Arizona Agricui^turai. Experiment Station 485 



were installed in the center of an alfalfa field. The duty of water 

 for alfalfa is to be obtained. 



Fuller descriptions will be given in a separate publication, but 

 the general results of the first year's observations are given herein 

 in P'ig. 13. The temperature, the wind movement in miles per hour, 

 and the evaporation in inches of water, in ten-day periods, are 

 shown by graphs. 



A close agreement between the temperature and the evapora- 

 tion graphs is very apparent for all three locations, but a careful 

 examination reveals that the agreement is modified by the wind 

 movement. Thus, the decreasing wind movement near Willcox in 

 May and June prevented so rapid an increase of evai)oration as in 

 March and April. The apparently erratic curve for Yuma in the 

 summer months is due to the raising of a seed crop. At Yuma an:l 

 at Willcox. in August, the wind movement was nearly equal and 

 the difference in evaporation-rate appears to be due to difference 

 in temperature. 



Reduced to lowest terms, the evaporation-rate depends upon 

 temperature and the vapor pressure directly at the water surface. 

 The first of these two factors is expressed here by the daily maxi- 

 mum temperature instead of by the daily mean temperature, which 

 is commonly used. The second factor depends upon the relative 

 humidity in the vicinity, and the air pressure, in addition to the 

 wind movement. Air pressure varies mainly with altitude, and the 

 extent and direction of this variation has been much debated. The 

 high evaporation-rate near Willcox is attributable in the main to 

 the rapid wind movement, but may be due in part to the fact that 

 the evaporation station near Willcox is in an isolated alfalfa field 

 surrounded by semi-desert conditions, while at Yuma and Mesa the 

 alfalfa fields are but parts of great irrigated valleys. Willcox lies 

 on the western edge of a region which some investigators have as- 

 serted to have the highest evaporation-rate for the United States. 



The importance of these investigations to agriculture is appre- 

 ciated when one recalls that the loss of water by transpiratic^i 

 from a well-develo])ed field of alfalfa is greater than the evaporation 

 loss from an open water surface. The average annual loss from a 

 well-irrigated alfalfa field cannot differ very greatly from that of a 

 standard evaporation pan i)laced within the field. Farmers in the 

 Sulphur Spring Valley have added incentive to ])lant windbreaks, 

 to cultivate after irrigating, and to raise crops having low water 

 requirements. 



