ESTIMATING GLOBAL SOLAR-RADIATION FROM COMMON METEOROLOGICAL OBSERVATIONS IN WESTERN CANADA

The interest in using crop growth simulation models for estimating lar ge area yields in western Canada has led to a requirement for daily va lues of solar radiation on an historical and a real time basis. Becaus e such data are usually not readily available, an equation was develop ed which relates solar transmissivity (zeta) (the ratio of incoming gl obal solar radiation at the earth surface (Q) to solar radiation at th e top of the atmosphere (Q0)) to daily observations of maximum and min imum air temperature and total precipitation (P): zeta=Q/Q0=a(DELTA T) b(1 + cP + dP2) where a, b, c and d are empirical coefficients which v ary with time of year and DELTA T is the range in daily temperature ex tremes. During the late fall-winter period, correlation coefficients b etween observed and calculated transmissivities were less than 0.5 wit h relative large root mean square errors (RMSE). However, during the g rowing season when the equation would be of most use, correlation coef ficients were 0.7 or higher with RMSEs of 0.12 or lower. The coefficie nts a, b, c and d were found not to be site-specific during the growin g season. No significant differences were found between wheat yields e stimated with observed solar radiation and those estimated with the ca lculated solar radiation from the equation.