has been widely used to estimate solar radiation from records of sunshine duration McEntee,
1980, where Q = global solar radiation MJ m
− 2
day
− 1
; Qa = the extraterrestrial radiation MJ m
− 2
day
− 1
, the global solar radiation in the absence of an atmosphere received on a horizon-
tal surface at the station; n = duration of bright sunshine in hours; N = day length in hours; and a
and b are constants. In this study Qa and N were calculated according to McMurtrie 1993. This
regression equation was applied to the mean daily data of the stations at Valentia, Malin Head,
Dublin and Kilkenny, where both sunshine hours and global radiation were recorded for the period
1984 – 1993. The overall values for parameters a and b for the four stations were, a = 0.22 and
b = 0.62. These compare favourably with those produced by McEntee 1980 where a = 0.21 and
b = 0.67.
2
.
3
.
2
. GIS and surface interpolation Degree days and incident radiation were calcu-
lated for each climate station. The model was run with parameters derived from the field measure-
ments for each climate station. The point data of mean degree days, incident radiation and pre-
dicted yields from the 23 meteorological stations across Ireland was incorporated into a geographic
information system GIS, IDRISI v4.1, Clarke University, Massachusetts, USA to produce a
digital elevation model DEM. Surface interpola- tion of the scattered point data on a regular grid
was done using a inverse distance weighting inter- polation routine in the GIS which calculates a
complete surface from point data according to distance weighted averages. Degree day, radiation
and yield values are displayed at a grid size of 1 × 1 km and referenced in Irish National Grid
co-ordinates.
3. Results
3
.
1
. Miscanthus growth and climate in
1994
and
1995
In 1994 the length of the growing season was 191 days. Air temperatures Fig. 2a were close to
average while radiation was below average Fig. 2b. Rainfall from May to September was 357.4
mm. The maximum soil moisture deficit reached 50 mm in June Fig. 2c. In 1995, the growing
season was 133 days 2 months shorter than in 1994. Rainfall between May and September was
201.1 mm. Air temperatures during the growing season were higher than normal in 1995 and, due
to the lower rainfall and higher temperatures dur- ing the growing season, the cumulative soil mois-
ture deficits in September were greater than 250 mm Fig. 2c.
Shoot densities in both 1994 and 1995 were highest in June Fig. 3a and decreased at the
beginning of July as the canopy reached a LAI Fig. 3b sufficient to intercept 95 of the incident
radiation Fig. 3c. In 1994, plant growth lead to steady biomass accumulation until the first frost
in autumn day 273 Fig. 3d. Measurements of the standing dry matter after the growing season
peak and before harvest in December 1994 showed that yields decreased by about 25 Fig.
3d. Drought conditions in 1995, which developed from the beginning of July day of year 190,
caused premature leaf senescence and a reduction in LAI Fig. 3b. However, LAI did not decline
sufficiently to reduce e
i
fraction of radiation in- tercepted by the crop Fig. 3c. The severe water
deficit in 1995 halted significant increases in above ground dry matter Fig. 3d in August.
3
.
2
. Model parameterisation
3
.
2
.
1
. Thermal leaf area coefficient and radiation interception
Although in 1994 there was no indication that water stress limited canopy development, the wa-
ter stress that developed in 1995 arrested leaf expansion in early August relationship up to 7
August 1995 was LAI = 0.0104 × DD
TB10
; r
2
= 0.99 and measurements after 7 August 1995 are
omitted from the model parameterisation. The inset graph in Fig. 4a shows the effect of
changing the base temperature for calculation of degree days on the correlation r
2
between LAI and accumulated degree days. The highest corre-
lation coefficient was found with a base of 10°C
r
2
= 0.97 and this is shown in Fig. 4a. Here,
from the regression, LAI = 0.0102 × DD
TB10
. For both years a LAI of above 3.0 was
sufficient to intercept 90 of incident radiation Fig. 4b. The radiation extinction coefficient k
from Eq. 1 for this canopy was calculated to be 0.68 9 0.031.
3
.
2
.
2
. Radiation use efficiency Radiation use efficiency e
c
of the crop was derived from the relationship between above
ground dry matter and intercepted radiation Fig. 4c but excluding data when the soil moisture
deficit was greater than 150 mm August 1995 when all plant available water was extracted 150
Fig. 2. a Monthly values for 1994 and 1995 of mean maximum and minimum temperatures °C, b mean monthly incident PAR MJ m
− 2
for 1994 and 1995 and the monthly values of the previous 10 year period 1984 – 1993. c Total monthly, long term mean precipitation, 1950 – 1980, monthly soil moisture deficit, and cumulative soil moisture deficit for 1994 and 1995. Climate data was
obtained from Kilkenny and provided by Met E ´ ireann. The unshaded area indicates the period of the growing season in the 2 years.
S
M – S
= x is the sum of precipitation between May and September.
Fig. 3. Growth parameters for M. × giganteus at Cashel, Co. Tipperary measured in the 1994 and 1995 growing seasons: a
shoot density, b leaf area index, c proportion of radiation interception e
i
and d above ground dry matter. Error bars = 9 1 S.E.M. n = 40 in a, b, d and n = 8 in c.
3
.
3
. Model outputs
3
.
3
.
1
. Validation Validation of the model with independent data
obtained in Essex, UK is shown in Fig. 5 Beale and Long, 1995. Radiation interception by the
canopy is well predicted by the model. However,
Fig. 4. Caption o6erleaf
mm is the approximate moisture storage capacity for soils in this region FAO, 1995. In 1994 and
1995 e
c
was 2.4 and 2.3 g MJ
− 1
, respectively r
2
= 0.97 and 0.99.
Fig. 5. a Predicted fraction of radiation intercepted solid line and measured triangles at Essex, E. UK. b Predicted
values of above ground dry matter yields solid and dotted lines when run with e
c
values indicated and a measured point near the end of the season in Essex, E. UK.
when an e
c
of 3.3 g MJ
− 1
derived at Essex is used, is close to that measured.
3
.
3
.
2
. Annual 6ariation Mean variation in annual radiation over all
sites ranged from 1583 to 1817 MJ m
− 2
12 between 1984 and 1994. In the same 10 years
variation in the frost free period was 89 days 33 variation in the length of the model’s ‘growing
season’. Mean degree days DD
TB10
calculated over the whole year for all stations ranged from
535 to 805°C day
− 1
33 over the 10 year period. Within the growing season, which is de-
pendent on the frost free period, degree days varied from 494 to 781°C day
− 1
36. The influence of annual variation in temperature
shown in terms of length of the frost free period Fig. 6a, degree days Fig. 6b and incident radi-
ation Fig. 6c conditions on the yield predictions are shown in Fig. 6d at Kilkenny meteorological
station. Predicted yields vary from 13 to 24 t DM ha
− 1
.
3
.
3
.
3
. Interpolation across Ireland Maps of annual incident radiation and degree
days show higher values in the south and south western areas of Ireland Fig. 7a,b, respectively.
Surface interpolation of results of the model run at each of the 23 meteorological stations showed
that potential peak productivity ranges from 16 to 26 t DM ha
− 1
in the MidlandsNorth Midlands region and South West region respectively Fig.
7c. The ‘bulls-eyeing’ of the data around stations is a result of the interpolation procedure which
accumulates distance weighted averages.
4. Discussion
