New Zealand Journal of Crop and Horticultural Science abstracts

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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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