Abstract
AbstractGlobal land surface models use spatially distributed soil information for the parameterization of soil hydraulic properties (SHP). Parameters of measured SHP are correlated with easy‐to‐measure soil properties to construct general pedotransfer functions (PTFs) used to predict SHP from spatial soil information. Global PTFs are based on a limited number of samples yielding highly variable and poorly constrained SHP. The study implements a physical constraint, soil‐specific capillary length, to reduce unphysical combinations of SHP. The procedure fits concurrently soil water retention and capillary length using the same parameters. Results suggest that meeting the capillary length constraint has minor effects on the goodness of fit to soil water retention data. Constrained SHP were applied to represent 4 years of lysimeter fluxes yielding evapotranspiration values in close agreement with measurements relative to slight overestimation by unconstrained SHP. The procedure was applied for testing constraint SHP at a regional scale in New Zealand using the surface evaporation capacitance model and Noah‐MP for detailed simulations of land surface processes. The use of constrained SHP in both models yields higher surface runoff in agreement with observations (unconstrained SHP severely underestimated runoff generation). The concept of constrained SHP could be extended to include other physical constraints to improve PTFs, for example, by consideration of vegetation cover and soil structure effects on infiltration.
Highlights
The quantification of near‐surface hydrologic processes requires information on spatially distributed parameters characterizing the soil hydraulic properties (SHP)
To overcome ambiguities in the determination of SHP based on goodness of fit to soil water characteristics (SWC) only, we report a method for injecting additional physical constraints to reduce occurrence of unphysical parameter combinations and potentially improve land surface representation
The specific objectives of this study were to (1) illustrate how the standard and unconstrained SHP (USHP) estimation may result in unphysical parameter combination, to (2) propose a parameter estimation procedure that links SWC and characteristic length information, and (3) to evaluate the potential improvement of constrained SHP (CSHP) for near‐surface process representation
Summary
The quantification of near‐surface hydrologic processes requires information on spatially distributed parameters characterizing the soil hydraulic properties (SHP). These parameters are inferred from fitting models to measured values of soil water characteristics (SWC) that relate volumetric soil water content θ with the capillary head h. The parameter values for the SHP are estimated either (i) by minimizing deviations between measured and parametrized water content capillary head (or conductivity) values or (ii) by using pedotransfer functions (PTFs) that link easy‐to‐measure soil properties (textural fractions, bulk density, organic carbon, or pH) with the hydraulic properties. Estimating SWC requires determination of parameter values that describe the pore size distribution and a characteristic pore size; these values play an important role in other physical processes (thermal conductivity, water infiltration, and capillary flow towards an evaporating surface)
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