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Title: CORRESPONDENCE AND UPSCALING OF HYDRAULIC FUNCTIONS FOR STEADY-STATE FLOW IN HETEROGENEOUS SOILS

Author
item ZHU, JIANTING - D BIOL&AG ENG, TEXAS A&M
item MOHANTY, BINAYAK - DEP BIOL&AG ENG,TEXAS A&M
item WARRICK, ARTHUR - DEP SWES, U AZ, TUCSON
item Van Genuchten, Martinus

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2004
Publication Date: 11/1/2004
Citation: Zhu, J., Mohanty, B.P., Warrick, A.W., Van Genuchten, M.T. 2004. Correspondence and upscaling of hydraulic functions for steady-state flow in heterogeneous soils. Vadose Zone Journal. 3:527-533.

Interpretive Summary: Simulations of unsaturated flow between the soil surface and the groundwater table typically use relatively simple functional relationships to represent the water-retention (water holding) capacity of the soil and the unsaturated hydraulic conductivity (permeability) as a function of the water content. The Brooks and Corey piecewise continuous model and the van Genuchten sigmoidal model are among the more widely used hydraulic conductivity functions. Conditions for which alternative forms of the hydraulic functions give the same or similar responses (e.g., for infiltration, evaporation, root water uptake, or recharge) are important in many applications. A related issue of concern for heterogeneous field soils is the upscaling of hydraulic parameters. Soil hydraulic functions are generally valid only at the point or local scale. When they are used in larger (plot, field, watershed or regional) scale models, major questions remain about how best to average the spatially variable hydraulic properties over a heterogeneous soil volume. The main objective of this study was to investigate how the hydraulic parameters of the above two functions correspond and what the correspondence implies in terms of averaging of the hydraulic properties for steady-state vertical flow in heterogeneous soils at the larger scale. Parameter equivalence was established such that the hydraulic funcitons would predict the same vertical water flux. Results show that the hydraulic parameters corresponded extremely well and that the same rules can be used for averaging the parameters of different functions when predicting evaporation from relatively dry soils in the presence of a shallow groundwater table. On the other hand, when the surface was relatively wet and/or the groundwater table deep, it was more difficult to obtain correspondence between the models. The hydraulic functions corresponded especially poorly when infiltration was considered. Results are important when trying the predict water flow in natural field soils that are generally very heterogeneous (e.g., layered versus depth).

Technical Abstract: Soil hydraulic parameters at relatively large scales (e.g., remote sensing footprints) are important for land-atmosphere interaction and general circulation models as well as other applications. Within this context, we investigated two major issues involving soil hydraulic properties: (i) hydraulic parameter correspondence among some of the more commonly used soil hydraulic conductivity functions (i.e., the Gardner [G], Brooks-Corey [BC], and van Genuchten [VG] equations) and (ii) their application to upscaling of hydraulic properties for steady-state flow in heterogeneous soils. We first establish parameter equivalence among the conductivity functions based on preserving macroscopic capillary lengths and predicting the same vertical water flux. Next we investigate the significance of parameter equivalence on averaging schemes for the hydraulic parameters to allow predictions of the ensemble characteristics for steady-state flow. Results show that the hydraulic parameters correspond very well and that the same rules can be used for averaging the parameters of different hydraulic conductivity functions when predicting ensemble evaporation rates from heterogeneous soils having a relatively large suction at the soil surface (e.g., a dry surface condition and/or a shallow groundwater table). On the other hand, when the surface suction is finite (especially when the suction is relatively small and/or the groundwater table is deep), it is more difficult to obtain correspondence between the parameters of the different conductivity models. The hydraulic functions correspond especially poorly when infiltration is considered. Parameter equivalence between the hydraulic functions is always satisfied for the case of evaporation from a shallow water table, as long as the macroscopic capillary length is preserved.