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Performance Evaluation of Models That Describe the Soil Water Retention Curve between Saturation and Oven Dryness

^a Dep. of Soil Management and Soil Care, Coupure Links 653, B-9000 Ghent, Belgium
^b Research Unit on Environment and Conservation of Natural Resources, National Institute of Agricultural Research, BP 415, Ave. de la Victoire, 10000 Rabat, Morocco
^c USDA, U.S. Salinity Lab., 450 W. Big Springs, Riverside, CA 92507

View larger version (35K): [in this window] [in a new window]   FIG. 1. Textural distribution of 137 soils from the UNSODA database (Nemes et al., 2001) used in model development: sandy (sand and loamy sand), loamy (sandy loam, loam, silt loam, and silt), and clayey (sandy clay loam, silty clay loam, clay loam, sandy clay, silty clay, and clay) soils.

View larger version (15K): [in this window] [in a new window]   FIG. 2. Variation of bulk density and organic matter content in the data set.

View larger version (26K): [in this window] [in a new window]   FIG. 3. Boxplots of (a) RMSE, (b) R2, and (c) adjusted R2 (R2adj) values for different soil-water retention curve models: Campbell and Shiozawa (CS); Rossi and Nimmo four-parameter sum (RN1); Rossi and Nimmo three-parameter junction (RN2); Fayer and Simmons modified Brooks–Corey (FS1); Fayer and Simmons modified van Genuchten (FD2); Webb (W); Groenevelt and Grant (GG); and Khlosi et al. (KCGS). The box plots summarize the distribution of RMSE, R2, and R2adj. The horizontal full line in each box signifies the median value and the mean in a dotted line, whereas the bottom and top of the box represent the 25th and 75th percentile. The whiskers display the 10th and 90th percentile, while the points indicate the outliers.

View larger version (35K): [in this window] [in a new window]   FIG. 4. Observed and fitted soil-water retention curves (SWRCs) for a loamy sand (LS), a silt loam (SiL), and a clay (C). Models are: Campbell and Shiozawa (CS); Rossi and Nimmo four-parameter sum (RN1); Rossi and Nimmo three-parameter junction (RN2); Fayer and Simmons modified Brooks–Corey (FS1); Fayer and Simmons modified van Genuchten (FD2); Webb (W); Groenevelt and Grant (GG); Khlosi et al. (KCGS); and van Genuchten (VG). The curve in the case of the GG model is only defined for potential values lower than –0.1 kPa (or –1 cm).