The Water Cycle

The Earth is called the blue planet because water covers 70 percent of Earth’s surface. The oceans contain 97.5 percent of Earth’s water; land holds 2.4 percent and the atmosphere holds less than .001 percent. The amount of water available on the earth has remained constant over time, with the same water used over and over. Water can be lost from an area, but it is not lost from the Earth; it has simply moved. The water cycle, also known as the hydrologic cycle, describes the movement of water on, in and above the surface of the Earth. Figure 1.1 shows components of the water cycle. Water is an essential resource. The abundance or scarcity of water in any given location affects the quantity and quality of life since water is critical for its existence.

While the Earth’s overall water budget doesn’t change, the form water takes (water vapor, liquid or solid) and the location of available water may change. The annual precipitation rate for Earth is over 30 times greater than the atmosphere’s capacity to hold it. For example, huge amounts of water evaporated from oceans may fall onto land as rain or snow, which can re-enter the ocean system through runoff carried in streams and rivers. While water in its various forms may be in flux, the overall amount of water on Earth (or water budget) remains the same.

The balance of the water budget is determined by the distribution of the water [1]. Water income is a gain of water from precipitation to surface and ground water sources. Water losses occur through evaporation and transpiration. Water incomes and losses are influenced by soil characteristics, topography of land, lake basins and river systems (how the watershed functions), seasons and climate.

Water is stored in the form of ice in glaciers or as liquid in groundwater, lakes and oceans. Current discussions in the scientific community are concerned about the losses of water in glaciers and gains of liquid water causing flooding rivers and increased ocean levels that may affect landforms and coastal communities worldwide. There is also concern about record-breaking precipitation rates causing flooding, while other areas may experience increased drought. Although high volume rain events may cause flooding, the water table (top of the unconfined aquifer) may still remain low because of storm water runoff to surface waters (rivers, lakes and oceans) without significant infiltration or groundwater recharge. The water cycle as we know it is changing and difficult to predict.

Public water uses can impact an aquifer. Though the Great Lakes are the largest freshwater system on Earth, a new basin-wide water availability assessment by the U.S. Geological Survey states that the basin has the potential for local shortages. For example, groundwater pumping has had relatively little effect on water in the basin as a whole; however, pumping in the Chicago and Milwaukee areas has caused local groundwater drawdown of as much as 1,000 feet. Moreover, if pumping were to increase as anticipated in the region, water levels in these areas are estimated to decline an additional 100 feet by 2040 [3].

Irrigation use also may lower the water table. One example of water table decline is in large portions of the Ogallala aquifer, which extends from Texas to South Dakota. Some areas of this aquifer have experienced water table decline by over 40 feet [2].

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