Comparing Net Radiation Estimation Methods: CIMIS versus Penman–Monteith

Net radiation $\left(R_n\right)$ estimated using the California Irrigation Management Information System (CIMIS) Penman and the Penman–Monteith (PM) methods are compared using hourly data from July 17, 2004 to October 24, 2004. Results showed that differences between the two estimates get larger towards higher values of $R_n$ because of the inability of the two methods to accurately estimate net outgoing longwave radiation $\left(R_{\mathrm{nl}}\right)$ . This was determined by comparing the two $R_n$ estimates against values measured using the Kipp and Zonen CNR1 net radiometer. On average, the CIMIS Penman $R_n$ was 4% lower and the PM $R_n$ was 8% higher when compared to the measured values. An attempt to modify the PM method for estimating $R_{\mathrm{nl}}$ resulted in significant improvements. $R_n$ values measured by four Kipp and Zonen NR-Lite radiometers are also compared with the corresponding values measured by the CNR1. The NR-Lites performed well under the given sets of surface and environmental conditions. Average values of slope, coefficient of determination $\left(r^2\right)$ , and standard error of estimates from the linear regressions of the four NR-Lites against CNR1 were 0.97, 0.99, and $18.79\mathrm{W}∕{\mathrm{m}}^2$ , respectively.