Spatial pattern of infiltration rate and its effect on hydrological processes in a small headwater catchment

Overland flow, which occurs when the rainfall intensity exceeds the infiltration rate, is an important factor in hillslope hydrological processes. Recent studies have suggested that a cause of Hortonian overland flow on forested hillslopes is the water repellency of surface soils. However, few studies have addressed the contribution of overland flow on storm runoff in such catchments. The overland flow generated on hillslopes needs to reach stream channels in order to contribute to storm runoff from a catchment. Therefore, the spatial pattern of the infiltration rate in a hillslope is essential for understanding the contribution of overland flow on storm runoff. To clarify the spatial pattern of infiltration for a hillslope, and its effect on overland flow generation and storm runoff from a small catchment with water repellent surface soil, we conducted artificial rainfall experiments along a hillslope transect (49 m) on 15 occasions over 20 months and measured the overland flow at a hillslope plot (8 × 20 m), stream flow at an outlet of a small catchment (0·43 ha), and the matric potential head along the hillslope transect. The replicated measurements suggest that the relationship between the infiltration rate and soil moisture was positive due to the impact of soil water repellency, and that the infiltration rate in the lower part of the hillslope was significantly higher than that of the upper and middle parts. Overland flow for individual storms measured at the plot scale was generally greater during dry periods than wet periods, suggesting that water repellency reduced the infiltration rate in dry periods as noted at the experiment plot scale. In contrast, stormflow for all events during wet and dry periods showed the opposite trend, indicating that the impact of soil water repellency during dry periods was not sufficient or continuous enough to cause measurable increases in stormflow. For a storm event < 100 mm in total precipitation, the comparison of hydrological responses during storm events between the dry and wet periods suggests that subsurface flow rather than Hortonian overland flow contributed to the storm runoff even though a substantial amount of overland flow was generated in the hillslope plot. Copyright © 2010 John Wiley & Sons, Ltd.