Estimation of Daily Evapotranspiration from Irrigated Cabbage Fields in the Mountainous Region of Northern Thailand

Recently, cultivation of cabbages by hill tribes has been rapidly expanding in the mountainous regions of northern Thailand. This leads to a, fear of water shortage caused by hill farming and irrigation. To estimate water consumption, predicting the amount of evapotranspiration (ET) that occurs in irrigated cabbage fields is required. In the present research, portable chambers were used to measure the actual ET occurring in irrigated cabbage fields. Since the measurement period was restricted in Thailand, measurements were also carried out in cabbage fields in Japan. The results showed that daily ET was the same despite differences in the sizes of cabbage plants with sufficient soil moisture. Furthermore, strong correlations existed between ET, solar radiation (Rs) and vapor pressure deficit (VPD). This tendency was the same in Thailand and Japan. The authors then derived an empirical equation using Rs and VPD data measured in both Thailand and Japan to estimate ET from irrigated cabbage fields. The empirical equation was validated using the data measured in Thailand and Japan, and it was determined that the root mean square error (RMSE) was 0.6mm/day^-1for a daily scale. Our results also indicated that ET decreased sharply with soil water content (SWC) less than 30% However, the field survey determined that cabbage fields are regularly irrigated with SWC higher than 30% in these regions. Therefore, it is possible to estimate ET from irrigated cabbage fields using the proposed equation.