Surface runoff as affected by soil water repellency in a Japanese cypress forest

Abstract

Recent studies have suggested that soil water repellency can be one of the important factors affecting hydrological processes on headwater catchments. In Japan, water repellency is known to occur under Japanese cypress (Chamaecyparis obtusa) forests, a typical plantation type in Japan, however, previous studies have not evaluated the severity of water repellency and its effects on surface runoff generation. To quantify water repellency and its effects, this study combined the critical surface tension (CST) test with a new spraying experiment in which the infiltration rates of water and ethanol solutions sprayed over 0·09‐m² plots were compared. Long‐term intensive hydrological observations of surface runoff from 2‐m² plots, soil water potential, and soil water content were conducted concurrently.

The spraying experiment revealed that strong water repellency in surface soils, as quantified by the CST test, caused Hortonian overland flow despite the high conductivity measured under saturated hydrophilic conditions. Generally, the surface runoff coefficient for a storm event was negatively correlated with initial soil moisture conditions. However, during a period of successive storm events separated by short intervals, the coefficient decreased gradually even when the initial moisture conditions were similar, indicating a weakening of water repellency by repeated wetting. On the other hand, a drying period with long inter‐rainfall intervals and increasing air temperature was associated with increases in the surface runoff coefficient. These results suggest that the water repellency and the resultant surface runoff depended on the history of rainfall at the site. Relationships between soil water potential and soil water content indicate that changes in the soil water repellency and consequently surface runoff coefficient could occur during a single storm event. Copyright © 2007 John Wiley & Sons, Ltd.