flow<- c(40, 39, 38, 37, 50, 49, 46, 44, 43, 45, 40, 30, 80, 75, 50, 55, 53, 51, 49, 100)
event<- c(1,1,1,1,2,2,2,2,2,3,3,3,4,4,4,5,5,5,5,6)
BF<- c(NA, NA, NA, NA, 39, 39, 39, 39, 39, 46, 46, 46, 45, 45, 45, 80, 80, 80, 80, 53)
a<- data.frame(flow, event, BF)

flow event BF  BF_1
1    40   1   NA   NA
2    39   1   NA   NA
3    38   1   NA   NA
4    37   1   NA   NA
5    50   2   39   39
6    49   2   39   39
7    46   2   39   39
8    44   2   39   39
9    43   2   39   39
10   45   3   46   37
11   40   3   46   37
12   30   3   46   37
13   80   4   45   45
14   75   4   45   45
15   50   4   45   45
16   55   5   80   30
17   53   5   80   30
18   51   5   80   30
19   49   5   80   30
20  100   6   53   53