Agricultural and Forest Meteorology 128 (2005) 81–92
www.elsevier.com/locate/agrformet

Annual water balance and seasonality of evapotranspiration
in a Bornean tropical rainforest
Tomo’omi Kumagaia,*, Taku M. Saitohb, Yoshinobu Satoc, Hiroshi Takahashid,
Odair J. Manfroie, Toshiyuki Morookae, Koichiro Kurajie, Masakazu Suzukie,
Tetsuzo Yasunarid, Hikaru KomatsufaUniversity Forest in Miyazaki, Kyushu University, Shiiba-son, Miyazaki 883-0402, Japanb
Research Institute of Kyushu University Forest, Sasaguri, Fukuoka 811-2415, Japanc
Research Institute for Humanity and Nature, Kyoto 602-0878, Japand
Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Nagoya 464-8601, Japane
Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Tokyo 113-8657, Japanf
Institute of Industrial Science, The University of Tokyo, Komaba, Tokyo 153-8505, Japan
Received 21 November 2003; received in revised form 12 August 2004; accepted 18 August 2004

Abstract
This study presents the results of 2 years combined field measurements of water vapor exchange with the atmosphere andsimplified model calculations at Lambir Hills National Park, Sarawak, Malaysia (48120 N, 1148020 E). The study site was
located in a lowland mixed dipterocarp forest, a major type of Bornean tropical rainforest, where there is no clear seasonality
in environmental factors such as radiation, temperature, vapor pressure deficits and precipitation; instead, unpredictable dry
spells often occur throughout the year. Modified [Priestley, C.H.B., Taylor, R.J. (1972). On the assessment of surface heat
flux and evaporation using large-scale parameters, Mon. Weather Rev. 100, pp. 81–92] and equilibrium evaporation
expressions enabled us to understand further the environmental control of water vapor exchanges with the atmosphere and to
produce a complete gap-filled data set of the hydrologic fluxes within this environment. The equilibrium evaporation from awell-watered surface sufficiently reproduced the transpiration rate (Tr); any discrepancies between the equilibrium andactual evaporation rates were caused by unpredictable intra-annual dry spells, which reduced transpiration. There were somediscrepancies during the study period because of exceptional dry sequences, but in normal years the annual transpiration rate
can be obtained from the equilibrium evaporation expression. The estimated annual Tr (1193.1 mm) and evapotranspiration(1545.0 mm) rates were nearly identical to the highest values reported for some humid tropical forests. Although, there were
exceptional dry sequences during the study period, the annual average fraction of available energy dissipated by Tr at this site
(0.69) was almost the same as the medium value between the dry and wet seasons in Amazonian tropical forests. This implies* Corresponding author. Tel.: +81 983 38 1116; fax: +81 983 38 1004.
E-mail address: kuma@forest.kyushu-u.ac.jp (T. Kumagai).
0168-1923/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.agrformet.2004.08.006

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T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

that the annual dynamics of the latent heat flux for this tropical rainforest are under more humid conditions than those ofother tropical rainforests.
# 2004 Elsevier B.V. All rights reserved.
Keywords: Eddy-covariance; Evapotranspiration; Equilibrium evaporation; Priestley–Taylor equation; Tropical rainforest; Southeast Asia

1. Introduction
Tropical rainforests are among the most importantbiomes because of their vast amounts of primary
productivity, and water and energy exchange with theatmosphere. Although these forests now cover only
12% of the global total land surface (FAO, 1993), they
contain about 40% of the carbon in the terrestrial
biosphere (Skole and Tucker, 1993) and are responsible for 50% of terrestrial gross primary productivity(Grace et al., 2001). Tropical rainforests are also amajor source of global land surface evaporation(Choudhury et al., 1998) and have profound influenceson global and regional climates and hydrologicalcycling (e.g. Lean and Warrilow, 1989; Nobre et al.,1991). These large latent energy fluxes from tropicalrainforests are known to influence global atmosphericcirculation patterns (Paegle, 1987). In the humid
tropics, climate change might drastically alter hydrological regimes, for example, because of alteredrainfall patterns and land cover transformation mainly
as a result of forest conversion (Bruijnzeel, 1996).
Consequently, such alterations might accelerate
further global climate changes.
A number of field studies have reported on thecomponents of the hydrologic cycle in Amazonian
tropical rainforests (e.g. Shuttleworth et al., 1984;
Shuttleworth, 1988; Hodnett et al., 1995; Grace et al.,
1995; Leopoldo et al., 1995; Malhi et al., 2002;
Vourlitis et al., 2002). These studies suggest that 50%of the incident precipitation in these areas is recycled
as precipitation. Water cycling in Southeast Asian
tropical rainforests is, however, less certain and fieldstudies on the exchange of water between Southeast
Asian tropical rainforests and the atmosphere havebeen poorly documented compared to the studies
performed in Amazonian tropical rainforests.
As Dykes (1997) pointed out, there is a need for
detailed long-term studies on the hydrologicalcomponents of the ‘‘maritime’’ environments of theSoutheast Asian tropics, which are quite different
from the environments of the South American andCentral African tropics. While there are definite
periodic dry periods in the South American andCentral African tropics, the Southeast Asian maritime
environments do not have such phase-locked dry
periods because their climate is the combined result ofa summer monsoon from the Indian Ocean, a wintermonsoon from the Pacific Ocean and South China Sea,and the Madden and Julian Oscillation (MJO).
Northern Borneo experiences only small seasonal
solar radiation and air temperature variations andannual rainfall is distributed evenly throughout theyear (Inoue et al., 1993). However, unpredictable
intra-annual dry spells (Kumagai et al., 2004b) and dry
sequences do occur (Kumagai et al., 2001). Intraannual dry spells, in particular, occur annually andoften throughout the year. The goal of this study is toprovide information regarding Southeast Asian tropical rainforest energy and water cycling. Towards thisgoal, measurements of hydrological components suchas transpiration rate, rainfall interception and soil
moisture content were initiated in 2001 as part of a
project entitled ‘‘Research and Observation on the
Mechanisms of Atmosphere–Ecosphere Interaction in
Tropical Forest Canopy’’ in a tropical rainforest in
Lambir Hills National Park, Sarawak, Malaysia(Nakashizuka et al., 2001). Transpiration rates,
canopy conductances and decoupling coefficientsduring dry spells and in wet conditions werepreviously compared (Kumagai et al., 2004b). Whilecanopy conductance and its sensitivity to micrometeorological variables were higher in wet conditions
than during dry spells, these higher values did notcause higher transpiration rates compared to the dry
spells. Thus, we expected transpiration in the studysite to be related to the equilibrium evaporation rate at
sufficiently large fetch under a steady energy supply(Slatyer and McIloy, 1961; McNaughton and Jarvis,
1983; Raupach, 2001). The present paper used theresults of a further 2 years of measurements to address
the following: (1) how transpiration rates are related toequilibrium evaporation in this tropical forest, (2) how
the possible environmental controls of variations in

T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

83transpiration respond to little climatic seasonality, and(3) how large the surface-atmosphere exchange ofwater vapor in a truly humid tropical rainforest is.

2. Study site and measurements
2.1. Site description
The experiment was carried out in a natural forest
in Lambir Hills National Park (48120 N, 1148020 E),
30 km south of Miri City, Sarawak, Malaysia (Fig. 1).
Mean annual rainfall at Miri Airport, 20 km from thestudy site, for the period 1968–2001 was around2740 mm with some inter-annual variation; forexample, the maximum and minimum annual rainfall
during this period were 3499 mm in 1988 and2125 mm in 1976, respectively. Fig. 2 shows theannual rainfall cycle obtained from monthly measurements above the canopy at the study site from July
2001 to June 2002. This data was compared with themean and standard deviations of the monthly rainfall
from 1968–2001. During this time period, the driest
months were February and March and the wettest
period was between October and January; however,March and January are not always the driest andwettest periods, respectively, and the timing of thedriest period is unpredictable. Exceptional dry
sequences were observed during the course of thepresent study from July to August 2001 and May toJune 2002 (Fig. 2).

Fig. 2. (a) Monthly rainfall at the study site from July 2001 to June
2002 (bars). The points with error bars show the means  standard
deviations of the monthly rainfall at Miri Airport from 1968–2001.
(b) Monthly mean maximum (thick line) and minimum (thin line)
daily air temperatures at Miri Airport from 1968–2001.

A global precipitation map was reproduced using
data from the Climate Precipitation Center (CPC)
Merged Analysis of Precipitation (CMAP; Xie andArkin, 1997) for the period 1979–2001. A low-pass filter
was used to eliminate all periods shorter than half a year
from these precipitation time series (Duchon, 1979).The
ratios of the variance of the low-pass filtered precipitation to that of the unfiltered precipitation are shown in
Fig. 3. The Lambir Hills National Park had one of theFig. 1. Location of the Lambir Hills National Park, Sarawak, Malaysia.

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T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

Fig. 3. Ratios of the variance of the low-pass filtered precipitation time series to that of the unfiltered precipitation. A global precipitation map
was reproduced using data from the Climate Precipitation Center (CPC) Merged Analysis of Precipitation (CMAP; Xie and Arkin, 1997) for theyears 1979–2001.

lowest ratios, that is, one of the weakest seasonalvariationsinprecipitation.Themean monthlymaximum
andminimumdailyairtemperaturesatMiriAirportfrom
1968–2001 are also shown in Fig. 2. Overall, this datasuggests that the mean annual temperature is around27 8C with little seasonal variation.
The rainforest in this park consists of two types oforiginal vegetation common to the whole of Borneo,that is, mixed dipterocarp (Dipterocarpaceae) forest
and tropical heath forest (Yamakura et al., 1995). The
former contains various types of dipterocarp trees andcovers 85% of the total park area. The soils consist of
red–yellow podzonic soils (Malaysian classification)
or ultisols (USDA Soil Taxonomy), with a high sandcontent (62–72%), an accumulation of nutrients in thesurface horizon, a low pH (4.0–4.3), and a highporosity (54–68%) (Ishizuka et al., 1998; Sakurai,1999). Borneo has a primarily hilly topography andmost of its soils are ultisols. Ultisols are also thetypical soil type throughout much of Southeast Asia,and its distribution corresponds to that of lowland andhill dipterocarp forests (Ohta and Effendi, 1992).

2.2. The crane facility
A 4 ha experimental plot at an altitude of 200 m,
gridded into 400 subplots or quadrats of 10  10 m2,
was used in this project. An 80 m tall (at the base of the
gondola) canopy crane with a 75 m long rotating jib
was constructed in the center of this plot to provide
access to the upper canopy. Observational stages atfour levels (23.5, 38.5, 59.0, and 75.5 m above theground) accessible by an elevator were also installed.
The 59.0 m high stage was devoted to Eddycovariance flux measurements. The subplots or
quadrats were used for in-canopy micrometeorological measurements, and throughfall and stemflow
measurements. The canopy height surrounding thecrane is about 40 m but the height of the emergent
treetops can reach up to 50 m.
2.3. Leaf area measurements
The leaf area index (LAI) was measured in
2002 using a pair of plant canopy analyzers

T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92(LAI-2000, Li-Cor, Lincoln, NE) at 5 m intervals in a
30  30 m2 subplot. The LAI ranged from 4.8 to
6.8 m2 m2 with a mean of 6.2 m2 m2 (Kumagai
et al., 2004b). The monthly amounts of litterfall were
evenly distributed throughout the year (M. Nakagawa,
unpublished data) perhaps suggesting small variationsin LAI.
2.4. Micrometeorological and soil moisturemeasurements
The following instruments were installed at the topof the crane, 85.8 m from the forest floor: a solar
radiometer (MS401, EKO, Tokyo, Japan), an infrared
radiometer (Model PIR, Epply, Newport, RI), and atipping bucket rain gauge (RS102, Ogasawara Keiki,Tokyo, Japan). Upward short-wave radiation and longwave radiation were measured at a separate tower
located 100 m south of the crane using an upsidedown solar radiometer (CM06E, Kipp & Zonen, Delft,
Netherlands) and infrared radiometer (Model PIR,
Epply, Newport, RI), which have been recording since
15 December 2001. Radiation was sampled every 5 s
and an averaging period of 10 min was used (CR10X
datalogger, Campbell Scientific, Logan, UT). These
measurements were used to compute the net radiation(Rn; Wm2); the net radiation time series prior to 15
December 2001 were calculated using the relationshipbetween solar radiation (Rs; Wm2) and net radiation(Rn = 0.87Rs35.0, R2 = 0.99).
The volumetric soil moisture content (u) and matricpotential (c) were measured in a subplot at 10, 20 and
50 cm below the forest floor at 10 min intervals(CR23X datalogger, Campbell Scientific, Logan, UT).
A time-domain reflectometer (TDR; CS615, Campbell Scientific, Logan, UT) was used to measure thetime series of u and a tensiometer (DIK3150, Daiki,
Tokyo, Japan) was used to monitor c at the same depth
as u thereby providing the necessary measurements tocompute in situ soil water retention curves at each
depth. The relative extractable water in the soil (Q;m3 m3) was calculated as follows using the average u
in the 0–50 cm soil layer: (u  ur)/(us  ur), where us
and ur are the saturated and residual water contents,
respectively. us was taken as the maximum average uobserved in the 0–50 cm soil layer during theexperimental period (June 15 2001–November 102002), and ur was estimated as the average u when c at

85

a depth of 10 cm was bounded by 0.7 m and0.01 m. The Q results range from 0 (soil dry) to 1
(soil saturated).
2.5. Evapotranspiration measurements by the Eddycovariance method
A three-dimensional sonic anemometer (DA-600(Kaijo, Tokyo, Japan) from 15 June 2001 to 4 August
2002, and USA-1 (METEK, Elmshorn, Germany)
since 4 August 2002) was installed at 60.2 m, that is,
20 m above the mean canopy height. An open-path
CO2/H2O analyzer (LI-7500, Li-Cor, Lincoln, NE)
was placed about 30 cm away from the sonicanemometer in an inclined orientation and with its
windows coated in wax to reduce problems associatedwith rain and dew droplets. In addition, a ventilated
psychrometer (MS020S, EKO, Tokyo, Japan) was
installed adjacent to the sonic anemometer and openpath analyzer to calibrate and correct the density
effects (Webb et al., 1980).
The sensors were mounted 2.5 m away from thecrane leg using a horizontal boom fixed on thesouthwest corner, so as to minimize flow distortion
when the wind was blowing from its prevailingnorthwesterly and southeasterly directions. When thecrane was constructed, a gap to hold the crane at its
center was formed. The largest radius of the gap was
about 25 m, and the fetch exceeded 1 km in thenorthwest sector; the minimum fetch, which was in thesouthwest sector, was about 200 m. The influences of
the gap and minimum fetch on the flux estimations
were examined using a two-dimensional analytic
solution of the diffusion equation proposed by
Schuepp et al. (1990) together with the stability
dependency proposed by Lloyd (1995). As a result, wefound that the wind almost always flowed from thenorthwest during the daytime, and that the maximum
80% effective fetch and the minimum 10% effective
fetch were about 1500 m (12:30 h, 18 June 2001) and
50 m (2:00 h, 13 June 2001), respectively. Therefore,
these influences were neglected for flux estimations
during the daytime.
Wind speeds and gas concentrations were sampled
at 10 Hz and the averages were computed at 30 min
intervals. The results were detrended using a movingaverage time constant of 200 s and the wind field (u, v,w) (ms1) coordinates were rotated so that the mean v

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T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

and w values were zero over 10 min periods. The
percentage corrections for the coordinate rotations
were less than 1%, probably because of the gentle
slope. We applied the density fluctuation correction
method described by Webb et al. (1980). In this study,
the flux data measured during rain events wereabandoned because data from the Eddy-covariancesystem obtained during rainfall events are not reliable.
However, as the durations of the rain events during thestudy period were short, gaps occurring in the flux data
because of this abandonment were small. Calibrationsof the open path analyzer were performed periodicallyusing a dew point generator (LI-610, Li-Cor, Lincoln,
NE): the zero and span were checked at intervals of
2–3 months.
2.6. Throughfall and stemflow measurements
A plot representative of the 4 ha experimental plot
was instrumented for throughfall and stemflow
measurements. Throughfall was measured using 40
collectors composed of manually-operated gauges
assembled with 20.6 cm diameter plastic funnels and
10 L capacity bottles, 20 of which were dispersed on agrid over the fixed plot. Samples were taken manually,
everyday except Sunday between 8:00 and 13:00 LST
using 1 L plastic cylinders with 10 ml graduationintervals.
For stemflow measurements, collars connected to
30 L bottles or 15.7 cc tipping bucket flow meters (no.
34, Ohta Keiki, Tokyo, Japan) were attached to thetrunks of 81 trees with various circumferences(ranging between 1 and 130 cm). Of these trees, 78
were located inside the fixed plot and three outside.
Some stemflow measurements were sampled as thetime of tip by the event data logger (KADEC-UP,
Kona System, Sapporo, Japan); all others were
measured manually using the same procedure as forthe throughfall measurements. Details on the throughfall and stemflow measurements are presented in
Manfroi et al. (2004)

3. Methods for estimating evapotranspiration
Equilibrium evaporation occurs when air passes
over an extensive wet surface and becomes saturated.
Then, the Bowen ratio (b = H/LE where H and LE are

the sensible and latent heat flux, respectively) takes thevalue g/D where D is the changing rate of saturationwater vapor pressure with temperature (PaK1), and gis the psychrometric constant (66.5 PaK1). Under
this condition, the daily equilibrium evaporation(LEeq, MJm2 d1) was obtained using the energybalance equation as follows:
LEeq ¼ 0:0864

D
Rn
Dþg

(1)

where Rn is the daily net radiation above the canopy(Wm2). Here, for the sake of simplicity we assumed
that the daily cumulative storage heat flux to the soil
and canopy could be neglected. The thermodynamic
variable D was calculated based on the mean air
temperature averaged over daylight hours. Daily net
radiation was obtained by averaging these hourly
values.
Although the precipitation, solar radiation, and airtemperature at the study site show small seasonalvariations, intra-annual dry spells that often occur
throughout the year have been reported (Kumagai etal., 2004b). Therefore, it is logical to expect thatenvironmental changes caused by this unpredictable
dry condition, especially, changes in soil moisture,
will affect the biological activities in this site; here theresultant changes in transpiration are examined. To
determine transpiration we need to link its limiting
factors to the variation in soil moisture in the rooting
zone. Hence, we used a modified Priestley and Taylor(PT) (1972) expression to compute the dailytranspiration rate (LE, MJm2 d1) according to
Scanlon and Albertson (2003) as follows:
LE ¼ hap LEeq

(2)

where ap is the PT coefficient (1.26) and h is thelimiting factor. De Bruin (1983a) found that ap varies
with surface resistance and the entrainment rate of dry
air at the top of the atmospheric boundary layer(ABL). By assuming that surface resistance is related
to soil moisture, we used the Eddy-covariance LE
measurement to obtain the daily h value then proceeded to derive a relationship between Q and h. Here,the averaged h was computed in bins of Q (incremented in 0.1) and a regression model describing Q andthe bin-averaged h was derived (R2 = 0.96) as follows:h ¼ 0:46Q þ 0:53; for 0  Q  1:0(3)

T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

87

The values of the ap and h products obtained here
ranged from 0.6 to 0.8 and appeared reasonable whencompared to other dry canopy studies (De Bruin,
1983a).
In Kumagai et al. (2004a), daily interception(Ic, mm d1) was related to daily rainfall P (mm d1)
using the following equation:
8
for
< P;
Ic ¼ 0:13P þ 0:78; for:
0:084P þ 0:96; for

0  P < 0:90
0:90  P < 3:91
3:91  P(4)

To estimate Ic, we assumed that a single storm event
does not exceed one day, and that multiple storm
events during 1 day can be lumped together as a single
daily storm. This assumption is necessary becauseinterception losses are related to daily precipitation
thereby forcing a daily minimum time step of integration. We believed this simplifying assumption was
reasonable here because single storm events exceeding 1 day seldom occur, at least in the historical
records of Miri Airport.

4. Results and discussion
4.1. Energy balance
Fig. 4 compares the sum of the daily cumulative
sensible (H) and latent (LE) heat fluxes (H + LE) to thedaily cumulative net radiation (Rn) for dry and wet
canopy conditions. The mean differences between
the H + LE and Rn values were 2.72 and 1.81 MJm2
day1 on days with no rainfall events and with both norainfall and rainfall events, respectively. The student’s
paired t-test indicated that the differences between
these quantities for both conditions were significantly
different from zero at a 0.01 probability level (ts =10.67 and ts0.01 = 2.63 on days with no rainfall events,
and ts = 9.82 and ts0.01 = 2.60 on whole observationdays). The H + LE value was regressed against theRn value, and the average slope was calculated as
0.78  0.03 (95% confidence) on days with no rainfall
events and 0.82  0.04 on whole observation days.
The H + LE values on days when rainfall occurred
tended to be larger than those on days when no rain
events occurred, despite discarding the Eddy-fluxes
measured during rainfall events. This might have beenFig. 4. The sum of the daily cumulative sensible (H) and latent (LE)heat fluxes (H + LE) vs. the daily cumulative net radiation (Rn) for adry (open circle) and wet canopy (closed circle).

caused by evaporation from the wet canopy, whichwere included in water vapor flux measurements
obtained on days when rainfall occurred.
On a one-dimensional scale, the energy balance
was not completely closed here; a possible source oferror in the energy balance might be attributed to thedifferent areas that were sampled for the H + LE and
Rn measurements. On the other hand, non-closure of
the energy balance is a common feature of Eddycovariance measurements above forest canopies, andthe closure deficit observed here (about 20%) was
comparable to that obtained at most forested sites (e.g.
Goulden et al., 1996; Ohta et al., 2004; Aubinet et al.,
2001; Wilson et al., 2002). This imbalance could be
explained by a variety of processes such as an
inadequate performance of the Eddy-covariancesystem or scale mismatches between the Eddy-fluxes
and micrometeorological samples (e.g. McMillen,1988). The energy balance relationship at this site
most likely indicates that the Eddy-fluxes were
underestimated by about 20%; an underestimationthat was caused in part by insufficient evaluation of thecontribution of the relatively large-scale Eddy (Lee,1998; Malhi et al., 2002). Although further analyses
are needed to resolve the reasons for this imbalance
and the Eddy-flux underestimations, the degree ofclosure was well within the range reported for other88

T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

forested sites. Therefore in this study, according toKumagai et al. (2004b), we assumed that the Eddyfluxes had been equally underestimated and thus
multiplied them by 1/0.78.

Fig. 5 compares the LEeq (see Eq. (1)) calculatedusing independent measurements of Rn and airtemperature against the measured LE for dry canopy
conditions. Note that the LEeq and LE from a dry
canopy are synonymous with transpiration (Tr). A
significant (ts = 3.09; ts0.05 = 1.99) but small(0.9 MJm2 d1 or 10% of the mean) difference
existed between the LEeq and measured LE. Overall,
LEeq clearly reproduced the measured LE value
despite its simplified form.
The first definition of LEeq was evaporation from awet surface into saturated air (Priestley, 1959).
However, air above extensive wet surfaces such as
the ocean is seldom saturated (Priestley and Taylor,1972). McNaughton (1976) presented a more generalidea of LEeq suggesting that it occurs whenever the ABL
has a constant height and is capped by a lid throughwhich no exchange of air occurs. Furthermore,
McNaughton and Jarvis (1983) showed that LEeq is
evaporation in the limit of complete decoupling.

A previous paper by the authors reported highdecoupling coefficient values under wet soil moisture
conditions due to relatively large surface conductancecompared with aerodynamic conductance (Kumagai
et al., 2004b). This might explain why in this study site
the LEeq can reproduce the measured LE value. Fig. 6
compares the LEeq and LE values for dry canopy
conditions modeled using the modified PT expression(see Eq. (2)). The LEeq values were clearly correlatedwith the modeled values (R2 = 0.74), but Fig. 6 also
reveals a systematic overestimate caused by thelimiting factor responding to dry soil moisture
conditions in the modified PT expression. However,despite the existence of dry conditions and theoverestimates, the mean difference between the LEeq
and modeled values was 0.8 MJm2 d1, or 8% of the
population mean.
Many researchers have suggested that LEeq is a
guide (even sometimes as a good as an approximation)of actual evaporation rates over well-watered vegetated surfaces, even though ideal field conditions
seldom occur (Raupach, 2001). While there weresome dry conditions in the observation period that
could have affected LE (see Fig. 2), the measured LE
somewhat conformed with the LEeq.
LE values measured on rainy days were excluded
from the Tr modeling. However, we assumed that

Fig. 5. Comparison between the measured latent heat flux
(LE_measured) and equilibrium evaporation.

Fig. 6. Comparison between the latent heat flux estimated using themodified Priestley and Taylor expression (LE_PT) and equilibrium
evaporation.

4.2. Estimating evapotranspiration

T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

modeled Tr rates could also be calculated on days
when rainfall occurs, because the duration of storm
events are short in this site and the modeled values
were calculated from the daily cumulative Rn. Thus,
the LE on rainy days was calculated as the sum of the
Tr modeled by Eqs. (1) or (2) and the Ic modeled by Eq.
(4). This calculation is analogous to the model used by
Shuttleworth and Calder (1979) for actual evaporationthat includes both the transpiration and evaporation
rates of intercepted water; we filled data gaps andreplaced unreliable data using this calculation.
4.3. Annual water balance
The annual variation in water balance is summarized in Table 1, which shows the monthly totals andaverages. The monthly total transpiration rates (Tr_eq
and Tr_pt) were estimated from the complete gap-filled
data set obtained using the equilibrium evaporation(Eq. (1)) and modified PT (Eq. (2)) expressions,
respectively.
During months with little rainfall, namely, August
2001, September 2001 and June 2002, the Tr_eq wereabout 20 mm larger than the Tr_pt. This implies that theTr_eq expression does not take into consideration theeffect of dry soil moisture conditions. As a result theannual total value of Tr_eq was 94 mm larger than thatof Tr_pt. However, the equilibrium evaporation
expression for estimating Tr is still effective because
the annual total rainfall in the study period (July 2001–
June 2002) was exceptionally low (see Fig. 2).

89

Furthermore, for practical estimations it has theadvantage of simplicity because it requires only net
radiation and air temperature as inputs.
Having verified that Eq. (3) describes the effect ofsoil moisture conditions on Tr reasonably well and that
the Tr_pt expression consequently reproduces themeasured Tr value, we proceeded to determine theseasonal and annual water cycles using Tr_pt. Tr wascontrolled by the ‘‘trade-off’’ between precipitation(P) and average daily net radiation (Rn); even if thereis plenty of rainfall, Tr is small when Rn is small, andeven if there are many dry days, if there is plenty of Rn,
Tr can be large.
Here, the monthly daily average latent heat flux
(LE) was estimated from the monthly total evapotranspiration rates, that is, Tr plus the monthly totalinterception (Ic). Therefore, since Ic effectively
increased with P, the Rn fraction dissipated by LE(fr) surpassed 100% in months with much rainfall(October and November 2001). fr ranged between
0.65 and 1.22, suggesting a close correlation between
P and fr (Table 3). While most fr results reported inother tropical forests did not take into consideration
evaporation from the wet canopy, our results included
the effect of the wet canopy in energy partitioning.
When we calculated fr using only Tr, it ranged
between 0.57 and 0.78 and the annual average value
was 0.69. In this study, the annual average fr estimatedfrom Tr was nearly identical to the value in thetransition period between the wet and dry seasons in atransitional tropical forest (0.65; Vourlitis et al., 2002)

Table 1Monthly summary of the total precipitation (P), transpiration (Tr), interception (Ic), the average daily net radiation (Rn), and fraction of Rn
dissipated by latent heat flux (fr)

Jul 2001
Aug 2001
Sep 2001
Oct 2001
Nov 2001
Dec 2001
Jan 2002
Feb 2002
Mar 2002
Apr 2002
May 2002
Jun 2002
Year Total

P (mm)

Tr_eq/Tr_pt (mm)Ic (mm)Rn (MJ/m2 d)

fr_eq/fr_pt

84.5
79.0
131.0
389.0
365.0
135.0
204.5
103.5
294.0
179.5
91.0
94.5
2150.5118.9/110.9
115.7/98.9
114.7/96.6
96.9/100.0
90.0/89.9
100.1/99.4
102.6/99.2
84.2/71.0
112.8/102.4
125.9/125.6
118.0/110.7
107.2/88.5
1287.1/1193.1

16.3
15.4
26.6
55.0
51.1
29.3
27.6
23.6
37.5
26.7
23.3
19.5
351.9

13.24
11.88
13.38
10.04
9.87
10.84
11.34
10.34
11.70
13.36
12.01
11.42
11.62

0.70/0.65
0.87/0.76
0.86/0.75
1.19/1.22
1.16/1.16
0.94/0.93
0.90/0.88
0.91/0.80
1.01/0.94
0.93/0.93
0.92/0.88
0.90/0.77
0.94/0.89

The subscripts eq and pt designate the values estimated using Eqs. (1) and (2), respectively.

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T. Kumagai et al. / Agricultural and Forest Meteorology 128 (2005) 81–92

and the mean value of the wet and dry seasons in an
Amazonian tropical rainforest (0.73 and 0.54 in the
wet and dry seasons, respectively; Malhi et al., 2002).
It is notable that although there are no clear seasonalvariations in rainfall in this site normally, because of
the exceptional dry months in the study period ourlowest estimated fr value was comparable to thatfound in Amazonian tropical forests during the dry
season (0.54: Malhi et al., 2002; 0.5: Vourlitis et al.,2002). Our results imply that the annual dynamics ofthe latent heat flux for this tropical rainforest are undermore humid conditions than for the above tropicalrainforests.
The annual Tr and Ic in 2001–2002 were estimatedas 1193.1 and 351.9 mm, respectively, suggesting thatIc can be up to 30% of Tr (Table 1). The ratio of Ic tototal P was 16%. Evapotranspiration, computed as thesum of Tr and Ic, was 1545.0 mm, which accounts forabout 72% of the total P. Evapotranspiration studies by
Bruijnzeel (1990) in humid tropical forests suggest
that: (1) the average annual Tr was 1045 mm (range
885–1285 mm), (2) the average Ic was 13% of theincident P (range 4.5–22%), and (3) the annual
evapotranspiration ranged from 1310 to 1500 mm.
While our interception losses were comparable to thisreported value, our Tr and evapotranspiration values
were on the highest boundary of their range fortropical forests (e.g. Calder et al., 1986; Bruijnzeel,
1990; Leopoldo et al., 1995). According to De Bruin(1983b), our annual evapotranspiration value can beseen in the evapotranspiration category from stations
with a long wet season and it can therefore be
determined with the PT method.

5. Conclusions
In this Bornean tropical rainforest, the equilibrium
evaporation from a well-watered surface sufficiently
reproduced the transpiration rate. The discrepancies
between the equilibrium and actual evaporation
rates were caused by unpredictable intra-annual dry
spells, which reduced transpiration. There were somediscrepancies in this study period because of exceptional dry sequences, but in normal years the annualtranspiration rate can be obtained from the equilibrium
evaporation expression. This also suggests that further
research is needed with regards to the effect of inter-annual variability in the length of dry periods,
especially at this site because it is a maritime
environment where the effects of El Niño-Southern
Oscillation (ENSO) and monsoon strength areprominent.

Acknowledgements
We thank H.S. Lee and L. Chong of the Forest
Research Center, Forest Department of Sarawak,
Malaysia, for providing the opportunity to carry out
this study. This work has been supported by CREST(Core Research for Evolutional Science and Technology) of JST (Japan Science and Technology Agency).
We thank T. Ichie for providing long-term climatic
data and G. Katul and two anonymous reviewers fortheir valuable suggestions.

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