WATER DEFICIT AS A CLIMATIC DISCRIMINANT it 



i.e. it is the difference between water going into the lysimeter- natural and 

 added-and what percolates through. Expressed as an algebraic equation 

 it is: 



VE = R+A-N±S 



where R is rainfall, A is added water, N is percolate, and 5 is the difference 

 in the amount of water stored in the lysimeter. There are various ways in 

 which 5 can be ehminated from the equation, and ^ and Ncan be measured 

 quite accurately, so the main element of doubt is due to uncertainty that 

 R is measured correctly. 



If, however, one considers the PWD or PWS, it is possible to ehminate 

 R, since 



(i) PWD = PE-R 



(ii) PWS=R-PE 



which equations are the same as 



(i) VWD = R+A-N-R 



(ii) VWS =R-R-A+N 



these simplify to 



(i) PWD=^-N 



(ii) PWS =N-A 



so long as A is always kept high enough to ensure that N does not sink to 

 nil-i.e. there must be enough irrigation water to ensure that percolation is 

 maintained, since otherwise it is not possible to allow for variations in 

 storage. 



PWS is potentially run-off, while PWD is an indication of water short- 

 age, or alternatively, of drying power. It is not necessarily the same as the 

 actual water deficit, because the potential evaporation may not be the same 

 as the actual evaporation. The latter (AE) differs from PE by amounts 

 which depend upon many factors, the nature of the soil, the vegetation 

 cover, the land use and treatment, and other differences in the surface. But 

 the PWD is an inescapable climatic factor, and so long as there is a potential 

 water deficit, there must be an actual water deficit of some sort, unless 

 water is supplied from elsewhere. 



PWD and PWS can be assessed according to different unit lengths of 

 time. If this time unit is one year, there will be either a single deficit or a 

 single surplus. To quote a year's surplus would be quite misleading how- 

 ever. In the example shown in Table i , although an annual assessment gives 

 a PWS of 1 0-0 in. an assessment made by calendar months shows this 

 PWS to be made up of a winter PWS of i6-o in. minus a summer PWD of 



