INFLUENCE OF PARTICLE-SIZE DISTRIBUTION MEASUREMENT METHOD ON THE PREDICTION OF SOIL WATER RETENTION CURVE

Application of the Arya and Paris (AP) model to estimate the soil water retention curve requires detailed particle-size distributions (PSD) that can be obtained by fitting a continuous model to traditional sieve-hydrometer (SH) data or using the laser diffraction (LD) method. The AP model was applied to 40 Sicilian soils for which PSD was measured by both SH and LD methods. The scale factor  was set equal to 1.38 (procedure AP1) or estimated by a logistic model with parameters gathered from literature (procedure AP2). To simplify application of the AP model, a relationship for estimating a soil-specific  value from clay content was developed (procedure OPT). For both SH and LD data, procedure AP2 allowed a more accurate prediction of the water retention than procedure AP1 confirming that a unique value of  for soils very different in texture is not suggested. Discrepancies in estimated water retention curves were attributed to underestimation of PSD by the LD method for the finer particle diameters. Overall, SH data resulted in more accurate predictions of the water retention curve than LD data. From comparison of the -distributions it was supposed that the PSDs determined by the traditional sieve-hydrometer method are in more close similarity with the measured water retention curve than those determined by the LD method. For a validation dataset of 170 Sicilian soils, the most accurate predictions of the water retention curve were obtained by procedure OPT that can be considered a valid alternative to the more complex logistic model.