174 P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186
3. Results
3.1. Sowing date Predicted sowing dates from the broad-scale model
for the mean 1961–1990 climatic period are com- pared with observed dates of sowing in Table 3.
The model reproduces the spatial pattern of ob- served sowing dates across Europe reasonably well.
In Fenno-Scandinavia winter wheat is sown from late August to late September. This corresponds with a
simulated range from early September to early Octo- ber. In central and eastern Europe sowing of winter
wheat is reported to occur from mid September to late October. This is encompassed by a simulated range
for this region of mid September to early Novem- ber. In northwest Europe sowing dates vary from
mid September to late November, which agree with simulated dates ranging from early September to late
Fig. 3. Simulated dates of a double ridges, and b anthesis for cv. Avalon for the mean 1961–1990 climatic period. Two suitability masks cold winters and wet autumns are overlain on model results see Harrison and Butterfield, 1996.
November. In southern Europe sowing occurs from mid October to late December. This is captured by
the broad-scale model, which simulates dates from late September to late December.
3.2. Development stages — qualitative validation The AFRCWHEAT2 model predicts seven stages
of wheat development. An example of output from the broad-scale model for the northwest European culti-
var Avalon is shown in Fig. 3 for the stages of dou- ble ridges and anthesis. The spatial distribution of the
timing of these stages for all cultivars is described. Dates of double ridges for winter wheat range from
late February to early April in western Europe for the range of cultivars. This corresponds with reported
dates from late February to early April for the UK Porter et al., 1987, from mid March to early April for
France Delécolle et al., 1989 and from early Febru-
P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186 175
ary to early March for Spain Narciso et al., 1992. In central, eastern and northern Europe double ridges is
predicted to occur from early April to mid May. This fits the observed average time of double ridges in Hun-
gary of early to mid April Zs. Harnos, personal com- munication, 1994. Dates of anthesis for winter wheat
range from early May to mid June in southern Europe for the range of cultivars. This concurs with reported
dates from late April to early June for Italy and Spain Miglietta, 1991; Narciso et al., 1992, from mid May
to late May in Greece Narciso et al., 1992 and from mid May to mid June for Albania, Bulgaria and Roma-
nia Russell and Wilson, 1994. In central and north- ern Europe anthesis occurs between early June and
early July. These predictions agree with observed dates of anthesis from late May to late June for Hungary,
Poland, the Czech Republic and Slovakia Zs. Harnos, personal communication, 1994; Russell and Wilson,
1994, from early June to late June for Germany, Den- mark, the UK and the Benelux countries Broekhuizen,
1965; Porter et al., 1987; Nonhebel, 1993 and from late June to early July in Fenno-Scandinavia Russell
and Wilson, 1994.
Simulated dates of maturity for all six winter wheat cultivars for the mean 1961–1990 climatic period are
shown in Fig. 4. Avalon, Riband, Slepner, Hustler and Caribo are all northwest European wheat varieties
and, hence, results are only described for this region. Maturity occurs from late July to early August for
Avalon a fast developing variety, from late July to early August for Riband a medium-fast developing
variety, in early August for Slepner a slow develop- ing variety and from early to mid August for Hustler
and Caribo both slow developing varieties. These predictions encompass the range of observed dates
in the UK Weir et al., 1984; Crofts, 1989; Hough, 1990, The Netherlands Reinink et al., 1986; Non-
hebel, 1993, Belgium Broekhuizen, 1965 and Ger- many Broekhuizen, 1965; Crofts, 1989.
Differences in the predictions for each cultivar can be explained with reference to their model calibra-
tions see Table 2. Model calibrations for Avalon, Riband and Slepner only differ in the thresholds
used for early development stages. Slightly longer thresholds are used for the period from emergence to
anthesis for the slower developing cultivars Riband and Slepner causing slightly later predicted dates
of maturity. Avalon, Riband and Slepner use a base temperature of 0
◦
C from anthesis to maturity, whilst Hustler and Caribo use a base temperature of 9
◦
C. The original version of the AFRCWHEAT2 model
ARCWHEAT1 assumed ‘a rather high base temper- ature of 9
◦
C’ for Hustler Weir et al., 1984. Later calibrations of the model were changed to assume a
lower base temperature of 0
◦
C because the 9
◦
C base temperature placed an unrealistic restriction on wheat
development in northern latitudes, such as Scotland, which are classified as unsuitable for Hustler and
Caribo. Cultivars Hustler and Caribo also have slightly longer thresholds for the early development stages,
which coupled with the higher base temperature for the later development stages, causes later maturity
dates compared with Avalon, Riband and Slepner. The model calibration for Caribo differs from Hustler
in three ways Reinink et al., 1986. Firstly, slightly shorter thresholds are used for early developmental
stages. Secondly, the vernalization curve and require- ment are stricter with only temperatures between −1
and 9
◦
C contributing to vernalization. Thirdly, the state of vernalization is fixed at the floral initiation
stage so that incomplete vernalization has a delaying effect on subsequent stages of development. These
differences result in later predictions of maturity dates compared with Hustler.
Alcala is the only southern European cultivar for which calibration data were available. The thresholds
for the phases between double ridges and anthesis are much shorter than for all the other cultivars. Predicted
dates of maturity in Spain for cultivar Alcala range from mid May to early July Fig. 4f. These match the
observed range of dates from late May to late July for this region as reported in Broekhuizen 1965, Narciso
et al. 1992 and Wolf et al. 1996.
3.3. Developmental stages — quantitative validation Quantitative validation of spatial models is re-
stricted by the availability of relevant data covering large regions at a detailed resolution. To address this
restriction, a database of observed dates of wheat de- velopment stages in Europe was constructed from in-
formation reported in the literature, from agricultural experimental stations and from experts. The database
covers most countries in the European Union, Scan- dinavia and eastern Europe and encompasses a range
of scales from point estimates to nationally-averaged
176 P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186
Fig. 4. Simulated dates of physiological maturity for six winter wheat cultivars for the mean 1961–1990 climatic period: a cv. Avalon; b cv. Riband; c cv. Slepner; d cv. Hustler; e cv. Caribo; and f cv. Alcala. Two suitability masks cold winters and wet autumns
are overlain on model results see Harrison and Butterfield, 1996.
statistics. Numerous observations of harvest date were available enabling this variable to be mapped
across Europe and statistically compared with mapped output on the date of physiological maturity from
the broad-scale model. However, maturity is not a well-defined stage and hence, these comparisons only
validate the approximate magnitude and spatial pat- tern of model predictions. The stage of anthesis is
better defined, but the number of observations avail- able for this stage is considerably less. However,
sufficient data on anthesis date were available to un- dertake comparisons at the national scale. Data for
other development stages were restricted to only a few specific countries.
An example of the output from this database for the average date of harvest for winter wheat is shown in
Fig. 5. A comparison between predicted dates of matu- rity, as shown in Fig. 4, and observed dates of harvest,
as shown in Fig. 5, indicates that the broad-scale model captures the observed spatial pattern of wheat devel-
P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186 177
Fig. 5. Observed average date of harvest in Europe. Data from Broekhuizen 1965, Bunting et al. 1982, Weir et al. 1984, Thompson and Stokes 1985, Reinink et al. 1986, Crofts 1989, Hough 1990, Narciso et al. 1992, Nonhebel 1993, Zs. Harnos personal
communication, 1994, Russell and Wilson 1994 and J. Wolf personal communication, 1994.
opment across Europe. When conducting this compar- ison, two factors must be taken into account. Firstly,
observed dates of harvest are the average value for a region and for all winter wheat varieties grown in
that region. Secondly, physiological maturity, as de- fined in the AFRCWHEAT2 model, generally occurs
about 1–2 weeks before harvest. For example, the ob- served average harvest date for Spain ranges from mid
June to late July. Hence, predicted dates of physiolog- ical maturity for an average cultivar in Spain should
be approximately 1–2 weeks earlier, i.e., from early June to mid July. The modelled cultivar Alcala, is a
fast developing variety and, hence, maturity might be expected to occur slightly earlier than for an average
cultivar. This agrees with predicted dates, which range from late May to early July in Spain.
Simulated dates of maturity have been subtracted from observed dates of harvest for three cultivars
in the UK in Fig. 6. There is reasonable agreement across the varieties taking into account the 1–2 week
displacement between physiological maturity and harvest. Specifically, the model predicts earlier dates
than observed for Avalon a fast developing cultivar, marginally earlier dates for Slepner and marginally
later dates for Hustler a slow developing cultivar.
A comparison of observed and modelled dates of anthesis is shown in Fig. 7 for selected European coun-
triesregions. In some countries, particularly Greece and France, the observed range of dates is consider-
ably less than that indicated by the spatial model. This may reflect an insufficient or unrepresentative sam-
ple of observations rather than errors in the model, because the date of maturity is reasonably simulated
for these countries. Nevertheless, predictions from the broad-scale model correspond to the range of obser-
vations in the majority of countries and the general spatial pattern of the observed occurrence of anthesis
across Europe is reproduced.
Predictions of four development stages from the broad-scale model for specific years in the 1961–
1990 time series were compared with observed dates from field experiments to test the model’s ability to
178 P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186
Fig. 6. Simulated date of physiologial maturity minus observed average date of harvest for three cultivars in the UK: a cv. Avalon; b cv. Riband; and c cv. Slepner.
reproduce inter-annual variability. It is not possible to conduct an experiment at the scale of a 0.5
◦
lati- tudelongitude cell, hence, such comparisons can only
broadly assess the ability of the broad-scale model to simulate year-to-year variability in wheat develop-
ment. For example, simulated dates from the spatial
Fig. 7. Comparison of simulated and observed dates of anthesis. S.E. Europe includes Bulgaria, Romania, Albania and the former Yugoslavia; Central Europe includes the Czech Republic, Slovakia and Hungary; Fenno-Scandinavia includes Finland, Denmark, Sweden and Norway.
Modelled dates for Spain, Italy, Greece and S.E. Europe are for cv. Alcala. Modelled dates for all other countries encompass cvs. Avalon, Riband, Slepner, Hustler and Caribo. Values in brackets refer to the number of observations.
model are compared with experimental observations and ARCWHEAT1 Weir et al., 1984 site model pre-
dictions over three growing seasons at Rothamsted, UK in Table 4. The climate at Rothamsted was sim-
ilar to that of the 0.5
◦
cell over the period from 1978 to 1981. Hence, differences in dates mainly reflect
P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186 179
Table 4 Comparison of simulated phenological dates from the broad-scale model against results from the ARCWHEAT1 site model and experimental
data for cv. Hustler for three consecutive growing seasons at Rothamsted, UK
a
Growing season Development stage
Experimental observation
b
ARCWHEAT1 site model
b
Broad-scale model 1978–1979
Sowing 287
287 287
Emergence 301
301 301
Double ridges 100
106 109
Anthesis 182
181 180
Maturity 237
230 228
1979–1980 Sowing
291 291
291 Emergence
307 310
308 Double ridges
93 94
96 Anthesis
168 174
171 Maturity
230 231
226 1980–1981
Sowing 302
302 302
Emergence 326
328 331
Double ridges 99
91 99
Anthesis 177
174 174
Maturity 226
228 222
a
Dates are in day of the year.
b
Data extracted from Weir et al. 1984.
differences in model performance rather than differ- ences in the input climatology. Predictions from the
broad-scale model are, in general, as close to the exper- imental observations as those from the ARCWHEAT1
site model. In the 1978–1979 growing season the spa- tial model predictions are slightly worse than those
from ARCWHEAT1, whilst in the 1979–1980 and 1980–1981 growing seasons both models perform
reasonably well. The broad-scale model captures dif- ferences in developmental rates between the growing
seasons, showing that 1978–1979 was the longest 315 days according to the experiment and 306 days
according to the spatial model and 1980–1981 was the shortest 289 days according to the experiment
and 285 days according to the spatial model of the three growing seasons. A slight over-prediction of the
duration from emergence to double ridges followed by a slight under-prediction of the duration from dou-
ble ridges to anthesis is evident. This concurs with the discussion of Fig. 1 in Section 2.1. The compensatory
nature of these errors means that the timing and dura- tion of the grain filling period is accurately predicted.
A comparison between the predicted date of matu- rity from the broad-scale model and three site models
for the climatic period 1975–1985 for cultivar Alcala at Seville is shown in Table 5. These models were cali-
brated and validated against experimental data sets for Seville Wolf et al., 1996 and thus provide an addi-
tional source of data for validation of predictions from the spatial model. Observed maximum and minimum
temperatures at the site and cell were similar over this period, hence, predictions of development stages
should also be similar between the models. The spa- tial model predicts a mean date of maturity of 141.5
for the cell containing Seville. This is slightly ear- lier than mean predictions from the site-based mod-
els, by approximately 2, 3 and 7 days for NWHEAT Groot, 1993; Wolf et al., 1995, SIRIUS Jamieson et
al., 1998 and AFRCWHEAT2, respectively. A similar standard deviation is predicted using the broad-scale
Table 5 Comparison of date of physiological maturity calculated at the site
of Seville, Spain using the AFRCWHEAT2, NWHEAT and SIRIUS models with that calculated at the associated 0.5
◦
latitudelongitude cell using the broad-scale model for the 1975–1985 climatic period
for cv. Alcala Model
Model predictions for date of maturity Mean
Standard devi- Range
DOY ation days
DOY Broad-scale model
141.5 3.75
131–148 AFRCWHEAT2
148.3 3.12
141–157 NWHEAT
143.7 4.27
136–152 SIRIUS
144.7 6.05
128–152
180 P.A. Harrison et al. Agricultural and Forest Meteorology 101 2000 167–186
model to AFRCWHEAT2 and NWHEAT, but a greater value was found for the SIRIUS model. The range
of predicted dates from the spatial model overlaps to a considerable degree with all the site-based models.
Differences in the predictions between the broad-scale model and the site models are not significantly greater
than differences between the site models themselves. The timing of other development stages at Seville also
fell within the range of the site model predictions.
4. Model sensitivity
