfields, lakes, rivers, aquifers) can supplement the scarce supply. During floods, water may become so abundant that natural ecosystems are damaged or destroyed.