Observation and estimate of net longwave radiation for improving evapotranspiration estimates

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I measured net longwave radiation on the well-irrigated grassland, reference surface, and showed the tendency of the radiation at Utsunomiya, Tochigi, JAPAN. In Addition, I examined the coefficients of net longwave radiation equation for improving the estimation accuracy of the reference evapotranspiration. As the result, I revealed the following:(1) net longwave radiation shows seasonal variation with higher radiation in winter and lower radiation in summer; (2) the well-known assumption that the daily surface temperature is equal to the daily air temperature was hard to apply throughout the year. The daily mean surface temperature of the reference surface is higher than the daily mean air temperature in summer and lower than the daily mean surface temperature in winter; (3) the coefficients of well-known net longwave radiation equations tends to be the climate dependent variables and are required to re-calibrate coefficients based on the observation data.