WATER DEFICIT AS A CLIMATIC DISCRIMINANT 17 



example, it has been shown by Alvin (i960) that moisture stress is an essen- 

 tial requirement for flowering of the coffee plant; it is no good irrigating a 

 coffee crop unless it has been allowed to experience a period of water 

 shortage. If there is continual water surplus, rank vegetative growth 

 appears to be encouraged, but maturation is inhibited. It is but common- 

 sense to expect that a crop which consists largely of water, such as a vege- 

 table marrow, will not be able to develop unless there is, as in Britain in 

 1958 or i960, a plentiful supply of water when the fruit is enlarging. But 

 there could very well have been no flowering, and thus no fruit to enlarge 

 at all, unless the period of plentiful water had been preceded by a period of 

 water stress. 



Turning from flora to fauna, it has been found that many invertebrates 

 exhibit variations in movement, and in population density, according to 

 whether or not there has been a moisture deficit in the soil or other habitat. 

 Thus Pearce, working in the Moor House NatureReserve in Westmorland, 

 found that Enchytraeid worms move upwards in the soil under conditions 

 of PWS and downwards in periods of PWD. 



If one accepts that the chmatic threshold described in this paper has a 

 vahd close correlation with the vegetational and agricultural features as 

 suggested, one can speculate about secular changes. It would require only a 

 very small change in 'total chmate'-i.e. the results of secular changes in 

 atmospheric circulation- to throw a place from one side of the threshold to 

 another. An attempt has been made at an individual station in Scotland, to 

 estimate monthly PWD in retrospect, back to the middle of last century. 

 Since this had to be done by subtracting measured rainfall from PE computed 

 by formula -both, values being suspect and valid only relatively -one could 

 not place very great confidence in the result. It is significant that no obvious 

 general trend was discernible over this period (though quite incidentally it 

 may be remarked that the individual years 1955 and 1959 had the highest 

 values of PWD in the whole 104 years). Indirect evidence exists however 

 that significant changes in PWD could be found over the preceding cen- 

 centuries and these can be correlated with existing knowledge of the 

 changes in vegetation and agricultural practices at places near the thres- 

 hold. 



Human interference has been referred to above, when it takes the form 

 of deUberate improvement of drainage for agricultural purposes, but 

 climatic bog can be affected by indirect improvement of drainage. Induce- 

 ment of increased grazing by animals, and moor-burning for various 

 purposes, will alter the surface sufficiently to intensify the natural effects of 

 short periods of PWD. The combined effects of a slow natural secular 



