12 F. H.W.GREEN 



6-0 in. Even a month is too short a unit for considering a water budget, and, 

 from the point of view of vegetation requirements, the unit of time should 

 be short enough for the plants concerned not to suffer from water shortage. 

 In an average soil (with average water retention properties) in a temperate 

 climate, all but extremely shallow-rooted plants will be able to endure lack 

 of rain for up to about five days. Table i thus gives assessments made, not 



Table i 

 Potential water deficit and potential water surplus at Achnagoichan, 1959 



Calculated by units of one calendar month: 



PWD — — — — 2-1 0-3 0-4 1-3 1-9 — — — 6-0 

 PWS 2-7 0-8 0-6 1-2 — — — — — 3-5 3-9 3-3 i6-o 



Calculated by wet and dry periods of five days or more: 



PWD — — 0-2 o-i 2-1 0-3 1-6 1-3 1-9 0-5 — — 8-o 

 PWS 2-7 0-8 0-8 1-3 — — 1-2 — — 4-0 3-9 3-3 i8-o 



(Values in inches.) 



only by whole calendar months, but by calendar months divided into wet 

 and dry spells, the dry spells being continuous periods of five days or more 

 with no rainfall (and/or rainfall far less than the PE over the same period). 

 Even this does not allow for isolated heavy thunderstorm downpours, 

 which may cause flash floods because the rate of fall is greater than the rate 

 of penetration into the soil. 



It will be seen in the example that some calendar months can, in total, 

 have both a surplus and a deficit. If the rainfall were distributed evenly 

 throughout the month, there could be only one or the other. If the rainfall 

 distribution is very uneven, there can be both, because, for instance, the 

 surplus at one time will have disappeared as run-off before a dry period 

 commences, and will not be available, necessarily, to make up for the deficit 

 in that period. 



Let us consider first what happens at a site where no period of PWD 

 occurs. Rain is always falling at a greater rate than it can be removed, either 

 by evaporation, run-off, or deep percolation, and this is virtually the case 



