David Kinner Xavier Ubeda John A. Moody 2009 <p>Heat from wildfires can produce a two-layer system composed of extremely dry soil covered by a layer of ash, which when subjected to rainfall, may produce extreme floods. To understand the soil physics controlling runoff for these initial conditions, we used a small, portable disk infiltrometer to measure two hydraulic properties: (1) near-saturated hydraulic conductivity,<span>&nbsp;</span><i>K_f</i><span>&nbsp;</span>and (2) sorptivity,<span>&nbsp;</span><i>S</i>(<i>θ_i</i>), as a function of initial soil moisture content,<span>&nbsp;</span><i>θ_i</i>, ranging from extremely dry conditions (<i>θ_i&nbsp;</i>&lt;&nbsp;0.02&nbsp;cm³&nbsp;cm⁻³) to near saturation. In the field and in the laboratory replicate measurements were made of ash, reference soils, soils unaffected by fire, and fire-affected soils. Each has a different degrees of water repellency that influences<span>&nbsp;</span><i>K_f</i><span>&nbsp;</span>and<span>&nbsp;</span><i>S</i>(<i>θ_i</i>).</p><p>Values of<span>&nbsp;</span><i>K_f</i><span>&nbsp;</span>ranged from 4.5&nbsp;×&nbsp;10⁻³<span>&nbsp;</span>to 53&nbsp;×&nbsp;10⁻³&nbsp;cm&nbsp;s⁻¹<span>&nbsp;</span>for ash; from 0.93&nbsp;×&nbsp;10⁻³<span>&nbsp;</span>to 130&nbsp;×&nbsp;10⁻³&nbsp;cm&nbsp;s⁻¹<span>&nbsp;</span>for reference soils; and from 0.86&nbsp;×&nbsp;10⁻³<span>&nbsp;</span>to 3.0&nbsp;×&nbsp;10⁻³&nbsp;cm&nbsp;s⁻¹, for soil unaffected by fire, which had the lowest values of<span>&nbsp;</span><i>K_f</i>. Measurements indicated that<span>&nbsp;</span><i>S</i>(<i>θ_i</i>) could be represented by an empirical non-linear function of<span>&nbsp;</span><i>θ_i</i><span>&nbsp;</span>with a sorptivity maximum of 0.18–0.20&nbsp;cm&nbsp;s^−0.5, between 0.03 and 0.08&nbsp;cm³&nbsp;cm⁻³. This functional form differs from the monotonically decreasing non-linear functions often used to represent<span>&nbsp;</span><i>S</i>(<i>θ_i</i>) for rainfall–runoff modeling. The sorptivity maximum may represent the combined effects of gravity, capillarity, and adsorption in a transitional domain corresponding to extremely dry soil, and moreover, it may explain the observed non-linear behavior, and the critical soil-moisture threshold of water repellent soils. Laboratory measurements of<span>&nbsp;</span><i>K_f</i><span>&nbsp;</span>and<span>&nbsp;</span><i>S</i>(<i>θ_i</i>) are the first for ash and fire-affected soil, but additional measurements are needed of these hydraulic properties for in situ fire-affected soils. They provide insight into water repellency behavior and infiltration under extremely dry conditions. Most importantly, they indicate how existing rainfall–runoff models can be modified to accommodate a possible two-layer system in extremely dry conditions. These modified models can be used to predict floods from burned watersheds under these initial conditions.</p> application/pdf 10.1016/j.jhydrol.2009.10.015 en Elsevier Linking hydraulic properties of fire-affected soils to infiltration and water repellency article