Experimental study on the influence of vegetation on the slope flow concentration time

Due to the steep slope of mountainous watersheds and large changes in vegetation coverage degree, flood response processes after rainstorms are complicated. The flow concentration time of the slope is a key parameter for the simulation of flood processes. The most widely used flow concentration time formula currently in the distributed hydrological model is T = L(0.6)n(0.6)i(-0.4)S(-0.3), which is derived from the kinematic wave theory (Melesse and Graham in J Am Water Resour As 40(4):863-879, 2004; Lee in Hydrol Sci 53(2):323-337, 2008). The flow confluence time T is characterized by the constant exponent of the slope length L, roughness n, effective rainfall intensity i and slope S, and the influence of vegetation on the flow concentration time is implied by the roughness. In this study, a series of heavy rainfall slope surface confluence tests under different slopes and vegetation coverage were carried out, a vegetation coverage factor, C, which was introduced, a statistical analysis method was used, and the vegetation coverage index was fitted. The results showed that the types of vegetation have a certain influence on the flow concentration time of slope, and the flow confluence time under turf vegetation was larger than the flow confluence time under shrubs vegetation; especially in the slope of the larger slope, the relative impact is more significant; at the same time, the influence of vegetation coverage on the flow concentration time of slope was more significant; no matter the condition of turf or shrub, the slope confluence time increased obviously with the increase in vegetation coverage. The index of vegetation coverage factor C varied with the slope and rain intensity. In general, the index of vegetation coverage factor C increased with the decrease in slope and decreased with the increase in rain intensity. In regard to the turf vegetation coverage index, when the slope is 45 degrees and 30 degrees, the decreasing trend of the vegetation coverage index a(0) is obvious with increasing rainfall intensity. When the slope is 15 degrees, the vegetation coverage index a(0) also decreases with increasing rainfall intensity. When the slope is 5 degrees, the vegetation coverage index a(0) basically has no change. In regard to the shrubs vegetation coverage index, when the slope is 45 degrees and 30 degrees, the decreasing trend of the vegetation coverage index a(0) is obvious with increasing rainfall intensity. When the slope is 15 degrees, the vegetation coverage index a(0) also decreases with increasing rainfall intensity. When the slope is 5 degrees, the vegetation coverage index a(0) basically has no change.