1. The Hydrologic Cycle and Freshwater Uses

1.1 Hydrologic Cycle

The focus of this module is on the interplay of water resources and agriculture, therefore, understanding the hydrologic cycle (fancy term for the water cycle) is important. All water on the planet is connected through the hydrologic cycle. This cycle is a closed system which means the volume of water on the planet today is the same as it always has been. No new water has been introduced to the planet and water has not left either – it has just changed where it is located and the form it has. 

Stores are locations where water “sits” in the hydrologic cycle. Flows are the processes that move water from one store to the next.

Water on the planet is constantly on the move. It travels from one location (called a reservoir or store) to the next through processes called flows. It is evaporated from the oceans, lakes, streams, the surface of the land, and plants (transpiration). It is transported in its gaseous form through the atmosphere by the wind and condenses to form clouds of water droplets or ice crystals. It falls to the Earth’s surface as rain or snow and flows through streams, into lakes, and eventually back to the oceans. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly flows through soils, surficial materials, and pores and cracks in the rock. The groundwater flow paths can intersect with the surface and the water can then move back into streams, lakes and oceans or be taken up by soil and plants (Figure 2A.1.1).

Illustration of the stores and flows in the water cycle. Ocean water evaporates to become atmosphere water which condenses to become precipitation (rain and snow) which falls on the ground and becomes surface water (streams and lakes) or glacial ice. Surface water either runs off into oceans, is evaporated into the atmosphere or infiltrates into soil or groundwater. Ice can melt and runoff into surface water or infiltrate into soil or groundwater. Groundwater flows and discharges into surface water or oceans. Vegetation and soil store water that has been taken up (plant uptake) from groundwater or surface water water in the soil. Plants transpire to return water to the atmosphere.
Figure 2A.1.1 The various components of the water cycle. Black or white text (accompanied by arrows) indicates the movement or transfer of water from one reservoir to another (flows). Yellow text indicates the storage of water (stores) Source: Steven Earle (2015), CC BY-SA 3.0. Found here.

There are 332.5 million cubic miles of water on the planet and most of this is salt water (the oceans hold close to 97% of all water on the planet). Of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use every day (Figure 2A.1.2).

Three bar graphs showing water storage on the planet as described in the figure caption.
Figure 2A.1.2 Bar charts showing percentages of where water exists on the planet. Left Bar: All water on the planet, 96.5% is oceans, 2.5% is freshwater, and the remaining 1% is saline lakes and groundwater. Center Bar: Breakdown of the freshwater portion of global water. 68.6% is frozen, 30.1% is groundwater, 1.3% is surface water and other freshwater. Right Bar: Breakdown of the surface and other freshwater portion of global water. The majority, 73.1%, is frozen as ice and snow, lakes have 20.1%, and rivers have only 0.49% even though rivers are where humans get most of their freshwater from. Source: USGS. Public Domain. Found here.

To put these percentages in perspective, pretend a one liter (1000 ml or about 4 cups) jug of water represents the entirety of Earth’s water supply (Figure 2A.1.3). The 970 ml in the jug is all the saltwater on Earth. One regular sized ice cube (~20 ml) represents frozen freshwater (glaciers/ice caps), and two teaspoons (~10 ml) are groundwater. All the visible surface water on the planet (lakes/rivers/streams) is represented by the three drops of water being added by the eyedropper. The saltwater, frozen water, and most of the groundwater is not easily accessible/readily available water, which is why humans mainly rely on surface water, but surface water is only a tiny fraction of Earth’s water resources. This is why taking care to keep our water resources clean and use them sustainably is so important!

Illustration to visualize the relative amounts of water in each store of the hydrologic cycle as described in the text and figure caption.
Figure 2A.1.3 All of Earth’s water in a 1 liter container: the small ice cube represents all the frozen water on the planet, two teaspoons represents the groundwater, and three drops represents all fresh water in lakes, streams, and wetlands, plus all atmospheric water. Left: Earth’s water reservoirs shown in a stacked bar chart. Source: Earth’s water reservoirs shown in a stacked bar chart, and using the analogy of a 1 L pitcher. Source: Karla Panchuk (2021), CC BY-SA 4.0. Found here.

Check your understanding: Hydrologic cycle

1.2 Freshwater Uses

Freshwater is used in a variety of ways. The way most people think of first is as everyday domestic use (washing dishes, brushing your teeth, drinking, etc.) which is often supplied by a public water supply. Other uses include industrial uses (manufacturing of consumer goods, washing of equipment, etc.), mining, water for livestock, water for aquaculture, thermoelectric water, and irrigation (agriculture).

Globally, the largest use of freshwater is for irrigation purposes. On average 70% of all water usage is used for irrigation, however, that varies significantly by country (Figure 2A.1.4) and income level. Low-income countries use on average 90% of water withdrawals for agriculture, while middle-income countries use 79%, and high-income countries use only 41%.

Map of the world showing agricultural (irrigation) water as a share of total water withdrawals in each country. Canada, the U.S., Russia, and most western European countries are low with percentages at 40% or lower. Mexico, Central American and South American countries are high with withdrawals above 60% and some above 80%. Through the Middle East and stretching east through to North and South Korea all countries have withdrawals over 80%, China is slightly lower at 60-80%. Through the Sahara region of Africa withdrawals vary between 60-80% and over 80%. The general pattern shows higher water withdrawals in areas where climate is drier (desert regions) and lower water withdrawals both in wetter climates and in more industrialized countries.
Figure 2A.1.4 Map showing agricultural (irrigation) water as a share of total water withdrawals in each country for 2019. The darkest blue countries use the highest percentage of their water for agriculture, while lightest blue countries use the lowest percentage of their water for agriculture. Source: Our World in Data. CC BY-4.0. Found here.

Freshwater usage has changed over time in the United States. The total volume of water being used for irrigation has stayed constant since the mid-1950s, with some small ups and downs, but the percentage of total water used for irrigation has declined since that time. In the 1950s the US used about the world average of 70% of its water for irrigation, but this declined as thermoelectric water usage increased and reached a peak in the 1970s through 2000s (Figure 2A.1.5). Presently between 40-50% of water usage is irrigation.

Bar graph of water withdrawals. Public supply, rural domestic and livestock, and other water usages are consistently low (less than 50 billion gallons per day) from 1950 through 2015. Irrigation water has remained between 100 and 150 billion gallons per day from 1950 through 2015 and was the largest water withdrawal of all categories from 1950 through 1960, after which thermoelectric became the largest water withdrawal. Thermoelectric was below 50 billion gallons per day in 1950 but steadily rose from 1955 through 1980 to reach a peak of 200 billion gallons per day in 180. It remained high, between 190-200 billion gallons per day from 1980 through 2005 and has declined since then back to 140 billion gallons per day. Total water withdrawals follow the same trend as thermoelectric water.
Figure 2A.1.5. Trends in total water withdrawals by water-use category, 1950–2015.Total water usage/withdrawals are shown in the blue line, and the colored bars are the breakdown of this water by category: public supply, rural domestic and livestock, irrigation, thermoelectric, and other (mining/industry/aquaculture). Source: USGS. Public Domain. Found here.

Thermoelectric water is water used in electricity production facilities (i.e., a coal fired power plants) for cooling purposes – this is not the same as a hydroelectric facility. The overall declines in water use since the mid-2000s have mainly been a result of a changing energy supply in the United States. As the US has shifted from coal fired power plants to natural gas and renewables as main sources of power, this has created a decreased need for cooling water (powerplant cooling water needs for natural gas are less than coal and many renewables require no cooling water at all).​

​Irrigation water use has also declined, although less so than thermoelectric. This has been the result of increased irrigation efficiency and more sustainable farming practices in relation to water usage.

Thermoelectric and irrigation continue to be the top two freshwater uses in the United States. Thermoelectric water use is highest, but it also has a major difference compared to irrigation water use.​ While there are various designs for power plants, just under half of the power plants in the US are designed with open systems where water is removed from a nearby river/lake/stream, used to cool the powerplant, and then returned directly back into the river/lake/stream. As a result, it ultimately stays in the same place in the hydrologic cycle – just with a brief stopover at an electrical facility. In this way, the water is largely not consumed (there is some water loss, around 2%, due to evaporation). This is termed non-consumptive water use.

In contrast, water used for irrigation purposes is used or consumed by the plants, the soil, evaporated, or runoff into a new location. In other words, it goes to a new place in the hydrologic cycle as a result of being used. This is consumptive water use​. 

Check your understanding: Consumptive and non-consumptive water uses

Water scarcity is already a major issue for the planet and will only get worse as climate change causes more changes to water availability (i.e., droughts), which then affects agriculture and food availability. The world demand for water is expected to increase significantly in the coming decades as growing populations drive increased consumption. However, the availability of water is finite, so it is not clear how this increased need for water can be met, particularly with climate change making some areas of the world wetter and others drier causing different regions to face increasing floods and droughts, respectively. Figure 2A.1.6 is a world map color coded to show water stress (measured by water usage compared to water availability) by country in 2040 if no changes are made regarding water usage and human impact on climate change. An estimated 3.2 billion people (46% of the global population) live under water stressed conditions for at least 1 month of the year every year. But this water stress is not distributed equally around the globe; around 80% of the water stressed population lives in Asia, dominantly in India, Pakistan, and northern China.​

Map of projected water stress by 2040 as described in the figure caption.
Figure 2A.1.6 World map color coded to show water stress by country in 2040. Dark red countries have extremely high water stress and red countries have high water stress. These are dominantly located through the Middle East, south Asia, China, northern Africa, southern Africa, western South America, Australia, Indonesia, United States, and Mexico. Many of these areas are already water stressed due to very arid climates, and climate change is expected to make these conditions worse. Source: WRI (2015). CC BY-4.0. Found here.

Balancing future water needs for the natural environment and humans will become more challenging as changes to climate, changing demographics (increasing size of urban centers), and changing socioeconomic conditions drive changes in water availability and water use. Ensuring these needs and the needs for agricultural water require sustainability practices to be emplaced.

References

Maddocks, A.; Young, R. S.; and Reig, P. (2015). Ranking the world’s most water stressed countries in 2040. World Resources Institute https://www.wri.org/insights/ranking-worlds-most-water-stressed-countries-2040

US Energy Information Administration (2011). Over half the cooling systems at U.S. electric power plants reuse water. https://www.eia.gov/todayinenergy/detail.php?id=3950

Vanhan et al., (2021). The number of people exposed to water stress in relation to how much water is reserved for the environment: a global modelling study. The Lancet Public Health. 5(11). https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(21)00234-5/fulltext#seccestitle160

 

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