The Interplay Between Soil Moisture and Water Repellency as a Control on the Temporal Trends in Infiltration Properties of Burnt Forest Soils, south-east Australia
Abstract
The infiltration capacity of burnt forest soil acts as an important control on the susceptibility of catchments to post-fire erosion events. The effect of water repellency on infiltration depends on a number of factors including i) the degree of water repellency, ii) its distribution within the soil profile, iii) its distribution across infiltrating hillslopes and iv) the availability of macropores and other pathways which allow infiltrating water to bypass an otherwise repellent soil matrix. This study focused on the hydraulic properties of dry eucalypt forests in south-east Australia that were burnt by intense wildfire in 2007 and 2009. The study sites were located in steep (>30 degrees) catchments on metamorphic and sedimentary geologies, which are particularly susceptible to post-fire erosion and high magnitude events such as flash floods and runoff-generated debris flows. We measured field saturated hydraulic conductivity (Kfs) and water repellency as a function of different moisture conditions in the field. Gravimetric water content and water repellency were measured on soil samples from five depth intervals in the top 10 cm of the soil profile which were sampled at four time steps along multiple transects that extended from the drainage lines to the ridge top of small headwater catchments. Using small disc infiltrometers the study obtained field measurements of Kfs under a small negative head and under ponding. The results indicate that water repellency displays stronger seasonal trends at the site burnt by wildfire in 2007 than at the sites burnt by wildfire in February 2009. In spring (November) 2009, when conditions were relatively wet, only 20% of the sampled points at the 2007 burn site were water repellent while at the sites burnt by the 2009 fire, more than 90% of the samples points were repellent. This was also reflected in the different soil moisture levels at the site burnt in 2007 which had on average 40% higher soil moisture than the site burnt in 2009. The increase in Kfs due to lower water repellency in spring was more pronounced for Kfs measured at small tension than at ponding. The hydraulic conductivity measured under ponding was on average 30-60% higher than the Kfs measured under a 5 mm tension, indicating that gravity dominated flow in macropores can contribute significantly to the infiltration process. However, the increase due to ponding is considerably less than what has been measured previously in wet eucalypt forests where macropore flow tends to reduce the impact of water repellency on infiltration rates. At all sites and at all time steps the strength of water repellence peaks at depths between 2 cm and 4 cm and diminishes at soil depths >7 cm. A 2-parameter function was fitted to a consistent trend in the water repellency distribution within the top 10 cm of the soil profile. The parameters in the function could then be linked to seasonality in rainfall and temperature in order to represent the water repellency as a dynamic property which integrates both the degree of water repellency and the distribution within the soil profile as a function of the climatic drivers.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.H53B1019N
- Keywords:
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- 1838 HYDROLOGY / Infiltration;
- 1865 HYDROLOGY / Soils;
- 1866 HYDROLOGY / Soil moisture