Speaking: Lisa Saperstein, USFWS, Eric Miller, BLM-AFS
Lisa Saperstein, USFWS - Landfire and the Interagency Fuel Treatment Decision Support System Updates for Alaska
Eric Miller, BLM-AFS - Evaluating the Drought Code Using in Situ Drying Timelags of Feathermoss Duff in Interior Alaska
Drying timelag is a concept that derives from the logarithmic drying rate equation that is useful in classifying components of a fuel bed based on speed of moisture loss. It is used in the Canadian Forest Fire Weather Index System to calculate current moisture content from a previous value based on weather and environmental conditions. The Drought Code of the system is underlain by a simple water balance model that is increased daily by rainfall and decreased by actual evaporation and features a drying timelag >50 d. An abstruse algorithm has impeded a common understanding of the water balance model, specifically that it is purely hydrological and does not require the presence of a soil at all. Thus the only way to determine the nature of a soil with a timelag equivalent to the DC is through empirical measurement. A dataset of volumetric moisture content measurements at four depths in a black spruce-feathermoss forest in Interior Alaska is used to estimate drying timelag. The seasonally averaged timelag of the 20 cm duff profile was 28 d while the DMC and DC averaged 16 and 58 d, respectively. Using this measured timelag, potential and actual evaporation at the forest floor were estimated at 1.09 and 0.60 mm/day. In turn, these values indicate that a soil profile of 37 cm depth with a water-holding capacity of 68 mm would have a timelag equivalent to the DC. Such a soil would be 44% mineral soil by depth and 96% by weight. The seasonal pattern of timelag suggests that the permafrost table shortens the timelag early in the season when the full water-holding capacity is partially frozen and not available, although the pattern is also correlated with potential evaporation. Timelag is shown to increase in a mostly linear manner with soil depth. A curve is used to revise our understanding of the correspondence of soil horizons with the moisture codes of the FWI. An understanding of how timelag varies according to the physical and hydrological properties of boreal soils allows a better interpretation of the DC in Alaska as well as the causes of its eccentricities.