Can the Dry-End (-1 to -1000 MPa) Soil Water Characteristic Curve be Well Characterized with a Single Point?.

It is well known in the literature that water content is linearly related to the logarithm of water potential at water potentials drier than about -1 MPa.  There is also evidence that the zero water content intercept, or the point where the line extrapolates to zero water content is a universal value among all soils, although this has been debated in the literature.  If the zero water content intercept is indeed a fixed value and the semi-log SWC curve is a straight line, then it should be possible to characterize the entire dry-end SWC curve with a single water content vs. water potential point in the dry region.  We used a new automated SWC curve generator (Vapor Sorption Analyzer) to collect dry region SWC curves on both the wetting and drying legs of the hysteresis loop for 13 well-characterized soils.  The zero water content intercept ranged from 5.7 to 6.3 log kPa for both adsorption and desorption SWC curves for all 13 soils, with an average value of 6.0 log kPa (-1000 MPa).  The relatively tight range argues that minimal error would be introduced into single point SWC curves by the use of a universal zero water content intercept.  However, hysteresis and non-linearity in the SWC curve (believed to be due to clay hydration state) add complication to the single point SWC curve method.