174 J. Og´ee et al. Agricultural and Forest Meteorology 106 2001 173–186
was to evaluate long-term energy budgets in order to understand how energy is shared, stored or released
throughout the year between the pine trees, the under- storey and the ground. This is of particular interest in
relatively open canopies with evergreen species like conifers: in such cases, the distribution of energy be-
tween the trees and the understorey is likely to vary significantly through the year.
In this context, night-time and day-time periods are of equal interest since it is impossible to perform cu-
mulative flux estimations without night-time values. As soil heat flux is a major term in the understorey
energy budget during the night and a significant term in the day, a good estimation of this flux is critical in
the context of long-term energy budgets.
For these reasons, soil heat flux measurements were performed continuously on our site. In the EU-
ROFLUX programme, no recommendation was given about the method to be used for this. We designed an
original method, well adapted to long-term measure- ments. It is derived from the null-alignment method of
Kimball and Jackson 1975, that was further modi- fied to avoid its main drawbacks, as pointed out by
de Vries and Philip 1986. This was made possible by the size of the data set collected during this exper-
iment nearly 17,400 temperature profiles recorded over almost 365 consecutive days are used here.
Several tests validate this method in our context. In particular, estimated soil heat flux values allow the
energy budgets of the whole stand and the under- storey to be closed see Lamaud et al., 2000. Also,
this large data set provides a good basis for modelling the soil heat flux from meteorological variables.
2. Experimental site and instrumentation
2.1. Experimental site The experimental site is located at about 20 km
from Bordeaux 44
◦
42
′
N, 0
◦
46
′
W, altitude 62 m in a maritime pine stand Pinus Pinaster Ait. planted in
1970. The trees are distributed in parallel rows along a NE-SW axis. The inter-row distance is 4 m and
the stand density is about 500 trees per hectare. The mean tree height is approximately 18 m. The under-
storey mainly consists of grass Molinia coerulea L. Moench. The roots and the clumps remain throughout
the year but the leaves are green only from April to late November, with a maximum LAI and height of 1.45
and 0.7 m, respectively Loustau and Cochard, 1991. In winter the soil is partly covered with dead leaves.
A 5 cm thick litter layer made of compacted grass and dead needles is present all year long. The soil is a
sandy and hydromorphic podzol, with a layer of com- pact sand located between 40 and 80 cm. The com-
paction and chemical composition of this layer make it hardly penetrable by the roots, so that organic mat-
ter is mainly located above it. During the 1997–1998 winter, the water table level went up to the soil sur-
face. In summer, it was at a depth of about 120 cm.
2.2. Soil measurements Soil temperature was measured at four different
locations and eight depths: 1, 2, 4, 8, 16, 32, 64 and 100 cm using 32 home-made thermocouples, i.e.
copper-constantan soldered joints coated with water- proof paint. All thermocouples had been previously
tested in a thermally-regulated water bath for 48 h. Only those giving similar response curves were kept
for our study. The leads were placed in plastic PVC tubes 2 cm in diameter and the thermocouples were
extracted at the measurement levels in such a way that their distance to the tubes was greater than 5 cm,
in order to minimise the influence of conduction along the tubes. The portions of the leads remaining
outside the tubes were then covered with heat-shrink tubing sheath. Insulation inside the tubes was in-
sured by glue around the sheath. The four tubes were then inserted in the soil at four different locations
as shown in Fig. 1. The thermocouples were con- nected to a multiplexer and the data was collected
on a Campbell 21X data-logger Campbell Scientific Ltd., Shepshed, UK. The measurements were made
every 10 s and only half-hourly means were recorded. The data-logger temperature was used as the refer-
ence temperature. Several checks using the amplitude differences and the phase shifts between the various
levels showed that temperature measurements were accurate within ±0.1
◦
C. In this paper, we only use temperature values av-
eraged over the four tubes. Standard deviations not presented here are always less than 0.5
◦
C and av- erage around 0.1
◦
C. The data set covers the period from September 1997 to September 1998. A mean
J. Og´ee et al. Agricultural and Forest Meteorology 106 2001 173–186 175
Fig. 1. Schematic representation of the site. Trees closed circles are aligned along a NE-SW axis with a 4 m inter-row distance.
The triangles and the open circles represent the locations of soil temperatures profiles and soil humidity profiles, respectively.
profile is available every 30 min, except for a few hours in early March 1998 when the transmission
system temporarily failed. Altogether we have about 17,400 temperature profiles.
Soil moisture was measured with a time domain reflectometer TRASE systems, Soil Moisture Equip-
ment Corp., Santa Barbara, CA using 20 cm long buri- able waveguides. The latter were placed horizontally
at three locations and four depths in the soil 5, 23, 34 and 80 m, and connected to a multiplexer Fig. 1.
The lower boundary 80 cm coincides with the bot- tom of the root zone. The measurements were auto-
matically recorded every 12 h. At each level humidity values were averaged over the three profiles. Battery
problems caused occasional loss of data. A grand total of 229 midday profiles are used in this study.
Profiles of soil bulk density had been previously measured by Diawara 1990 and more recently by Ar-
rouays personal communication. Soil samples from different depths and three locations in the vicinity of
the other soil measurements were extracted, dried and weighed. Fig. 2 shows the mean profile of porosity
over the top 40 cm, along with the standard deviation at each depth. Soil porosity ε is related to bulk density,
ρ
, by ε =
1 − ρ
ρ
m
1 where ρ
m
≈ 2660 kg m
− 3
is the density of minerals. The experimental profile has been smoothed with a
Fig. 2. Profile of porosity, ε, in the top 40 cm: experimental data closed circles and adjusted profile with spline functions solid
line. The bars represent the standard deviations calculated over three experimental profiles.
spline function in order to get interpolates between the experimental points.
2.3. Micrometeorological measurements Eddy fluxes of momentum, sensible heat, water
vapour and CO
2
were measured above and occasion- ally below the tree crown with the eddy-covariance
technique. Heat storage in the air, the vegetation and the litter was also estimated using more than 60
thermocouples. All relevant details can be found in Lamaud et al. 2000. Above the canopy, at 25 m,
global radiation was measured with a thermopile Cimel CE 180, Paris, France and net radiation was
measured with a Q-7 net radiometer Radiation En- ergy Balance System, Seattle, WA that was viewing
a representative area. Here also, samples were taken every 10 s, and 30 min means were recorded. The data
was collected on a Campbell 21X data-logger.
In order to estimate net radiation above the un- derstorey we use a radiative transfer model that has
been previously calibrated at this site Berbigier and Bonnefond, 1995. The original model assumes that
the ground, the canopy and the air all have the same temperature. In the present study, a term has been
added to the longwave component of the model to ac- count for the difference between ground and canopy
176 J. Og´ee et al. Agricultural and Forest Meteorology 106 2001 173–186
Fig. 3. Variation of mean canopy leaf area index during the ex- periment. Experimental data triangles and parameterisation used
to calculate transmitted net radiation solid line. The bars repre- sent the standard deviations calculated over a different number of
points at each date.
temperatures. We use the litter temperature at the air-litter interface for the ground and the air tempera-
ture at 14 m for canopy temperature. The transmitted net radiation is then computed with
R
n,t
= R
′ n,t
+ 1 − f
2
σ {T
4 a,14m
− T
4 a,l
} 2
where σ = 5.67 × 10
− 8
W m
− 2
K
− 4
, T
a,14m
is the air temperature at 14 m above ground, a surrogate for the
mean foliage temperature, T
a,l
is the air temperature at the air-litter interface, and f and R
′ n,t
are given by equations 9a and 11a of Berbigier and Bonnefond
1995, respectively. The mean canopy leaf area index was measured reg-
ularly by an optical method based on the interception of the solar beam Lang, 1987. We observed contin-
uous variations from about 2.8 in winter to almost 3.0 in summer Fig. 3. The standard deviations shown
in Fig. 3 are a consequence of the heterogeneity of the canopy rather than measurement uncertainties: at a
given time of the year, different days of measurement would provide almost the same means and standard
deviations.
3. Methodology
