408 THE FUTURE OF ARID LANDS 
limited volume of water (about 10,000 m*/hr). This serves a 
double purpose. First, this controls the maximal pressure of water 
reaching the channel, and secondly, this constantly reduces the 
water pressure on the detention dam, and thus helps to preserve it. 
The earth dam is built sufficiently strong to detain the total 
flow until it is completely drained off by the outlet pipe at a con- 
trolled rate. 
Flood Plains 
In many places in the Negev small or large level plots of land 
can be found surrounded on all sides by gently inclined slopes. 
After a rainfall, the runoff water of these slopes is collected on the 
flood plain, where it slowly sinks into deeper soil layers. Small, 
stone-reinforced channels, built obliquely on the surrounding 
slopes, greatly increase the amount of catchment water. 
Before concluding, we should like to emphasize that all our 
experience so far bears out the well-known truth that there is no 
soil conservation or flood control without permanent plant cover 
on the water-spreaded lands. 
Agrotechnical Problems (6) 
There are a great number of agrotechnical problems involved in 
desert agriculture. One of the main agrotechnical difficulties en- 
countered in desert agriculture is the rapid formation of a hard 
crust on the upper surface of the loess soil, whenever the soil has 
been wetted and starts to dry out. This crust greatly obstructs 
reseeding by preventing seedling emergence, except from cracks 
formed in the drying out crust, and by tearing of roots when the 
crust separates from the subsoil. This problem is being attacked by 
trying to prevent crust formation by adding soil conditioners 
(Kryllium and other artificial soil conditioners specially developed 
in Israel (26) and natural soil conditioners like straw and sand) to 
the soils. Another type of attack is made via germination. The 
study of the germination mechanisms of each seed often leads to 
methods of greatly accelerating germination, so that uniform 
seedling emergence is obtained before crust formation starts. 
This is another example of the need for close cooperation between 
various research agencies, as under these extreme conditions it is 
