P. van der Keur et al. Agricultural and Forest Meteorology 106 2001 215–231 221
soil moisture content θ is calculated as an average of DAISY simulated water content at nodes within
0–100 cm. The minimum resistance r
min c
, i.e. r
min s
L in Eq. 24 is the parameter of interest for linking energy
balance modeling, e.g. latent heat flux evapotranspi- ration, to remote sensing data. However, as such data
is not yet available for this study, minimum canopy resistance is estimated from r
c
in Eq. 29 Allen et al., 1989; FAO, 1990
r
c
= r
day
0.5L =
200 L
29 where r
day
is the average daily 24 h stomatal resis- tance of a single leaf. Sellers et al. 1992a,b estimated
r
min c
to be between 40 and 120 s m
−1
for crops, cor- responding to LAI values from 5 to 1.7, respectively,
in Eq. 29.
3. Study area and 1997 field campaigns
Field scale data for the RS-model project was collected at selected sites with agricultural crops at
the Research Centre Foulum RCF, 56
◦
30
′
N, 9
◦
36
′
E, altitude 45 m above sea level in Jutland, Denmark.
For the plot-scale study here focus was on a winter wheat crop. Height of the crop throughout the grow-
ing season varied from 0.31 m 16 May, 0.41 m 26 May, 0.70 m 9 June, 0.80 m 24 June, 0.87 m 30
June and 0.85 m until 8 August. Schjønning 1992 investigated the soils surrounding RCF and found for
most of the profiles a rather homogeneous distribution of texture with depth. Soil content of clay generally
increases from 7 in the topsoil to about 10–15 in the deepest part of the profile. The fine sand fraction
20–200 mm was found to make up about half the particles in all depths. Meteorological data including
radiation, precipitation, air humidity, air temperature and wind profiles, as well as water and carbon dioxide
fluxes by means of eddy covariance equipment have been monitored at a winter wheat and spring barley
site. Canopy related measurements such as spectral reflectance, leaf angle distribution, cover fraction,
leaf area index, biomass and water content were specifically designed for accommodating the study
of combining crop modeling and remote sensing. In addition soil moisture at various levels was measured
by means of time domain reflectometry TDR using horizontal and vertical probes. Data on soil tempera-
ture and ground heat fluxes were also collected.
3.1. Meteorological data During the 1997 experiment all relevant meteoro-
logical data was measured locally at the winter wheat site, see Fig. 2, and was used as forcing data for the
DAISY model. However, during the ‘spin-up’ period prior to the 1997 campaign meteorological data from a
nearby climate station distance approximately 1 km at the Foulum Research Centre is applied. Since the
crop in question is a winter wheat, the ‘spin-up’ period was from June 1996 to approximately the beginning of
April 1997 when some of the field campaign measure- ments were initiated. Missing local values from the
climate mast at the winter wheat site were replaced by data from the nearby Foulum climate station. On-site
precipitation data, available during the period of inter- est, was used for simulation purposes. Applied time
step was 1 h.
3.2. Soil moisture measurements Continuous soil profile measurements of water con-
tent by TDR using single horizontal probes at 5, 10, 15, 20, 30, 40 and 50 cm on half hourly basis were
compiled for the period 14 May to 8 August and av- eraged to hourly values. Soil water content by verti-
cally inserted TDR probes for depths 0–20, 0–50 and 0–100 cm, were also available for the same period. The
applied TDR system includes a 1502BC Tektronix ca- ble tester Tektronix Inc., Beaverton, OR operated by
a portable PC, a TSS 45 Tektronix multiplexer, inter- face electronics Thomsen and Thomsen, 1994, and
two wire TDR probes. Measured dielectric constants by the Tektronix cable tester were converted to volu-
metric water content by the Topp et al. 1980 equa- tion. The software used for the analysis of TDR traces
is discussed by Thomsen 1994.
3.3. Energy flux measurements Eddy covariance measurements at the winter wheat
site were made using an open path system comprising an Ophir IR-2000 Optical Hygrometer and a METEK
USA-1 1D ultrasonic anemometer operating at 20 Hz.
222 P. van der Keur et al. Agricultural and Forest Meteorology 106 2001 215–231
Fig. 2. Daily averaged meteorological variables at winter wheat plot from April to September 1997.
Instruments were mounted on a mast at a height of 2.5 m.
The eddy covariance and meteorological station was placed close 50 m to the northern edge of the long
and flat field of winter wheat. Fetch conditions to the N and NE of the station were considered inadequate due
to the presence of a farm surrounded by tall trees min- imum distance 135 m and a forest edge minimum
distance 290 m. Therefore, a bad fetch sector was de- fined comprising 129
◦
to the N and NE of the sta- tion. The positive fetch sector, comprising 231
◦
, was to the W, S and SE of the station. Borders of the field
were windbreaks 3–5 m high situated at minimum and maximum distances of 200 and 750 m from the
station, respectively. Fetch conditions were adequate throughout the day on 12 June, 19–21 June, 25–29
June, 9–23 July, and 5–7 August 1997. The closure of the energy balance R
n
− B − G − H − λE ∼ 0 was generally not good. A regression of heat fluxes
H + λE versus available energy R
n
− G, using all available 30 min concurrent measurements, yielded
H +λE = 0.71R
n
−G+5 W m
−2
R
2
= 0.87. A similar regression that included only data on days with
a good fetch as defined above, produced H + λE = 0.70 R
n
− G + 5 W m
−2
R
2
= 0.90, indicating that the closure problems were not related to fetch
requirements. This leaves some 30 of the available energy to be accounted for by photosynthesis, canopy
energy storage and measurement errors. The compo- nents whose order of magnitude could be evaluated
were analyzed as described below.
The net radiometer REBS Q
∗
7.1, Seattle, WA was new at the start of the measurement period
and the factory calibration was applied. Net radia- tion measured at the winter wheat field was in the
same order of magnitude as net radiation measured above short green grass at the nearby 3 km Foulum
meteorological station. We found a similar agree- ment between soil heat flux observed at the winter
wheat and at the grass cover. Even if the energy bal- ances at the two different surface covers were not
likely to be identical, we have confidence that wheat net radiation and soil heat flux measurements were
sound.
P. van der Keur et al. Agricultural and Forest Meteorology 106 2001 215–231 223
The measurements of latent heat flux were evalu- ated by comparing to changes in water content mea-
sured using the automated TDR station. Over periods with no rainfall, the difference from start to end in
water content in the top 50 or 100 cm soil eight repli- cates of each probe length equals the amount of wa-
ter lost as soil evaporation and crop transpiration to the atmosphere. We assume, in agreement to model
predictions, that there is no significant drainage from the soil profile during such dry intervals. During the
period 10–13 June 1997, the accumulated amount of water lost to the atmosphere according to the eddy
correlation measurements was 11 mm, while the water content decline recorded by the 50 and 100 cm TDR
probes corresponded to 11 and 13 mm of water, re- spectively. During the equally short dry period 18–20
June 1997, latent heat loss was equivalent to 9 mm of water, while the 50 and 100 cm TDR probes recorded
water deficits of 9 and 12 mm, respectively. 9–23 July 1997 was a long dry spell. Observed water depletion
during this period in the 0–50 and 0–100 cm soil depth was 49 and 69 mm, respectively. Accumulated latent
heat loss during the dry spell was equivalent to 45 mm of water. Since the root zone of the wheat crop proba-
bly exceeded 50 cm in July, and since we assume the TDR technique to be accurate in estimating relative
changes in soil water content, the results indicate that eddy covariance estimates of latent heat flux could be
underestimated in July.
4. DAISY model simulations
