400
With the existing data available to land managers and the STSM framework as a
tool to investigate alternative scenarios and management actions, the above research
needs can be addressed. With time, it is hoped that these modeling strategies will
assist management decisions and result in desired outcomes with higher efficiency
and reduced resources.
Acknowledgments Any use of trade, product, or firm names is for descriptive purposes only and
does not imply endorsement by the US Government.
Appendix
Calculating the Palmer Drought Severity Index
The Palmer Drought Severity Index (PDSI) time series was used to calculate the
temporal multipliers for replacement fire, drought, annual grass invasion, and tree
invasion. Drought is a major influence for these disturbances. PDSI measures long-
term soil drought and is updated monthly (Palmer 1965 ; Heddinghaus and Sabol
1991 ). Positive values indicate above average soil moisture (>3 is very wet), whereas
negative values represent droughty soil (<−3 is very dry). A PDSI of zero is average
soil moisture. The formula for PDSI at time t (month) is as follows:
PDSItt= ́^0 .(^897 PDSI-^1 + ́kPtt/)^3 ()-Pt (13.1)
where Pt is precipitation during month t, Pt is average (historic) precipitation for
month t, and k is a monthly climatic coefficient that weighs the local importance of
(Pt − Pt) (Palmer 1965 ). For example, k might imply that (Pt − Pt) in January does not
contribute as much to PDSI as the same deviation in precipitation observed in
August (Palmer 1965 ). Although we downloaded monthly precipitation values and
obtained monthly Pt from historic precipitation data (respectively, month, precipita-
tion [mm/day]: January, 0.8004; February, 0.8368; March, 1.0234; April, 0.9310;
May, 0.9612; June, 0.6130; July, 0.6356; August, 0.7394; September, 0.6876;
October, 0.7502; November, 0.7476; December, 0.6858), the value of kt is unknown
and requires complicated field estimation based, among others, on evapotranspira-
tion (Palmer 1965 ). (To remove this complication and need for a heuristic equation,
future projects will use the Standard Precipitation Index [Hayes et al. 1999 ]).
Therefore, we made several arbitrary assumptions to imitate k using the month’s
temperature differential. Specifically,
ket
/. 31 = ́ 51 ()- - ́^01 .(^5 MaxT-Tt)
(13.2)
where MaxT = 31 (°C) is the maximum temperature observed, and Tt is the average
temperature during month t. In this heuristic equation, higher temperatures cause
smaller values to multiply (Pt − Pt) when monthly precipitation is higher and thus
L. Provencher et al.