MEASUREMENT OF ENERGY BUDGET COMPONENTS DURING THE INTERNATIONAL RICEEXPERIMENT (IREX) IN JAPAN

A field study was conducted to measure the fluxes of energy budget com ponents above a rice canopy in Okayama, Japan, from 5 to 16 August 199 6. During this period, the canopy height was, on average, 0.72 m and t he leaf area index was about 3.1. The study site was approximately 300 m x 300 m, and was surrounded by similar rice fields. Our footprint a nalysis indicated that > 90% of the measured flux (at a height of 2.2 m above the ground) was expected from within the nearest 300 m of upwi nd area. The closure of the energy budget was examined to ensure the d ata quality. Owing to poor thermal conductivity of the heat transducer s, soil heat fluxes were corrected for the difference in the thermal c onductivities of the paddy soil and the transducer. Eddy covariance fl uxes were corrected for the effects of line averaging and the sensor s eparation. Latent heat flux was further corrected for the density fluc tuations. The magnitudes of the total corrections were not small and a mounted to up to 20% of the raw flux values. With these corrections th e surface energy budget of a rice stand was closed within 1% under dra ined conditions and 5% under flooded conditions. The daily evapotransp iration (ET) rate was near the potential rate of open water evaporatio n and ranged from 4.2 to 5.8 mm d(-1). In terms of changes in crop wat er use with irrigation, no difference in ET rates was found between th e drained and flooded conditions. (C) 1998 John Wiley & Sons, Ltd.