30 



BULLETIN 1017, U. S. DEPARTMENT OF AGRICULTURE. 



A study of the annual and monthly records indicates that no com- 

 bination of excessive monthly precipitation is likely to occur which 

 will produce a maximum annual precipitation of more than 30 inches 

 over that part of the Red River watershed south of the international 

 boundary. It appears also that the maximum annual precipitation 

 for the extreme western portions of the watershed will probably not 

 exceed 25 inches, and that the maximum for areas of 2,000 square 

 miles or greater in the eastern sections of the watershed will probably 

 not exceed 37 inches. The maximum annual precipitation on the 

 watershed above Fargo and on the Red Lake River watershed above 

 Crookston will probably not exceed 36 inches. From the maximum 

 of 37 inches south of Lake Traverse and 36 inches south of Red Lake 

 intensities diminish toward the north by about 2 inches per degree of 

 latitude, and toward the west by about 3 inches per degree of longi- 

 tude. These maximum conditions are likely to occur but once in a 

 long period, perhaps 50 to 75 years. 



The records further show that maximum annual and monthly pre- 

 cipitation on the smaller watersheds have not occurred simultaneously 

 on the different sub-areas. It is improbable, therefore, that the esti- 

 mated maximum precipitation will occur on all the sub-areas in any 

 one year. 



STORM PRECIPITATION. 



Records were compiled of all storms within the Red River water- 

 shed with precipitation of 2 inches or more in 24 hours. For each 

 storm the depths of precipitation for one day, two days, three days, 

 and for the duration of the storm were determined for all observa- • 

 tion stations within or near the watershed. The records cover the 

 26-year period 1894 to 1919, and include 130 storms of from two to 

 eight days' duration (see Table 8). By plotting isohyetals on a map 

 of the Red River watershed the intensity and extent of the storms 

 were made evident. The areas covered by storms during the years 

 1894 to 1904 are somewhat indefinite, as the number of records is 

 small. For the period 1905 to 1919 many more records are available 

 and the areas covered can be more accurately determined. During 

 many of the storms excessive precipitation occurred in but few locali- 

 ties and covered but small areas. The relative importance of the 

 storms was determined from the isohyetal maps. 



Table 8. — Storm periods during which precipitation of 2 inches or more occurred 

 on Red River ivatershed in the United States. 



Year. 



Date. 



Length 

 ofstorm 

 period. 



Year. 



Date. 



Length 

 ofstorm 

 period. 



1894 



Apr. 15-20 



Days. 

 6 

 3 

 4 

 5 

 5 



1896 





Days. 

 4 





June 26-28 





Apr. 8-13 



6 



1895 





Apr. 2.5-30 



6 





Jane 6-10.. 



May 11-17 



7 





July 4-8 



June 4-8 



5 



