a condition of unsaturation existing above it. Dur- 

 ing and immediately following periods of precipi- 

 tation or irrigation, water moves downward 

 through the soil to the water table. At other times, 

 water losses through evaporation from the soil 

 surface and transpiration from plants (evapotrans- 

 piration) may reverse the direction of flow in the 

 soil so that water moves upward from the water 

 table by capillarity. The rate of movement is de- 

 pendent upon water content, soil texture, and 

 structure. In humid and subhumid regions, this 

 capillary rise of water in the soil is a valuable 

 water source for use by crops during periods of 

 drought. 



Evapotranspiration removes pure water from 

 the soil leaving the salts behind. Since salt uptake 

 by plants is negligible, salts accumulate in the 

 soil. A favorable salt balance in the root zone can 

 be maintained by leaching through the use of irri- 

 gation water in excess of plant needs. Good drain- 

 age is essential to prevent a rising water table 

 and salt accumulation in the soil surface and to 

 maintain adequate soil aeration. 



Soils vary greatly in their physicochemical 

 properties; therefore, the resultant effect of a given 

 irrigation water quality on the plant root environ- 

 ment will also be quite variable. 



Plant 



Plants can be affected in two ways by irrigation 

 water quality. First, where sprinkler irrigation is 

 used, foliar absorption or adsorption of constitu- 

 ents in the water may be detrimental to plant 

 growth or consumption of aifected plants by man 

 or animals. Secondly, where surface or sprinkler 

 irrigation is practiced, the effect of a given water 

 quality on plant growth is determined by the com- 

 position of the equilibrium soil solution. This is 

 the growth medium available to roots after soil 

 and water have reacted. 



Plants vary considerably in their tolerance to 

 water quality constituents. Genetic considerations 

 apply not only to differences between species, 

 but to varietal differences as well. Many species 

 and varieties of plants have been observed for 

 tolerances to salinity, trace elements, pesticides, 

 and pathogens. A good start has been made in 

 classifying plant tolerance to salinity, but much 

 remains to be learned regarding the effects of 

 irrigation water. 



Climate 



Irrigation is necessary for intensive crop pro- 

 duction in arid and semiarid areas and is used to 

 supplement rainfall in humid areas. The need for 



irrigation is determined to a large extent by rain- 

 fall and snow distribution; but temperature, radia- 

 tion, and humidity are also significant factors. 



The effects of water quality characteristics on 

 soils and on plant growth are directly related to the 

 frequency and amount of irrigation water applied. 

 The rate at which water is lost from soils through 

 evapotranspiration is a direct function of tempera- 

 ture, radiation, wind, and humidity. Soil and plant 

 characteristics also influence- this water loss. Aside 

 from water loss considerations, water stress in a 

 plant as affected by the rate of evapotranspiration 

 will determine the plant's reaction to a given soil 

 condition. For example, in a saline soil at a given 

 water content, a plant will usually suffer more 

 in a hot, dry climate than in a cool, humid one. 

 Considering the wide variation in these climatic 

 variables over the United States, it is apparent that 

 water quality requirements also vary considerably. 



Rainfall and snowmelt are also significant be- 

 cause they affect not only the amount of available 

 water in the soil, but may also be a factor in leach- 

 ing constituents applied in irrigation water out 

 of the plant root zone. Because precipitation pat- 

 terns are so variable, they influence the degree of 

 hazard presented by use of water of a given quality. 



The soil, plant, and climate variables must be 

 considered in developing criteria for evaluation of 

 irrigation water quality. A wide range of suitable 

 water characteristics is possible even when only 

 a few variables are considered. Even under favor- 

 able conditions of soil, drainage, and environ- 

 mental factors, too-sparing applications of high 

 quality water with total dissolved solids of less 

 than 100 mg/1 would ultimately damage sensitive 

 crops such as citrus fruit, whereas with adequate 

 leaching water containing 500 to 1,000 mg/1 might 

 be used. Under the same conditions, certain salt- 

 tolerant field crops might produce economic re- 

 turns using water with more than 4,000 mg/1. Cri- 

 teria for judging water quality standards must take 

 these factors into account. 



Past and current trends in 

 water quality classification 



From the very beginning of irrigation in the 

 United States, farmers have observed differences 

 in water quality that have influenced their crops. 

 In some areas, they soon learned to bypass water 

 that contained excessive amounts of sediment or 

 that originated from tributaries known to be saline. 



Means {106) observed in 1903 that safe salin- 

 ity limits previously set for irrigation water were 



145 



