Soil water repellency index prediction using the molarity of ethanol droplet test

The profound impact of soil water repellency (WR) on vadose zone processes makes accurate characterization of this phenomenon paramount. Numerous WR measurement techniques exist, each having advantages and disadvantages with regard to laboriousness, resolution, and accuracy. The molarity of ethanol droplet (MED) test quantifies WR as the lowest ethanol concentration permitting droplet penetration within 5 s, or alternatively, the 90° liquid surface tension of the infiltrating droplet (γ _ND). This method is simple and rapid but poorly represents soil wetting behavior across measurement intervals. Although time consuming, water/ethanol sorptivity ratio calculation of the repellency index (R) generates a continuous, linear scale of WR that intrinsically isolates the effect of WR on infiltration. This study compared MED and R measurements of sand samples displaying varying degrees of WR. Each technique was performed at 20°C and 1.78 kPa H ₂O vapor pressure using duplicate subsamples of oven-dried (55°C) sands. A nonlinear association between R and γ _ND or MED was observed. Regressing log ₁₀ R by γ _ND revealed a statistically significant model, yet the 95% log ₁₀ R prediction interval included values less than the theoretical lower limit of R. Alternatively, regressing log ₁₀ R by MED generated the following model (P < 0.0001, r ² = 0.727): log ₁₀ R = 0.705 + 0.5144(MED), capable of predicting R within the operation bounds of R theory. While the predicted R values are distributed across a wide interval, their availability offers cautious users an intuitive scale for enhanced interpretation of more commonly generated MED data.