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 basin in Thailand by using two methods. One method is that evaporation was estimated by the heat balance model using daily mean ground surface temperature (0 cm depth), (hereafter GST), The second is that it was estimated by using the Force-Restore Method for estimating daily mean soil temperature (hereafter FRM). Seasonal and annual evaporation were estimated by using agrometeorological station data of the Thai Meteorological Department in 1989 at Chiang Mai (lawn) and Bangkok (bare soil). At Chiang Mai, annual precipitation 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 temperature at 5 cm depth) and 1140 mm by the GST method. The results of these two methods were comparatively close to each other, however the estimated evaporation by the GST method was slightly larger than that by the FRM. This difference was caused by the differences between the ground surface temperature and soil temperature at 5 cm depth. The reason considered was the difficulty of accurate ground surface temperature measurement by the stem thermometer. The estimated evaporation at Chiang Mai was comparatively close to the results of the complex area of vegetation obtained by Toda et al. (2000b) and paddy field obtained by Toda et al. (2000a).