Water resources. Global and local
by
Giuseppe Alonzo
Giuseppe Alonzo
ITAF Dept. – Faculty of Agronomy – University of Palermo, Viale delle Scienze,
90128 Palermo, Italy
Water distribution on Earth
Earth appears from the space as a “blue” planet. The color is due to the water covering about 71% of the whole Earth surface. Water also is present in appreciable amounts under the land surface and as water vapor in the atmosphere. Because of the temperature and pressure ranges available on the Earth, water is encountered in all the three states of the matted and this has been considered of outmost importance for the development of the life on this planet.
The Earth planet is an essentially closed system which means that the matter leaving or entering the system is rather small. Water is also therefore a finite resource which is cleaned and replenished in its different forms through the hydrologic cycle.
The earth has an abundance of water, but unfortunately, only a small percentage, about 0.3%, has characteristics usefull for its use by humans. The other 99.7% is in the oceans, soils, icecaps or present in the atmosphere. Moeover, much of the previously mentioned 0.3% is unattainable. Most of the water used by humans comes from rivers, lakes or the underground provided these met the required quality parameters. The rivers and lakes which are visible bodies of water are referred to as surface water.The majority of fresh water is actually found underground as soil moisture and in aquifers.
Following the U.S. Geological Survey, 1984, the different forms in which the water is found in our planet are the following:
Ocean water: 97.2%
Glaciers and other ice: 2.15%
Ground water: 0.61%
Fresh water lakes: 0.009%
Inland seas: 0.008%
Soil Moisture: 0.005%
Atmosphere: 0.001%
Rivers: 0.0001%
About 300 Km3 per day of water are needed for human uses. Surface water is the easiest to reach and the most common source of water, including potable water. Surface water provides as much as the 80 percent of the water used on a daily basis. Problems, however, exist due to the increasing contamination of the water bodies thus further limiting the amount of water available for human consumption. Water is found in many different forms and in many different places. While the amounts of water that exist seem to be plentiful, the availability of the water for human necessities is limited.
Oceans, which are the largest source of surface water, comprise approximately 97 percent of the earth’s surface water. However, since the oceans have high salinity, the water is not useful as drinking water. Efforts have been made to remove the salt from the water (desalination), but this is a rather costly endeavor. The oceans also play a vital role in the hydrologic cycle, in regulating the global climate, and in providing habitats for thousands of marine species.
Glaciers and icecaps cover 10% of the earth’s surface. Glaciers melting can have a tremendous effect on the sea level. If all of the glaciers were to melt today, the rise of the sea would be about 78 meters.
Ground water is a very plentiful source of fresh water, it must be a protected resource. Once an aquifer is contaminated with chemicals or petroleum, it is difficult, if not impossible, to clean up. Many times, surface water also is in direct contact with underground water and this can create a problem with contamination of the ground water. There is also the problem of saltwater intrusion (present in coastal regions) where over-pumping of the ground water draws the denser saltwater up into the aquifer. So, protection of the ground water should be a high priority as the population on Earth continues to rise and potable water becomes a valuable resource.
The Water Cycle
The hydrologic cycle or more simply the “water cycle” is a pictural representation of how water is recycled through the environment. Water molecules remain constant, though they may change between solid, liquid, and gas forms. Evaporation can occur from water surfaces, land surfaces, and snow fields, into the air as water vapor. Moisture in the air can condensate. Water vapor condenses on tiny particles of dust, smoke, and salt crystals to become part of a cloud. After a while, the water droplets combines with other droplets and fall to Earth in liquid (rain) or solid (for example snow) forms. Once the drop has fallen to Earth, it may go into an aquifer as ground water, or the drop may stay above ground as surface water.
One problem facing the cycle of water on Earth is water contamination. Chemicals that go into the water often are very difficult, if even possible to remove. One potential source of contamination of water is runoff, the overland flow of water. While precipitation causes the runoff to occur, stripping vegetation from land can add to the runoff in a particular area. The sediment and soil from these areas, not to mention any pesticides or fertilizers that are present, are washed into the streams, oceans, and lakes.
What happens to the rain after it falls depends on many factors, such as the intensity and duration of rainfall, the topography of the land, soil conditions, amount of urbanization, and density of vegetation.
Water on Earth is a finite source. Protecting the water means protecting all forms of the water found on Earth. Water at the surface, under the ground, in vapor form, and as precipitation. Pollution from using fossil fuels can impact all forms of water (from crude oil leaks to acid rain generated from coal burning). Acid rain falls onto the land and flows into the surface water, back into the ground, and back into the air. It can be an endless cycle. As contamination infiltrates the water cycle, more water will be impacted. Most of the water on Earth is saline. Fresh water is and will be in demand and become a very valuable resource. Care must be taken to prevent overuse of potable water sources. Care must also be taken to protect the earth’s waters from contamination. Water is indeed a valuable resource.
Water resources in Sicily
A variety of hydro-orographical factors influence the climatic characteristics of Sicily. Recent data confirm that Sicily is going to a semi-arid behaviour characterized by an increasing drought frequency.
The surface of Sicily is 25.708 Km2. The flat land is 14.2%, hills contribute for 61.4% and mountains result to be 24.4%. The overall population is a bit more than 5.000.000 people.
There are no major rivers in Sicily, there are however several minor, of which the Simeto and the Alcantara on the Ionian side of the island are the most interesting. The Salso (South Imeria), Belice and Platani are long rivers but have very low water volumes. The absence of major rivers is one of Sicily's greatest problems; lack of water. The few rivers and lakes that exits are by no means sufficient to sustain the needs of the population mass.
If we consider that at least 50 liters of water, including civic, industrial and agricultural activities, are required per person and per day, following FAO recommendations, it can be easy obtained that 0.25 Km3 of usable water would be required by the whole sicilian people. During one year, the amount of required water would be about 90 Km3.
Meteoric water in sicily contributs for 17.5 Km3 (700 mm medium value for the whole sicilian surface) and, therefore, from rain we get about 20% of what is really needed.
Plants transpire water and thus requires replenishment at rates which are dependent on the plant microclimate (leaf temperature, solar radiation, air humidity, wind speed), the plant age, morphology, health, and the ease at which the water is available within the root zone. In Sicily, as much as 63% of the whole water received from sky is lost through evaporation and evapotranspiration fenomena. Therefore, each year only 6.2 Km3, corresponding to the remaining 37% will be available for uman uses. Considering run-off fenomena and that water fraction becoming underground water, only 2.3 Km3/Year.will be available for human uses and they result sub-divided as: 0.7 for civic uses, 0.4 for industrial uses and 1.2 for the agriculture.
Sicily suffers therefore of an acute water scarcity. On the other hand, one of the characteristic aspects of urban development is the widespread availability of piped water, a sewage system and the resultant production of urban wastewater. Because water is in the short run even more critical than food for human survival, it seems necessary to plan the reuse of urban wastewaters especially during meteorological drought. This rather stable production of urban wastwater should be perceived by planners as a real asset not to be wasted!
This lecture was delivered by the writer during the project meeting in Carini-Sicily-Italy (October 2003)
posted by Stergios Nastopoulos @ 10:23 PM
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