Of the estimated 1.4 x 10 18 m 3 of water on the Earth, more than 97% is in the oceans. Approximately 35 x 1015 m3 of the Earth’s water is freshwater, of which about 0.3% is held in rivers, lakes, and reservoirs. The remainder of freshwater is stored in glaciers, permanent snow, and groundwater aquifers. The earth’s atmosphere contains about 13 x 1012 m 3 of water, and is the source of all the rain that falls on earth.
Quantity of Evaporation
Yearly, about 151,000 quads (quad = 1015 BTU) of solar energy cause evaporation and move about 577 x 1012 m3 of water from the earth’s surface into the atmosphere. Of this evaporation, 86% is from oceans. Although only 14% of the water evaporation is from land, about 20% (115 x 1012 m3 per year) of the world’s precipitation falls on land with the surplus water returning to the oceans via rivers. Thus, each year solar energy transfers a significant portion of water from oceans to land areas. This aspect of the hydro-logic cycle is vital not only to agriculture but also to human life and natural ecosystems.
Availability of Water
Although water is considered a renewable resource because it depends on rainfall, its availability is finite in terms of the amount available per unit time in any one region. The average precipitation for most continents is about 700 mm/yr (7 million liters/ha/yr), but varies among and within them. In general, a nation is considered water scarce when the availability of water drops below 1,000,000 liters/capita/yr. Thus Africa, despite having an average of 640 mm/yr of rainfall, is relatively arid since its high temperatures and winds that foster rapid evaporation.
Regions that receive low rainfall (less than 500 mm/yr), experience serious water shortages and inadequate crop yields. For example, 9 of the 14 Middle Eastern countries (including Egypt, Jordan, Israel, Syria, Iraq, Iran, and Saudi Arabia) have insufficient rainfall.
Substantial withdrawals from lakes, rivers, groundwater, and reservoirs used to meet the needs of individuals, cities, farms, and industries already stresses the availability of water in some parts of the U.S. When managing water resources, the total agricultural, societal, and environmental system must be considered. Legislation is sometimes required to ensure a fair allocation of water. For example, laws determine the amount of water that must be left in the Pecos River in New Mexico to ensure sufficient water flows into Texas.
Ground Water Resources
Approximately 30% (11 x 1015 m 3) of all freshwater 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. Most groundwater has accumulated over millions of years in vast aquifers located below the surface of the earth. Aquifers are replenished slowly by rainfall, with an average recharge rate that ranges from 0.1% to 3% per year. Assuming an average of 1% recharge rate, only 110 x 1012 m3 of water per year are available for sustainable use worldwide. At present, world groundwater aquifers provide approximately 23% of all water used throughout the world. Irrigation for U.S. agriculture relies heavily upon groundwater, with 65% of irrigation water being pumped from aquifers.
Population growth, increased irrigated agriculture, and other water uses are mining groundwater resources. Specifically, the uncontrolled rate of water withdrawal from aquifers is significantly faster than the natural rate of recharge, causing water tables to fall by more than 30 m in some U.S. regions. The overdraft of global groundwater is estimated to be about 200 x 10 9 m 3 or nearly twice the average recharge rate. For example, the capacity of the U.S. Ogallala aquifer, which underlies parts of Nebraska, S. Dakota, Colorado, Kansas, Oklahoma, New Mexico, and Texas, has decreased 33% since about 1950. Withdrawal from the Ogalla is 3 times faster than its recharge rate. Aquifers are being withdrawn more than 10 times faster than the recharge rate aquifers in parts of Arizona.
Similar problems exist throughout the world. For example, in the agriculturally productive Chenaran Plain in northeastern Iran, the water table has been declining by 2.8 m/year since the late 1990s. Withdrawal in Guanajuato, Mexico, have caused the water table to fall by as much as 3.3 m per year. The rapid depletion of groundwater poses a serious threat to water supplies in world agricultural regions especially for irrigation. Furthermore, when aquifers are mined, the surface soil area is prone to collapse, resulting in an aquifer that cannot be refilled.
Stored Water Resources
In the U.S., many dams were built during the early 20th century in arid regions in an effort to increase the available quantities of water. Although the era of constructing large dams and associated conveyance systems to meet water demand has slowed down in the U.S, dam construction continues in many developing countries worldwide.
Given that the expected life of a dam is 50 years, 85% of U.S. dams will be more than 50 years old by 2020. Prospects for the construction of new dams in the U.S. do not appear encouraging. Over time, the capacity of all dams is reduced as silt accumulates behind them. Estimates are that 1% of the storage capacity of the world’s dams is lost due to silt each year.