Estimating Solar Irradiance for Crop Modeling Using Daily Air Temperature Data

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Abstract
Crop growth models require solar irradiance as input data, yet there are few places where such data are routinely measured. For locations where measured values are not available, solar irradiance can be estimated using empirical models such as the Bristow–Campbell (B–C) model. This study was conducted to assess the spatial and seasonal accuracy of the B–C model for midcontinental locations in Kansas. A 30‐year data set from Manhattan, KS, was used to calibrate and evaluate unmodified and modified forms of the B–C model. The effect of seasonality was investigated by subdividing the yearly data into two subsets, a high noontime solar elevation angle period, ranging from DOY 121 to 273, and a low noontime elevation angle period comprising the remainder of the year. The B–C model was also evaluated without seasonal division of the year. The calibrated models were then tested against measured solar irradiance values for 10 sites distributed across the state of Kansas. Results indicate that, for the calibration site at Manhattan, irradiance was more accurately estimated using a modified form of the B–C model. For the yearly data, root mean square error (RMSE) was 3.9 MJ m−2 d−1 (25% error), compared with 5.2 MJ m−2 d−1 (24% error) for the high solar elevation angle period and 3.6 MJ m−2 d−1 (32% error) for the low solar elevation angle period. The RMSE for the 10 test sites ranged from 2.0 to 6.2 MJ m−2 d−1; percentage error ranged from 26 to 47%. Neither latitude nor distance from the calibration site significantly affected the accuracy of irradiance estimates at the evaluation sites. Results suggest that the modified B–C model provides reasonably accurate estimates of irradiance at noninstrumented sites and that the model can successfully be used at sites away from the calibration site. Seasonal subdivision of the data adds little to the accuracy of estimates.