146 
IOWA ACADEMY OF SCIENCES. 
the more imperative is the demand for fire protection; all 
this at the very time when the supply is near its minimum. 
It is evident that the minimum precipitation is the pre- 
cipitation that must be considered. The government 
report"^ already mentioned gives us the relation of run-off 
and evaporation to precipitation in the storage and grow- 
ing periods but not in the replenishing period; for this the 
average of thirty -seven records given in the tables in the 
same pamphlet is assumed to be correct. For the rainfall 
at Indianola the figures are as follows: 
RUN-OFF AND EVAPORATION. 
Run-Off. 
Evaporation. 
Run-Off. 
Minimum. 
Average. 
Minimum. 
Average. 
During Storage Period 
During Growing Period . . . 
During Replenishing Period. 
80 % 
10 % 
23. 7% 
5. 09 in. 
. 26 in. 
. 70 in. 
9.98 in. 
1.19 in. 
1.52 in 
1.27 in. 
2. 38 in. 
2.25 in. 
2. 50 in. 
10.69 in. 
4. 89 in. 
The loss by evaporation of water stored in a reservoir is 
at a different rate than the loss by evaporation from the 
surface of the ground. The record for Lake Cochituatef 
for thirty-eight years gives the following averages: 
Mean 
Precipitation. 
Mean 
Evaporation. 
Evaporated. 
Storage 
23.15 inches. 
8,23 inches. 
35.5 per cent. 
82 per cent. 
73 per cent. 
Growing 
11.59 inches. 
9.51 inches. 
Replenishing 
12.38 inches. 
9.06 inches. 
Indianola is situated on a divide which is thoroughly 
drained. The nearest places toward which the drainage is 
sufficient to supply even the demand for half a million 
gallons daily are the river bottoms, one, a mile south of 
the city, the other, six miles north. The location already 
spoken of as a mile east of the city is at a point where the 
rock formation is unusually good for a dam; but the area 
drained past that place is only one square mile. As this 
location is close to a railroad by which a supply of coal 
♦Water Supply and Irrigation Paper No. 80. 
tWater Supply and Irrigation Paper No. 80, page 89. 
