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Contacts: Key Question: Following wildfire, there is a risk of surface erosion and gully formation. The degree of risk is in part associated with the extent and continuity of fire-induced water repellency. To best assess and design mitigation addressing this risk, answers to the following questions would be helpful:
Study Design: To address the above questions we propose two linked sets of observations, one on downslope continuity of water repellency, and the second on structure performance in the same area. Water repellency will be measured along 100-m transects going down the hillslope at 0.5 m spacing. This will give yes-no data along transects to indicate the degree of water repellency. Others (e.g. DeBano, 1981) have suggested penetration time, depth of the repellent layer, and drop entry angle as measures of the degree of repellency. We have found that it is easier in the field to measure a binary response based on penetration time, allowing many samples to be taken along a downslope transect quantifying the continuity of water repellent areas. We propose that this continuity is itself a useful measure of the degree of repellency and its potential hydrologic and geomorphic effects. At the Trail Creek Fire site (granitic soils), we will measure 20 transects, 10 in an area with relatively coarse soils and 10 in an area with somewhat higher silt and clay content. We will measure 10 additional transects in an area to be determined that has contrasting soils (e.g. Belt or Borderzone geologies). Candidate areas are on the Lolo, Bitteroot, and Salmon-Challis National Forests. The purpose of the second site is to provide some ability to generalize to other areas in the two regions. All transects will have experienced high fire severity. We propose to take these measurements annually over the course of five years to measure the change over time. In the initial year, we will collect soil samples along the transects to measure soil texture. The second set of observations will take place in areas around the transects where staked wattles and contour-felled logs were placed following the fire. Soil capture and performance of a sample of these structures will be monitored over the five years to see how soil capture and probability of overtopping or diversion around the end of the structures relates to:
Precipitation gages (tipping bucket gage with tipping bucket data loggers) will be placed in the areas of the transects and structures. If severe sediment transport or structure failure occurs during some event, we can estimate the return period of the storm based on a long-term record from nearby National Weather Service 15-minute precipitation gages. Process for Field Data Collection: Rocky Mountain Research Station and Boise National Forest personnel will carry out field data collection at the Trail Creek Fire site during August. Trainee NFS staff from the second field area will participate for a portion of the data collection period at Trail Creek to learn the data collection process. RMRS staff will travel to the second field site to participate in early field data collection there to verify similarity in techniques. Products: Expected products are basic science relating soil properties and time following fire to the continuity of water repellency. In addition, we will provide observations on how that continuity affects failure rate of hillslope sediment retention structures given particular contributing areas and storm frequencies. Such observations would constitute only a case study at this time; however, they would also serve as an example for other BAER monitoring that could broaden the database.
Lead Scientists and Forest Personnel: Charlie Luce from RMRS Boise will be the lead scientist for the investigations and will cooperate with T.J. Clifford, Forest Hydrologist (Boise NF), Terry Hardy, Forest Soil Scientist (Boise NF), Cavan Maloney, District Hydrologist (Boise NF), and specialists and managers from the site yet to be selected. Part of the selection criteria for the second site will be on interest and willingness of the local specialists and managers to collaborate in the monitoring. Reference:
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