Improved prediction of drought for wildland fire danger rating in Canada

Abstract

Canadian fire management agencies track drought conditions using the Drought Code (DC). The DC is one of three fuel moisture codes in the Fire Weather Index System, which is part of the Canadian Forest Fire Danger Rating System. The DC represents the moisture of deep organic layers (15-18 cm nominal depth) and is used operationally to assess potential lightning ignition holdover, persistent deep smoldering, and mop-up problems. As the climate changes and drought conditions arise more frequently, our understanding of drought and how to measure it become more important. Determining what the DC means in areas without deep organic soils is a question commonly proposed by fire operations personnel. Recent studies have indicated that some more complex models (e.g. the Canadian Land Data Assimilation System – CaLDAS) may provide added intelligence about the fire environment and drought conditions, something that has not been explored in Canada. To shed light on these questions we carried out field studies in the provinces of Alberta and Ontario. Four field sites were included in our study, two in Alberta near Edson and Red Earth Creek, and two in Ontario near Dryden and Chapleau. At each of the seven plots within these four sites, we installed 8-12 water content reflectometry (WCR) probes at two different depths. The probes were installed from the surface through the organic layers, and in some cases, into the mineral soil. Overall, our results indicated that the simple DC model predicted the moisture content of the deeper organic layers (10-18 cm depths) well, even compared to the more complex CaLDAS model. The WCR probes at these depths, exhibited good agreement with how the DC model estimated moisture changes. The DC may therefore be representative of changes in moisture content in a wide range of depths and soil horizons. Issues with model inputs, particularly missed precipitation events and incorrect DC spring starting values, had a greater influence on DC model fit than other factors. Calibration and validation of the CaLDAS model to mineral soils may be the cause of its consistent under prediction of organic layer moisture.