4.1 The water cycle and fresh water supply

In chapter 3, we saw that accessible freshwater resource is a tiny fraction of the water on Earth. Fortunately, water is a renewable resource and is difficult to destroy. Evaporation and precipitation combine to replenish our fresh water supply constantly; however, water availability is complicated by its uneven distribution over the Earth. Arid climates and densely populated areas have combined in many parts of the world to create water shortages, which are projected to worsen in the coming years due to population growth and climate change. Human activities such as water overuse and pollution have significantly compounded the problem of providing clean freshwater to the world. Freshwater availability is central to UN Sustainable Development Goals for clean water, life below water, and sustainable cities and communities.

The water cycle

The water (or hydrologic) cycle shows the movement of water through different reservoirs or pools, including oceans, the atmosphere, glaciers, groundwater, lakes, rivers, and the biosphere. Solar energy and gravity drive the motion of water in the water cycle. Simply put, the water cycle involves water moving from oceans, rivers, and lakes to the atmosphere by evaporation, forming clouds. From clouds, it falls as precipitation (rain and snow) on both water and land. The water on land can either return to the ocean by surface runoff, rivers, glaciers, and subsurface groundwater flow or return to the atmosphere by evaporation and evapotranspiration (sometimes just transpiration; loss of water by plants to the atmosphere directly from their leaves and during respiration) (Fig 1).

Sublimation occurs when water in solid form as ice or snow goes directly into water vapor, by evaporating. Evapotranspiration is evaporation that takes place when plants breathe and lose water vapor as they exchange oxygen and carbon dioxide.

Living things are affected both by (1) surface water and soil water available for drinking and root uptake and (2) water in the atmosphere. The need for drinking water is familiar to all of us, but the atmospheric aspect is less obvious. You may have experienced your nose and mouth and eyes and even skin feeling dry when the air is dry – when relative humidity is low. Because atmospheric humidity is low, moisture is moving from you into the atmosphere, by diffusion. You represent a body of highly concentrated moisture, and the atmosphere represents a body of low-concentration moisture. Diffusion moves water along the gradient from you, on the high end of the gradient, to the atmosphere, on the low end of the gradient. You don’t breathe in order to lose moisture, but rather to exchange gases. But you breathe through a system of moist membranes in your lungs and nasal passages, and you can lose water from these surfaces. Similarly, when plants exchange gases, they breathe through small pores in their leaves (called stomata) and can lose water during this process – part of evapotranspiration. And just as your skin can lose moisture directly, many plant leaves can lose moisture directly – the other part of evapotranspiration. Plants in arid areas often have leaf surfaces modified to minimize such losses. But they still need to exchange gases and still lose water to dry air.

For living things, relative humidity is more important than absolute humidity. “Relative” means “as a percent,” whereas “absolute” refers to actual quantity. Cold air can hold very little absolute moisture. A cubic meter of air at freezing temperature (32°F, 0°C) can hold a maximum of about 3.5 grams of water in vapor form (less than 1 teaspoon) Air at room temperature can hold more absolute moisture – a maximum of about 17 grams of water, or 3.4 teaspoons. Saturated air – air that is holding all the moisture it can hold – has 100% relative humidity. Air during a rainstorm is saturated and has 100% relative humidity. If the air is holding half of what it can hold at maximum, then it has 50% relative humidity (considered a comfortable indoor humidity).

Living things deal with relatively humidity, because that is the measurement that indicates how much more moisture the air could hold than it presently does. The lower the relative humidity, the greater the difference between living things (as somewhat permeable bags containers of moisture with some evaporative surfaces) and the atmosphere. If the atmosphere could hold much more water than it has – if the relative humidity is low – then moisture will move out of living things and into the atmosphere by diffusion, through evaporation, as described above.

An important part of the water cycle is how water varies in salinity, which is the abundance of dissolved ions in water. The saltwater in the oceans is highly saline, with about 35,000 mg of dissolved ions per liter of seawater. Evaporation (water changing from liquid to gas at ambient temperatures) is a distillation process that produces nearly pure water with almost no dissolved ions. Water vaporizes and leaves the dissolved ions in the original liquid. Eventually, condensation  (water changing from gas to liquid) forms fog and clouds and sometimes precipitation (rain and hail and snow). After rainwater falls onto land, it dissolves minerals in rock and soil, which increases its salinity. Most lakes, rivers, and near-surface groundwater are called freshwater and have a relatively low salinity. The next sections discuss important parts of the water cycle relative to freshwater resources.

Take a moment and watch the pattern of precipitation through the months over the Sahara in northern Africa, the west coast of central South America, and over the Himalayas (north of India). Watch the differences in different places (eastern compared to western North America, for example). Which places are always wet? Which are always or very often dry? Compare this to information about precipitation related to atmospheric cells from Chapter 1.

 

Flowing water from rain and melted snow on land enters river channels by surface runoff (Fig 3) and groundwater seepage. River discharge describes the volume of water moving through a river channel over time (Figure 4). The relative contributions of surface runoff vs. groundwater seepage to river discharge depend on precipitation patterns, vegetation, topography, land use, and soil characteristics. Soon after a heavy rainstorm, river discharge increases due to surface runoff. The steady normal flow of river water, when storms aren’t involved, is mainly from groundwater that discharges into the river. Gravity pulls river water downhill toward the ocean. Along the way, the moving water of a river can erode soil particles and dissolve minerals. Groundwater also contributes a large amount of the dissolved minerals in river water.

The geographic area drained by a river and its tributaries is called a drainage basin or watershed. The Mississippi River drainage basin includes approximately 40% of the U.S., an area that includes the smaller drainage basins, such as the Ohio River and Missouri River, that help to comprise it. Rivers are an important water resource for cropland irrigation and drinking water for many cities worldwide. Rivers that have had international disputes over water supply include Colorado (Mexico, southwest U.S.), Nile (Egypt, Ethiopia, Sudan), Euphrates (Iraq, Syria, Turkey), Ganges (Bangladesh, India), and Jordan (Israel, Jordan, Syria).

Although glaciers represent the largest reservoir of fresh water, they generally are not used as a direct water source because they are located too far from most people (Figure 4). However, meltwater from glaciers provides a natural source of river

water and groundwater, due to the cycle of melting during summer and building during winter. Under climate change, the melt-build cycles are increasingly pushed to more melt and less build, as less and less precipitation falls as snow and more falls as rain. Obviously, this can only last until glaciers melt completely.

Water use in the U.S. and world – withdrawals from surface and groundwater

Humans use water to produce the food, energy, and mineral resources they use.  Consider, for example, these approximate water requirements for some things people in the developed world use every day: one tomato = 3 gallons (11.4 L); one kilowatt-hour of electricity from a thermoelectric power plant = 21 gallons (79 L); one loaf of bread = 150 gallons (568 L); two pounds of beef (0.9 kg) = 3,200 gallons (12,100 L); and one ton (or tonne) of steel = 63,000 gallons (238,500 L). Human beings require only about 1 gallon (3.8 L) per day to survive. Still, a typical person in a U.S. household uses approximately 100 gallons (380 L) per day, which includes cooking, washing dishes and clothes, flushing the toilet, and bathing – indoor uses.  The water demand of an area is a function of the population and other uses of water.

In the US, In the years immediately after World War II, agriculture was still the primary undertaking using water (Fig 5). As municipal and industrial demand for energy increased, use of water for cooling mostly coal-fired power plants increased, passing agricultural water use in the mid-1960s. In the 21st century, as the nation’s energy portfolio shifted from coal-fired power plants to natural-gas power plants that need less cooling, overall water use began to decline, helped along by the economic downturn in 2008. In 2015, use of water for power plants was again only slightly higher than use for agriculture. The decline in water use in the US has come largely in surface water use, with groundwater use holding steady in the 21st century (Fig 6).

Water use declined after 2005 for several reasons, including reduced irrigation demand for fruits and vegetables during the 2008 economic downturn, reduction in coal-fired power plants and associated water use for cooling, and increased efficiency of cooling technology.

 

Although use of water in the energy sector has declined in the past, the anticipated expansion of electrical energy use, in response to increases in electrical vehicles, expansion of US manufacturing, the development of hydrogen fuels, growth of AI, and general increases in electrification will likely increase water demand. Water use to cool AI data centers is anticipated to increase 2-3 times between 2024 and 2028, for example, causing concern for local water availability.^[2]

The sectoral use of water in the US varies geographically (Fig 7), with some regions having higher agricultural withdrawal, and others higher industrial withdrawal. In the wetter eastern part of the country, rain-fed agriculture is more common, and irrigation is less necessary than in the drier, western part of the country. Power plants tend to cluster near population centers, and industrial use has both historical and recent components to its distribution. Note that water used for agricultural purposes mostly returns to the atmosphere through evaporation and evapotranspiration, whereas a significant proportion of water used for many other uses, including cooling of power plants, industrial use, and municipal use, is often returned to surface water bodies.

Global total water use is steadily increasing at a rate greater than world population growth (Fig 8). During the 20th century, global population tripled, and water demand grew by a factor of six. The increase in global water demand beyond the population growth rate was due to an improved standard of living without an offset from water conservation. Increased production of goods and energy entails a large increase in water demand.

 

Rates of increase in global water use are slowing in the 21st century, as water use is decoupled from economic growth by improvements in technology in all sectors, from irrigation, to industry, to municipal use. Globally, the US, India, and China have the highest level of water withdrawal (Fig 9).

The largest water users shown are the US, India, and China. Despite the aridity of many countries, water withdrawal in Africa is comparatively low.

Continents with more agriculture on drier land rely heavily on irrigation, and agricultural water use continues to be the largest use of water, worldwide (Figure 10). Although domestic use in countries like the US may be much higher than the global average, owing to watered lawns, water-intensive appliances, etc., water for agriculture is a much larger share of the country’s water portfolio.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.