Water infiltration and hydrophobicity in forest soils of a pine–beech transformation chronosequence

Abstract

Summary Soil water repellency is widespread in forest soils and has important hydrological implications. The relation of soil water repellency to hydraulic properties is of interest to assess the overall hydrological behaviour of a forest ecosystem. Both soil water repellency and hydraulic properties depend on tree species and age, forest management practices, and the season of the year. In this study, the variation of soil water repellency and hydraulic properties during the season of the year and between different forest stands are analysed for a pine–beech forest transformation chronosequence. Soil water repellency is assessed with the water drop penetration time (WDPT) test, the critical surface tension (CST) test, and the sessile drop contact angle method. Hydraulic conductivity is measured with a hood infiltrometer, a special form of a tension infiltrometer, and a ponded ring infiltrometer. Soil water repellency for the upper 10 cm of the soil exhibited a pronounced seasonal variability, both for field moist and oven dry samples, whereas the variability between the different forest transformation stages seemed to be less significant. For hydraulic parameters, the differences between the forest stands and the measurement dates were lower than the variabilities within the stands and within the dates, but also the hydraulic conductivities revealed a seasonal trend with higher values during the summer months. Saturated hydraulic conductivities determined with the hood infiltrometer were by a factor of 10 lower than saturated hydraulic conductivities estimated with the ponded ring infiltrometer. Presumably, Ks values estimated with the hood infiltrometer are underestimated due to an incomplete wetting of the soil beneath the infiltrometer. This would limit the suitability of this device for water repellent soils, unless the soil is thoroughly wetted prior to infiltration measurements.