Water Resources: Water Volume and Cycle
Water serves a range of productive, environmental and social purposes in the agricultural sector and wider economy. Governments, water managers and consumers/users have a role to ensure that mechanisms and actions are in place to make certain that water is allocated and used to achieve socially and economically beneficial and efficient outcomes in a manner that is environmentally effective and sustainable.
But management of water resources is being severely tested with rising food and energy prices, growing competition for water resources between different users, an expanding global population, and concerns related to climate change.
The anticipated growth in world population from 7 billion currently to 9 billion by 2050, will involve a major expansion in demand for water, not only for use in agriculture but for drinking, sanitation, industry, the energy sector, as well as to meet demands for environmental improvements of ecosystems and associated recreational and cultural uses.
Water Volume and Cycle
An estimated 1.4 × 1018 cubic meters (m3) of water on Earth, is more than 97% is in the oceans (Shiklomanov and Rodda 2003).
Approximately 35 × 1015m3 of Earth‘s water is fresh water, of which about 0.3% is held in rivers, lakes, and reservoirs (Shiklomanov and Rodda 2003). The remainder of the fresh water is stored in glaciers, permanent snow, and groundwater aquifers.
Earth‘s atmosphere contains about 13 × 1012m3 of water and is the source of all the rain that falls on Earth (Shiklomanov and Rodda 2003).
Yearly, about 151,000 quads (159,300 exajoules) of solar energy cause evaporation that moves about 577 × 1012 m3 of water from Earth‘s surface into the atmosphere. Of this evaporation, 86% is from the oceans (Shiklomanov 1993).
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Although only 14% of the water evaporation is from land, about 20% (115 × 1012 m3 per year) of the world‘s precipitation falls on land, with the surplus water returning to the oceans through rivers (Shiklomanov 1993).
Thus, each year, solar energy transfers a significant portion of water from oceans to land areas. This aspect of the hydrologic (water) cycle is vital not only to agriculture but also to human life and natural ecosystems (Pimentel et al. 2004a).
Ground water Resources
Approximately 30% (11 × 1015 m3) of all fresh water on Earth is stored as groundwater. The amount of water held as groundwater is more than 100 times the amount collected in rivers and lakes (Shiklomanov and Rodda 2003).
Most groundwater has accumulated over millions of years in vast aquifers located below the earth‘s surface. Aquifers are replenished slowly by rainfall, with an average recharge rate that ranges from 0.1% to 3% per year (Pimentel et al. 2004a).
Assuming an average recharge rate of 1%, this leaves only 11 × 1013 m3 of water per year available for sustainable use worldwide. World groundwater aquifers provide approximately 23% of the water used throughout the world (USGS 2003a).
Water Availability
Although water is considered a renewable resource because it is replenished by rainfall, its availability is finite in terms of the amount available per unit of time in any one region.
The average precipitation for most continents is about 700 millimeters (mm) per year (7 million liters [L] per hectare [ha] per year), but this amount varies among and within continents (Shiklomanov and Rodda 2003).
In general, water in a nation is considered scarce when its availability drops below 1 million L per capita per year (Engleman and LeRoy 1993).
Thus, Africa is relatively arid, despite its average rainfall of 640 mm per year, because its high temperatures and winds foster rapid evaporation (Pimentel et al. 2004a).
Regions that receive low rainfall (less than 500 mm per year) experience serious water shortages and inadequate crop yields. For example, 9 of the 14 Middle Eastern countries (including Egypt, Jordan, Saudia Arabia, Israel, Syria, Iraq, and Iran) have insufficient fresh water (Myers and Kent 2001, UNEP 2003).
When managing water resources, the total agricultural, societal, and environmental system must be considered (Pimentel et al. 2004b)
Uses of Water
Some researchers have described the green and blue concept of water.
― Green water is used to denote effective rainfall or soil moisture that is used directly by plants, while blue water‖ denotes water in rivers, lakes, aquifers, or reservoirs.
― Blue water generally refers to water that can be delivered for irrigation or made available for alternative uses, while green water must be used directly from the soil profile.
Other uses of water are shown in Figure 1. The blue-green concept has helped increase public awareness of an important dimension of water resource management. The terms green water and blue water generate easily recallable images of soil moisture and stored surface water in a manner that is likely to be helpful to many public officials and agency staff members.
Figure 1: Uses of water. Source: Adapted from Moran and Dann (2008).
In summary, green water is used to denote effective rainfall or soil moisture that is used directly by plants. Blue water denotes water in rivers, lakes, aquifers, or reservoirs.
The anticipated growth in world population from 7 billion to 9 billion by 2050, will involve a major expansion in demand for water.
Approximately 30% (11 × 1015 m3) of all fresh water on Earth is stored as groundwater. The amount of water held as groundwater is more than 100 times the amount collected in rivers and lakes (Shiklomanov and Rodda 2003).
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Approximately 35 × 1015m3 of Earth‘s water is fresh water, of which about 0.3% is held in rivers, lakes, and reservoirs (Shiklomanov and Rodda 2003). The remainder of the fresh water is stored in glaciers, permanent snow, and groundwater aquifers.
