The increasing demand of Europeans and environmental organizations for cleaner rivers, lakes, groundwater, and beaches has been evident for a long time. This was confirmed by the result of a representative opinion called “Eurobarometer” which was conducted in all 25 countries – now 27, since January 1, 2007 – of the European Union. The respondents were asked to list five environmental issues that they are worried about, and nearly half of them(47 percent) included water pollution in their lists.
The Water Framework Directive (WFD) 2000/60/EC of the European Parliament and the European Council addresses this issue. This directive ensures that polluted waters will be clean again and that clean waters are kept clean. It also introduces novel tools and instruments to European Union water law as an ecology-based and holistic approach to water status assessment and river basin planning.
The participation of citizens and organizations is crucial in achieving these goals because it is apparent that legislation alone cannot solve the worldwide problem on water pollution. And as part of their participation, citizens should have a better understanding of the importance of water and how human activities affect the water cycle.
SOME FACTS ABOUT WATER
Water is the source of all life, a unique component that cannot be replaced by any chemical compound. It can exist in three forms – as a liquid, solid (ice) and gas (vapor). In its purest state, water is colorless, odorless, and tasteless.
As a chemical compound, water is represented by the chemical formula H2O. This means water is made up of two atoms of hydrogen and one of oxygen.
Approximately three-quarters of the Earth’s surface are covered by water. The oceans, containing 97.5 percent of the Earth’s water, are salty and it is too costly to make this water potable. On the other hand, the land, atmosphere, and the glacier in the North and South poles host 2.4 percent, 0.001 percent, and 2 percent of all the water on earth, respectively. But the fresh water that people could use makes up less than 1 percent of all the water in the world.
The distribution of water on the Earth is quite varied; many locations have plenty of it while others have very little. Oceans, rivers, clouds, and rain, are in a frequent state of change – surface water evaporates, cloud water precipitates, rainfall infiltrates the ground, etc. – but the total amount water on Earth does not change.
The atmosphere holds less than 0.001 percent water, which may seem surprising because water plays an important role in weather. The annual precipitation for the whole world is more than 30 times the atmosphere’s total capacity to hold water. This fact indicates the rapid recycling of water that must occur between the Earth’s surface (sea and land) and the atmosphere.
Seawater covers 70 percent of the Earth’s surface, but it is difficult to comprehend the total amount of water when we only see a small portion of it. The diagram below shows the volumes of water contained on land, in oceans, and in the atmosphere. Arrows indicate the annual exchange of water between these storages. Exchange rates are expressed in quadrillions (10^12) of cubic meters (m3) per year and storage capacities are expressed in quintillions (1015) of cubic meters (m3).
THE HYDROLOGIC CYCLE
The circulation and conservation of Earth’s water is called hydrologic cycle or water cycle. But before we discuss it, let us first discuss the phenomena that contribute in the cycle.
Evaporation. This is the transformation of water from a liquid into a gas, a process that humidifies the atmosphere. Approximately 80 percent of all evaporation comes from the oceans, and the remaining 20 percent coming from inland water and vegetation. Winds transport the evaporated water around the globe, influencing the humidity of the air throughout the world.
Most evaporated water exists as a gas outside the clouds, diluted in the air. Evaporation is more intense when the temperature is warmer. This is why the evaporation occurring over the oceans and near the equator is intensive.
Transpiration. This is the evaporation of water into the atmosphere from the leaves and stems of plants. Plants pump the water up from the soil to deliver nutrients to their leaves. This pumping is driven by the evaporation of water through small pores called stomates, which are found on the undersides of leaves. Transpiration accounts for approximately 10 percent of all evaporating water.
Condensation. Condensation is the change of gas into liquid. It generally occurs in the atmosphere when warm humid air rises, cools, and decreases its capacity to hold water vapor. As a result, excess water vapor condenses to form tiny cloud droplets that stay in the air. Sometimes, if the temperature of the higher air layers is very low, these droplets become ice. In fact, the white lines that airplanes left in the sky are clouds of iced water droplets, produced by the combustion of the plane fuel.
Transport. Advection of transport is the movement of water through the atmosphere. Atmosphere is not a big storage of water. Rather, it functions as an “expressway” for water transportation.
Some of the Earth’s moisture transport is seen as clouds, which consist of ice crystals and/or tiny water droplets. A 1-kilometer thick cloud contains only enough water for a millimeter of rainfall, whereas the amount of moisture in the atmosphere is usually 10 to 50 times greater.
Humid air does not always contain clouds. Most water is transported in the form of water vapor, the third most abundant gas in the atmosphere. Water vapor may be invisible to us, but not to satellites, which are capable of collecting data about the moisture content of the atmosphere, and forming images, of moisture status in the atmosphere.
Precipitation. It is the primary mechanism for the return of the water from the atmosphere to the surface of the Earth. There are several forms of precipitation. and the most common of which is rain. Other forms of precipitation are hail. snow, sleet, and freezing rain.
Amounts of precipitation can vary by location. For example, the desert of Nevada in USA averagely receives less than an inch of total precipitation per year, while tropical rain forests in Hawaii can receive more than 100 inches of precipitation per year.
Amounts of precipitation also vary by year. In 1988, an intense drought gripped the Midwestern United States, disrupting agriculture because there was not enough rain to sustain crops. Five years later in 1993, the same area was flooded.
Runoff and Surface Waters. Runoff is the movement of surface landwater to the sea. Runoff includes the variety of ways by which water moves across the land surface. This includes both surface runoff and channel runoff. As it flows, the water may infiltrate into the ground, evaporate into the air, stored in lakes or reservoirs, or be extracted for agricultural or other human uses. Even the smallest streams are connected to larger rivers that carry billions of liters of water back to the oceans.
Excess runoff can lead to flooding, which occurs when there is too much precipitation. Overland runoff from disturbed areas often contains excessive sediment (see fig. 7).
Groundwater. This is the water that has penetrated the Earth’s surface. It is sometimes thought that groundwater flows through underground rivers. Although this can happen sometimes, groundwater in general is not like surface water that is concentrated in streams and lakes. Rather, it is found underground in the spaces between particles of rock and soil, gravel or sand layers, or in crevices and cracks in rock.
When the entire area below the ground is saturated or the penetration is very slow, flooding occurs because all subsequent precipitation remains on the surface.
Water that infiltrates the soil . flows slowly downward until it encounters impermeable rocks and then travels laterally. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually continue its underwater travel and be discharged directly into the oceans. The flow of groundwater is much slower than runoff, with speeds usually measured in centimeters per day, meters per year, or even centimeters per year.
The water cycle describes the continuous movement of water on, above, and below the surface of the Earth.
The sun drives the water cycle. It heats water in the oceans and on the land surface. Some of the water evaporates as vapor into the air. Ice and snow can sublimate directly into water vapor. Rising air currents take the sea vapor up into the atmosphere, along with water from evapo-transpiration, which is the combination of water transpired from plants and evaporated from the soil. Approximately, 90 percent of atmospheric water is coming from evaporation, while the remaining 10 percent is from transpiration.
The vapor rises into the air where the cooler temperature causes a portion of it to condense into clouds. As winds move vapor and clouds around the globe, cloud droplets collide, grow, and fall out of the sky as precipitation.
The water falls back on the Earth’s surface and due to gravity, the water either enters the ground as groundwater or flows over the ground as surface runoff. Some runoff flows into the rivers, and the streamflow moves water towards the sea, while some runoff and groundwater is stored as freshwater in lakes. Some water also infiltrates deep into the ground and replenishes aquifers (saturated subsurface porous rock), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies as ground-water discharge, and some ground water flows into the openings in the land surface and emerges as freshwater springs. Over time, the water continues to flow to reenter the ocean where the water cycle starts again.
CHANGES OVERTIME
Compared to the amount of water used in the hydrologic cycle, much water is stored for long period of time. The major “storehouses” of water are the oceans, while minor storehouses are the lakes, glaciers, and the underground aquifers.
During colder climatic ages, more ice caps and glaciers are formed, and enough of the global water supply becomes ice. The reverse happens during warm periods. About three million years ago, the oceans could have been up to 50 meters higher. During the last global “warm spell”, about 125,000 years ago, the seas were about 5.5 meters higher than they are now and during the last ice age – it began about 70,000 years ago and ended 10,000 years ago -glaciers covered almost one-third of Earth’s land mass.
These changes were caused by the forces of nature, and were developed very slowly. Human life causes more changes, especially during the last centuries. In fact, human activities such as agriculture, alteration of the chemical composition of the atmosphere, construction of dams, deforestation and afforestation, removal of groundwater from wells (overpumping), water abstraction from rivers, and urbanization, affect the water cycle.
The disturbance of the hydrologic cycle has something to do with the global climate changes, the major threat today. Human activities affect both the quantity and quality of the water, and the causes are overuse of water (extravagance) and pollution.
EFFECTS OF WATER CYCLE ON BIO-GEO-CHEMICAL CYCLING
While the water cycle itself is a biogeo-chemical cycle, the flow of water over and beneath the Earth is essential in the cycling of other bio-geo-chemicals. For example, runoff is responsible for almost all of the transport of eroded sediment, phosphorus and other fertile substances from highlands to lowlands. Even the salinity of the oceans is mainly caused by erosion and transport of dissolved salts from the land. Moreover, the eutrophication of lakes is primarily due to phosphorus, which is excessively applied along with fertilizers to agricultural fields, and then transported by water.
Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies. For instance, the dead zone at the outlet of the Mississippi River is caused by nitrates from fertilizers that water transported from agricultural fields to the river system and then to the Gulf of Mexico. Runoff also plays a part in the carbon cycle, and this is because of the transport of eroded rock and soil.
The oceans and all the other liquid forms of water on Earth are also very good “balancer” of the increasing amount of carbon dioxide (CO2) in the atmosphere, as big quantities of CO2, are absorbed from the atmosphere and diluted in the waters.
to be continued…
