Sandro Lanfranco
Department of Environmental Science, GF Abela Junior College, University of Malta, Msida

Introduction
The presence of abundant liquid water on Earth has profoundly affected the geological and biological history of the planet. Water is responsible for weather and much of climate; it has given rise to various landforms; it is an important link in the regulation of the average temperature of the Earth and of the gases in our atmosphere. However, the principal significance of water lies in the fact that it permits biological process to occur. It is both an ingredient as well as a medium for life. It acts as a habitat, as a trap for nutrients that may be required by living organisms, and as a medium within which chemical interactions may occur.
 

Distribution of water resources
The surface area of the Earth is approximately 510 million km². Of this, some 360 million km² , equivalent to 71% of the surface, is covered by water, principally oceans and marginal seas. Much of the water on Earth is stored in the oceans, collectively referred to as the world ocean. It is estimated that the world ocean holds 97% of the total water available on Earth. The remaining 3% is freshwater that is mainly stored in solid form in glaciers. Only a very minor proportion (0.75%) of the total water of the Earth is present as liquid freshwater. Much of this liquid freshwater is maintained in rocks as groundwater whilst the rest is present as water in lakes, ponds, streams, rivers and other similar repositories.
 

The Hydrologic Cycle
Water on or above the surface of the Earth is therefore stored in four principal compartments: oceans, surface waters, groundwater and atmospheric water. Exchange of water between any of these compartments occurs continually and follows a broadly predictable path known as the Hydrologic Cycle (or Water Cycle). The starting point is generally taken to be the world ocean, which is the dominant source of liquid water on the surface of the Earth. Evaporation of water from the surface of the oceans leads to the accumulation of water vapour in the atmosphere. When the concentration of water vapour exceeds the saturation point of the atmosphere (the maximum amount of water vapour that can be held in a mass of air at any one time at a given temperature), this condenses into liquid form and forms clouds. Such water precipitates, generally as rain, although the form may vary depending on temperature. Approximately 90% of the water evaporated from the ocean, falls back to the ocean as rainfall. The remaining 10% falls on land. Once water has entered the terrestrial environment, it may contribute to surface waters (lakes, rivers, ponds etc), it may infiltrate rock and settle as groundwater or it may be absorbed by the soil. Water that contributes to lotic (moving) freshwaters, such as rivers and streams, re-renters the oceans as surface runoff. Water that would have entered lakes or ponds would generally reside for a longer period of time, usually until it evaporates once again and falls back into the ocean as rain. Water that infiltrates rock is generally characterised by a much longer residence time and may not re-renter the ocean compartment for several thousand years. Water that infiltrates soil is returned to the atmosphere by evaporation and through transpiration by vegetation.

 
The World Ocean
The oceans are not distributed uniformly over the surface of the Earth. The Northern Hemisphere is characterised by water coverage of 61% whilst 81% of the Southern Hemisphere is covered by ocean. The volume of the oceans is many times greater than the volume of land above sea level. In fact, the volume of all land above sea level is only 5.6% that of the oceans. The world ocean is subdivided into three major oceans: The Pacific Ocean, Atlantic Ocean and Indian Ocean. Although they are essentially continuous, the bathymetry of the sea floor tends to isolate one basin from another, reducing exchange of water between different oceans. The Pacific Ocean is by far the largest of the three, being nearly as large as the Atlantic ocean and Indian ocean put together. It contains just over half the water in the world ocean, and, because it includes few shallow seas along its margins, it has the greatest average depth (3490m). The Atlantic is a relatively narrow ocean which comprises several marginal seas (Baltic Sea, Caribbean Sea, Mediterranean Sea). The presence of wide continental shelves, reduces the average depth to 3310m, making it the shallowest of the three main oceans. The Indian Ocean is largely confined to the Southern Hemisphere ocean and is the smallest of the three.
Seawater contains several dissolved chemicals which together comprise roughly 3.5% of its weight. The proportion of dissolved salts in the ocean is termed salinity. The average salinity of the world ocean is roughly 35ppt (parts-per-thousand), although this tends to vary from place to place depending on local conditions. The principal mineral present in solution in seawater is sodium chloride (NaCl) contributing 68.1% of the average mineral content. Other minerals include magnesium chloride (MgCl₂) comprising 14.4% and sodium sulphate (Na₂SO₄) with 11.4%. The salt in the ocean may have been derived from chemical weathering of rocks on the continents and by outgassing of chemicals residing in the interior of the Earth through volcanic eruptions.

The water of the world ocean is generally stratified. Differences in the temperature and salinity of water masses generate a three-layered structure that comprises a shallow zone of surface water, a central zone termed a thermocline and a zone of deep water. The surface layer forms due to the warming of surface water by the solar energy. This layer is warmer and less dense than the underlying water. The surface water therefore floats on the heavier water beneath it. Wave action and turbulence from currents ensure that this zone is relatively well-mixed. The depth of this shallow zone varies but is generally in the region of 450m. Its temperature ranges between 21°C and 26°C. Below this shallow zone, temperature decreases sharply with depth. This layer of rapid temperature decline is the thermocline. Below the thermocline, at depths of more than 1500m, lies the deep water, where temperatures are consistently below 4°C.

The water of the oceans is in constant motion producing various forms of currents. At the surface, these are mainly induced by winds (e.g. upwelling) whilst at deeper levels, currents are mainly driven by gravity (e.g. thermohaline circulation).

 
Groundwater
Water that has soaked into rock accumulates in pores and spaces that may be present. This represents the largest reservoir of water that is readily available to humans and is therefore of considerable value. Groundwater is held in porous rocks. Such rock would have sufficient pore space to permit the accumulation and passage of substantial volumes of water. The quantity of water that can be stored depends on the porosity of the rock which refers to the percentage of pore space relative to the total volume of a rock. Rocks may be very porous and still not permit water to move through them. The permeability of a material indicates the ability of water to move between the various pore spaces within it. If the spaces between particles are too small, water will move very slowly or not at all. Clay, for instance, is impermeable. Although its ability to store water is often high, its pore spaces are so small that water is unable to move through it.. Impermeable layers such as clay are called aquicludes or aquitards. Rock strata that are permeable are called aquifers.
 

Management of water resources
Much of the water that is used for human consumption is freshwater and is therefore derived from surface waters and groundwater. Whenever freshwater is utilised, its use may be consumptive or non-consumptive. Water usage that loses water for further use is termed consumptive. Irrigation is one such use. When an area of land is irrigated, the irrigation water is lost to the atmosphere by evaporation and transpiration and cannot be recovered for further use. Water usage in which the water can be recovered for further use are termed non-consumptive. Washing is a non-consumptive use, since the water used for washing may be recovered for further uses (e.g. removal of waste products in domestic or industrial settings).
As is the case with all other resources, water can be overused. Overdrawing of surface waters leads to shortages of supply, when demand exceeds rates of replenishment and may also have ecological consequences. Groundwater too may be overdrawn, resulting in depletion of this resource. However, overexploitation of groundwater can result in the disappearance of some surface waters that may be derived from subterranean sources. Removal of groundwater from aquifers sometimes results in subsidence of land overlying the groundwater reserve and, at coastal sites, may result in the infiltration of seawater into groundwater supplies.