Vietnam War: Forest Fire as a Military Weapon - Paperless Archives

Precipitation Regimes:
SECRET
The fireclimate precipitation classes show: (i) the relative length and intensity of the dry season havingweather
suitable for burning, and (2) the relative amount and frequency of rainfall during that portion of the non-burn season having
temperatures favorable for plant growth. Four lengths of burn season are recognized: none, short, long, and yearlong.
The non-burn portion of the year is classed as either humid or nonhumid.
The distinction between a humid and a nonhumid month commonly is made by comparing average precipitation of
the month with the potential evapotranspiration for the month. Potential evapotranspiration-a concept introduced by
Thornthwaite _2/is the estimated amount of water needed for both evaporation losses and plant transpiration losses from
a vegetation-covered land surface, assuming that plant growth is not inhibited by water shortage. Papadakis has published
monthly potential evapotranspirational losses for 2400 locations over the world. INThe monthly loss can be approximated
from average maximum daily temperature and average afternoon relative humidity as shown by the curves in Figure 28,
which were adapted from the Papadakis data.
Months with average precipitation equal to or greater than estimated potential evapotranspiration are considered
humid, designated as "H". Papadakis has described a dry month, designated as "D", as one in which water stored in the
soil during previous humid months plus average precipitation of the month together equal less than one half of the estimated
potential evapotranspiration for the month. Maximum water storage capacity of th.. soil is assumed to be 3.9 inches
of water. At most locations a few months of the year are intermediate between humid and dry.
The data obtained during calculation of the burning odds at any location-as illustrated in Table 4-also can be used
to designate months of adequate precipitation and months of moisture stress. For example, "H" months always show positive
(+) values from Figure 27: and "D" months, with few exceptions, are also burn months, designated as "B", which
have negative (-) values. In addition, the driest of the intermediate months are apparent because they have negative (-)
values, which usually are small.
By combining length of burn season and adequacy of precipitation during the non-bum season, the 7 fireclimate
precipitation regimes are:
Humid yearlong (HUYL)-No "B" months and no (-) values; yearly (+) values from a minimum of 200 down to 50,
depending on temperature regimes as shown in Table 5. These locations have no "D" months, and usually have 6 to
12 "H" months.
Nonhumid. no burn season (NHYL)-No "B" months and no "D" months; but otherwise not in the humid class (see
Table 5).
Humid, short bum season (HUSB)- 1-4 "B" months, with yearly (-) values of 1-25; (for yearly (+) values see Table
5). Most locations have 4 to 10 "H" months 0 to 4 "D" months.
Nonhumid, short burn season (NHSB)- 1-4 "B" months, with yearly (-) values of 1-25; yearly (+) values too low for
humid (see Table 5).
12! Thornthwaite, C. W. An approach towards a rational classification of climate. Geogr. Rev. XXXVIII, pp. 55-94. 1948.
1_J3 Papadakis, J. Climatic tables for the world. Published by the author Cordoba 4564, uenos Aires, Argentine. 1961.
SECRET 34