A forward looking definition of drought

Drought is typically defined by measures which characterise the deficit of water relative to normal conditions. Whilst undoubtedly useful for ranking moisture deficits in the historical context, such measures fail to provide much useful information regarding the future impacts of a given shortfall. This paper describes an alternative view which characterises drought in a forward looking sense by defining drought severity as a function of the probability that the water requirements of a given process will be satisfied within some future window of time from the present. These probabilities are fundamentally different from the implied climatological probabilities that underpin the traditional view of drought severity in that they are conditional upon the existing state of drought. The new approach is explored through a case study which investigates the effects of the 1976 drought on the production of winter wheat in the UK.