PROBLEMS AND RESULTS IN ISRAEL 403 
The first problem in the utilization of rain water in the desert 
is how to deal with desert floods and transform them from a 
destructive force into a creative agent. Even though the rainfall 
in our deserts is very scant, the limited percolation of water in our 
desert soils, and the resulting excessive runoff, cause sudden 
tremendous flash floods in the wadis. After a rainfall of as few as 
10 mm on the watershed, for instance, it is no rarity to see a flood 
of 30,000 m*/hr crashing through a wadi draining a 50-sq km 
catchment basin for four to five hours. These great quantities of 
water are practically the only source of irrigation water. But 
unless they are harnessed, they are lost, and all they cause is exten- 
sive soil erosion. Various types of dams, dykes, terraces, and spill- 
ways were, and still are, the answer to this problem. Topographical 
considerations are the decisive factor in the choice of the system 
to be employed. 
After ten years of observation we have reached the conclusion 
that this type of flood causes a rainfall of a few millimeters to in- 
crease soil moisture in the wadis to the equivalent of several 
hundred millimeters of precipitation. This transforms the wadis 
and the catchment basins, however small, into productive soil, 
especially when the natural moistening by floods is augmented by 
various flood control measures, as described later. 
The second problem is an edaphic one. Real soil in our desert 
area is rare. Most of our terrain cannot be used at all for lack of 
soil (21). Only where loess, sandy loess, or sand is to be found, or 
can be induced to deposit by flood control, is agriculture possible. 
This again means that desert agriculture 1s possible in strips and 
patches found only in the above-mentioned wadis and catchment 
basins, or in sandy plains. 
The utilization of water and the agricultural technique are very 
different in tributary wadis, in the major drainage channels, and 
in flood plains. Accordingly, we shall treat each one separately. 
Tributary Wadis 
The first consideration with this type of wadi is that, owing to 
its narrowness and steepness, a tributary wadi has to be controlled 
as a unit from the watershed down. Only thus may the destructive 
