Text 1.3 Water Cycle

abundance	изобилие, избыток
province	область, провинция (геологическая, климатическая)
evaporation	испарение
precipitation	выпадение осадков, осадки
transpiration	активное испарение воды листьями растений, транспирация
transportation	транспорт, перенос
runoff	сток
detention	удержание, задержка
polar caps	полярные шапки
glacier	ледник
fresh water	пресная вода
uneven	неровный, неуравновешенный
reverse	обратное, противоположное (чему-либо)
aloft	наверху, в воздухе
turnover	оборот, кругооборот

Water cycle

About 1.4 billion cubic kilometers (1.4xl0⁹ Gigatons) of water in liquid and frozen form make up the oceans, lakes, streams, glaciers, and groundwater. The oceans and seas cover roughly 71 percent of the Earth's surface, with an average depth of 3,795 meters. An abundance of water in liquid form makes the Earth unique; no other planet in the solar system has this feature.

Water is the medium of life: without the cycling of water biogeochemical cycles could not exist, ecosystems could not function and life could not be maintained. Life began in the salty oceans and steadily developed there for billions of years in spite of the salt. But the sea dwellers had never utilized the salty water they lived in because nature had fitted them with a remarkable "de-salting apparatus" based on the capability of biologic diaphragms or membranes to let pure water get through but to hold back the salty substances. This natural device is being studied today as a promising way to make drinking water from the salty oceans for human needs.

Water is important for living things for several reasons:

it is the medium by which nutrients are introduced into plants;

all of the hundreds of thousands of chemical reactions that go on within the living beings to sustain life must be carried on in water solutions;

it is an important part of living tissue, either as liquid water or as part of essential organic molecules (relatively little water found in living tissues is chemically bound; living organisms are nearly 71% of H₂0, the human body itself is three fourth water);

it serves as a means of thermal regulation for both plants and animals.

The following terms are used to define the ways in which water is moved or stored within the province of the land, oceans, or atmosphere:

EVAPORATION: release of water as vapor into the atmosphere;

precipitation: release of water from the atmosphere as rain or snow;

TRANSPIRATION: evaporative water loss from leaves of plants (the most transpiration takes place during the day);

transportation: movement of water with winds or as surface runoff;

detention: temporary storage of water on land or in oceans.

Over many millions of years, the amount of water on Earth has remained the same. Almost 95% of total water on Earth is chemically bound in rocks and does not cycle. Of the remainder, about 97.3% is salt water contained in oceans, about 2.1% exists as ice in the polar caps and permanent glaciers. Fresh water in lakes and streams makes up 0.036 percent, while groundwater is 10 times more abundant at 0.365 percent. The amount of water vapor in the atmosphere is sufficient to cover the entire globe to a depth of 2.5 cm. In the atmosphere at sea level, water vapor is present (by volume) in 0.3% in tropical regions and in 2xl0"⁵ % in Antarctic region. Each of the above is considered to be a reservoir of water. Water continuously circulates between these reservoirs in what is called the hydrologic cycle.

The hydrologic cycle is driven by solar energy and gravity. Evaporation, precipitation, movement of the atmosphere, and the downhill flow of river water, glaciers, and groundwater keep water in motion between the reservoirs and maintain the hydrologic cycle. Water is returned continually to the atmosphere by evaporation from land areas and from the surface of oceans, lakes, and streams. Large amounts are also transpired by passing up the root system of plants and then through the leaves into the atmosphere. This system of circulation brings water back to land and oceans, after it is released from the atmosphere as precipitation - either rain or snow.

The rate of cycling between surface and atmosphere is very rapid. Water molecules do not stay aloft for more than ten days, on average, so the turnover rate is rapid for the airborne part of the cycle. Water released as precipitation remains on land for about 10 to 120 days, depending on the season and where it falls. Then some of it evaporates or is transpired by plants or is carried by rivers and streams to seas and the cycle begins again.

Plants actively recycle the rainwater back to the atmosphere. The total power of plant transpiration is 3xl0³TW (1 Terawatt = 10¹² Watt). This power is about thirty times more than the total power of photosynthesis in the biosphere, and it is of the same order of magnitude as wind power or power of oceanic waves. Net gain of water from Oceans to Land approximately equals the surface runoff and makes about 1/3 of the total water cycle over the land. Practically 2/3 of the cycle provides plants transpiration (water flow evaporated from the Earth's surface, lakes and rivers is negligibly small compared with the flow of transpiration). In tropical forests up to 90 percent of incoming solar radiation goes to transpiration, a process so efficient that ecologists refer to tropical forests as "rain machines". With deforestation, this vigorous recycling of water will weaken and could lead to dramatic climate change.