Measurement of energy budget components during the International Rice Experiment (IREX) in Japan

A field study was conducted to measure the fluxes of energy budget components above a rice canopy in Okayama, Japan, from 5 to 16 August 1996. During this period, the canopy height was, on average, 0·72 m and the leaf area index was about 3·1. The study site was approximately 300 m×300 m, and was surrounded by similar rice fields. Our footprint analysis 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 upwind area. The closure of the energy budget was examined to ensure the data quality. Owing to poor thermal conductivity of the heat transducers, soil heat fluxes were corrected for the difference in the thermal conductivities of the paddy soil and the transducer. Eddy covariance fluxes were corrected for the effects of line averaging and the sensor separation. Latent heat flux was further corrected for the density fluctuations. The magnitudes of the total corrections were not small and amounted to up to 20% of the raw flux values. With these corrections the surface energy budget of a rice stand was closed within 1% under drained conditions and 5% under flooded conditions. The daily evapotranspiration (ET) rate was near the potential rate of open water evaporation and ranged from 4·2 to 5·8 mm d−1. In terms of changes in crop water use with irrigation, no difference in ET rates was found between the drained and flooded conditions.