Quantifying the Effect of Subcritical Water Repellency on Sorptivity: A Physically Based Model

• Published in: Water resources research 2020-11, Vol.56 (11), p.n/a
• Main Authors: Shillito, R. M., Berli, M., Ghezzehei, T. A.
• Format: Article
• Language: English
• Subjects: Contact angle; Debris flow; Emergency preparedness; Emergency response; Fires; Flood predictions; Flooding; Floods; Hydraulic conductivity; Hydrologic models; Hydrology; Hydrophobicity; Infiltration; Laboratory experiments; Mathematical models; Model matching; Moisture content; Mountain regions; Mountainous areas; Pest control; post‐fire runoff; Rain; Raindrops; Rainfall; Repellency; Repellents; Runoff; Runoff models; Sand; Silica; Silicon dioxide; Soil; Soil water; soil water repellency; Soils; sorptivity; Water; Water droplets; Water drops; Water repellent soils; Wettability; Wildfires
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2020WR027942

Soil water wettability or water repellency is a phenomenon that can affect infiltration and, ultimately, runoff. Thus, there is a need to develop a model that can quantitatively capture the influence of water repellency on infiltration in a physically meaningful way and within the framework of existing infiltration theory. The analytical model developed in this study relates soil sorptivity (an infiltration parameter) with contact angle (a direct measure of water repellency) for variably saturated media. The model was validated with laboratory experiments using a silica sand of known properties treated to produce controlled degrees of water repellency. The measured contact angle and sorptivity values closely matched the model‐predicted values. Further, the relationship between the frequently used water drop penetration time test (used to assess water repellency) and sorptivity was illustrated. Finally, the direct impact of water repellency on saturated hydraulic conductivity was investigated due to its role in infiltration equations and to shed light on inconsistent field observations. It was found that water repellency had minimal effect on the saturated hydraulic conductivity of structureless sand. A quantitative model for infiltration incorporating the effect of water repellency is particularly important for post‐fire hydrologic modeling of burned areas exhibiting water repellent soils. Plain Language Summary Wildfires can cause soils to become water repellent, which means that raindrops may not immediately enter the soil (infiltrate) and may instead flow over the land surface. In this way, water repellency is believed to contribute to the sometimes‐devastating flooding and debris flows that follow rainfall events on burned mountainous areas. Currently, however, the effect of water repellency on infiltration and runoff cannot be accounted for directly. Through our research, we were able to quantitatively describe not only the degree of water repellency but also the effect of water repellency on infiltration. This quantification will allow for the improvement of runoff models to aid in post‐fire flood prediction and emergency response. Key Points The developed model shows the effect of water repellency on sorptivity and was validated with experiments using a well‐characterized sand The developed model accounted for the effects of contact angle, pore radius, porosity, and moisture content on sorptivity Simulations showed repellency‐altered sorptivity had