POTENTIAL WATER DEFICIT AS A 

 CLIMATIC DISCRIMINANT 



F. H.W.Green 

 Nature Conservancy 



This paper consists of two parts. In the first I will explain what is meant by 

 'Potential Water Deficit', and in the second I will suggest how useful the 

 concept can be to ecologists as a chmatic discriminant. 



It may be stated simply that Potential Water Deficit or Potential Water 

 Surplus (PWD and PWS, respectively, for short) is the difference between 

 rainfall (and/or snowfall) and potential evaporation; it is a deficit or surplus 

 of water according to which of these quantities is the greater. One need not 

 at the moment say much about rainfall measurement beyond pointing out 

 a few of the known difficulties in measurement. For instance, 'rainfall' is 

 not always rainfall; it may be precipitation of water in other forms, e.g. 

 snow, hail, or ice pellets, all of which it is normally attempted to measure 

 in rain-gauges. Then there is so-called 'occult precipitation', which includes 

 dewfall and condensation from fog. Again, what is measured in a standard 

 rain-gauge may not be quite the same amount as in fact falls on the 

 neighbouring ground. Those difficulties will be referred to again later. For 

 the moment it may first be said that in fact, in this as in most countries, an 

 attempt is made to measure rainfall under certain 'standard' conditions, 

 which are inevitably arbitrary. 



It needs now to be explained what is meant by potential evaporation (or 

 evapo-transpiration). Broadly, it is the amount of water which would be 

 evaporated if there were always water available to be evaporated. It thus 

 may differ from actual evaporation which is dependent upon availabiUty of 

 water. Measurement of potential evaporation (or 'PE') presents difficulties, 

 so that, since it is related, like air temperature, to incoming radiation (direct 

 and indirect), it has commonly been computed, by various formulae, from 

 other chmatic elements which are widely measured. It is, however, possible 

 to measure PE by means of irrigated lysimeters, provided various precau- 

 tions are taken, and certain 'standard' conditions are accepted. Then, as 

 with rainfall, comparison can, with caution, be made between values at 

 different places. 



The measurement of PE by lysimeter involves the remainder method, 



