and salt balance in all parts of the fields. The ini- 

 tial flush of tail water may be somewhat more 

 saline than later, but rapidly approaches the same 

 quality as the applied water (141). 



In many projects, however, a large part of the 

 unused water supply does get into the soil, through 

 seepage from ditches and from amounts entering 

 the irrigated soil in excess of that utilized in evapo- 

 transpiration. Such waters that have entered the 

 soil are more saline and do return to the down- 

 stream supply by one means or another. It is 

 axiomatic that water is actually used and reused 

 numerous times in a river system and there will be 

 progressive concentration of salts except as the 

 mainstream is diluted by tributaries. 



Drainage and leaching requirement 



Addition of irrigation water in excess of that 

 required for plant use is necessary to prevent salt 

 accumulation in the soil. This is referred to as the 

 leaching requirement. It is possible to predict the 

 approximate salt concentration that would occur 

 in the soil after a number of irrigations by estimat- 

 ing the proportion of applied water that will perco- 

 late below the root zone. In any steady state leach- 

 ing formula, the following assumptions are made: 



(1) No precipitation of salts occurs in the soil; 



(2) Ion uptake by plants is negligible; 



(3) Uniform distribution of soil moisture 

 through the profile and uniform concentra- 

 tion of salts in the soil moisture; 



(4) Complete and uniform mixing of irrigation 

 water with soil moisture before any of the 

 moisture percolates below the root zone; 

 and 



(5) Residual soil moisture is negligible. 



A steady state leaching requirement formula has 

 been developed by the U.S. Salinity Laboratory 

 (181) designed to estimate that fraction of the 

 irrigation water that must be leached through the 

 root zone to control soil salinity at any specified 

 level. This is given as: 



Dj^, ECdw 



where LR = leaching requirement; 



Dd„ = depth of drainage water; 

 Diw = depth of irrigation water; 

 EQw = salinity of irrigation water; 

 ECd„ = salinity of water percolating past root 

 zone. 



Hence, if ECdw is determined by the salt toler- 

 ance of the crop to be grown, and the salt content 



of the irrigation water ECi^ is known, the fraction 

 can be calculated. This will then determine the 

 relationship between the depths of irrigation and 

 drainage water which must be applied. Since ECe 

 (electrical conductivity of the soil solution extract) 

 is a diluted index value relative to the actual EC of 

 the soil water, and since ECdw is the permissible 

 salt concentration at the bottom of the root zone 

 with the mean level of soil salinity being consider- 

 ably less, the ECe value for 50-percent yield reduc- 

 tion for a particular crop has been recommended 

 as a guide for ECdw. The actual yield reduction 

 probably would be less than 50 percent (75). 



Bernstein (16) has developed a leaching frac- 

 tion formula which takes into consideration factors 

 that control leaching rates such as infiltration rate, 

 climate (evapotranspiration), frequency and dura- 

 tion of irrigation, and, of course, the salt tolerance 

 of the crops. He defines the leaching fraction as 

 LF= 1 — ETc/ITj where LF= the leaching fraction 

 or proportion of applied water percolating below 

 the root zone; E= the average rate of evapotrans- 

 piration during the irrigation cycle, Tc; and 1= the 

 average infiltration rate during the period of infil- 

 tration, Ti. By utilizing both the required leaching 

 derived from the steady-state formula 



LR = 



and the leaching fraction based upon infiltration 

 rates and evapotranspiration during the irrigation 

 cycle, it is possible to estimate whether adequate 

 leaching can be attained or whether adjustments 

 must be made in the crops to be grown to permit 

 higher salinity concentrations. 



In addition to determination of crops that should 

 be grown, leaching requirements may be used to 

 indicate the total quantities of water that will be 

 required. For example, irrigation water with a 

 conductivity of 2 millimhos requires one-sixth 

 more water to maintain root zone salt concentra- 

 tions within 8 millimhos than would water with a 

 salt concentration of 1 millimho under the same 

 conditions of use. In other words, where 600 acre- 

 feet of less saline water would suffice, 700 acre- 

 feet of the more saline water would be required to 

 accomplish the same result. 



There are problems with applying the leaching 

 requirement concept in actual practice. In the first 

 place, it is not practical to apply water with com- 

 plete uniformity. In surface irrigation, the objec- 

 tive is to apply the same amounts of water to all 

 parts of the field; but particularly in view of the 

 ever-increasing cost of skilled labor, some parts of 

 a field may receive more water than others. Gen- 

 erally, land is not sufficiently leveled to achieve 



169 



