Thome (57) raise a similar question regarding the 

 bicarbonates. Waters high in bicarbonate relative 

 to other anions can affect permeability more than 

 their SAR would indicate. Coachella Valley of 

 California was formerly irrigated with well water. 

 Some well waters low in salt and low in SAR but 

 relatively high in bicarbonate created highly im- 

 pervious soils. The problem disappeared upon in- 

 troduction of Colorado River water, higher in 

 salts than the well water used, but having a positive 

 Langelier index — a strong tendency to deposit 

 calcium carbonate. 



In summary, water with SAR values between 

 eight and 18 may have an adverse effect on per- 

 meability of soils containing an appreciable 

 amount of clay. The specific SAR value that has 

 this effect increases as the salinity increases. Low 

 salt water high in bicarbonates may present a 

 permeability hazard even at low SAR values. 



specific irrigation water 



quality considerations 



for humid regions 



Suspended Solids 



Suspended organic solids in surface water sup- 

 plies seldom give trouble in ditch distribution sys- 

 tems except for occasional clogging of gates and 

 for carrying weed seeds onto fields where subse- 

 quent growth of weeds can have a severely adverse 

 effect on the crop, but also may have a beneficial 

 effect by reducing seepage losses. Where surface 

 water supplies are distributed through pipelines, it 

 is often necessary to have self-cleaning screens to 

 prevent clogging of the pipe system appliances. 

 Finer screening is usually required where water en- 

 ters pressure-pipe systems for sprinkler irrigation. 



There are waters diverted for irrigation that 

 carry heavy inorganic sediment loads. The effects 

 that these loads might have depends in part on the 

 particle-size distribution of the suspended material. 

 The ability of sandy soils to store available mois- 

 ture has been greatly improved after being irri- 

 gated with muddy water for a period of years. 

 More commonly, sediment tends to fill canals and 

 ditches, causing serious cleaning and dredging 

 problems. It also tends to further reduce the 

 already low infiltration characteristics of slowly 

 permeable soils. Kennedy (76) developed criteria 

 to keep sediments moving in irrigation canals to 

 prevent deposition. These criteria worked very 

 well with the somewhat coarser sediments of the 

 Indus River system where they were developed, 

 but are not universally adaptable. In most waters 

 carrying appreciable amounts of sediments, provi- 

 sion is usually made now for most to be settled out 

 and be bypassed back to the mainstream near the 

 point of diversion. 



Environmental factors 



Climate 



The most striking feature of the climate of the 

 humid region that contrasts with that of the far 

 West and intermountain areas is the larger amoimt 

 of, and less seasonable distribution of, the precipi- 

 tation. Rainfall, rather than lack of it, is the normal 

 expectation. There are perhaps more cases in 

 which crops are damaged because of too much 

 water than because of too little. Yet, droughts are 

 common enough to require that attention be given 

 to supplemental irrigation. These times of shortage 

 of water for optimum plant growth can occur at 

 irregular intervals and at almost any stage of plant 

 growth. 



Water demands per week or day are not as high 

 in humid as in arid lands. But rainfall isn't easily 

 predicted. Thus a crop may be irrigated and imme- 

 diately thereafter receive a rain of one or two 

 inches. Supplying the proper amount of supple- 

 mental irrigation water at the right time is not easy 

 even with adequate equipment and a good water 

 supply. There can be periods of several successive 

 years when supplemental irrigation is not required 

 for most crops in the humid area. There are times, 



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