Water is a powerful insulator. When water is heated or cooled, its temperature changes more slowly than other liquids. This resistance to gaining or losing heat is due to the high specific heat of water. Because of water’s unique molecular and bonding structure, heat must first break the hydrogen bonds between the molecules, rather than immediately speeding up the molecules and thereby increasing their temperature. This allows water to serve as a “heat sink”; it absorbs heat without rapid changes in temperature which helps buffer aquatic organisms from large swings in temperature.

Figure 5: Photosynthesis. The process of photosynthesis requires light energy, water and carbon dioxide (CO2) as resources for the plant. In the chloroplasts of the green leaf and stem tissues, the plant utilizes these three resources to produce glucose and oxygen. The chemical equation for photosynthesis is the opposite of respiration: 6CO2 + 6 H2O + light = C6H12O6 + 6O2 1

Water’s high specific heat allows large bodies of water to absorb significant heat during the summer without a rapid increase in water temperature. At night, large bodies of water cool very little, providing a relatively stable temperature, and during the winter, water’s gradual release of heat energy warms the air, giving coastal areas a milder ‘maritime’ climate. The moderation of water temperature is vitally necessary for the plants and animals living in aquatic and marine ecosystems.

Water is required for many chemical reactions necessary to life. It is a key component, for instance, in metabolism—or the series of chemical reactions that allow organisms to live, grow, and reproduce. The type and purpose of each metabolic process is unique, but photosynthesis (see Figure 5) is one well-known example. In this process, plants use radiant energy from the sun and carbon dioxide (CO2) from the atmosphere, and water from the soil to produce chemical energy in the form of sugar (glucose, C6,H12O6). Plants release oxygen (O2) as a by-product of photosynthesis. The oxygen in our atmosphere enables plants, animals, and micro-organisms to live and undergo cellular respiration. The ultimate source of this life-giving oxygen is the water molecules that are absorbed by the plants’ roots. You will learn more about the importance of photosynthesis in Chapter 3 on energy.

Questions to Consider

  • Imagine how the fresh water supply in your community will be modified by a warming climate. What effect would this have on water availability in your community?
  • Imagine that the fresh water supply in your community became so polluted that it lost a good deal of its solvency. What effect would this have on the use of water in your community?
  • What are the properties of water that make it essential for life on earth?

The Hydrologic Cycle

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.

Hydrologic Cycle
Figure 6: The Hydrologic Cycle. 2

Looking Ahead


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 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.

Figure 7: Evapotranspiration. 3

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.

Looking Ahead


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 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.

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. 

Closer Look


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.

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.

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.