267
A laboratory study of weather effects on
the drying rate of jack pine litter
C. E. VAN WAGNER
Canadiall Forestry Service, Petawawa Forest Experimelll Station, Clwlk River, OIll.,Cal/ada KOJ 1JO
Received May 9, 1978'
Accepted February 13, 1979
VAN �AGNER, C. E. 1979. A laboratory study of weather effects on the drying rate of jack pine
lItter. Can. J. For. Res. 9: 267-275.
This paper describes the variation in the drying rate of jack pine (Pillus banksiana Lamb.)
litter with external conditions. Temperature, relative humidity (RH), and wind were varied
separatel� . Drying runs were carried out in a cabinet with controlled temperature and humidity
and also 111 the open laboratory. Most runs followed the exponential pattern, and the drying
rates were measured in terms of the slope of the semilog graph of free moisture content versus
time. The logarithm of this slope was found to be inversely proportional to the reciprocal of
absolute temperature. The drying rate varied with 100
RH at high RH but increased only
slightly below 60% RH owing to the limiting effect of the wax and resin content on the rate of
internal diffusion. Drying in wind proceeded in two stages, the first faster than the second.
Wind was important at low speeds, and the drying rate increased but little above 2 km/h.
These principles were used in the design of the drying equations of the Fine Fuel Moisture Code
of the Canadian Forest Fire Weather Index.
VAN WAGNER, C. E. 1979. A laboratory study of weather effects on the drying rate of jack pine
litter. Can. J. For. Res. 9: 267-275.
L'auteur decrit la variation du taux de sechage de la litiere du Pin
(Pinlls bal/ksian(l
Lam ) soumise It des conditions externes. Sont traites separement la temperature, I'humidite
relatlve (HR) et Ie vent. On crea des conditions artifieielles de sechage en cabinet 11 temperature
et humidite controlees, et aussi en laboratoire a eiel ouvert. Le cours de la plupart des secheres­
ses changea exponentiellel11ent, les taux de sechage furent l11esures en termes de la pente de la
courbe semi-Iogarithl11ique representant la teneur en humidite libre versus Ie temps. Le loga­
rithme de cette pente se revela inversel11ent proportionnel It la reciproque de la temperature
absolue. Le taux de sechage lorsque I'HR etait elevee etait semblable a celui lorsque I'HR faisait
100 (ou presq ue). Par contre il augmentait peu sous 60% de HR vu l'effet freinant de la eire et de
la resine sur Ie taux de diffusion interne. Le sechage au vent s'effectua en deux stades, Ie premier
plus rapide que Ie second. C'est surtout It basse vitesse que Ie vent eut Ie plus d'effet et Ie taux
de sechage augl11enta bien peu au-dessus de 2 km/h. L'auteur utilisa ces prineipes pour preparer
des equations de "the Fine Fuel Moisture Code of the Canadian Forest Fire Weather Index."
?
Introduction
The traditional purpose of fuel moisture research
in the Canadian Forestry Service has been to supply
background information for the design of forest fire
danger rating systems. Generally, the first step was
to predict the moisture content of the important fuel
from daily weather observations, the second step
being the prediction of fire behaviour from fuel mois­
ture and wind. The main Canadian work on fuel
moisture has been in the outdoors, based on tech­
niques first used by Wright (1932) and carried on
for several decades in many locations throughout
Canada. Prior to 1970, Canadian fire danger systems
were based exclusively on these field results, and the
current danger index, the Fire Weather Index (Anony­
mous 1976), also depends heavily on them. How­
ever, during the 1960's a sequence of indoor drying
experiments on forest litter was perfonned at the
Petawawa Forest Experiment Station, and some of
'Revised manuscript received February 9, 1979.
these findings on variation in drying rate were in­
corporated into the new fire danger system. Recently,
a few additional experiments were carried out to
complete the picture. The aim throughout this work
was to develop a set of principles by which the field
results could be analyzed and better understood,
rather than a system of fine fuel moisture prediction
based on indoor results only. The main results from
the whole indoor program are presented in this paper.
In fuel moisture research, two properties are im­
portant: the equilibrium moisture content (EMC),
toward which the moisture content tends, and the
rate at which the moisture content changes. Some
Station work on EMC has been previously reported
(Van Wagner 1972), and this paper deals with the
drying rates of one typical fine fuel, namely jack pine
(Pinus banksiana Lamb,) litter. Atmospheric wet­
ting, or adsorption, is not dealt with.
A working principle established early in this re­
search was that forest fuel dries according to an ex­
ponential relationship; in other words, from a wet
0045-5067/79/020267-09$01.00/0
© 1979 National Research Council of Canada/Conseil national de recherches du Canada
268
CAN. J. FOR. RES. VOL. 9, [979
condition it loses moisture quickly at first and then
at a gradually decreasing rate as it approaches an
EMC. If the amount of moisture above EMC when
plotted against time yields a descending straight line
on semilog paper, the process can be called "ex­
ponential." Since the time to reach true equilibrium
is indefinite, the rate of such a drying process is oftcn
" the time required
quoted in terms of the "time
for the initial free moisture to fall to lie or 0.368 of
its original value. The time lag is in fact the reciprocal
of the semilog slope of the drying graph, called here
the logarithmic drying rate or simply "log drying
not considered to vary with temperature or humidity.
The section that follows is a brief discussion of
the basic equation used in the present work and the
theory of how moisture Teacts to variation in the
atmospheric environment. Accounts of methods
used, results obtained, and a discussion of how the
results fit the theoretical principles.follow. The paper
closes with a description of how these principles
were used in the Canadian forest fire danger rating
system and some conclusions.
Theory
rate." It is in terms of the log drying rate that drying
rates will be quoted in this paper.
The assumption of "exponential" drying is as
much empirical as theoretical. The principle has been
familiar to chemical engineers concerned with the
drying of moist materials for many decades (e.g.,
Lewis 1921) and is the subject of a wide literature in
that field. Exponential drying is also a feature of fire
danger rating in both the United States (Lancaster
1970; Fosberg and Deeming 1971) and Canada
(Van Wagner 1974). With respect to forest fuels,
Nelson's (1969) paper is the only one combining a
fairly complete account of exponential drying theory
with sound experimental evidence of how time lag
is affected by temperature, humidity, and fuel size or
thickness. King and Linton (1963) measured the
drying and wetting rates of several kinds of wood and
leaves at variou<; combinations of temperature,
humidity, and wind. Their individual runs followed
the exponential pattern, but the effects of variation
in external conditions on drying rate in their work
cannot readily be analyzed. Britton et al. (1973)
studied the drying rate of grass as it varies with
temperature, humidity, and wind but included no
underlying theory with their paper. As their results
are expressed as regression equations, interpretation
Modes of Drying
As the purpose of these experiments was primarily
practical, only enough theory was formulated to
permit a logical analysis of the data. The main source
was Chapter 15 of Chemical Engineers' Handbook
(Perry et al. 1963) from which the following ideas
were derived.
According to Perry et al. (1963), if a layer of
material is wet enough, the drying process may first
pass through a "constant-rate period." The instan­
taneous rate of moisture loss remains constant, and
the process is controlled by the same factors that
affect evaporation from a free water surface. It is
doubtful if forest fuels are ever wet enough to dry i n
this manner for any appreciable length o f time.
Once the surface, all or in part, can no longer be
maintained in a saturated state, the "falling-rate
period" begins and the instantaneous rate of moisture
loss falls continuously. Perry et al. (1963) divide
this period into two zones: (1) the "zone of un­
saturated surface drying" in which the drying rate
depends both on (a) "factors affecting the diffusion
of moisture away from the evaporating surface" and
(b) "factors affecting the rate of internal moisture
movement" and (2) the "zone of control by internal
moisture movement" in which the plane of evapora­
tion may retreat into the solid and the influence of
as a reference for other similar work is not easy.
Mutch and Gastineau (1970) measured the drying
and wetting time lags of reindeer lichen at a single
external variables diminished.
combination of temperature and humidity but did
not deal with their variation. Using theory alone,
Fosberg (1970) presented evidence based on funda­
mental diffusion theory that the assumption of ex­
The present drying experiments were all certainly
within the "faIling-rate period." Whether the two
zones described above can be positively identified is
not so certain. Nevertheless, it proved useful to pos­
ponential drying for wood may not be quite correct
but is close enough for practical purposes. He later
tulate two different modes of drying for jack pine
litter that, if not the same as the zones of Perry et al.
(Fosberg 1975) devised a model for moisture varia­
(1963), are at least analogous to them: (1) the
tion in litter and duff based on basic theory in the
fields of heat and moisture transfer. In his model, the
"evaporation mode" in which the drying rate appears
to be controlled by all the weather variables affecting
both surface evaporation and internal diffusion,
initial vertical profile of moisture content within the
fuel piece or layer is the main factor affecting the
drying rate of a given fuel, but the time lag itself is
namely temperature, humidity, and wind, and (2)
the "diffusion mode" in which the drying rate ap­
treated as a constant property of the material and is
pears to be controlled by temperature alone through
VAN WAGNER
its effect on the internal saturation vapor pressure
and hence on the diffusion coefficient.
The Drying Equation
If during the falling rate period the instantaneous
rate of moisture loss remains proportional to the
current moisture content above EMC, then an ex­
ponential relationship between moisture content and
time applies. The concept of exponential drying
used here considers the log
rate ( defined
earlier) as simply a measure of the rate of moisture
change. The log drying rate certainly depends on
fuel properties such as particle
density, and
structure and on the depth and bulk density of fuel
beds; these were held constant or nearly so in the
present work. It also depends on temperature and
to some extent on relative humidity, wind, and
possibly initial moisture content as well. When all
variations in drying rate are incorporated in one vari­
able, the basic equation becomes
[1 J
(M - E)/(Mo - E)
where Mo is initial moisture content, M is current
moisture content after time t, E is equilibrium mois­
ture content (EMC), Fo is initial moisture content
above EMC, F is current moisture content above
EMC, t is time from the start of the process, and k is
log drying rate. This is in fact the classical exponen­
tial equation y = aeb.1! in which Fo takes the part of
the constant a, the value of y at zero x. If initial mois­
ture content Mo were less than EMC, then F would
be a moisture deficit below EMC and the equation
would describe atmospheric wetting rather than dry­
ing. Equation 1 thus provides a complete and unique
description of any true exponential moisture-change
process.
Actually, k is quoted in logarithm to base lOin this
paper, time being in hours. The time lag then equals
the reciprocal of 2.303 k, with dimension hours.
269
should result when log k is plotted against 1 ITo and
that this relation should apply equally well for both
drying modes.
(ii) Variation with Relative Humidity
If, for a fuel drying according to the evaporation
mode, the temperature (T) is held constant, then
the ambient vapor pressure deficit should depend
only on the relative humidity ( RH) . Thus, the log
drying rate should be proportional to 100
RH
and should be zero at 100% RH. In the diffusion
drying mode, however, little or no effect of RH on
k is expected. As no drying is conceivable at 100%
RH, any drying process proceeding at low humid­
ity by the diffusion mode would be expected to
switch to the evaporation mode if RH were increased
toward 100%.
(iii) Variation with Wind
A wind effect is expected during the evaporation
mode only. However, once the internal diffusion
rate falls below the evaporation rate, the diffusion
mode should presumably take over, rendering the
wind speed irrelevant. As the wind speed is reduced
during an evaporation mode process, the limiting
situation is simple diffusion of vapor into the sur­
rounding atmosphere. Even at zero wind, therefore,
the log drying rate should have a finite value under
either drying mode.
Initial Moisture Content
The assumption of exponential drying implies that
the end points of the process are immaterial, i.e.,
that the log drying rate under given atmospheric
conditions will be the same regardless of the initial
moisture content or the duration of the run. How­
ever, the number of possible joint effects of three
weather variables on two modes of drying renders
unlikely such a simple picture. The manner in which
this initial moisture is distributed internally is, as dis­
cussed by Fosberg (1970, 1975), of great impor­
Weather Effects on Log Drying Rate
tance for fuels of substantial size or depth, though
probably not significant for material the size of pine
(i) Variation with Temperature
needles. One possible effect of variable Mo is that
In the evaporation drying mode ( defined above) ,
under a given set of external conditions, the starting
if relative humidity ( RH) is held constant, then the
RHI
ambient vapor pressure deficit, equal to P,( 1
drying mode may depend 011 whether the material is
100), should vary in simple proportion to the satu­
ration vapor pressure (PJ. Similarly, in the diffusion
drying mode, the diffusion coefficient is known to
vary with P, (Stamm and Nelson 196]); hence, so
should the log drying rate k as well (Nelson 1969).
However, it is further known by the Clausius­
Clapeyron equation that the logarithm of P, is lin­
early related to the reciprocal of the absolute temper­
ature Tn. Thus, all theory suggests that a straight line
quite wet or nearly dry. Drying in a strong wind is
such a possible situation.
Experimental Methods
The basic procedure was to add a specific amount of
water to each sample, to condition it in a closed container
to an even high moisture content, and then to dry it under
controlled conditions weighing the sample at intervals. At
the end of a run, the sample was oven-dried for 24 h at
100°C and the moisture content sequence was calculated.
Three weather factors affecting drying behaviour were in-
270
CAN. J. FOR. RES. VOL. 9, 1979
vestigated: temperature, relative humidity (RH), and wind.
200
The effect of direct solar radiation was not included, al­
though some laboratory work on the joint effect of wind and
sun on Jitter surface temperature was carried out (Van
Wagner 1969a). The effects of temperature and RH on the
drying rate were studied in a cabinet with constant tempera­
ture and humidity. At first, the equipment was capable of
maintaining conditions to rather rough tolerance only; later
a new apparatus was obtained that could hold temperature
within
1°C and RH within ±3% over a range of 25 to
95% and could be used at as low as 10%. The sample was
suspended by a wire attached (at first ) to a manual balance
on the eabinet roof; later a load cell was substituted and
the sample weight was recorded continuously on a strip
chart. Fan speed was held constant during these runs, equiv­
alent to a wind of about 2 km!h at the sample.
To study the effect of variable wind, samples of litter in
open trays were held against one side of a lab bench flush
with the surface and a fan was placed so as to blow air
across the bench. Several samples were run at a time, the
wind at each sample depending on its position with respect
to the fan. Wind was measured 2 em above the sample sur­
face with a hot-wire anemometer. Room eonditions during
10��-2��3--4��5--6��7��8�9�1�0�1I�1�2�13�1�4�15
Time, h
the wind flIns were kept fairly close to 2rc and 30% RH.
To study the effect of wax and resin content on drying.
subsidiary experiments were done on jack pine litter pre­
viously extracted in boiling xylene and on blotting paper.
The litter was collected in several different years, mainly
in midsummer. Part way through the work it was realized
that the drying rate of pine litter changes throughout the
FIG. 1. Semilog graphs of four drying runs, two straight,
one with double slope, and one curving downward:
16cC, 70% RH, k
=
0.022; (B) 27°C. 12% RH, k
and 0.078; (C) 49°C, 50% RH, k
litter) 27°C, 12% RH, k
=
=
(A)
0.125
0.170; (D, extracted
0.216. M, moisture content (%);
EMC, equilibrium moisture content ( % ).
year, being slowest after needle fall in autumn and fastest
in late summer (Van Wagner 1969b). Unfortunately, sam­
ples collected throughout the whole period were not all
quite comparable, although reasonable unifolmity was
achieved at least within separate portions of the work. The
standard sample was 15 g dry weight, set in a plastic tray
11.5 em square and 3 em deep. Sam'ple bulk density was
thus about 0.04 g!cm", and water evaporated from the top
surface only.
The whole work ean be divided into a number of separate
series, each consisting of 5 to 10 drying nlns in which one
the drier section was sometimes steeper, sometimes less
steep. Because of this confused pattern, the sections were
simply pooled as separate data for the same drying condi­
tions (except in series W). This weakness in the data is
acknowledged.
Once ealeulated. the log drying rates in each series were
first plotted against the variable factor. Then, if possible. a
further analysis according to some theoretical form was
attempted.
factor was varied. The series reported in this paper are de­
scribed in Table 1. Temperature was originally varied in
steps of 10°F; these are quoted here to the nearest degree
Celsius.
After each drying run, the sequence of moisture contents
M was calculated. and the equilibrium moisture content
EMC was subtracted from each M to yield the "free" mois­
ture contents F, i.e. the moisture available for further loss.
In many eases, the runs were not carried down to equilib­
rium level; EMC was then estimated from other data (e.g..
Van Wagner 1972). The initial moisture contents of most
samples were between 50 and 100% and most end points
were between 2 and 8 points above EMC. A few slow runs
were cut short as high as 15 points above EMC.
Results
The best way to present the results is in graphical
form. Accordingly, each series of runs listed in Table
1 is represented by one or more figures, depending
on the method of analysis. These figures tell most of
the story and are listed below with notes on those
features worthy of observation.
Temperature
Figure 2
Free moisture F was then plotted as log F against time,
The basic results of series Tl and T2 are plotted
using semilog paper. Most of these graphs were remarkably
straight, or if not completely so, then two or more straight
as k over T. An obvious anomaly is the higher trend
of k at 50% RH than at 40%; this is due to a differ­
ence in the litter samples, collected at different sea­
sections could be identified. A few graphs were curvilinear
on semilog paper throughout their entire course, e.g., some
runs of series HX and HB (Table I). In such cases, k was
computed from the time for F to drop to 5% of its original
value F". Four example runs are shown in Fig.
1. Each
straight section was described by its hourly log drying rate
k, i.e., the decrease in the logarithm (to base 10) of free
moisture content per hour. In cases where the slope changed.
sons in different years.
Figure 3
The k's of Fig. 2, averaged by T within series, are
plotted over 1/1'" on semilog paper. Straight lines
fit the data well and demonstrate the predicted rela-
VAN WAGNER
271
TABLE 1. Details of the experimental drying runs with jack pine litter by series
Series
Date
Tl
T2
1963
1972
1963
1963
1965
1975
1975
1961
1975
Hi
H2
H3
HXb
HBc
W
M
Factor
varied
Temperature
Temperature
RH
RH
RH
RH
RH
Wind
I nitial moisture content
Range of
variation
4-49 °C
4-49°C
Constant
factors"
RH 50(;�
RH 40:�
Temperature 16°C
Temperature 27°C
Temperature 38°C
Temperature 27°C
Temperature 27°C
0-11 km/h Temperature 27°C. RH 30';{
21-132%
Temperature 2l°C, RH 40o/c
aWind constant at 2 km/h except in series W.
bJack pine litter preboiled in xylene.
cThree layers of blotting paper.
.20
2
. 0
:; .15
Serle. T!
S,rle. Tl,50 '% RH
.10
x
x
.!
"
a: .O�
{
.04
�
,03
.,.
"
....
�
�
° �---LI --- - L ----�� --- - 4 ---- 5
O
0
0
2O
0
0
Temperafur., ·C
o
o
o
.02
.01 .L:-----:�--___::':-----=:':- --- -=L---__=':__--__:'
M
�
U
U
U
U
U
ReCPTQ1;CI 'Ot Ab.oh,lte Temperafure (tOOO/Ta)
FIG. 3. Graphs of logarithm of log drying rate vs. recip­
rocal of absolute temperature for series Tl and T2. Plotted
points are averages of values at each temperature in Fig. 2.
5.,1..
T 2. 40 % RH
= .10
"
a:
'"
c
>;
Ci
,05
8'
..J
lines represent the expected trends of k if k were
simply proportional to 100 - RH. (The series H
samples came from the same lot as those of series
T1.) The results of the three H series exhibit a
double pattern. First, at high humidity, k varied
directly with 100
FIG. 2. Graphs of log drying rate vs. temperature for
series TI at 50% RH and series T2 at 40% RH. Curves are
transcribed from the straight lines in Fig. 3.
RH. Second, at humidities below
about 60%, the data are erratic; some k 's at low RH
are as high as the trend above 60% RH would sug­
gest but some k s
' at 20% RH are no higher than
should be inversely proportional to the reciprocal
those at 60%. This erratic pattern is most evident
in series HI (16°C) and H3 (38°C) but is present
to some extent in series H2 (27°C) as well.
of the absolute temperature. (These straight lines
were then transcribed as curves on to the graphs of
The k s
' of series HX and HB are plotted over RH.
tion, namely that the logarithm of the log drying rate
Fig. 2.)
Relative Humidity
Figure 4
Figure 5
The significant feature of these results is the con­
tinuously rising trend of k as RH decreases. In con­
trast with the confused pattern at low RH in Fig. 4,
these data plot smoothly to nearly zero humidity.
The k s
' of series HI, H2, and H3 are plotted over
RH. To aid interpretation, broken lines were in­
Furthermore, the materials of Fig. 5 dried much
faster than natural jack pine litter. For example, at
scribed on each graph to pass through the value of
k at 50% RH as given by the curve in Fig. 2, series
with k
T1 for each temperature in Fig. 4 in turn. These
untreated litter.
27°C and 50% RH, xylene-extracted litter dried
0.15, twice the value of 0.08 obtained for
CAN, J, FOR, RES, VOL. 9, 1979
272
,10
�
Seri"
'"
0
go,OS
;:,
�
5
"
.3
0
HI,16·C
'iI-, x
0
20
60
40
Relative
Wind,
'm/"
�
80
100
Humidity, .,.
.I�
0,75
-
� .10
2,34
'"
"
�
0.05
'"
0
.
.
.::
..J
9,62
54"
0
0
20
40
80
Relative Humidity) %
'
,x
,
'6), '\ S.ri.. H 3. 3S*C
X 'lI.
.10
X'
x
't
0
20
40
60
Relolive HumiditYt %
80
!
I
4
3
I
5
j
6
f
6
i
9
!
10
!
12
t
13
Tim., h
!
14
t
15
FIG, 6. Graphs of four drying runs of series W at various
wind speeds illustrating two-stage drying at high wind, (See
Fig. 1 for M and EMC.)
'-X ,
g'.05
I
0
x
..J
0
100
,
.15
�
.
'6
'"
"
�
c;
60
100
FIG, 4. Graphs of log drying rate vs. relative humidity for
series HI at 16"C, H2 at 27°C, and H3 at 38°C. The plotted
circles at 50% RH are values for the respective tempera­
tures taken from the series Tl curve in Fig , 2.
,35
o
,30
o
o
0
o
25
SIr." HX-X
Sorln HS-O
,20
o
o
i
�
�
2
0 �--�----- 1� ----�----�----� L--�
I
1
0
4
Wind.
20
40
60
80
100
Rtlollve Humidity. %
FIG, 5, Graph of log drying rate vs. relative humidity for
series HX (xylene-extraeted litter) and HB (blotting paper),
both at 27¢C.
krn/n
FIG. 7, Graph of log drying rate vs. wind speed for series
'
W, Two drying stages are plotted separately, (See Fig. 6
for relative duration of each.)
and final log drying rates (k 1 and k�) were therefore
analyzed separately.
Wind
At zero and very low wind speeds, the drying
runs of series W followed a single exponential trend
right from the start. At moderate and high wind
speeds, a fast-drying first stage occurred, generally
over within I h, after which the moisture content de­
creased exponentially at a slower rate. The initial
Figure 6
Four typical drying runs from series W are shown
as F plotted against t. The short, steep initial drop in
M is noteworthy, most pronounced at the highest
wind speed but negligible at zero wind.
Figure 7
The k1's and k'!,'s of series W are plotted separately
VAN WAGNER
273
3), and the slope of this effect was common to series
3
x
x
,06
.
o
a:
i,04
x
x
x
x
x
T1 and T2 in spite of the difference between their
absolute ranges of k. The failure of k to respond
smoothly to variation in humidity below 60% RH
was a puzzle in the first years of this research, but
the series HX and HB experiments with extracted
<5
:;'
litter and blotting paper offer a reasonable explana­
",I ,02
tion, namely the limiting effect of waxy cuticle and
resin on the needle 's internal diffusion rate. The
Inltlol Moisiuro Content. %
FIG. 8. Graph of log drying rate
tent for series M.
VS.
initial moisture con­
over wind W. Some points at very low W are com­
mon to each set, and the two-stage drying behaviour
is more and more evident as wind speed increases.
The log drying rates in each stage appear to reach
a limiting maximum at fairly low wind speed, two
to three times as fast in the initial stage as in the
final stage. In still air, k was about 0.05, substantially
less than the value of 0.09 obtained at similar T and
RH in the controlled cabinet. This suggests that the
rate of air movement in the cabinet (about 2 km/h)
was above the range at which k is most sensitive to
W.
series H data, with their variable reaction to humid­
ity ( Fig. 4) , illustrate the evaporation and diffusion
drying modes described earlier. Thus, k responds
smoothly to variation in humidity at high RH as ex­
pected in the evaporation mode; in contrast, at low
RH, k appears nearly independent of humidity as fits
the diffusion mode. The erratic pattern of k at low
RH is a weakness in the data and in part may be due
to variation in the litter samples. The temperature
effect, however, as shown in Fig. 3 is smooth over
the whole temperature range, as befits either drying
mode.
The drying behaviour of extracted litter (series
HX) and blotting paper (series HB) , with its smooth
variation in k throughout the whole humidity range
( Fig. 5) , suggests that diffusion mode drying should
be found only in fine materials with a substantial
resinous content or waxy cuticle. Once this barrier
Initial Moislllre Content
Figure 8
The k's of series M are plotted over initial mois­
ture content Mo.
The individual runs of this series, at 21°C and
40% RH, all yielded good straight semilog graphs.
However, the pattern of k against Mo is rather er­
ratic. There is a slight suggestion of higher k at M o's
over 100% but of no discernible trend below this
level.
to the escape of water vapor is removed, the evapo­
ration mode appears to prevail, with generally faster
drying rates as well. In a previous experiment (Van
Wagner 1969b) , natural pine litter even after a
year 's weathering was found to dry more slowly than
wood splints several times thicker.
The above discussion of cuticle effect treats the
litter samples as a collection of discrete particles.
This is reasonable because the sample bulk density
was only 0.04 g/cm3 and the individual needles were
quite intact. Conversely. because the sample was 3
Discussion
The assumption of exponential drying in fuel
moisture prediction depends ultimately on the ex­
perimental evidence, bolstered by theory. Certainly,
the semilog straight line nature of most of the drying
runs in this work, for several other kinds of fine
material as well as jack pine litter, provides justifica­
tion for this working principle, common to all the
cited literature on forest fuel drying. It is true that
cm deep, the drying environment was certainly less
favourable at the bottom than at the top; thus, for
some purposes, the samples can be treated as layers
in which drying rate depends on depth and bulk
density. There is no doubt that a single exposed
layer of pine needles will dry faster than one 3 cm
deep. The latter depth was chosen to simulate as
well as possible realistic forest litter.
Of all the cited similar work, that of Nelson
( 1969) , who measured the exponential drying rates
curved or double-sloped semilog graphs occur and
this remains to be explained.
of sawdust layers and wooden rods, is most com­
With respect to temperature and relative humid­
ity, the experimental results can be fitted reasonably
effect in his Fig. 5 can be replotted to yield a de­
scending straight line as in Fig. 3 of this paper, al­
well into the theoretical picture presented earlier.
Thus, log k was inversely proportional to liT" (Fig.
though with a steeper slope. Similarly, the humidity
effect in his
6 matches the dependence of k on
parable with the present results. The temperature
274
100
CAN. J. FOR. RES. VOL. 9, 1979
-
RH shown by series HX and HB in Fig. 5 of
this paper. Nelson tested no materials with appre­
ciable wax or resin content; thus his humidity ellect
is smooth throughout the whole applied range of RH
(18% to 83%). With this dillerence, t here is good
agreement between Nelson's results and these.
The two-stage drying process starting at high Mo
in moderate to strong wind (Fig. 6) was also ob­
served by Britton et al. (1973) in their experiments
on the drying rate of tobosa grass. Their first stage
was likewise rather short, about t h. They assumed
a simple constant-rate mechanism during this stage
rather than an exponential one. In fact, this
is
short enough that very detailed measurements would
be required to identify the process exactly. The
present results (Fig. 7) show that in both stages k
is quite sensitive to variation in wind at low wind
speed whereas increasing wind speed has little addi­
tional effect on drying rate at higher levels. The firstdata are rather erratic, but the second-stage k's
demonstrate well that the drying rate is nearly inde­
pendent of wind speeds above about 2 km/h close
to the fuel surface. (The equivalent wind speed as
measured 10 J11 aboveground at fire weather stations
would be about 10 km/h.) These wind results sug­
gest evaporation mode drying for a short period
when initial fuel moisture is high, switching to dif­
fusion mode at moderate moisture content. In other
words, wind speed is most important in the estima­
tion of fine fuel moisture content in the forest just
after rain.
The effect of initial moisture content Mo on k, if
any, cannot be regarded as settled by the present
results. The graph of thc series M k's was disappoint­
inaly erratic, illustrating the difficulties of control in
�
m isture content work with natural materials. Nel­
son (1969) felt, admittedly on incomplete evidencc,
that k may vary with (MOl - EMC)
a fairly strong
relation that did not show up in the present results.
At least series M provides no evidence of an initial
moisture effect on k important enough to be
icant in fire danger rating. A definitive answer to
this question will have to await further work.
It is worth pointing out that most of the drying
runs in this work began well above fibre saturation
point (FSP) and ended well below it. (FSP is about
30% moisture content for woody and related mate­
rials.) Most of the semilog graphs were straight
throuahout
the entire moisture range, and in cases
b
where a double slope occurred, the break pomt was
not obviously related to the FSP. No evidence was
•
Drying in the Fine Fuel Moisture Code
The general principles found in this laboratory
work are reproduced fairly well in the Fine Fuel
Moisture Code (FFMC) of the Forest Fire Weather
Index. The drying cquations (Van Wagner and Pick­
ett 1975) are based primarily on the analysis of field
data, extended and modificd by the methods de­
scribed here and in Van Wagner (1974). Thus, the
exponential drying pattern was clearly evident in the
field data. Also, the derived temperature effect on
log drying rate is analogous to the inverse straight
line relationship shown in Fig. 3, while the humidity
effect is a curve that starts from zero at 100% RH,
rises relatively steeply at first but flattens off grad­
ually below 60% RH according to the pattern in
Fig. 4. The wind effect is a compromise in which k
is given a base value at zero W with an increase
above that in proportion to the square root of W. In
practice, the drying rate is most important at high
moisture content and during the 1st or 2nd day after
rain. Once the moisture content has fallen to within
several points of equilibrium, the EMC itself be­
comes the dominant influence on fine fuel moisture
content and the drying rate is of secondary conse­
quence.
From the earliest days of Canadian research on
fire danger rating, fine fuel (the litter layer in the
forest) has been assumed to dry at a rate slow
enough that each afternoon's moisture content de­
pends partly on the previous day's value (Beall
1947). For example. a typical log drying rate in
2 and 4 at say 25°C and 40% RH is about
0.07/h, corresponding to a time lag of 6.2 h. Start­
ing with a free moisture content of say 100%, such
a fuel would still be 35 % above EMC after one
time lag period, about the duration of good drying
time in 1 day. l11lls, the FFMC has an appreciable
time lag from day to day, the aim being to match
moisture changes from afternoon to afternoon with
noon weather conditions. The well known diurnal
cycle of weather and fine fuel moisture was bypassed
by this empirical approach, and the problem of esti­
matina fine fuel moisture at short intervals around
the cl ck has been dealt with elsewhere (Van Wag­
�
ner 1977).
Conclusions
In brief, the conclusions to be drawn from this
found for any difference in the drying behaviour of
research, together with the existing literature, are
as follows. (1) Fine forest fuels dry exponentially
for practical purposes. (2) The log drying rate
fine fuels whether above or below FSP.
varies according to the external environment, and
VAN WAGNER
this variation can be accounted for in terms of tem­
perature, relative humidity, and wind speed. (3)
The effect of temperature is such that the logarithm
of the log drying rate is inversely proportional to the
reciprocal of the absolute temperature. (4) The log
drying rate is inversely proportional to 100
-
RH,
but waxy cuticle or extractives may interrupt this
trend at low humidities by limiting the rate of in­
ternal diffusion. (5) Drying from high initial mois­
ture content in moderate or strong wind is a two­
stage process, the first stage being shorter and faster
than the second. Drying rates are sensitive to wind
at low wind speed but reach limiting values at about
2 km/h (measured close to the drying surface) with
little further increase at higher wind speeds. (6) The
drying rates of leaf and needle litter are slowed con­
siderably by resin content and waxy cuticle. These
rates may then be slow enough that account must be
taken of yesterday'S moisture content for realistic
prediction of today's. (7) Depending on current
moisture content, extractive content, and external
conditions, fine forest fuel may dry in one or other
of two different ways called here the evaporation
and diffusion drying modes. In the evaporation dry­
ing mode, all three weather factors, temperature,
humidity, and wind, act jointly to defermine the log
drying rate. In the diffusion drying mode, tempera­
ture is the controlling factor and humidity and wind
have little effect.
The evidence for some of these conclusions is cir­
cumstantial rather than absolute, but the results as
a whole fit together fairly well and have provided a
framework for the design of practical fuel moisture
schemes in the Canadian forest fire danger rating
system.
Acknowledgment
J. W. Bell carried out the drying runs in this
program, kept the research equipment in good run­
ning condition, and compiled the results.
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