Joel R. Gat

Center of Water Science and Technology, Ben Gurion University of the Negev, Israel

Drylands are what they are due to a negative water balance at the surface; the precipitation is not sufficient to overcome the soilwater deficit resulting from both direct evaporation and transpiration, both on a long term basis and usually also for the event based accounting. As a result a continuous plant cover cannot be maintained and groundwater recharge occurs only under exceptional circumstances, namely when some degree of surface flow causes the confluence of the precipitation from a larger area onto a focused infiltration site, producing locally a sufficient water depth for a positive hydrological balance. Additional sources of groundwater can be found where these originate outside of the arid region, either in space (e.g. in rain-shadow deserts such as the Judean Desert) or in time (e.g. the paleowaters of the Sinai and Negev formed during a wetter period during the Pleistocene).

This hydrological situation is inherently associated with the surface buildup of salinity during the interval between rain events, as well as in the groundwater due to the long residence time and slow movement of the waters. Given these circumstances, the local recharge as well as the flushing of the surface salinity depends critically on the surface structure and land use and is extremely sensitive to any change in both the climatic regime as well as the environmental situation, especially that which would affect the rain/runoff relationship.

For decades the focal point of research and development in the arid zone was based on the full exploitation of the indigenous water sources and the adaptation of the development activities to the quantitative and qualitative constraints of the region, including the pursuit of runoff farming, the adoption of salt resistant crops, the utilization and treatment of brackish waters, etc. Based on the ideal of "making the desert bloom" the marshalling of any additional water was always considered a step in the right direction. Moreover, many of the development projects such as the siting of hazardous industrial operation in the arid zone were made on the (erroneous) assumption of the slow spread of material within the hydrologic system due to the low amounts of precipitation and minor recharge rates.

In effect most of the development schemes disregarded the hydrologic characteristics of the desert, its drainage and self-cleaning capacities and thus finally resulted in further stressing the water sources, worsening the aridity mainly as a result of further salinity buildup and pollution and cannot therefore be considered sustainable operations.

With the increasing population pressures, the growth of major urban centers and the relocation of industrial and other activities to the drylands, the Negev being a prime example, it became obvious that the indigenous water sources were quite inadequate. Further development relies more and more on extraneous water, either pumped in by conduits from afar or by desalination of seawater. One then begins to realize that the problem has turned around. Instead of asking how much water the system can provide one is now faced with the question of the amount and quality of excess waters with which the system can cope without leading to ecological or hydrologic stresses and disasters. Foremost is the problem of the natural drainage capability, the prevention of the mobilization of the huge amounts of salinity stored in the soil and geologic formation, so as to prevent problems such as rising water tables flooding cellars and creating sabkha conditions or the secondary salinisation which is plaguing the whole southern region of Australia.

Obviously the development schemes need to be in tune with respect to the eco-hydrological situation , not only for the sake of the preservation of the fragile desert environment but, indeed, for the practical reason of achieving a livable and sustainable existence.