118 THE ROYAL SOCIETY OF CANADA 
Tucson, Arizona, a rainfall of less than -15 of an inch does not affect 
the soil moisture. As an example of the amount of rain required 
to saturate the soil after a considerable period of drought it may be 
stated that at Ottawa rain fell for fourteen hours on the 4th of August, 
1915. Altogether one inch of rain fell, but it only penetrated into the 
soil to a depth of seven inches. Below this depth the soil was quite 
dry and hard. Some rainfall figures are given in Table VIII. 
TABLE VIII. 
RAINFALL AND SNOWFALL, 1909-12 





Rain Rain Snow Snow 
Nov.—Apr. | May.—Oct. | Nov.—Apr. May.—Oct 
Inches. | Inches. Inches. Inches. 
Clayoquotemia sy NE 70-83 31-24 7-6 0 
WATIEOUIVEEEN carci Re AT 5-95: 4! 17-68 32-4 0 
Glacier ee Meare aad ae oe 18 100 18-60 435-2 20:7 
Fort Vermilion 1910-12... 43° | 6-54 32-2 2-7 
OwAppelle $25, min et -89 | 12-72 49-6 12-8 
Toronto ERA ru 8-91 | 16-88 64-9 +5 
Haas ete me nr 24-56 | 24-14 79-6 -2 




Not only should the total rainfall for the two halves of the year 
be given for any particular locality but its duration in time should 
also be known. As rain washes the pollen of many flowers on to the 
ground or in many cases affects its vitality injuriously, the duration 
of the rainfall is important from this point of view. In fact very 
little pollination either by wind or insects can take place while rain 
is falling. The usual method of expressing the number of days on 
which one hundredth of an inch or more of rain falls is not satisfactory 
as thisamount isso small as tohave very little effect. Probably a better 
way would be to measure the duration of the rainfall in hours instead 
of days. While the moisture requirements of different species of 
plants vary greatly it may be safely assumed that the amount of rain- 
fall should be at least equal to the evaporation from the surface of 
water. Using the five-day period a fair estimate of the distribution of 
the rainfall during the season of growth may be made by adding 
up the number of five-day periods with a rainfall of half an inch 
or more. These figures are given in Table IX. 
