Unit 4

Water Resources

Introduction

The presence of water is a feature that makes Earth unique in the solar system, and makes life as we know it possible.

Water is so common that we take it for granted. It covers nearly three-quarters of the surface of the earth and appears to be an inexhaustible resource, but in fact the amount of water available for human use is a very small fraction of the total.

As shown in Table 1, most water on earth (~97%) is in the oceans, but seawater is not generally feasible as a large-scale water source because the energy required to purify and treat (desalinate) it is too great. Approximately 2% of the water on earth is permanently frozen in glaciers and at the polar ice caps, and not generally available to most humans. So the usable sources, groundwater and rivers and lakes, form less than 1% of the total present on the planet.

In comparison with other global resources such as coal and oil it is perhaps comforting to know that projected water demand is unlikely to exceed potential global supply for a long time. However its importance increases each year, and there are indications that by the year 2000 we will be nearing the limits of the accessible water. Since 1950 water use has tripled and the World-watch Institute, an environmental research organisation in Washington D.C. believes that;

"Water scarcity may be to the nineties what the oil price shocks were to the seventies--a source of international conflicts and major shifts in national economies".

Water resources are unfortunately not evenly distributed across the planet, and those who experience shortages now are likely to be exposed to increased depravation.

Our focus tends to be on the availability of clean drinking water, (not surprising, we can live approximately 1 month without food, but only 1 week without water). However water forms the basis for all life on earth and we rely on it for much more than just drinking. The list below indicates some of the broad areas of water use in the USA.

• Public supply
• Domestic
• Hydroelectric Power
• Livestock
• Irrigation
• Industrial
• Thermoelectric Power
• Mining
• Commercial
• Waste water treatment

Agriculture is the main user of water world-wide (~73%) with industry accounting for another 21% and domestic use the remaining 7%. Irrigation of crops is by far the biggest agricultural use of freshwater but usually highly inefficient. For example in the USA, industry is the largest withdrawer of water but agriculture is the largest consumer, i.e. of the water used in irrigation approximately half is lost to the local system usually through evaporation.