Notes

1 Soil water here is defined as the water intercepted by the root systems of plants and crops. It is the water that exists in soil profiles after a period of rainfall. It is not the water provided by an irrigation system from surface or groundwater resources. This rain-fed soil water is extremely difficult to quantify. Soil water is, however, very important globally as most of the agricultural production of the world, and almost all the forest products, are raised with soil water. Engineers are not comfortable with the concept as soil water cannot be pumped. They are also unhappy with attempts to quantify soil water as it normally moves downwards in response to gravity. The significant soil water is that intercepted by plants and crops. Economists are disposed to ignore soil water as it is even more difficult to value than to quantify. For this chapter it has been estimated that soil water available annually in the 18 economies considered averages to about 40 billion cubic metre. This volume is about 30% bigger than the renewable groundwater available (28 billion cubic metre) in these economies.

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