402 THE FUTURE OF ARID LANDS 
ecotypes can be tested under controlled conditions like those 
available in the Earhart greenhouses in Pasadena. The old meth- 
ods of testing under natural conditions are much too laborious 
and slow for the rapid tempo needed in a progressive desert agri- 
culture. 
Use of Natural Precipitation (6, 30, 31, 40) 
As already mentioned, the only source of irrigation for our 
desert agriculture is the natural precipitation, and the runoff 
water derived thereof. We learned how to utilize this, mainly by 
studying the remnants of the ancient agricultural systems of the 
Nabataeans and Byzantines who once populated the central and 
southern Negev. There are today six dead cities (1) which were 
once thought to have thrived on caravan trade only. After an 
archaeological survey we know that they practiced a highly devel- 
oped desert agriculture and possessed a very elaborate system of 
utilizing practically every drop of precipitation for agricultural 
purposes.* After the destruction of these desert cities in the sev- 
enth century A.D. (they were founded in the second century B.c.) 
the intricate system of perfectly and laboriously constructed 
dams, spillways, and terraces slowly crumbled, and for many 
centuries they were maintained very amateurishly by desert no- 
mads. Today we partly use the same reconstructed dams and 
terraces, the same rock-hewn cisterns which the Nabataeans had 
built over 2,000 years ago. It is thrilling to see time and time 
again how the present day dispositions of highly complex irriga- 
tion systems, calculated by trained specialists, with the latest 
technical aids, coincide with remnants of ancient irrigation sys- 
tems on the same spot. 
There is today a consensus that in historical times there was 
no fundamental change of climate in our region (31) and that the 
Nabataean and Byzantine agriculture in the Negev faced essen- 
tially the same problems we are facing today. 
* Similar agricultural systems are known from ancient civilizations in 
the desert areas of southern Arabia and North Africa. A comparative 
study of such systems all over the world might provide valuable practical 
and historical information. 
