Dynamic runoff connectivity of overland flow on steep forested hillslopes: Scale effects and runoff transfer

Abstract

[1] Both scaling effect and connectivity of overland flow were examined in steep hillslopes covered by (1) Japanese cypress (hinoki, Chamecyparis obtusa) plantations with sparse understory vegetation, (2) hinoki plantations with fern understory vegetation, and (3) deciduous forests. Two sizes of plots were installed for monitoring overland flow: small (0.5 Â 2 m) and large hillslope scale (8 Â 24-27 m). For all hillslopes, measurable amounts of overland flow occurred during storms. Runoff coefficients of large plots (0.1-3%) were consistently smaller than those of small plots (20-40%). Estimated runoff flow lengths at the hillslope scale were based on runoff coefficients from small plots and were used to calculate runoff volume from large plots. Then we compared the differences between observed and estimated runoff volumes of large plots. Estimated runoff from large plots was smaller than observed runoff in hinoki slopes with sparse understory vegetation. Greater amounts of observed compared to estimated overland flow suggest that more runoff occurred from hillslopes with sparse understory. In contrast, estimated overland flow was larger than observed runoff from the deciduous forest, implying greater opportunities for infiltration compared to hinoki hillslopes. Comparison of estimated versus observed overland flow for successive 5 min intervals during storms indicates that runoff networks expand upslope during short and intense precipitation periods. Our examination and comparison of storm runoff from small and large plots facilitate better understanding of runoff mechanisms, scaling effects in hillslopes, and connectivity of the overland flow network.

1-D steady state runoff production in light of queuing theory: Heterogeneity, connectivity, and scale

“…For q < 1, the coefficient decreases rapidly, as the inverse of the domain size. Such a decrease has not been observed by the authors who support the scale effect theory [Cerdan et al, 2004;Gomi et al, 2008]. We suggest that the slow decrease observed on-site and the apparent independence from the rainfall rate may come from the influence of the zero flow upper boundary condition on the mean runoff flow rate, near the top of the domain.…”

“…Connectivity is used to describe the coalescence of runoff patterns during a rainfall event. This mechanism is important because it explains the rising limb of a hydrograph, as the patterns connect to the stream [Gomi et al, 2008]. It is also helpful in explaining the scale effect and the rainfall intensity threshold mentioned previously [Hopp and McDonnell, 2009].…”

“…[7] Several authors have suggested that the observed scale dependency of hillslope runoff (i.e., decrease of the runoff coefficient with domain size) is controlled by spatial and temporal variability in infiltration and, thus, by the runoff-runon process [Wood et al, 1988;Cerdan et al, 2004;Gomi et al, 2008]. Others invoke the spatiotemporal variability in the rainfall rate [Wainwright and Parsons, 2002], vegetation [Lesschen et al, 2009], topography or a combination of these [Wood et al, 1988].…”

“…They showed that an appropriate representation of these connectivities in a distributed runoff model enables it to simulate realistic flow patterns and hydrographs. Based on runoff experiments performed on steep forest hillslopes, Gomi et al [2008] discussed the relationships among scale effect, connectivity, land-cover, and rainfall. Sen et al [2010] delineated spatially and temporally variable runoffgeneration areas in an instrumented pasture hillslope.…”

Quantifying spatial and temporal discharge dynamics of an event in a first order stream, using distributed temperature sensing

“…To observe the spatial and temporal distribution of lateral inflows, several researchers excavated trenches (Woods and Rowe, 1996;Weiler et al, 1998;Uchida et al, 2005;Retter et al, 2006;Gomi et al, 2008;Tromp-van Meerveld et al, 2008). Although these were able to give spatial and temporal flow information, installation of trenches is destructive and limited in size (2-60 m).…”