PPP) THE FUTURE OF ARID LANDS 
physical properties as far as irrigation agriculture is concerned. 
The hydraulic conductivity of soil is related to texture. It is 
generally higher in coarser soils, but it is also influenced by struc- 
ture and can be profoundly altered by soil management operations 
and the exchangeable cation status. 
Open ditches or covered drains can be used if necessary to 
control the water table, but the cost of such facilities will be 
prohibitive if the water conductance of the soil is too low. In 
subhumid climates, a shallow, stable water table may be a great 
economic asset for carrying crops through occasional drought pe- 
riods but, for irrigation agriculture in an arid climate, a shallow 
water table is a serious salinity hazard. Given an adequate water 
supply and reasonably favorable soil texture, it might well be 
argued that in arid climates the hazards from a water table near 
the root zone considerably outweigh any advantages connected 
with subirrigation. 
The drainage requirements of any given irrigation project will 
relate to the permissible depth and mode of variation of the water 
table and the volume of water that the drains must convey in a 
given time. These, of course, will depend on a whole complex of 
factors related to soil, crop, climate, quality of irrigation water, 
and farm management. In any case, the underground drainage 
must be adequate to carry away the excess water and salt added 
during irrigation if long-time profitable operation is to be attained. 
Water Quality 
The quality of irrigation water enters importantly in deter- 
mining irrigation feasibility and permanence. There have been 
no recent discoveries of new hazards to irrigation agriculture 
among the soluble constituents of irrigation water. Some constitu- 
ents like boron are toxic in minute concentrations. The main prob- 
lems, however, appear yet to be the accumulation of soluble salts 
and exchangeable sodium in soil. 
The salinity of irrigation water has a direct bearing on such 
factors as crop selection, method of application of the water, and 
the leaching required to control salt accumulation in the soil. All 
these in turn are subject to constraints imposed by drainage 
conditions. 
