4 
COLORADO EXPERIMENT STATION. 
evaporation were correspondingly more important. Without in- 
tending, or desiring, in this place, to enter upon a discussion of the 
mass of evaporation observations, this fact has led to a fuller state- 
ment of the observations of evaporation than were at first thought 
. necessary or desirable. 
§ 2. The loss from the lakes m.ay be due to evaporation from 
* the water surface, and seepage or filtration through the dam and 
■ bottom of the reservoir. The leakage through imperfectly fitting 
gates can be prevented or remedied by better construction. Gain 
may come from rainfall on the lake, the drainage from the water 
shed tributary to the lake and the seepage from irrigated lands 
above the lakes. The aggregate of these gains and losses is desired 
by the companies as much, or more, than a knowledge of each, but 
the aggregate can best be told when a knowlege of the amount of 
.loss or gain from each cause is determined. The losses from seep- 
age in many cases deserve most attention, as they may vary between 
wide limits, and to some extent are preventable. The amount of 
'Joss from. evaporation maybe estimated with considerable certainty. 
The loss from seepage is more uncertain, as it must vary with the 
' conditions of each basin, and the amount is peculiar to that particu- 
. far site. Its determination is surrounded with difficulty, and re- 
‘ quires accessory investigations, so that attempts to determine the 
, loss by seepage from reservoirs seem not to have been made, or if 
-30, I have been unable to find any records of the attempt or of the 
results. But while evaporation depends upon various circumstances 
3ome of which can be controlled, the 'amount of evaporation cannot 
be materially modified at any practicable cost. 
§3. Generally the scarcity of sites for ■ reservoirs makes the 
selection depend on their availability, nevertheless the possibility 
of an undue loss from seepage needs to be borne in mind in making 
tbe selection. In most of the sites found in Colorado, the strata 
form a natural basin from which the loss by seepage is small. There 
are places in which the strata of rock incline both ways from the 
reservoir, an anticlinal in the term of the geologist-. This condition 
or when the strata dip in one direction from the site should be 
avoided, although when the rocks are deeply covered with soil un- 
desirable effects may be small. I have seen reservoirs in Algeria 
with the strata inclining from the reservoir, where all the exposed 
rock has been cemented in order to prevent the loss of water through 
the strata. With a bottom of sand, the loss may be large for a time, 
but the action rapidly grows less after the sand layers are once sat- 
urated. 
EVAPORATION. 
§ 4. The general conditions of evaporation are well known. 
The amount of evaporation depends upon the temperature of the 
