kbdindex: Keetch-Byram drought index and Mac Arthur Drought Factor

Description Usage Arguments Details Value Note Author(s) References

View source: R/kbdindex.R


Implementation of the Keetch Byram Drought Index and Mac Arthur's Drought Factor for vector data


kbdindex(dates, t, p, wrs = 5, start.date = NULL, what = "kbdi")



Vector of dates. This is a character string in the form d/m/y


Vector of daily temperature (in deg Celsius)


Vector of daily accumulated preciptation (mm)


Minimum total weekly precipitation value used to define a "rainy" week (in mm). Default to 5 mm. Ignored if start.date is supplied.


Starting date for computation. Default to NULL, meaning that the computation starts since the first input record. Otherwise, the calculation starts the given date, that assumes a state of saturation of the soil (and hence KBDI=0, see Details), for instance just after snow melt. This argument overrides wrs.


What index should be returned by the function?. Current options include:

  • kbdi: Keetch-Byram Drought Index. This is the default.

  • madf: Mac Arthur's Drought Factor. Derived from the latter. See Details.


The physical theory for the Keetch-Byram Drought Index (Keetch and Byram, 1968) is based on a number of assumptions: The first assumption is that soil moisture is at field capacity with a water depth equivalent to about 200 mm. The second assumption is that the rate of moisture loss in an area depends on the vegetation cover in the area, and vegetation density is a function of the mean annual rainfall. Hence, daily transpiration is approximated by an inverse exponential function of the mean annual rainfall. Finally, the evaporation rate of soil moisture with time is assumed to be an estimation of relative evapotranspiration from exponential function of the daily maximum air temperature. Sensitivity analyses from earlier researchers have revealed that KBDI decays exponentially with an assumed maximum soil water deficit and is sensitive to the daily maximum air temperature (Dennison et al., 2013). Its values range from 0 to 800 (inches), with 800 in (203.2 mm after conversion) indicating extreme drought and zero indicating saturated soil.

The McArthur's Drought Factor was developed to predict the amount of fine fuel which would be available to be consumed in the flaming front of a fire. The predictive model used by McArthur was based on a combination of the Keetch Byram Drought Index, and the amount, and time since fall, of recent rain. That is the reason both indices are calculated by the same function.


A numeric vector containing the (daily) KBDI (or MADF) time series


The original equations of the code presented by Keetch and Byram (1968) were later corrected for two significant typographical errors affecting the index output Alexander (1990).


Joaquin Bedia-Jiménez


ClimInd documentation built on April 10, 2021, 1:06 a.m.