Water resources yield assessment 1751 Definitions of yield

The yield is a generic term for the reliable supply of water from various sources or groups of sources. For example, the yield of a reservoir can be defined in terms of the

Fig. 17.7 Geographical variation in relative security of public water supplies in England and Wales expressed in terms of supply as a percentage of demand (Environment Agency, 2008, reproduced with permission).

volume of water regularly available over a unit period of time; it is often expressed in units of millions of litres (megalitres) per day (ML day-1). Techniques for reservoir yield analysis were mentioned in Section 11.12 and are covered in detail by McMahon and Adeloye (2005).

In studying the yield of a reservoired catchment area, it is important to appreciate more fully the implications of the term with respect to the variabilities in the catchment runoff. A definition of yield suggested by the Institution of Water Engineers and modified by Law (1955) is as follows:

The uniform rate at which water can be drawn from the reservoir throughout a dry period of specified severity without depleting the contents to such an extent that withdrawal at that rate is no longer feasible.

Thus given a fixed storage capacity in the reservoir, over a drought period of greater severity than that of the design, the regular amount of water available for supply, the yield, would have to be reduced. The occurrence of any such drought governs the yield for a given storage. Alternatively, if a constant yield is required beyond the capabilities of a reservoir designed to fulfil requirements over a drought of specified severity and duration, the capacity of the storage would need to be increased to meet the demand. To ensure adequate storage for a constant yield or to evaluate the yield of an existing reservoir, the study of low flows is therefore fundamental.

In water resources planning, the deployable output is a more specific assessment of yield that can be sustained during a dry year from a commissioned water source, or group of sources, subject to constraint by abstraction licences, required water quality standards and the capacity of the water treatment and supply systems.

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