too low. Since that time, various schemes for clas- 

 sification of the suitability of water for irrigation 

 have been developed. Scofield (152) discussed 

 increasing soil solution salinity through the use 

 of saline irrigation water and recognized the need 

 for use of leaching water and adequate drainage. 

 He established a classification system ranging from 

 excellent to unsuitable based upon the concentra- 

 tion of chlorides and sulfates. He strongly em- 

 phasized that the classification was applicable to 

 a specific region considering crops, soil, climate, 

 and the quantity of irrigation water relative to 

 rainfall. 



Some of the later schemes of classification at- 

 tempted to establish ratings of "average" condi- 

 tions having general applicability. In all cases, 

 the interpretation of the suitability of water for 

 irrigation use was largely empirical. Current trends 

 in research are based upon relating quality of 

 irrigation water to specific soils and crops for 

 specific irrigation methods (195, 196, 197, 181). 

 A knowledge of the basic mechanisms involved is 

 fundamental to the prediction of irrigation water 

 effects on soils and plants. No single set of cri- 

 teria can currently be established to evaluate water 

 quality characteristics for irrigation purposes. It 

 is the purpose of the following discussion to point 

 out the various soil-plant-water-climate interrela- 

 tionships and how they apply to specific water 

 quality characteristics. Where possible, criteria or 

 guidelines will be designed for specific character- 

 istics. 



water quality 

 considerations 

 for irrigation 



Effects on plant growth 



Irrigation is practiced primarily for the pur- 

 pose of increasing economic returns from agricul- 

 ture. Successful sustained irrigated agriculture, 

 whether in arid regions or in subhumid regions, 

 requires skillful water application based upon the 

 characteristics of the land and the requirements 

 of the crop. Through proper timing and adjust- 

 ment of frequency and volumes of water applied, 

 detrimental effects of poor quality water may often 

 be mitigated. 



Undesirable water quality characteristics can 

 affect plant growth either directly or indirectly. 

 Plants may be affected directly by either the de- 

 velopment of high osmotic conditions in the plant 

 substrate, or by the presence of a phytotoxic 

 constituent in the water. In general, plants are 

 more susceptible to injury from dissolved con- 

 stituents during germination and early growth 

 stages (77) than at maturity. Plants affected dur- 

 ing early growth stages may result in complete 

 crop failure or severe yield reductions. Effects of 

 these undesirable constituents may be manifested 

 in suppressed vegetative growth, reduced fruit de- 

 velopment, impaired quality of the marketable 

 product or a combination of these factors. The 

 presence of sediment, pesticides, or pathogenic 

 organisms in irrigation water, which may not spe- 

 cifically affect plant growth, may affect the ac- 

 ceptability of the product. Another aspect to be 

 considered is the presence of elements in irrigation 

 water which are not detrimental to crop produc- 

 tion, but may accumulate in crops to levels which 

 may be toxic to animals or humans. 



146 



