Linking water vapor sorption to water repellency in soils with high organic carbon contents

Water repellency (WR) significantly affects the hydraulic behavior of soils. Although WR often is regarded as a phenomenon with implications for dry soils, it is prevalent at water contents (w) exceeding the wilting point water content. Because the measurement of the WR–w relationship is laborious, alternative more time-efficient methods are desirable to estimate parameters of the WR-w curve. Using 32 high organic carbon (OC) soils from Denmark and South Greenland, we characterized the water vapor sorption isotherms (WSIs), investigated the interrelated effects of OC and clay contents on WSIs and the WR–w relationship, and further evaluated if parameters of the WR-w curve may be derived directly from WSIs. The samples exhibited OC and clay contents ranging from 0.014 to 0.369 kg kg^–1 and from 0.02 to 0.16 kg kg^–1, respectively. The WSIs measured for relative humidity (RH) values between 3 and 93%, were strongly hysteretic, were OC dependent, and could be accurately characterized with the Guggenheim, Anderson, and de Boer model. Further, the WR_area and w_non parameters, derived from WR measured for several w, were well estimated with linear regressions based on OC content and multiple linear regressions based on OC and clay contents. Estimations for WR_area and w_non based on the WSI parameter w_m-a were superior to OC and clay content. Finally, we established mathematical expressions that estimate WR_area or w_non from any air-dry w obtained from either the desorption or adsorption isotherms between 10 and 90% RH.