(groundwater). An important aspect of the water systems is an aquifer which is a water-bearing (water-rich) subsurface formation (a subsurface zone) that yields water to wells. An aquifer may be porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. Aquifers are important reservoirs storing large amounts of water which, in theory, should be relatively free from evaporation loss or pollution. However, in practice, this is not always the case.
The aquasphere consists of a variety of non-oceanic water systems (Table A-23) that are essential to life, and all are interrelated, and the many interactions between the systems of the Earth are complex, and they are occurring constantly and simultaneously although the effects are not always obvious.
Table A-23 Sources of non-oceanic water and water systems.
| Source | Description |
|---|---|
| Surface water | Water in a river, lake or fresh water wetland; the water is naturally replenished by precipitation and naturally lost through discharge to the oceans, evaporation, evapotranspiration, and groundwater recharge. |
| Groundwater | Fresh water located in the subsurface pore space of soil and rocks; also water that is flowing within aquifers below the water Table. |
| Aquifer water | Fresh water in a layer of sediment or rock that is highly permeable; usually water in a layer of sand and gravel that have high permeability. |
| Unconfined aquifer | An aquifer that is overlaid by permeable earth materials and which is recharged by water seeping down from above in the form of rainfall and snow melt. |
| Confined aquifer | An aquifer which is sandwiched between two impermeable layers of rock or sediments and are recharged only in those areas where the aquifer intersects the land surface. |
The oceans play a key role in the water cycle insofar as the oceans hold 97% v/v of the total water on the Earth and 78% v/v of the global precipitation occurs over the oceans, and it is the source of 86% v/v of global evaporation. Besides affecting the amount of atmospheric water vapor and hence rainfall, evaporation from the sea surface is important in the movement of heat in the climate system. Water evaporates from the surface of the ocean, mostly in warm, cloud-free subtropical seas. This continuing event cools the surface of the ocean, and the large amount of heat absorbed the ocean partially buffers the greenhouse effect from increasing carbon dioxide and other gases. Water vapor carried by the atmosphere condenses as clouds and falls as rain, mostly in the intertropical convergence zone (ITCZ), far from where it evaporated. Condensing water vapor releases latent heat which drives much of the atmospheric circulation in the tropics. This latent heat release is an important part of the heat balance of the Earth, and it couples the energy and water cycles of the Earth.
The intertropical convergence zone, known by sailors as the doldrums or the calms because of the monotonous, windless weather, is the area where the northeast and southeast trade winds converge. The zone encircles Earth near the thermal equator, although the specific position of the zone can vary on a seasonal basis. When the zone lies near to the geographic equator. it is referred to as the near-equatorial trough. When the intertropical convergence zone is drawn into and merges with a monsoonal circulation, the zone is sometimes referred to as a monsoon, a usage that is more common in Australia and parts of Asia.
The major physical components of the global water cycle include the evaporation from the ocean and land surfaces, the transport of water vapor by the atmosphere, precipitation onto the ocean and land surfaces, the net atmospheric transport of water from land areas to ocean, and the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes.
On land, the situation is more complex, and includes the deposition of rain and snow on land; water flow in runoff; infiltration of water into the soil and groundwater; storage of water in soil, lakes and streams, and groundwater; polar and glacial ice; and use of water in vegetation and human activities. Processes labeled include precipitation, condensation, evaporation, evapotranspiration (from tree into atmosphere), radiative exchange, surface runoff, ground water and stream flow, infiltration, percolation, and soil moisture. Furthermore, in the water systems (particularly in the lakes) the term eutrophication becomes important. Eutrophication is the deterioration of the esthetic and lifesupporting qualities of lakes and estuaries, caused by excessive fertilization from effluents high in phosphorus, nitrogen, and organic growth substances. Algae and aquatic plants become excessive, and when they decompose, a sequence of objectional features arises.
Water for human consumption (and, in many cases as on farms for animal consumption) from such lakes must be filtered and treated. Diversions of sewage, better utilization of manure, erosion control, improved sewage treatment, and harvesting of the surplus aquatic crops alleviate the symptoms.
There are some extremely dramatic examples of Earth systems interacting, such as volcanic eruptions and ocean tsunamis, but there are also slow, nearly undetectable changes that alter ocean chemistry, the content of the atmosphere, and the microbial biodiversity in soil. Each part of the Earth, from inner core to the top of the atmosphere, has a role in making Earth suitable for the existence of billions of lifeforms (1 billion = 1 x 109). Within the aquasphere, the phenomenon known as (i) precipitation, (ii) evaporation, (iii) freezing, (iv) melting, and (v) condensation are part of the hydrological cycle – also known as the water cycle – which is a continuous global process of water circulation from clouds to land, to the ocean, and back to the clouds.
The water cycle describes the means by which water evaporates from the surface of the Earth, rises into the atmosphere, cools, condenses to form clouds, and falls again to the surface as precipitation. This system of the cycling of water is intimately linked with energy exchanges among the atmosphere, the ocean, and land that determine the climate of the Earth and cause much of natural climate variability. For example, approximately 75% of the energy (or heat) in the global atmosphere is transferred through the evaporation of water from the surface of the Earth. On land, water evaporates from the ground, mainly from soils, plants (through transpiration), lakes, and streams. In fact, approximately 15% v/v of the water entering the atmosphere is from evaporation from the land surfaces and evapotranspiration from plants which (i) cools the surface of the Earth, (ii) cools the lower atmosphere, and (iii) provides water to the atmosphere to form clouds.
The major physical components of the global water cycle include (i) the evaporation from the ocean and land surfaces, (ii) the transport of water vapor by the atmosphere and precipitation onto the ocean and land surfaces, (iii) the net atmospheric transport of water from land areas to ocean, and (iv) the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes.
Water pollution has become a widespread phenomenon and has been known for centuries, particularly the pollution of rivers and groundwater. By way of example, in ancient time up to the early part of the 20th century, many cities deposited waste into the nearby river or even into the ocean. It is only very recently (because of serious concerns for the condition of the environment) that an understanding of the behavior and fate of chemicals, which are discharged to the aquatic environment as a result of these activities, is essential to the control of water pollution. In rivers, the basic physical movement of pollutant molecules is the result of advection, but superimposed upon this are the effects of dispersion and mixing with tributaries and other discharges. Some of the chemicals discharged are relatively inert, so their concentration changes only due to advection, dispersion, and mixing.