Quantification of Soil Moisture Effects on Runoff Formation at the Hillslope Scale

Pre-rain-event soil moisture (preSM) plays a crucial role when evaluating runoff formation during heavy rainfall. Using sprinkling experiments and numerical modeling, this study investigates the impact of preSM on runoff formation at the small hillslope scale (<= 100 m(2)). Sprinkling experiments were conducted on three sites in the Austrian Alps and observed hydrological behavior was simulated by using the hillslope model HILLFLOW. For each site, runoff was modeled with different preSM scenarios, covering the whole soil moisture range between permanent wilting point and saturation. Depending on the dominant runoff processes, the results showed impacts of preSM on both the maximum runoff and the total runoff. A predominant threshold was observed near field capacity (35 and 32% by volume). In general, the results clearly showed that the consideration of preSM is a prerequisite to reproduce runoff formation and total runoff amounts accurately. Field capacity turned out to be a good indicator (threshold) for surface runoff estimates. (C) 2015 American Society of Civil Engineers.