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Identification of optimal soil hydraulic functions and parameters for predicting soil moisture
Citation:
Pan, F., R. B. MCKANE, AND M. Stieglitz. Identification of optimal soil hydraulic functions and parameters for predicting soil moisture. Hydrological Sciences Journal. IAHS LIMITED, Oxford, Uk, 57(4):723-737, (2012).
Impact/Purpose:
We examined the accuracy of several commonly used soil hydraulic functions and associated parameters for predicting observed soil moisture data.
Description:
We examined the accuracy of several commonly used soil hydraulic functions and associated parameters for predicting observed soil moisture data. We used six combined methods formed by three commonly used soil hydraulic functions – i.e., Brooks and Corey (1964) (BC), Campbell (1974) (CA), and van Genuchten (1980) (vG) – and two methods to determine soil hydraulic parameters – i.e., a soil hydraulic parameter look-up table given in Rawls et al. (1982) (RA), and soil pedotransfer functions (PTF) developed by Cosby et al. (1984) (CO). We also used a novel data analysis and modeling approach that eliminated the effects of evaporation and transpiration so that specific sources of error among the six combined methods could be identified and quantified. By comparing simulated and observed soil moisture at six sites of the Soil Climate Analysis Network (SCAN), we identified the BC-RA and CA-RA methods as most accurate for predicting soil moisture. Main reasons were that (1) the RA soil parameters proved more accurate than those using the CO PTFs; (2) the BC and CA functions provided essentially the same accuracy (root mean square error for simulated vs. observed soil moisture < 0.5%V/V); and (3) although the vG function provided similar overall accuracy as the BC and CA functions, the vG function did not simulate observed high frequency variations in soil moisture nearly as well. Through a series of sensitivity tests, we also show that adjusting only the soil saturated hydraulic conductivity parameter, Ks is insufficient for representing important effects of macropores on soil hydraulic conductivity. Our analysis illustrates that, in general, soil hydraulic conductivity is less sensitive to Ks than to the soil pore size distribution parameter, b (the scaling exponent of the power function of soil hydraulic conductivity).
