Response time of forested mountainous watersheds in humid regions

This study proposes an analytical method for the estimation of time lag for forested mountainous watersheds. The water flow in a watershed is separate d and analyzed in two phases, the land or hillslope phase and the stream ch annel phase. In many areas around the globe the flow in a forested high gra dient watershed is generated through subsurface pathways as several field e xperiments have shown. The kinematic wave theory is used to describe the ge neration of flow from steep forested hillslopes. This hillslope runoff is, then, used as input to the stream channels. The equations were developed by assuming kinematic conditions in the stream channel and that the stream sl ope and the roughness coefficient i) vary according to a second order polyn omial with the distance from the mouth of the watershed, ii) vary linearly with the distance from the outlet of the watershed, and iii) are constant t hroughout the watershed. Comparison of the results of the proposed equation with data from two experimental watersheds in Coastal British Columbia ind icates that the three expressions of the proposed equation, even the simple st one assuming constant stream slope and roughness coefficient, are reliab le and give good approximation of the observed time lag.