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Linking hydraulic properties of fire-affected soils to infiltration and water repellency
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 m...
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Published in: | Journal of hydrology (Amsterdam) 2009-12, Vol.379 (3), p.291-303 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Subjects: | |
Citations: | Items that this one cites Items that cite this one |
Online Access: | Get full text |
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Summary: | 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,
K
f
and (2) sorptivity,
S(
θ
i
), as a function of initial soil moisture content,
θ
i
, ranging from extremely dry conditions (
θ
i
<
0.02
cm
3
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
K
f
and
S(
θ
i
).
Values of
K
f
ranged from 4.5
×
10
−3 to 53
×
10
−3
cm
s
−1 for ash; from 0.93
×
10
−3 to 130
×
10
−3
cm
s
−1 for reference soils; and from 0.86
×
10
−3 to 3.0
×
10
−3
cm
s
−1, for soil unaffected by fire, which had the lowest values of
K
f
. Measurements indicated that
S(
θ
i
) could be represented by an empirical non-linear function of
θ
i
with a sorptivity maximum of 0.18–0.20
cm
s
−0.5, between 0.03 and 0.08
cm
3
cm
−3. This functional form differs from the monotonically decreasing non-linear functions often used to represent
S(
θ
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
K
f
and
S(
θ
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. |
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ISSN: | 0022-1694 1879-2707 |
DOI: | 10.1016/j.jhydrol.2009.10.015 |