SECTION OF IRRIGATION AND METEOROLOGY. 
water, upon the dryness of the air (not directly upon the tempera- 
ture of the air), upon the wind. The wind brings fresh, unsaturated 
air in contact with the water surface and gives opportunity for 
more vapor to be absorbed. Unless the temperature of the water 
surface is warmer than the dew point of the air, evaporation cannot 
proceed ; if lower, condensation may take place. The wind als© 
causes waves and increases the area subject to evaporation. 
§ 5. The temperature of the water affects the evaporation 
much more than is usually realized. A shallow lake evaporates 
faster than a deep one, because its temperature is higher. Likewise 
the evaporation from the shallow parts of a lake is greater than from 
the deep portions. I have often found the temperature of the water 
in the shallow areas much higher than at the deep places. The 
temperature of the water, and the wind exposure, may differ so 
much between bodies of water in the same neighborhood that a gen- 
eral statement must be accepted with reserve. It is entirely possible 
for two tanks side by side to have very different losses from evapo- 
ration. In the evaporation tank, which has now been maintained 
for eleven years, the loss from evaporation averages 41 inches per 
annum. From lakes during the summer months the evaporation 
has been found to be as much as twice that from the tanks, an in- 
crease of temperature of ten degrees, or enough to change the tem- 
perature from 70° to 80°, may be sufficient to double the amount. 
§6. Evaporation proceeds from ice, but at a diminished rate. 
When our tanks are frozen they show a loss of from 1 to IJ inches 
per month, solely from the frozen surface. 
The evaporation at night, contrary to common opinion, is almost 
the same as during the day, and this is nearer equality as the body 
of water is larger. Even in our tanks, the evaporation during the 
nights of a month is often found to be more than during the days 
for the same period. 
THE LOSS FROM SEEPAGE. 
§ 7. For two winters observations were made to determine the 
loss from reservoirs by seepage. In many cases water runs into the 
reservoirs until late in the fall and the filling begins early in the 
spring, hence the period during which the losses can be found with- 
out measurements of inflow and outflow is short. Nearly a dozen 
reservoirs were visited. Bench marks were established, and levels 
run to the surface of the water. Some of those selected were filled 
during the winter, and the record was of no value. Perhaps half a 
dozen gave some basis for estimating the loss. 
Most of the reservoirs under observation are natural basins sit- 
uated within twelve miles of Fort Collins, and at an elevation from 
5,000 to 5,500 feet above sea level. The sites have been ponds in wet 
