100 THE FUTURE OF ARID LANDS 
water collect in streams and recovering it downstream on suitable 
lands. The former method needs no special study of the runoff. 
It presents only a limited interest because the quantity of water 
reserved by evapotranspiration for dry farming will be increased 
rather insignificantly, and also because in the arid zone, the places 
where rainfall is most abundant are often mountains with poor 
soil. 
The second method is of much greater interest. It permits 
utilization of soils of good quality, on which the complement of 
water, as compared with the rainfall per surface unit watered, is 
important enough to increase the frequency of harvest years. 
It is the method of water spreading. To be applied scientifically, 
it requires a study of the soils, in order to choose those which are 
most likely to give the best agricultural eficiency and the best 
retaining of the water. It is also obviously necessary to be familiar 
with the conditions of runoff. 
Relationship between Annual Runoff and Annual Rainfall 
In humid regions, runoff is mostly dependent on soil saturation. 
Since, during long periods the rainfall is greater than the potential 
evapotranspiration, the runoff is characterized by great regularity. 
It has been studied in numerous countries. Formulas have been 
established for annual average runoff which in general give 
satisfaction in the regions for which they were established. 
Even more general formulas have been established (19) or the 
documentation condensed into curves of very wide application 
(13). 
Let us consider a simple formula: 
I == Aller” 
where R = annual average runoff in meters, 
H = annual average rainfall in meters, 
a = a coefficient greater than 2. 
To apply this formula we must have a map of the precipitation 
of a given watershed in order to divide it into surfaces of equal 
depth of precipitation and to arrive at the total of the partial 
runofts thus computed. 
