Water repellency by volcanic ash interacting with organic matter: Incubation response and effect on infiltration

Abstract

Volcanic ash deposition disrupts soil surface hydrology. Our previous study showed that soil infiltration was reduced eightfold after a volcanic eruption in various land-use systems adjacent to Mount Kelud (Indonesia). Yet, soil macroporosity was relatively unchanged, indicating soil hydrophobicity. We tested the hypothesis that hydrophobicity or water repellency (WR) can be induced by volcanic ash interacting with organic matter and quantified its effect on surface water infiltration.We combined volcanic ash with leaf litter from coffee, durian, pine and mixed sources and tested for WR as a function of incubation time (0–16 weeks) and soil water content (θ, %), with Water Drop Penetration Time (WDPT, s) and Contact Angle (CA, o) as WR metrics. Lipids content (%) and pH were also analysed during incubation. Water droplets were placed onto a material tested for WR in a slope-adjustable stage to determine the critical angle for droplet runoff before penetration. Finally, to quantify the effect of WR on water flow, we layered 5 and 10 cm of ash with organic matter additions on top of a control soil column and performed infiltration (hydraulic conductivity) measurements.Among litter sources, pine litter induced the highest WR. There were significant relationships between WR, lipid, and pH. However, these relationships became weaker with material water contents > 7%. A significant relationship was also found between CA and critical slope of both small and large drops. A higher CA lowered the minimum critical slope for droplet runoff to 7° and 10° for the diameters of large and small water droplets, respectively. The mixture of volcanic ash and organic matter layered into the soil surface resulted in a five-times lower average hydraulic conductivity, with indications of air entrapment limiting water infiltration in a thicker ash layer. A higher CA was strongly associated with a lower hydraulic conductivity ratio, particularly under the 5 cm organic matter treatment. The WR effects on hydraulic conductivity are equivalent to at least a ten-fold reduction in effective soil porosity. By reducing infiltration, WR may contribute to ash movement in the volcanic landscape.