Seasonal variation in evapotranspiration from mat rush grown in paddy field

Abstract Evapotranspiration from mat rush (Juncus effusus L.) was observed using the Bowen ratio method based on heat balance in a commercial paddy field located on the Yatsushiro Plain (32 deg 33.9' N, 130 deg 38.4'E, 1 m above sea level). Observations were conducted from transplanting (December) to harvest (July) in 1999/2000 (2000 season) and 2000/2001 (2001 season). Seasonal variation of evapotranspiration (E) showed an increasing trend with air temperature and solar radiation. Average and total values of E during the entire growth period in the 2000 and 2001 seasons were 2.02, 2.16 mm day**-1 and 491, 499 mm, respectively. Daily mean values of E were compared with the reference crop evapotranspiration (E sub(0)) and potential evaporation (E sub(p)). Seasonal variations of E/E sub(0) showed an increasing trend from O.46 to 1.OO during the growth of mat rush. Similarly, E/E sub (p) showed an increasing trend from 0.42 to 0.84 during the growth of mat rush. E/E sub(0) and E/E sub(p) of mat rush could be expressed as a function of plant area index (PAI). Root mean square error (RMSE) of estimating E using the function was 0.5 mm day**-1 on a 1-day scale, 0.2 mm day**-1 on a 5-day scale and 0.1 mm day**-1 on a 10-day scale. Variations in E/E sub(0) and E/E sub(p) were conjectured to be also influenced by soil moisture, wind speed and saturation deficit as the result of change in transpiration from mat rush. However, it was difficult to explain the difference in E/E sub(0) and E/E sub(p) that was found between 2000 and 2001 seasons, based only on the difference in environmental factors described above. It was suggested that the difference in the physiological activity of the mat rush, which was indicated by differences in stern color between 2000 and 2001 seasons, affected the E/E sub(0) and E/E sub(p).