the volume of the water applied by sprinkler as 

 supplemental irrigation is much less than that 

 applied by furrow or flood irrigation in arid re- 

 gions. In assessing trace element concentrations in 

 irrigation water, therefore, total volume of water 

 applied and the physicochemical characteristics 

 of the soil must be taken into consideration. Both 

 of these factors could result in different standards 

 or criteria for supplemental irrigation as compared 

 with surface irrigation in arid regions. 



Other considerations 



Organic Compounds 



These are primarily pesticides, but may also con- 

 tain other types of organic quality deterrents origi- 

 nating from domestic and industrial sources. Here 

 again, quantity applied, soil characteristics, and 

 plant sensitivity must be taken into consideration. 



BOD and Aeration 



Although not a problem in normal irrigation 

 waters, it could be a problem where certain proc- 

 essing plant effluents are involved. Using sprinkler 

 application for supplemental irrigation, the com- 

 bined effects of the sprinkling and infiltration into 

 the soil provide considerable aeration which would 

 minimize this hazard. Where sprinkler irrigation is 

 used for effluent disposal and where the soil re- 

 mains excessively wet for long periods of 

 time, BOD may become a deterring factor, but 

 no specific information is available to enable 

 quantification. 



Suspended Solids 



Two factors regarding suspended solids must be 

 taken into consideration for sprinkler irrigation, 

 which are not significant for surface irrigation. The 

 first deals with the plugging up of sprinkler noz- 

 zles by these sediments. Size of sediment is a defi- 

 nite factor, but no specific particle size limit can be 

 established. Of the larger sediment particles that 

 do pass through the sprinkler, much of these can 

 be washed off certain leafy vegetable crops. Some 

 of the finer fractions, suspended colloidal material, 

 could accumulate on the leaves and, once dry, are 

 extremely difficult to wash off, thereby impairing 

 the quality of the product. These hazards increase 

 with frequency of irrigation and volume of water 

 applied. 



Adequacy and achievability of criteria 



Of all the criteria discussed, most information is 

 available regarding salinity. Yet a careful review 

 of this material indicates that it is most difficult to 

 assign tolerance limits, or even ranges of values, 

 for irrigation water. All research points to the 

 interactive effects of water table depth, soil type, 

 plant tolerances, and climatic conditions. Soil 

 salinity itself is not only a function of salinity level 

 of the irrigation water, but also the volume and 

 rate of application and leaching effects of inter- 

 vening rainfall. The same is true for the sodium 

 hazard involved in certain saline waters. Adequate 

 guidelines do exist regarding salinity; and, although 

 a specific limit cannot be set, these guidelines can 

 be used to judge the suitability of a given water for 

 irrigation. 



Our knowledge of the effect of trace elements in 

 irrigation water on plant growth is extremely lim- 

 ited. Work cited as being done in nutrient solutions 

 seldom provides sufficient information on toxic 

 limits for a variety of crops. Even if this were 

 available, the gap must be bridged between the 



175 



