ABSTRACT
The total volume of water in the world remains constant. What changes is its quality and availability. Water is constantly being recycled, a system known as the water or hydrological cycle. Hydrologists study the chemical and physical nature of water, and its movement both on and below the ground. In terms of total volume, 97.5% of the world ’ s water is saline with 99.99% of this found in the oceans, and the remainder making up the salt lakes. This means that only 2.5% of the volume of water in the world is actually non-saline. However, not all of this freshwater is readily available for use by humans. Some 75% of this freshwater is locked up as ice caps and glaciers, although this is slowly changing in response to climate change, with a further 24% located under ground as groundwater, which means that less than 1% of the total freshwater is found in lakes, rivers and the soil ( Table 1.1 ). Only 0.01% of the world ’ s water budget is present in lakes and rivers, with another 0.01% present as soil moisture but unavailable to humans for supply. So while there appear to be lots of water about, there is in reality very little which is readily available for use by humans. Within the hydrological cycle, the water is constantly moving, driven by solar energy. The sun causes evaporation from the oceans, which forms clouds and precipitation (rainfall). Evaporation also occurs from lakes, rivers and the soil; with plants contributing signifi cant quantities of water by evapotranspiration. While about 80% of precipitation falls back into the oceans, the remainder falls onto land. It is this water that replenishes the soil and groundwater, feeds the streams and lakes, and provides all the water needed by plants, animals and, of
course, humans ( Fig. 1.1 ). The cycle is a continuous one and so water is a renewable resource ( Franks, 1987 ). In essence, the more it rains, the greater the fl ow in the rivers and the higher the water table rises as the underground storage areas (i.e. the aquifers) fi ll with water as it percolates downwards into the earth. Water supplies depend on rain, so when the amount of rain decreases, the volume of water available for supply will decrease, and in cases of severe drought will fall to nothing. Therefore, in order to provide suffi cient water for supply all year round, careful management of resources is required.
