Incipient subsurface heterogeneity and its effect on overland ?ow generation – insight from a modeling study of the ?rst experiment at the Biosphere 2 Landscape Evolution Observatory

摘要

Evolution of landscape heterogeneity is controlled by coupled Earth system dynamics, and the resulting pro-cess complexity is a major hurdle to cross towards a uni?ed theory of catchment hydrology. The Biosphere 2 Landscape Evolution Observatory (LEO), a 334.5 m~2 arti?cial hillslope built with homogeneous soil, may have evolved into hetero-geneous soil during the ?rst experiment driven by an intense rainfall event. The experiment produced predominantly seep-age face water out?ow, but also generated overland ?ow, causing super?cial erosion and the formation of a small chan-nel. In this paper, we explore the hypothesis of incipient het-erogeneity development in LEO and its effect on overland ?ow generation by comparing the modeling results from a three-dimensional physically based hydrological model with measurements of total mass change and seepage face ?ow. Our null hypothesis is that the soil is hydraulically homoge-neous, while the alternative hypothesis is that LEO developed downstream heterogeneity from transport of ?ne sediments driven by saturated subsurface ?ow. The heterogeneous case is modeled by assigning saturated hydraulic conductivity at the LEO seepage face (K_(sat,sf)) different from that of the rest (K_(sat)). A range of values for K_(sat), K_(sat,sf), soil porosity, and pore size distribution is used to account for uncertainties in estimating these parameters, resulting in more than 20 000 simulations. It is found that the best runs under the hetero-geneous soil hypothesis produce smaller errors than those under the null hypothesis, and that the heterogeneous runs yield a higher probability of best model performance than the homogeneous runs. These results support the alternative hy-pothesis of localized incipient heterogeneity of the LEO soil, which facilitated generation of overland ?ow. This model-ing study of the ?rst LEO experiment suggests an important role of coupled water and sediment transport processes in the evolution of subsurface heterogeneity and on overland ?ow generation, highlighting the need of a coupled modeling sys-tem that integrates across disciplinary processes.