an even depth of water application. With sprinkler 

 irrigation, there is a common need, particularly in 

 the arid and semiarid regions, of keeping applica- 

 tion rates low. This need is in conflict with attempts 

 to approach complete uniformity of coverage. Op- 

 timum sprinkler uniformity of coverage is about 

 85 percent under still conditions and less with 

 wind. 



Secondly, soils are far from uniform, particularly 

 with respect to vertical hydraulic conductivity. 

 Considerable nonuniformity must be expected, far 

 more in areas of discontinuous stratification than 

 elsewhere. 



Thirdly, the effluent from tile or ditch drains 

 may not be representative of the salinity of water 

 at the bottom of the root zones. The streamlines of 

 flow from the water table to the tile go to consid- 

 erable depths; and in a newly reclaimed area par- 

 ticularly, the ground water below the tile system 

 may be undergoing considerable freshening. Re- 

 cent studies in the San Joaquin Valley of Califor- 

 nia indicate that this freshening will go on for 50 

 years {134). 



Fourthly, there is a considerable variation in 

 drainage outflow which has no relation to leaching 

 requirement when different crops are irrigated 

 (131). This results from variations in irrigation 

 practices for the different crops. 



The leaching requirement concept while very 

 useful should not be used as a sole guide in the 

 field. The leaching requirement is a long-period 

 average value which can be departed from for 

 short periods with adequately drained soils to make 

 temporary use of water poorer in quality than 

 customarily applied. 



The exact manner in which leaching occurs and 

 the appropriate values to be used in leaching re- 

 quirement formulae require further study. The 

 many variables and assumptions involved preclude 

 a precise determination under field conditions. 



Specific problem areas 



Salinity Hazard 



Waters with TDS less than about 500 mg/1 are 

 used by farmers without awareness of any salinity 

 problem, unless, of course, there is a high water 

 table (97). Also, without dilution from precipita- 

 tion or an alternative supply, waters with TDS of 

 about 5,000 mg/1 usually have little value for irri- 

 gation (130). Within these limits, the value of the 

 water appears to decrease as the salinity increases. 

 Where water is to be used regularly for the irriga- 



tion of relatively impervious soil, its value is lim- 

 ited if the TDS is in the range of 2,000 mg/1. 



The following classification as to salinity hazard 

 is suggested: 



TDS mg/l EC mmhos/cm 



Water for which no detri- 

 mental effects will usually 

 be noticed <500 <0.75 



Water which can have det- 

 rimental effects on sensi- 

 tive crops 500-1,000 0.75-1.50 



Water that may have ad- 

 verse effects on many 

 crops and requiring care- 

 ful management practices- 1,000-2,000 1.50-3.00 



Water that can be used for 

 tolerant plants on perme- 

 able soils with careful 

 management practices — _ 2,000-5,000 3.00-7.50 



Permeability Hazard 



There are two criteria that are used to evaluate 

 the effect of certain salts in the irrigation water on 

 soil permeability. One of these is the sodium ad- 

 sorption ratio (SAR) and its relation to the ex- 

 changeable sodium percentage. The other of these 

 is the bicarbonate hazard which is particularly 

 applicable to arid region irrigation agriculture. 

 Eaton (47) developed the concept of "residual 

 sodium carbonate" (RSC) for characterizing water 

 quality. More recently, Bower, et al. (23, 24) 

 found that the hazard is related to the tendency 

 of calcium carbonate to be precipitated from the 

 soil solution, as indicated by the Langelier index 

 (83) and to the fraction of inflow water evapo- 

 transpired. In other words, the greater the tend- 

 ency of the soil water to precipitate CaCOa during 

 the evapotranspiration concentration process be- 

 tween irrigations, the more rapidly SAR of that soil 

 water increases. Thus, there is a relationship be- 

 tween SAR and bicarbonate hazard, as suggested 

 by Doneen (41, 42), but any specific relationship 

 is affected by irrigation management practices. In 

 general, the bicarbonate hazard presents the great- 

 est problem at low salt concentrations. 



Another problem with a permeability hazard is 

 that permeability tends to increase and the stability 

 of a soil to any ESP level increases as the salinity 

 of the soil water increases (135). The work of 

 Rollings gives the most recent information on the 

 interrelationships of EC, SAR, and soil structure 

 stability (144). 



Doneen (41) has long suggested that precipi- 

 table calcium carbonate and the precipitable cal- 

 cium sulfate be deducted from total salinity to get 

 what he calls "effective salinity." Christiansen and 



170 



