Earth is home to over 1,386,000,000 km3 (the amount of water to fill 565 trillion Olympic sized swimming pools) of precious water. Water is abundant, and can be readily found in all three of its physical states: as ice, liquid, and gaseous water vapor. However, most water occurs as saltwater in the oceans, which cover over 70% of the Earth’s surface. Saltwater cannot be consumed by humans because the salinity causes our body’s cells to dehydrate. The left column in Figure 11 shows the distribution of all water on Earth.
Only 2.5% of the total water supply is fresh water, the form needed to support animal and plant life including essential human activities like drinking, cooking, cleaning, bathing, agriculture, and industry (figure 12).
You will learn in the Water and Ethics section that water is a ‘human right’. You can imagine why this would be the case as you see here how water is essential for life and in short supply.
As you study the middle and left-hand columns in Figure 11 you see that almost 69% of the Earth’s freshwater is frozen in glaciers and ice caps. The remaining 31.4% is found in underground aquifers and surface bodies of water. With the rapid melting of glaciers and polar ice caps, much of the Earth's stored fresh water is melting and mixing with the oceans, making it unavailable for human use.
Other than frozen water stored in glaciers, there are three primary sources of stored liquid freshwater on Earth. The first is surface water, or water found in lakes, wetlands, or rivers. Surface water is created by precipitation collecting in a drainage basin or watershed. The second source is ground water, which collects in the small spaces between gravel and soil or within underground aquifers. Ground water is recharged by both precipitation and the third source of freshwater: under-river flow. Under-river flow is the water moving through the hyporheic zone which lies just beneath the sediment surface of a floodplain or riverbed. Under-river flow is very dynamic. When the ground water table is low, under-river flow recharges it or replenishes water to the water table. When ground resources are completely saturated, water is forced back up through the hyporheic zone and into the river itself, a process called discharge.
In the upcoming Action section you will learn about actions being taken in Indonesia to deal with flooding caused by deforestation.
Clean, accessible freshwater is rare and unevenly distributed across the globe. We have already encountered this problem in the Ganges case study that opened this chapter. The availability of clean water varies widely from place to place for two reasons. The first is natural variation in the hydrologic cycle, which was discussed above. The second is human intervention which will be discussed below. Human activities have greatly impacted the natural distribution of water. These interventions include division of water for agriculture, deforestation, industry, and the burning of fossil fuels. Over time, human activity has also contributed to global climate change, which also greatly alters water distribution. Higher global temperatures cause:
- increased evaporation from the surfaces of oceans and land
- increased precipitation over oceans and land; increased rain intensity
- ocean acidification due to dissolving of carbon dioxide from the atmosphere into H2O
- less snow; less rebuilding of glaciers and high altitude snow pack
- frequent, intense storms; hurricanes, tornadoes, monsoons, typhoons
- ice cap melting; higher sea levels
Of course, human communities have also directly manipulated water flow for socio-economic reasons. Technologies built to ensure reliable water access for human use have changed the paths of rivers, created and depleted surface resources, and drained aquifers, as we have seen with the River Ganges. The following chapter on food systems will thoroughly discuss how modern industrial agriculture has impacted water resources in these ways.