S. Khabba et al. Agricultural and Forest Meteorology 106 2001 131–146 137
• Finally the model was tested using experimental
observations made in Morocco June 1997 and in Belgium September 1998.
3. Experiment
Data to test the model were collected from two field experiments: the first was in Marrakech, Morocco
31
◦
37
′
N, 7
◦
52
′
W, altitude = 643 m and mea- surements were taken between 11 and 15 June
1997, 75 days after planting. The second field was in Louvain-la-Neuve, Belgium 50
◦
40
′
N, 4
◦
40
′
E, altitude = 130 m and observations were made from
12 to 23 September 1998, 138 days after planting. Ear development was assessed by grain moisture content
M wet basis. In Belgium, it varied from 52.8 ± 1.5 at the beginning of observations to 49.7 ± 2.4
at the end. In Morocco, the variation in moisture content was negligible, M = 67.6 ± 1.8. The up-
permost 11 from 16 leaves in Morocco, and 9 from 15 leaves in Belgium were still green at the time
of measurements. Canopy height, h
c
, and leaf area index were, respectively, 1.5 m and 3.6 in Morocco
and 2.2 m and 5.2 in Belgium. In Morocco, the field dimensions were 70 m × 40 m
and the plant density 70 000 plants ha
− 1
. The rows were oriented north–south. Micrometeorological ob-
servations of total and diffuse downward solar ra- diation flux density on a horizontal surface as well
as of wind speed at 6 m were made near the maize field ≈300 m from maize field. The following mea-
surements were made within the field: air tempera- ture at ear level and at 2 m above the crop using two
thermocouples TTC10; air relative humidity at 2 m above the field using a capacitive hygrometer Vaisala,
Helsinki, Finland; soil surface temperature, average of two chromel alumel thermocouples 1.5 mm diam-
eter; ear temperatures were measured at mid-length: three thermocouples were inserted in the middle of
grains at three polar positions north-east, south and north-west. These measures were made on three ears
for which the heights above the ground, z
e
, were mea- sured. The thermocouples wires were connected to a
multichannel digital electronic thermometer with an accuracy of 0.1
◦
C. All these data were recorded ev- ery 30 min from 6 to 19 h UT 27 observations per
day. In Belgium, the experiment was performed over a
large maize field 1.5 ha located near the Catholic University of Louvain in Louvain-la-Neuve. The
plant density was about 90 000 plants ha
− 1
. The rows 75 cm apart were oriented east–west. Micrometeoro-
logical measurements of total and diffuse downward solar radiation flux density, wind speed at 3.5 m, wet
and dry bulb temperatures were made at a nearby meteorological station ≈500 m from the field. In
the field, the following observations were made: 1 air temperature at ear level using copper–constantan
thermocouple AWG 24 placed in a double shielded aspirated screen; 2 soil temperature with infrared
thermometer model Everest 4003 from 14 to 23 September 1998; 3 net radiation, at ear level, with
one net radiometer TRL, Delta-T Devices, England, and upward radiation with one radiometer TSL,
Delta-T Devices, England; the sum of these two mea- surements gives the downward and upward all-wave
radiation at ear level: 4 ear temperatures were also measured at mid-length of the ear: four thermocouples
AWG 30 were inserted in the middle of grains at polar positions corresponding to north, east, south and
west. These measurements were made on four ears for which the heights about the ground, z
e
, were mea- sured. All these data were recorded on a data-logger
Campbell Scientific, Shepshed, UK every minute 1440 observations per day. The values of R
s
were used to calculate the day length when R
s
is positive and length of night R
s
is zero. The soil was sandy in Marrakech and a clay loam in Louvain-la-Neuve.
Soil reflectances were, respectively, 0.16 and 0.14 0.13 on rainy days. Following Davies and Buttimor
1969, ear albedo was chosen as a
e
= 0.29.
The geometrical structure of the canopy was mea- sured on two 10 and 14 June 1997 and three 14,
18 and 21 September 1998 occasions during the experimental period in Morocco and in Belgium,
respectively. In total, 12 and 22 plants, randomly chosen, were used in Morocco and Belgium, respec-
tively. Structure parameters leaf area density and leaf inclination distributions of each cell were esti-
mated by the plant silhouette method modified for a two-dimensional description Sinoquet and Bon-
homme, 1989. Vertical and cross-row distributions of these two parameters are shown in Figs. 3 and
4, respectively. The number of horizontal layers N
z
was 6 in Morocco and 10 in Belgium, and 5 vertical
138 S. Khabba et al. Agricultural and Forest Meteorology 106 2001 131–146
Fig. 3. Vertical and a cross-row distributions of leaf area density. The number N
z
of layers was 6 in Morocco and 10 in Belgium, the number N
x
of slices was 5. The geometrical structure of the canopy is assumed symmetrical on both sides of inter-rows line.
The layer numbers refer to heights from the bottom to the top. The slice numbers refer to a cross-row, from the row to the inter-row
Fig. 2.
slices were taken between two rows for the two ex- periments. The geometrical structure of the canopy
was assumed symmetric on both sides of the row.
4. Results and discussion
