OVERFLOW OF RED RIVER OF THE NORTH. 43 
Horton. 6 These authorities show that evaporation as high as 2.2 
inches may occur during a month just before the snow and ice is 
changed into water. Snow melting slowly is subject to greater 
evaporation than snow melting more rabidly because of rains or 
high atmospheric temperatures. 
ABSORPTION AND STORAGE. 
Absorption by the soil, which is in effect a storage of water, 
affects run-off in two ways. In retarding the escape of water it 
decreases the rate of run-off by increasing the time required for 
water to reach drainage outlets, and the amount of run-off by increas- 
ing opportunity for evaporation and plant transpiration. Perco- 
lation also tends to reduce run-off in much the same manner as does 
absorption. In the spring the amount of percolation depends upon 
the condition of the ground when thaw begins. It has been found 
where the' ground, even if frozen, is not saturated, and where water 
or slush covers it for a time, that a surprisingly large amount of 
water will seep away. Thus the percolation factor, the dryness of 
the ground, and the depth to which it was frozen before being cov- 
ered with snow may affect run-off very materially. 
The effect of storage in lakes, ponds, and reservoirs is similar to 
that of storage by soil absorption. The stored water is delayed in 
reaching drainage outlets, and thus greater opportunities for evapo- 
ration and transpiration are afforded. 
RELATION BETWEEN PRECIPITATION AND RUN-OFF. 
For each of those watershed areas on which precipitation and 
run-off records are coextensive the probable effect of extreme pre- 
cipitation conditions was determined. Moreover, various relations 
which are more or less indefinite were indicated by data compiled. 
Thus certain data on precipitation were adapted for use to a limited 
extent in connection with other data in estimating run-off for all 
the watershed areas listed in Table 5. As the run-off records for 
the watershed above Grand Forks cover a much longer period than 
is covered by records for any other area the relation of precipitation 
to run-off was investigated in considerable detail for that water- 
shed. 
ANNUAL AND MONTHLY PRECIPITATION AND RUN-OFF. 
For the period 1882 to date annual and monthly precipitation 
records are available for all the watershed areas on which records' 
have been kept of the run-off. On four watersheds, namely, Red 
River above Grand Forks, Red River above Fargo, Red Lake River 
above Crookston and Thief River above Thief River Falls, the ratio 
6 Monthly Weather Review, U. S. Department of Agriculture, February, 1914. 
