Nature manages her water resources extremely efficiently with techniques for self-purification and for managing on a finite budget by recirculation of moisture. The recirculation technique is known as the hydrologie cycle, which is best explained diagrammatically (Figure 4.1). About 97 per cent of Earth’s water is stored in the oceans in a saline form. Almost the entire amount of fresh water is locked frozen in Antarctica and Greenland. A significant amount of the remainder lies at a considerable depth in the subsurface. The water that is normally available to us comes from the atmosphere, the land surface and the shallow subsurface; and constitutes an extremely small part of the total water inventory. Technologically it is feasible to acquire, process and utilize water from the oceans, ice sheets and deep underground, but the cost makes such efforts uneconomic. One of the favourite expressions in hydrology is that people expect water to be cheaper than dirt.
Part of the precipitation is interrupted by the vegetation, and is either evaporated back to the atmosphere or runs down the plant stems or trunks to reach the ground. The water that reaches the ground surface enters into the soil (infiltration), or fills small (1-2 cm deep) surface depressions and, if the rain is intense, starts to flow on the surface (surface runoff) to rills and gullies, and ultimately to larger streams. The water that has infiltrated into the soil fills the smaller pores in it, and drains downwards by gravity (gravity drainage) via the larger pores to the groundwater below. Most of the water stored in the soil (soil moisture) is available to plants via their root systems. The groundwater table is raised by gravity drainage, which accelerates the lateral flow of the