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Potable water treatment
Potable water treatment

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2 The hydrological cycle

2.1 Introduction

The hydrological cycle, the continuous cycling of water between land, open water surfaces and the sea, either directly or indirectly, is an extremely complex process which has been known for a long time (Figure 1). The identifiable mechanisms of the cycle are complicated not only by the characteristics of air-water-land interfaces across which the cycle operates, but also by climatic factors which vary in both time and space. The various operations and mechanisms within the cycle are illustrated in Figure 2 and are described over the next few pages.

Figure 1
Figure 1 Probably the oldest reference to the hydrological cycle, the Chandogya, one of the principal Upanishads, says 'rivers … lead from sea to sea'. It reveals that as early as 1000 BCE, attempts were being made to interpret and explain recurrent phenomena on the basis of direct experience. (Cartoon by Ajit Nunan)
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Figure 2 The hydrological cycle (volumes are in Tm3 = 1012 m3)
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