USING MULTIPLE RAIN RATES TO EXAMINE SPATIAL VARIABILITY IN INFILTRATION

Infiltration rates may change after a fire due to physical and chemical alteration of the soil surface during burning. Predicting the runoff response of these soils under variable rain rates will help indicate the amount of water available for related debris flow bulking processes during different rain storm conditions. Shortly after the Overland fire, near Jamestown, CO, we completed a series of rainfall simulation experiments on recently burned, granitic soils. The soils consisted of two layers: an ash layer and the underlying sandy soil. To examine the affect of rain rate on runoff response, we applied three rain rates (18, 30 and 46 mm hr^-1) for 30-minute intervals on four different 1-m² plots. Two plots were randomly positioned on north-facing slopes and two on south-facing slopes. For each experiment, we measured a range of variables including rainfall, runoff, plot topography, overland flow velocities and depth of the surface ash layer.

The plots generally reached steady flow conditions during the 30-minute experiments. Steady flow responses varied between north-facing and south-facing aspects particularly at low rain rates (18 and 30 mm hr^-1). North-facing plots produced greater amounts of steady runoff than south-facing plots. However, the overall pattern of runoff generation depends largely on individual plot characteristics, particularly the characteristics of the ash layer. These burned, multi-layered soils exhibit two types of relations between steady flow conditions and rain rate. One set of plots appear to have a threshold relation between the two variables, with the runoff amount increasing linearly with rainfall rate above an approximate threshold rain rate of 20 mm hr^-1. The other set of plots have a relation between steady flow and rain rate that is consistent with an underlying continuous lognormal distribution of point-scale infiltration rates over the plot area, with point scale being infinitely small segments of the plot. Analytical modeling of the experiments indicates that the combined affect of ash and soil capillarity has to be small in order to match the observed hydrographs.