Discussion of “Flood frequencies and sedimentation from forest watersheds”

Abstract

The history of flood studies might be said to consist of seeking answers to: “How big can floods get?”, then “How often will floods come?”, and now “What causes floods?” ‐ or design flood studies, flood frequency studies, and flood cause studies. Hydrologists have lately devoted more and more time to the last. Peak discharges and sedimentation from watersheds as related to storm and watershed variables are one part of these flood cause studies.Schiff's discussion [Trans., v . 30, pp. 584–586, 1949] of our report on such a study brings out some important points. In the present primitive state of development of the relationship of peak discharges to meteorological and watershed variables such suggestions are sure to be helpful. He suggests and presents data to show that for two Ohio watersheds the effects on discharges of increasing forest cover varied considerably between two areas, one of .001 and one of .068 sq mi. In the smaller of these (his Watershed 134) he reports significant reductions in peak discharges occurred when forest cover was changed from a density of ten to 95 per cent. This was due to a reduction in surface runoff. Of the larger (his Watershed 172) he makes the statement that, “Had the actual peak discharges been used to establish an equation for Watershed 172, increases in cover would seem to have little influence. Better cover increased permeability rates and consequently infiltration rates so that most of the flow of Watershed 172 is now subsurface flow which reappears above the measuring flume. Peak discharges have not materially reduced and reflect the particular soil and geologic conditions of Watershed 172.” This is an interesting interpretation, and the factors which he talks of are probably operating; however, the results on peak discharges seem to be not at all what he hypothesizes. First, we would expect a reduction in peak discharges when surface runoff is changed to subsurface flow. And second, that such a reduction occurs and is of large magnitude may be shown when Schiff's data are actually “used to establish an equation.] Taking the data from his Table 1 and testing the relation of Q172, the peak discharges in inches depth per hour to the associated per cent cover density (C), with the maximum hourly precipitation (P1) in inches per hour as the meteorological control we find thatequation imageThe regression coefficient for cover density is found to be highly significant and the magnitude of the effect indicated is such that associated with the increase in cover from ten to 95 per cent there was a reduction of 61 per cent in the peak discharges. This is certainly not an insignificant decrease. With more storm data it would be possible to test other meteorological parameters and improve the equation but probably the cover density effect would remain about the same, since no obvious correlation appears between cover density and any of the meteorological characteristics listed in his Table 1.