Keetch-Byram Drought Index Research Articles

Relationships among Vegetation, Surficial Geology and Soil Water Content at the Pocono Mesic Till Barrens

EBERHARDT, R. W. AND R. E. LATHAM (Department of Biology, Swarthmore College, Swarthmore, PA 190811390). Relationships among vegetation, surficial geology and soil water content at the Pocono mesic till barrens. J. Torrey Bot. Soc. 127:115-124. 2000.-We examined relationships among vegetation, surficial geology, and soil water content in the Pocono till barrens, an assemblage of pitch pine-scrub oak-heath communities rich in rare species near Long Pond, Pennsylvania. Soil water content was measured using time domain reflectometry over three time periods in summer 1997 in four vegetation types (forest and three barrens communities) on two parent materials (Illinoian glacial till and upper Devonian sandstone bedrock). Wide variation in drought conditions among the time periods (quantified using the Keetch-Byram drought index) allowed for strong tests of differences in soil water-holding capacity among vegetation and parent material types. Soils of barrens and forests did not differ in water content at any depth or drought condition, which confirmed floristic evidence and earlier research that suggested pitch pine-scrub oak vegetation is not associated with xeric conditions. Disturbance history, and possibly the hypothesis of alternative community states, better explain the distinction between barrens and forests in the study area. Soils derived from till and bedrock had similar water contents at each time period, although bedrock soils may have been slightly wetter in the top 15 cm of the soil profile. This contradicts a presupposed relationship between parent material and soil water content and suggests that pitch pine-scrub oak vegetation might occur more extensively on mesic sites in northeastern Pennsylvania than previously thought. Contrary to expectation, barrens types did not differ in soil water content under any drought condition. These results suggest that rhodora barrens, which include diverse and abundant hydrophytes, are not restricted to sites relatively immune from drought. A testable alternative explanation is offered for the distribution of the rare rhodora barrens vegetation type.

A Method to Initialize the Keetch-Byram Drought Index

Abstract The 1988 National Fire Danger Rating System implements the Keetch-Byram Drought Index. This index is output both as an estimator of drought in its own right and is used to modify fire danger calculations to account for deep drying of dead vegetation and duff. A method initializes this index for those fire danger rating stations that do not run the National Fire Danger Rating System all year long. West J. Appl. For. 8(4)109-115.

A simpler empirical expression for actual evapotranspiration rates — a discussion

Live Fuel Moisture Content (LFMC) is a critical variable affecting fire ignition, behavior and severity in many ecosystems. Although the use of meteorological drought indices as proxies for LFMC is a straightforward and widespread approach, it is largely unknown whether it can provide reliable estimates of LFMC, either for local or spatial applications. We address this issue by evaluating the capacity of drought indices to predict LFMC quantitative variations and critical values. LFMC observations used for reference were measured on six different Mediterranean shrub species for 15 years in 20 different sites in Southern France. Six drought indices were evaluated: the Duff Moisture Code (DMC) and Drought Code (DC) of the Canadian Forest Fire Weather Index System, the Keetch-Byram Drought Index (KBDI), the Nesterov Index (NI) and the Relative Water Content (RWC) of the soil derived from a forest water balance model for low (80 mm) and high (160 mm) field capacities. The species were classified in two groups according to their seasonal variability: high and low responding species. We found large differences in the capacity of drought indices to predict LFMC, with indices that simulate long-term drought dynamics (DC, RWC and KBDI) generally performing better than others (NI and DMC). Once calibrated at stand scale, drought indices showed a good potential for predicting LFMC of high responding species, although large variations between sites were observed. In contrast, spatial predictability was limited with a RMSE and R2 on the order of 20% and 0.3, respectively (for high responding species). Our results suggest that drought indices should therefore be used with caution for spatial applications in wildfire research and operational fire management. Because they can explicitly consider environmental (soil, climate) and biological (species traits related to dehydration) factors, mechanistic indices have a great potential to improve LFMC predictions.