Environmental Engineering Reference
In-Depth Information
Plate 27.7
A termite mound built around a thorn bush, South
Africa. Photo: D. J. Briggs.
2 Seasonal rivers which are dammed to provide adequate storage to survive the dry
season (and perhaps several drier-than-average years if the dam is large).
3 The diversion of water in aqueducts from wetter areas, such as the Los Angeles
aqueduct.
4 Ground water, pumped to the surface artificially, by artesian pressure or through
underground tunnels from the water table.
Let us have a look at these elements in turn to examine how the varying availability of
water can affect the nature of water resources and their exploitation in an area.
RIVERS
Many rivers begin their course in relatively wet mountain areas which provide sufficient
discharge for them to flow into dryland areas. Not all of them reach the sea, as natural
loss or human exploitation depletes the flow until it disappears into its dry river gravels
or swamps, like the Okavango in Botswana, or an internal lake basin such as the Jordan
in Israel.
The use of rivers as a source of water goes back to biblical times or even earlier.
Systems appear to have been developed independently in at least three locations - south-
west Asia by 5500 BC, Peru by 1200 BC and China in 350 BC. The amount of water in
the river would determine how much could be used for agriculture. Variations in levels
would be reflected in variations in crop yield. Figure 27.6 shows how this relationship
