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 rapidly 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 material^. 



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. 



