content of that element in a given irrigation and 

 the resultant content in the soil solution following 

 irrigation. Here, again, availability of a given ele- 

 ment to plants will vary with soil characteristics. 

 In general, criteria for trace elements are inade- 

 quate and guidelines previously described are the 

 best generalizations that can be made with existing 

 information. 



The BOD or COD value of water is important 

 for many uses, but its significance in irrigation 

 water has not been fully assessed. As previously 

 mentioned, it is not likely to be a problem where 

 sprinkler irrigation is used predominantly and ade- 

 quate soil drainability is maintained. For these 

 reasons, no specific criteria are prescribed. 



It is evident that there is a great lack of informa- 

 tion regarding quality deterrents in water for irri- 

 gation in general. Guidelines are available for 

 some naturally occurring deterrents; but as the 

 pollution pattern of our water sources changes, 

 additional research will be necessary to evaluate 

 effects of these wastes on various crops and for a 

 range of soil conditions. This information is neces- 

 sary before adequate and achievable criteria can 

 be developed. 



In view of the above discussion, it is apparent 

 that judicious use of water of impaired quality may 

 be more practical than water treatment. Adequate 

 guidelines are available for salinity, but additional 

 research is needed to develop comparable guide- 

 lines for other mineral and organic contaminants. 



Steps to improve water quality 



It is outside the scope of this report to discuss 

 water treatment in detail. Limited water treatment 

 possibilities must be reconciled with the economic 

 value of the crop being produced. For field irriga- 

 tion in general, treatment is not usually practical. 

 Where good quality water is necessary for high 

 value crop production in greenhouses, water treat- 

 ment may be feasible. Each case must be consid- 

 ered on its own merits. 



Nevertheless, since good water management is 

 so germane to quality characteristics of irrigation 

 water, brief mention will be made of several meth- 

 ods whereby the quality of irrigation water can be 

 maintained or improved. 



contents in streams are frequently high during low 

 flows and low during periods of high flow, inter- 

 mixing in the reservoir and strategic releases of 

 water can provide more uniform salinity levels in 

 the irrigation water. 



Evaporation of water from reservoirs tends to 

 increase the salt content. Continuing studies are 

 being conducted on new materials and application 

 techniques to minimize this effect. 



Elimination of nonbeneficial uses of water by 

 phreatophytes not only lessens the concentration of 

 salts through transpiration, but conserves water as 

 well. Lowering the water table and developing 

 mechanical and chemical techniques for elimina- 

 tion of phreatophytes will insure more efficient 

 water use and minimize salt hazards. 



Salts are frequently added to irrigation water 

 from mineral springs, oil wells, industrial enter- 

 prises, mine waters, and urban areas. Each of these 

 sources must be considered individually to deter- 

 mine effective control measures. 



Regulation of return flows according to quantity 

 and quality is another means of maintaining and 

 improving irrigation water quality. Utilization of 

 ponds and reservoirs to control streamflow can be 

 helpful in this respect. 



Drainage water from irrigated lands in arid re- 

 gions is commonly more saline than the applied 

 water. This is especially true where reclamation of 

 saline soils is in progress. In coastal areas, irriga- 

 tion water quality can be maintained by bypassing 

 saline return flows directly to the ocean. 



Desalting water may be a potential in the future 

 when technology permits production at a relatively 

 low cost. Desalted water can be used directly for 

 irrigation, for augmenting low flows, or for mixing 

 with poor quality water. For the use of desalted 

 water to be feasible, adequate opportunities for 

 disposal of the resulting brine must be available. 



Water shortages in some areas emphasize the 

 need for conjunctive use of ground and surface 

 waters. One aspect of this involves more effective 

 use of ground water storage potential. Increasing 

 ground water recharge to make use of this potential 

 would be most beneficial. The threat of gradual 

 deterioration in ground water quality through diffi- 

 culties in achieving basin salt balance could be 

 mitigated by greatly expanded utilization of ground 

 water storage resources. 



Total Dissolved Solids 



Sediment 



Uniformity of irrigation water quality can be 

 achieved through stream regulation by controlling 

 release of water from storage reservoirs. Since salt 



One of the major ways of minimizing the sedi- 

 ment burden of streams is through proper water- 

 shed management. Practices designed to provide 



176 



