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Biomat Flow: From Field Experiments with Dyes to Pore-Scale Modeling of Transport Properties and Flow Models
Citation:
Gerke, K., R. Sidle, D. Mallants, R. Valisyev, M. Karsanine, E. Skvortsova, AND D. Korost. Biomat Flow: From Field Experiments with Dyes to Pore-Scale Modeling of Transport Properties and Flow Models. Presented at AGU Fall Meeting, San Francisco, CA, December 09 - 13, 2013.
Impact/Purpose:
Presented at AGU Fall Meeting 2013.
Description:
Recent studies highlight the important role that the upper litter layer in forest soils (biomat) plays in hillslope and catchment runoff generation. This biomat layer is a very loose material with high porosity and organic content. Direct sampling is usually problematic due to limited layer thickness. Conventional laboratory measurements can mobilize solids or even cause structure failure of the sample thus making measurements unreliable. It is also difficult to assess local variation in soil properties and transition zones using these methods; thus, they may not be applicable to biomat studies. However, if the physics of flow through this layer needs to be quantified and incorporated into a model, a detailed study of hydraulic properties is necessary. Herein we show the significance of biomat flow by staining experiments in the field, study its structure and transition to mineral soil layer using X-ray micro-tomography, assess hydraulic properties and structure differences using a pore-scale modeling approach, and, finally, use conventional variably-saturated flow modeling based on Richards equation to simulate flow in the hillslope. Using staining tracers we show that biomat flow in forested hillslopes can extend long distances (lateral displacement was about 1.2 times larger than for subsurface lateral flow) before infiltration occurs into deeper layers. The three-dimensional structure of an undisturbed sample (4 x 3 x 2.5 cm) of both biomat and deeper consolidated soil was obtained using an X-ray micro-tomography device with a resolution of 15 um. Local hydraulic properties (e.g., permeability and water retention curve) for numerous layers (e.g., transition zones, biomat, mineral soil) were calculated using Stokes flow FDM solution and pore-network modeling. Anisotropy, structure differences, and property fluctuations of different layers were quantified using local porosity analysis and correlation functions. Current results support the hypothesis that small-scale structural differences alone can explain the lateral transport observed in field. Possible water repellent behaviour at the biomat-mineral soil interface may also be contributing to extended surface flow. Based on conventional modeling with HYDRUS-2D we show how pore-scale modelling-derived hydraulic properties can improve large scale simulations and clarify how local heterogeneities in wetting and hydraulic properties affect infiltration into deeper soils and likely magnify preferential flow.
