Estimation of Daytime Net Radiation Over Well‐Watered Grass

Net radiation $\left(R_n\right)$ is an important component of the modified Penman equation used to calculate reference evapotranspiration $\left({\mathrm{ET}}_0\right)\text{.}$ Net radiometers, however, require continual maintenance to ensure that the data they generate are reliable. An equation is developed that estimates hourly $R_n$ over well‐watered grass from meteorological data, such as solar radiation, vapor pressure, and air temperature, collected by weather stations in various regions in California. The equation is based on Monteith's daily net radiation equation. Empirical equations are also developed to quantify surface albedo and clear sky global transmissivity. The modified Monteith formula for net radiation is tested in coastal, desert, mountain, and interior valley regions within California. After examining the 1984–1989 data from weather stations in the various regions, the absolute mean error of calculated hourly $R_n$ is within 10% of the measured hourly $R_n$ The absolute mean error of daytime $R_n$ during $\theta >10$ degrees is within 8% of the measured $R_n$ These errors represent an even smaller error in ${\mathrm{ET}}_0$ estimates. The modified Monteith equation for $R_n$ does not require site specific correction factors.