New Zealand Journal of Crop and Horticultural Science
abstracts
Estimating daily solar radiation in New Zealand using air temperatures
A. L. Fletcher*
email: fletchera@crop.cri.nz
D. J. Moot
Agriculture and Life Science Division
P.O. Box 84, Lincoln University
Lincoln 7647
Canterbury, New Zealand
*Present address: Agronomy Physiology Laboratory, University of Florida, P.O.
Box 110965, Gainesville, FL 326110965 United States.
Abstract The amount of solar radiation received at the earth’s
surface (Rs, MJ m–2 day–1) sets the potential for crop growth, is
a key driver of evapotranspiration and, consequently, is an important input variable
for crop simulation models. However, values for Rs are seldom included in historical
weather records. This paper assesses two methods proposed to estimate Rs as the
product of extraterrestrial radiation (Ra) and a transmissivity coefficient (Tt),
using data from 10 New Zealand sites (37.5°S–46.1°S). Both the
Hargreaves-Samani (HS) and the Weiss et al. (WS) models use the difference between
daily maximum and minimum temperature (DT) to estimate Tt. The HS model uses
the square root of DT and a single empirical coefficient, k. Reported
values of k are 0.16 for continental and 0.19 for coastal locations.
The WS model uses the DT squared and a single empirical coefficient, b,
to estimate Rs. Reported values of b range between 0.20 and 0.75. The
current research used a sensitivity analysis of k and b to determine
the accuracy of the two models at the 10 sites and to determine the optimum
values for the coefficients k and b. Sensitivity analyses showed
the HS model gave more accurate estimates of Rs than the WS model. Root mean
squared deviations (RMSD) were 2.43 and 3.50 (MJ m–2) across all 10 sites for the
HS and WS models, respectively. The range of values of k in the HS model
was 0.14–0.17, which is typical of continental sites. The mean value (k =
0.15) is suggested as appropriate for other New Zealand sites. The range for b in
the WS model was 0.35–0.65. The principal use for these estimated Rs values
would be in crop simulation models. Therefore, a simulation analysis was undertaken
with these data. This compared simulated wheat (Triticum aestivum) biomass
and grain yields with measured yields from an experiment at Lincoln, New Zealand.
Simulations used observed Rs values and those estimated with the HS and WS
models. Simulated biomass and grain yields from the HS model were virtually
identical to those using the observed Rs data, whereas those from the WS model
were 5–6%
less accurate.
Keywords crop modelling; weather data
New Zealand Journal of Crop and Horticultural Science, 2007, Vol. 35:
147–157
0014–0671/07/3501–0147 © The Royal
Society of New Zealand 2007
H06098; Online publication date 12 April 2007. Received 27 October
2006; accepted 7 February 2007
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