Water Flow Modeling with Dry Bulk Density Optimization to Determine Hydraulic Properties in Mountain Soils

Abstract

The GEOtop model was used to calculate water flow in mountain soils. Dry soil bulk density was optimized to determine soil hydraulic properties. Soil water contents for two watersheds were used to test the optimization procedure. A new method for determining profile‐average and depth‐wise hydraulic properties in heterogeneous mountain soils is presented using the GEOtop watershed model in 1‐D vertical mode. Dry soil bulk density–converted volumetric soil water retention data are used to determine van Genuchten soil water retention parameters, and the Kozeny–Carman equation is used to determine saturated soil hydraulic conductivity. Optimum dry soil bulk densities are identified by minimizing the sum of squared error between measured and calculated soil water content time series. The new method was tested using soil moisture data from soil profiles at the Dry Creek Experimental Watershed, Boise, ID, and the Libby Creek Experimental Watershed, Laramie, WY. Results of different scenarios showed that the optimization of a single profile‐average dry soil bulk density is a good option for describing soil water flow in the heterogeneous mountain soils. Soil water content modeling efficiency (ME) values of 0.084 ≤ ME ≤ 0.745 and –2.443 ≤ ME ≤ 0.373 were found for the Dry Creek and Libby Creek sites, respectively. Relatively low ME values for the deepest sensor depths for some scenarios were attributed to the overestimation of soil water freezing and uncertainty in the soil water retention function near saturation. The resulting calibration procedure is computationally efficient because only one parameter (dry soil bulk density) is optimized.

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

“…This study uses the bulk density optimization method of Fullhart et al () to determine the hydraulic properties of the upper and lower regolith in small catchments in Idaho and Wyoming. A bimodal subsurface with either porous media or coarse fragments is assumed.…”

“…The modifications pertain to how turbulent fluxes in the soil–plant–atmosphere system are handled. Specifically, the leaf boundary resistance was modified, as well as the two coefficients in the soil evaporation equation, as detailed in Fullhart et al (). A new modification made to the model for this study is the inclusion of osmotic head in the soil‐freezing algorithm to incorporate the impact of dissolved ions on ice formation (GEOtop's equation for soil freezing is given in its original form without osmotic head by Dall'Amico, Endrizzi, Gruber, & Rigon, ).…”

“…Soil texture and gravimetric water retention were measured in the laboratory using sieved (<2‐mm diameter) grab samples from the northeast and southwest facing slopes. The texture and retention data showed little variation and were pooled (see Fullhart et al, , for details). The simulation period was August 1, 2011, to August 1, 2014.…”

“…Vegetation height was 0.4 m, leaf‐stem area index was 3.3 m 2 m −2 , root depth was 1.25 m, and the minimum stomatal resistance was 170 s m −1 (Kumar et al, ). The dimensionless eddy decay coefficient, regulating within‐canopy turbulent exchange (Endrizzi & Marsh, ; Fullhart et al, ), was set to 2.0 based on a sensitivity analysis. Values for this coefficient are generally between 0.4 and 4.0 (Brutsaert, ).…”

“…A novel bulk density optimization method was recently developed by the current authors to determine subsurface hydraulic parameters for 1‐D vertical water flow in heterogeneous mountain soils. Briefly, bulk density‐converted water retention data are used to determine water retention parameters, and the Kozeny–Carman equation is used to estimate saturated hydraulic conductivity (Fullhart, Kelleners, Chandler, McNamara, & Seyfried, ). It is the purpose of this study to demonstrate the use of bulk density optimization to determine subsurface hydraulic properties for 3‐D water flow in small mountainous catchments using an integrated watershed model.…”