Estimation of seasonal and annual evaporation using agrometeorological data from the Thai meteorological department by the heat budget models

Seasonal evaporation on bare soil and lawn was estimated using only routine meteorological data at Bangkok and Chiang Mai in the Chao Phraya river bas in in Thailand by using two methods. One method is that evaporation was est imated by the heat balance model using daily mean ground surface temperatur e (0 cm depth), (hereafter GST). The second is that it was estimated by usi ng the Force-Restore Method for estimating daily mean soil temperature (her eafter FRM). Seasonal and annual evaporation were estimated by using agrome teorological station data of the Thai Meteorological Department in 1989 at Chiang Mai (lawn) and Bangkok (bare soil). At Chiang Mai, annual precipitat ion was 1000 mm, annual pan evaporation was 1540 mm and estimated value of annual evaporation (lawn) was 760 mm by the FRM (using soil temperature at 5 cm depth) and 815 mm by the GST method. At Bangkok, annual precipitation was 1320 mm, annual pan evaporation was 1560 mm and the estimated value of annual evaporation (bare soil) was 1080 mm by the FRM (using soil temperatu re st 5 cm depth) and 1140 mm by the GST method. The results of these two m ethods were comparatively close to each other, however the estimated evapor ation by the GST method was slightly larger than that by the FRM. This diff erence was caused by the differences between the ground surface temperature and soil temperature at 5 cm depth. The reason considered was the difficul ty of accurate ground surface temperature measurement by the stem thermomet er. The estimated evaporation at Chiang Mai was comparatively close to the results of the complex area of vegetation obtained by Toda et al. (2000b) a nd paddy field obtained by Toda et al. (2000a).