The water balance in six selected crops (mung beans, tomatoes, maize, cassava, yam, and sweet
potatoes) was determined at the field block scale in the Vietnamese Mekong Delta for two agricultural growing
areas. A novel approach for the calculation of water balance was employed, which combined the reference crop
evapotranspiration (ETo), derived from four methods, namely, Penman-Monteith, Blaney-Criddle, Thornthwaite,
and Turc. The equation of Penman-Monteith is generally considered to be the most representative to calculate the
ETo value. The reference crop evapotranspiration for some crops was calculated. The daily ETo ranges from 1.8
to 7.5 mm, while a monthly ETo ranges from 3.5 to 5.0 mm. The calculations of the water balances show that
irrigation is necessary for crops grown on raised beds crops during the dry season. The results show a promising
approach for evaluating the water regime to satisfy crop water requirements.