Assessing soil hydrophobicity and its variability through the soil profile using two different methods

Soil water repellency (hydrophobicity) and its heterogeneity in field soils under natural conditions can cause unstable wetting fronts, preferential flow, and accelerated solute leaching. For assessing possible effects of water repellency and its heterogeneity on flow processes in a given soil, investigations of both overall levels of repellency and its variability are necessary. The purpose of this study was to assess water repellency levels and its variability in sandy soils under a pine-beech forest transformation chronosequence. Water repellency was quantified at four plots for soil depths between 0 and 160 cm on disturbed and oven-dried samples with the water drop penetration time (WDPT) test and the sessile drop method (SDM) (contact angles [CAs]). Intrasample variability was quantified with a heterogeneity index (RI) which is based on the difference between the 90 and 10% quantile, divided by the overall range of encountered values. For both methods and all plots, repellency levels were highest in the topsoil layer (0- to 10-cm depth) and decreased clearly with increasing depth. Larger maximum values of intrasample variability were determined with the WDPT method compared to the SDM. When the proportion of estimated measurement error is subtracted from heterogeneity values, the average heterogeneity is higher for log(WDPT) (mean 8.9%) than for CAs (mean 6.7%). The preferential flow which was observed at this site despite the ostensible homogeneity of the soil may be due to the high variability of hydrophobicity, although other factors (e.g., runnel flow) may contribute to this as well.