210 Bulletin 88 



tive tests have shown the water requirement on sand and clay to 

 be reduced more than fifty percent by the addition of fertilizers. On 

 rich loams, however, the addition of fertilizer appears to have little 

 effect upon the ratio of water transpired to crop produced. 



It is not likely that the actual transpiration ratio in broad fields 

 is so high as given in the table. Nevertheless, the table indicates 

 the relative rates of different crops and the possibility of tremen- 

 dous demands for water by plants under adverse soil and climatic 

 conditions. 



The water requirements of crops in Arizona have not been de- 

 termined in an adequate manner. Observations and meager records 

 indicate that, for the Salt River Valley, alfalfa that is grown con- 

 tinuously through the summer should have about eight 6-inch 

 irrigations per year on medium loam soil, and about twelve 4-inch 

 or 5-inch irrigations on sandy soils. Cotton requires from two acre- 

 feet per acre on fertile loam soil to three acre-feet on light sandy 

 soil and new desert soil, and milo maize about one and a half or 

 two. The yield of alfalfa increases almost in proportion to the 

 amount of water applied, at the rate of one ton of hay per acre- 

 foot of water, even up to seven or eight acre-feet per acre ; but with 

 grain and other crops the yield is reduced by applying more than 

 the optimum amount. The duty of water is higher in some cases 

 due to subirrigation. 



There is one school of irrigators in the Valley who do not irri- 

 gate alfalfa during part of July and through August. Allowing 

 the alfalfa to rest during this period tends to keep out water grasses 

 and tends to avoid damage by insects. Under this system the 

 water requirements are less than the amount stated above, and the 

 total yield also is reduced. 



WATER LOSSES 



The water transpired by plants constitutes, ordinarily, but a 

 small part of the total water diverted for irrigation. Beginnino- at 

 the point of diversion, the supply stream suffers continuous losses. 

 The sequence of these losses is as follows : seepage (and evapora- 

 tion) from canals; seepage from the field laterals; evaporatir.n 

 upward from the irrigated fields ; seepage downward from the 

 fields ; and wilful or careless waste. These losses will be discussed 

 separately and suggestions will be offered as to how they can be 

 reduced. 



