10 
IRRIGATION AND METEOROLOGY. 
was during the prevalence of a western, or “chinook” wind, which 
had already lasted two days. The mean temperature of the water 
surface was 53 degrees, and the mean dew point 94 degrees. 
During the months of March and April, evaporation was very ir¬ 
regular, being greatest between the hours of 2 and 6 p. m.. while 
frequently for several days there would be no perceptible loss. 
Since May, evaporation has gone on as rapidly during the night as 
during the day. The explanation which suggests itself is this: 
Irrigation began on the grounds surrounding the evaporation, 
apparatus in May. During the day, when evaporation would 
otherwise be most rapid, the air is loaded with moisture by evapo¬ 
ration from the saturated soil. At night, owing to its more rapid 
cooling, evaporation from the soil ceases sooner than from the 
tank. To illustrate this, I have chosen a day at random, which 
* 
happened to be July 23rd. The dew point for the hours of 7 a. 
m., 2 p. m. and 9 p. m., were 51, 62 and 55 degrees, respectively, 
and the temperature of the water surface for the same hours was 
68, 82 and 74 degrees. The difference between the two tempera¬ 
tures, on which the rate depends, was respectively, 17, 18 and 19 
degrees, showing that at 2 p. m. and 9 p. m. the rate should have 
been, and was, the same. 
It is not probable that our mountain streams suffer any appre¬ 
ciable loss from evaporation, as their mean temperature is us ally 
helow that of the mean dew point. To test this matter, daily 
observations were taken at Fort Collins, of the temperature of the 
water of the Poudre River during June and July. For June, the 
mean temperature of the water was 59 degrees, while the mean 
dew point was 63 degrees. For July, the mean water temperature 
was 63. and the mean dew point 59 degrees. So that in the 
vicinity of Fort Collins, the river probaMv gained by its contact 
with the air during June, and lost during July. Below Fort Col¬ 
lins the increasing temperature of the water would cause the loss 
to be greater, while above, the reverse would be true. 
The loss in canals varies with their location, size and velocity. 
The main loss frmu this source comes, however, after the water 
reaches the laterals here, and in flowing over the heated ground in 
irrigation the temperature is soon raised to a point where evapora¬ 
tion is greater tluan any here recorded. - 
That irrigation materially increases the humidity of oiir cli¬ 
mate during the summer months is, I think, susceptible of proof. 
