Bulk density optimization to determine subsurface hydraulic properties in Rocky Mountain catchments using the GEOtop model

Authors: FULLHART, A - University Of Wyoming; KELLENERS, T - University Of Wyoming; CHANDLER, D - Syracuse University; MCNAMARA, J - Boise State University; Seyfried, Mark

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2019
Publication Date: 4/18/2019
Citation: Fullhart, A., Kelleners, T., Chandler, D., McNamara, J., Seyfried, M.S. 2019. Bulk density optimization to determine subsurface hydraulic properties in Rocky Mountain catchments using the GEOtop model. Hydrological Processes. 33(17):2323-2336. https://doi.org/10.1002/hyp.13471.
DOI: https://doi.org/10.1002/hyp.13471

Interpretive Summary: A new method for determining layer-wise subsurface hydraulic properties in mountainous catchments is presented using the integrated watershed model GEOtop. Bulk density – converted volumetric water retention data are used to determine van Genuchten water retention parameters, and the Kozeny-Carman equation is used to determine saturated hydraulic conductivity. Optimum bulk densities for the upper and lower regolith are determined by minimizing an objective function that contains profile-average soil-water content and stream discharge data. The new method was tested in two small catchments in the Dry Creek Watershed in Idaho and the Libby Creek Watershed in Wyoming. Modeling efficiencies for profile-average soil-water content for the two catchments were between 0.55 and 0.77. Modeling efficiencies for stream discharge were lower with values of 0.23 and 0.24 for the Idaho and Wyoming catchments, respectively. The calculated hydraulic properties suggest that stormflow is an important driver of streamflow in the Idaho watershed while lateral flow through the lower regolith is an important driver of streamflow in the Wyoming watershed. The overall calibration procedure is computationally efficient because only two bulk density values are optimized.

Technical Abstract: A new method for determining layer-wise subsurface hydraulic properties in mountainous catchments is presented using the integrated watershed model GEOtop. Bulk density – converted volumetric water retention data are used to determine van Genuchten water retention parameters, and the Kozeny-Carman equation is used to determine saturated hydraulic conductivity. Optimum bulk densities for the upper and lower regolith are determined by minimizing an objective function that contains profile-average soil-water content and stream discharge data. The new method was tested in two small catchments in the Dry Creek Watershed in Idaho and the Libby Creek Watershed in Wyoming. Modeling efficiencies for profile-average soil-water content for the two catchments were between 0.55 and 0.77. Modeling efficiencies for stream discharge were lower with values of 0.23 and 0.24 for the Idaho and Wyoming catchments, respectively. The calculated hydraulic properties suggest that stormflow is an important driver of streamflow in the Idaho watershed while lateral flow through the lower regolith is an important driver of streamflow in the Wyoming watershed. The overall calibration procedure is computationally efficient because only two bulk density values are optimized.