Dirksen, C., Dept. of Water Resources, Wageningen Agricultural Univ., Wageningen, The Netherlands.
Dasberg, S., Dept. of Water Resources, Wageningen Agricultural Univ., Wageningen, The Netherlands.

The purpose was to compare the Topp-Davis-Annan calibration equation with a theoretical (Maxwell-De Lor) and an empirical (fitting exponent α) mixing model for the four components: solid phase (s), tightly bound water (bw), free water, and air. Water content permittivity were measured, gravimetrically and by time domain reflectometry (TDR), on packed columns of 11 soils ranging from loess to pure bentonite. Topp yielded accurate values only for four soils, including illite. Maxwell-De Loor gave similar accuracy for seven soils, including attapulgite. Deviations from Topp can be accomodated by the Maxwell-De Loor equation. This makes it a better calibration equation than Topp. The empirical α model is sensitive to the unpredictable value of α and cannot accomodate anomalous behaviour. -from Authors