or precipitated out in the subsoil. The subsoil con- 

 stitutes a horizon below the surface and usually is 

 of finer texture and more compact. The surface 

 horizon varies in thickness from a few inches to a 

 few feet and can provide a good environment for 

 plant roots if deep enough; but the subsoil often 

 cannot, and water often does not penetrate it 

 readily. Soils with restrictive horizons are not the 

 best for irrigated agriculture; but if in an elevated 

 position relative to surrounding land, the night air 

 currents may make them more frost-free and suit- 

 able for sensitive crops that cannot be grown 

 elsewhere. 



Water 



Each river system within the arid and semiarid 

 portion of the United States has quality character- 

 istics peculiar to its geologic origin and climatic 

 environment. In considering water quality charac- 

 teristics as related to irrigation, both historic and 

 current data for the stream and location in question 

 should be used with care because of the large 

 seasonal and sporadic variations which occur. 



Both chemical composition and sediment load 

 in surface waters will vary with the stage of flow. 

 Salt concentration during low-flow periods is us- 

 ually greater than during peak flows. Storm runoff 

 not only affects the salt content, but frequently 

 tends to increase the sediment burden. Because of 

 variations in rainfall distribution, water quality 

 characteristics will differ significantly. Where rain- 

 fall is more uniformly distributed, the maximum 



concentrations of dissolved solids are two to three 

 times the minimum, whereas this factor may vary 

 from 5 to 10 where rainfall distribution is more 

 sporadic. 



The range of sediment concentrations of a river 

 throughout the year usually is much greater than 

 the range of dissolved solids concentrations. Maxi- 

 mum concentrations may be 10 to more than a 

 thousand times the minimum concentrations. Usu- 

 ally the sediment concentrations are higher during 

 high flow than during low flow. This differs in- 

 versely from dissolved-solids concentration which 

 is usually lower during high flows. 



Four general designations of water have been 

 used {136), based on their chemical composition: 

 calcium-magnesium, carbonate-bicarbonate; cal- 

 cium-magnesium, sulfate-chloride; sodium-potas- 

 sium, carbonate-bicarbonate; and sodium-potas- 

 sium, sulfate-chloride. Although a listing of data 

 for each stream and tributary is beyond the scope 

 of this report, an indication of ranges in dissolved- 

 solids concentrations, chemical type, and sediment 

 concentration are given in table IV-19 (136). 



Customarily, each irrigation project diverts 

 water at one point in the river and the "return 

 flow" comes back into the mainstream somewhere 

 below the system. This return flow consists in the 

 main of: (1) regulatory water, the unused part of 

 the diverted water required so that each farmer 

 irrigating can have the exact flow he has ordered; 

 (2 ) tail water, that portion of the water which runs 

 off the ends of the fields; and (3) underground 

 drainage, required to provide adequate application 



TABLE IV-19. 



Variations in Dissolved Solids, Chemical Type, and Sediment (136). Rivers in 

 Arid and Semiarid United States 



Dissolved solids 

 concentrations, mg/l 

 From To 



Prevalent chemical type > 



Sediment concentrations, 



mg/l = 



From To 



Columbia River Basin <100 300 



Northern California <100 700 



Southern California <100 +2,000 



Colorado River Basin <100 +2,500 



Rio Grande Basin <100 +2,000 



Pecos River Basin 100 +3,000 



Western Gulf of Mexico Basins.. 100 +3,000 



Red River Basin <100 +2,500 



Arkansas River Basin 100 +2,000 



Platte River 100 +1,500 



Upper Missouri River Basin 100 -|-2,000 



Ca-Mg, 

 Ca-Mg, 

 Ca-Mg, 

 Ca-Mg, 

 Ca-Mg, 

 Ca-Mg, 

 Ca-Mg, 



S-C. 

 Ca-Mg, 

 Ca-Mg, 



S-C. 

 Ca-Mg, 

 Ca-Mg, 



C-b. 



C-b <200 300 



C-b <200 +500 



C-b; Ca-Mg, S-C <200 +15,000 



S-C; Ca-Mg, C-b <200 +15,000 



C-b; Ca-Mg, S-C +300 +50,000 



S-C +300 +7,000 



C-b; Ca-Mg, S-C; Na-P, <200 +30,000 



S-C; Na-P, S-C +300 +25,000 



S-C; Ca-Mg, C-b; Na-P, +300 +30,000 



C-b; Ca-Mg, S-C +300 +7,000 



S-C; Na-P, C-b; Na-P, <200 +15,000 



■■Ca-Mg, C-b= Calcium-magnesium, carbonate-bicarbonc 

 Ca-Mg, S-C= Calcium-magnesium, sulfate-chloride. 

 Na-P, C-b= Sodium-potassium, carbonate-bicarbonate. 

 Na-P, S-C= Sodium-potassium, sulfate-chloride. 



- Sediment concentration 



Annual Load 



Annual Streamflow 



168 



