Reed, D.W. 1995b. Rainfall assessments of drought severity and centennial events. Paper to CIWEM Centenary Conference, London, 25-26 October 1995, 17 pp.
The analysis of monthly rainfall data plays an important role in judging drought severity. Whereas water supply problems may be aggravated by operational factors or high demand, calculations of rainfall rarity can offer an independent and objective measure of drought severity.
Some professionals view rainfall rarity assessments with suspicion. Monthly rainfall totals do not fully reflect the impact that drought has on a water resource system; factors such as air temperature and the detailed temporal pattern of rainfall are also influential. However, there are other reasons why rainfall assessments of drought severity enjoy a mixed reputation. First, those typically obtained are intrinsically unsuited to assessing drought impact on a water resource system. Second, the estimates generally err toward overstating the severity of an ongoing drought.
The paper begins by explaining why assessments of drought severity are relevant to the UK water industry. A recent review of existing methods is discussed and me limited value of "Tabony tables" explained.
Centennial droughts and "wets" are defined by reference to a long-term monthly rainfall series for Norwich, and their properties explored for durations from one to 48 months. A new method of assessing drought severity is presented and illustrated for a resource sensitive to 5 to 9-month droughts.
Suggestions are made as to how the technique can be elaborated to provide drought severity assessments for a range of resource systems.