THE RELATION BETWEEN SOIL-WATER RETENTION AND PARTICLE-SIZE DISTRIBUTION PARAMETERS FOR SOME PREDOMINANTLY SANDY WESTERN-AUSTRALIAN SOILS

The soil water retention curve (WRC) may be estimated from soil textur e if there is shape similarity between the normalised cumulative parti cle mass size distribution curve (PSD) and the WRC. For similar shaped curves, parameters describing the shape of the PSD may also describe the shape of the corresponding WRC. We studied the relation between PS Ds and WRCs at 4 sites in Western Australia with predominantly sandy s oils and fitted a sigmoidal function to all normalised PSDs and WRCs. The model gave an excellent description of all PSD and WRC data sets. The parameter describing the slope of the WRC was correlated with the slope of the PSD. However, the asymptotic minima of the PSDs differed from the WRCs leading to poor estimates of the WRC residual water cont ents, theta(r). This difference was most marked in 2 clay subsoils and resulted in progressively greater errors in prediction of water conte nt with increasing negative pressure head.