Testing the BEST procedure to estimate the soil water retention curve

The BEST (Beerkan Estimation of Soil Transfer parameters) procedure is attractive for simple soil hydraulic characterization but there is the need to test the reliability of the predictions. In this investigation, the BEST procedure to predict water retention of 199 Sicilian soils was evaluated. The BEST water retention model performed well (relative error, Er≤0.05) for approximately 80% of the soil samples. Low errors were obtained in soils with a high clay, cl, content (≥44%), whereas both high and low Er values were obtained in soils with a lower cl content. The BEST particle size distribution (PSD) model was accurate for 50% of the samples and the fitting accuracy increased with cl, with Er≤0.05 for cl≥45.2%. Alternative models, allowing an improved description of the PSD data, were also characterized by a fitting accuracy that increased with cl. Differences among alternative estimates of the particle-size shape index, PM, decreased as the cl content of the soil increased. The laboratory determined water retention shape index, pm, did not coincide with that estimated according to BEST (mean factor of discrepancy=1.33, maximum=4.6), and absolute discrepancies were particularly noticeable in soils with clb10%. Better results (mean and maximum discrepancy by a factor of 1.29 and 2.3, respectively) were obtained with the indirect procedure by Minasny and McBratney (2007), using the soil's clay and sand percentages. In conclusion, the BEST water retention model can be considered appropriate for most soils. Checking the soil textural characteristics before conducting the BEST experiment is suggested to establish if the expected performance of the water retention model is good or there is the possibility of a poor description of the data. Using a reduced information on soil textural characteristics does not compromise prediction of water retention characteristics in Sicily. Testing the results of this investigation with other databases including a larger number of clay soils is desirable.