On the Links Among Forest Structure, Throughfall Patterns, Soil Permeability, and Overland Flow Connectivity

Abstract:

Throughfall - rain that drops through forest canopies - greatly varies in space. Yet, this spatial variation is not similar across forests; instead, recent research provides evidence that forest structure influences the strength and range of throughfall autocorrelations. Moreover, previous studies showed that throughfall patterns can be temporally persistent, which hints at their potential hydrological relevance. Whether or not these patterns indeed influence the hydrological response of forested hillslopes remains an open question. Some studies reported an influence of throughfall spatial structures on runoff generation whereas others indicate that throughfall patterns are of secondary importance. Most of the previous studies considered only a few realizations of throughfall spatial structures and neglected soil variability. In this study I try to overcome these limitations by combining multiple high-resolution, throughfall and soil permeability datasets from different forest ecosystems. Based on stochastic simulations I investigate the effect of throughfall and soil permeability spatial patterns on overland flow connectivity by calculating the size of connected patches where throughfall intensities exceed the permeability in the topsoil. An increasing connectivity of the patches indicates an increasing likelihood of overland flow generation. So far, the simulations provide strong evidence that spatially structured throughfall patterns decrease overland flow connectivity. That is, an increasing autocorrelation in throughfall always decreases the connectivity of those patches in which throughfall intensities exceed soil permeability. Interestingly, the simulations further indicate that throughfall patterns in old-growth forests have more impact on overland flow connectivity than patterns typically found in young forests. Based on these results I discuss the hypothesis that forest succession, which typically results in an increasingly structured canopy, involves a decrease in overland flow connectivity.