Tomoyoshi HIROTA・Masato FUKUMOTO Hokkaido National Agricultural Experiment Station We studied the properties of the ground surface temperature and the exchange speed CHU (reciprocal of atmospheric resistance) on bare soil. These are both important factors for estimating the sensible heat flux by the bulk transfer method. The ground surface temperature is difficult to observe accurately in the daytime because a large temperature gradient often exists near the surface and the landsurface

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... s are inhomogeneous. On the other hand, the daily mean value of ground surface temeprature is not difficult to observe because it is not sensitively dependent on the soil depth and it is not sensitive to inhomogeneous properties of the landsurface. We explained the features by using the Force-Restore Model (FRM) which has been developed for simple prediction of the ground surface temperature. According to the FRM, the surface layer thickness for the ground surface temperature is affected the damping depth that is the function of not only the thermal diffusivity of soil but also the period. With regards to the exchange speed CHU, we obtained almost the same results between CHU calculated from 30 minutes averaged value and CHU calculated from daily mean value. If the soil surface is sufficiently wet, the exchange speed CHU can be estimated by using daily mean values obtained from routine meteorological data without needing the observed sensible heat flux. We estimated the daily mean value of sensible heat flux well by using routine meteolorogical data. The root mean square error was about 10 W• m-2 .