SENSITIVITY OF SELECTED WATER-RETENTION FUNCTIONS TO COMPACTION AND INHERENT SOIL PROPERTIES

It is known that field-scale variations in subsurface hydraulic charac teristics are influenced, to a large extent, by soil properties. Limit ed information, however, exists on the sensitivity of hydraulic functi ons to field-scale variations in soil properties. The sensitivity of 4 soil water retention functions, theta(h), to variations in soil prope rties and changes in bulk density (rho) across and within soils along a 500-m transect has been assessed in this study. The theta(h) functio ns compared are those of van Genuchten, Brooks and Corey, Campbell, an d Gardner. Water retention characteristics for 7 soils, each packed to 2 relative rho, were established for each function. The coefficient o f determination, R-2, for the best fit of water retention ranged from 0.79 to 0.98 for the Gardner and Campbell functions, from 0.92 to 0.99 for the Brooks and Corey function, and from 0.83 to 0.99 for the van Genuchten function. Simple linear regression analysis indicated the no nlinear slope parameters of the 4 functions were more strongly correla ted with soil properties. However, only the van Genuchten slope parame ters were sensitive to changes in rho. No consistency existed between the sensitivity of the linear parameters of the 4 functions and soil p roperties, and none were sensitive to changes in rho. Except for the a lpha parameter in the van Genuchten function, all the parameters in th is function can be predicted with satisfactory confidence from soil pr operties and rho. The results indicate that, of the 4 functions assess ed, the van Genuchten theta(h) function is the most sensitive to field -scale variations in soil properties along a transect in a landscape u nit and to changes in rho.