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a Dep. of Plant Sci., Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK S7N 5A8, Canada
b Dep. of Crop, Soil, and Environ. Sci., Univ. of Arkansas, 1366 W. Altheimer Dr., Fayetteville, AR 72704
c Dep. of Geogr., Univ. of Saskatchewan, 9 Campus Dr., Saskatoon, SK S7N 5A5, Canada
* Corresponding author (lpurcell{at}uark.edu).
Received for publication February 11, 2003. Solar radiation data at the earth's surface (Rs, MJ m–2 d–1) are not typically recorded at weather stations, but they may be predicted from other meteorological measurements. For one location, Keiser, AR, we developed an empirical equation for predicting Rs. The mechanistic models of Hargreaves–Samani (HS) and two forms of the Bristow–Campbell model, described by Thornton and Running (TR) and Weiss et al. (WS), were also evaluated for predicting Rs at 13 sites, covering a 23° range in latitude and a 42° range in longitude. For the HS, TR, and WS models, we used coefficients as they were originally published, and for the HS model, a site-specific coefficient (HS-SS) was derived and evaluated for each site. Regression of predicted vs. observed Rs values using the empirical equation for Keiser gave r2 values (0.77) similar to the best of the mechanistic models. The HS-SS model had the lowest root mean square error of 3.50 MJ m–2 d–1, followed by the TR (3.56), the HS (3.86), and the WS (4.33) models. Predicted vs. observed values gave r2 values ranging from 0.72 (TR model) to 0.56 (WS model). There was a slight superiority of the TR model over the HS-SS and HS models. Similar fits (r2 > 0.87) and errors were found among the TR, HS-SS, and HS models when Rs values were averaged over a 7-d period, and it was concluded that these three models provided accurate and precise Rs estimations for our sites without further model modification.
Abbreviations: DOY, day of year • HS, Hargreaves–Samani model • HS-SS, site-specific Hargreaves–Samani model • kRs, Hargreaves–Samani coefficient • MAE, mean absolute error • Ra, total extraterrestrial radiation • Rs, total solar radiation at the earth's surface • RMSE, root mean square error • Tmax, maximum air temperature • Tmin, minimum air temperature • Ttmax, maximum transmittance that occurs for a given site under clear-sky conditions • TR, Thornton–Running model • WS, Weiss et al. model
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