Peculiarities of Infiltration Measurements in Water-Repellent Forest Soil

Abstract

The paper deals with measurements of water infiltration carried out on a well-developed forest floor formed by needle-leaf litter of Norway spruce. Three field methods (tension disk permeameter, single-ring infiltrometer and Guelph permeameter) were used to determine the soil hydraulic conductivity. The results were strongly influenced by the water repellency at the interface between the O- and A-horizons. This interface was severely water repellent during the hot and dry summer season, regardless of the generally humid mountain climate of the High Tatras foothill. The single-ring method paradoxically provided lower hydraulic conductivity (3.2 × 10−4 ± 1.3 × 10−4) compared to the tension disk permeameter (8.5 × 10−4 ± 3.3 × 10−4) due to the presence of the water-repellent O/A-interface. This effect was also observed with the Guelph permeameter method, which gave the lowest value (5.6 × 10−5 ± 4.3 × 10−5). Abrupt retardation of infiltration on the water-repellent interface may generate shallow subsurface runoff (as was proved by the irrigation experiment) or litter splash during extreme rainfall events and promote water flow to deeper soil horizons through preferential pathways. The observed effects of the forest floor on rainfall infiltration will depend on the seasonal variability of soil water repellency. Although the forest floor is a source of hydrophobic substances that cause water repellency at the O/A-interface and can trigger runoff generation, at the same time its cohesive duff layer protects the forest soil from erosion.