Soil water repellency - a concern for groundwater recharge and quality?
Plant & Food Research
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Water repellency has been shown to occur in many soil orders under different types of vegetation and climate. It is now understood that the transient development of water repellent soil conditions is the rule rather than the exception. While it is easy to measure how repellency impedes the infiltration of single water droplets into soil, the effect of repellency on water dynamics at larger scales and over longer time periods has hardly been quantified. When water infiltrates into repellent topsoils, preferential flow can develop and bypass large parts of the soil matrix. Repellency could thus also accelerate contaminant transport to aquifers and could negatively affect groundwater recharge rates. Lack of mechanistic process understanding limits prediction of how soil water repellency affects infiltration, runoff generation, contaminant transport and groundwater recharge at larger spatial scales in different seasons. Our research aims to isolate the effects of repellency on soil water dynamics by directly quantifying how repellent soil conditions affect water sorptivity, infiltration and runoff at the mesoscale. We developed a fully automated tension disc infiltrometer and a runoff measurement apparatus. Using these in laboratory infiltration and runoff experiments with intact soil cores and slabs we studied water fluxes in a water repellent pastoral soil from New Zealand. In all our experiments, we compared the behaviour of water to that of an aqueous ethanol solution to directly quantify the effects of repellency. New insight into the transitory character of water repellency was inferred from early- and late- time infiltration and runoff patterns as well as analysing the behaviour of soil collected at different times of the year. In addition, in the infiltration experiments we compared flux dynamics at different effective tensions. Results highlighted the fractional wettability of soil. We showed that pore size classes differ in wettability. We also measured the soil’s repellency dynamics over a range of volumetric water contents using the sessile drop method. Our novel measurement technologies were combined with models based on the resulting soil water repellency characteristic curve. Our research highlights the opportunity to develop new approaches for gaining insight into these complex transitory processes, particularly over time scales of infiltration and groundwater recharge. Our preliminary results emphasize that soil water repellency should be included in hydrological and solute transport modelling. It also highlights that research is required to assess the effects of repellency at field and catchment scales.