Estimation of monthly solar radiation from measured air temperature extremes A. Bandyopadhyay a, *, A. Bhadra b , N.S. Raghuwanshi c , R. Singh c a Centre for Flood Management Studies (Brahmaputra Basin), National Institute of Hydrology, Sapta Swahid Path, G.S. Road, Dispur, Guwahati, Assam 781006, India b Department of Agricultural Engineering, North Eastern Regional Institute of Science and Technology, Nirjuli, Itanagar, Arunachal Pradesh 791109, India c Department of Agricultural and Food Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India agricultural and forest meteorology 148 (2008) 1707–1718 article info Article history: Received 13 August 2007 Received in revised form 6 June 2008 Accepted 15 June 2008 Keywords: Solar radiation Air temperature Sunshine hours Meteorological data Data substitution abstract Solar radiation (R s ), a very important variable in agricultural meteorology, is measured at a very limited number of meteorological stations worldwide. However, a number of methods are reported in the literature for estimating R s from routinely measured air temperature extremes, the accuracy of which needs to be tested. R s was thus estimated by different methods for 29 stations distributed throughout India. The methods compared were Har- greaves [Hargreaves, G.H., 1994. Simplified coefficients for estimating monthly solar radia- tion in North America and Europe. Dept. Paper, Dept. Biol. and Irrig. Eng., Utah State Univ., Logan, Utah], Annandale et al. [Annandale, J.G., Jovanovic, N.Z., Benade ´ , N., Allen, R.G., 2002. Software for missing data error analysis of Penman–Monteith reference evapotranspiration. Irrig. Sci. 21, 57–67], Allen [Allen, R.G., 1995. Evaluation of procedures for estimating mean monthly solar radiation from air temperature. Rep., Food and Agriculture Organization of the United Nations (FAO), Rome, Italy], Samani [Samani, Z., 2000. Estimating solar radiation and evapotranspiration using minimum climatological data. J. Irrig. Drain. Eng., ASCE, 126(4), 265–267], Allen [Allen, R.G., 1997. Self-calibrating method for estimating solar radia- tion from air temperature. J. Hydrol. Eng., ASCE, 2 (2), 56–67], and Bristow and Campbell [Bristow, K.L., Campbell, G.S., 1984. On the relationship between incoming solar radiation and daily maximum and minimum temperature. Agric. Forest Meteorol., 31, 159–166]. The estimated R s values were then compared to measured R s (or R s estimated from measured sunshine hours with locally calibrated A ˚ ngstro ¨ m coefficients), to check the suitability of these methods under Indian conditions. Three statistical indicators were used for compar- ing the performances of different methods, namely, modelling efficiency (ME), coefficient of residual mass (CRM), and root mean squared error (RMSE) expressed as a percentage of the arithmetic mean of observed values. To understand the effect of R s estimation on reference evapotranspiration, ET 0 , the internationally accepted FAO-Irrigation and Drainage Paper 56 Penman–Monteith based method (Allen et al. [Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop evapotranspiration—guidelines for computing crop water requirements. Irrig. and Drain. Paper 56, Food and Agriculture Organization of the United Nations (FAO), Rome, Italy]), employed using both measured R s and estimated R s , were also compared. The methods performed differently for different stations. In general, the original Hargreaves methods, both with [Annandale, J.G., Jovanovic, N.Z., Benade ´ , N., Allen, R.G., 2002. Software * Corresponding author. Tel.: +91 3612331150; fax: +91 3612228823. E-mail address: arnabbandyo@yahoo.co.in (A. Bandyopadhyay). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/agrformet 0168-1923/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.agrformet.2008.06.002