In order to construct a simple and practical equation calculating the shallow groundwater temperature (Tg) near the water table in the paddy fields from the air temperature (Ta) in the same area, we analysed the data of Tg that was obtained from our field investigations. The results obtained were as follows:
1) We found that the Tg-Ta relationship could be expressed by three straight lines (or linear equations) when a year (12 months) was divided into three periods -- 1,2 and 3.
2) To show the three straight lines, I, II and III, we constructed Eqs. (1), (2) and (3) respectively, and calculated the coefficients and constants of Eqs. (1), (2) and (3) by using the method of least squares. The results were listed in Table 1.
3) The calculated values of Tg (using Eqs. (1), (2) and (3) with the values listed in Table 1) were close to the observed values (Figs.3,4,5 and 6). The deviation of the calculated values from the observed ones calculated by using Eq. (8) was only a few per cent in the yearly average values.
4) Therefore, the linear equations Eqs. (1), (2) and (3) with Eqs. (4), (5) and (6) can be used- to show. the relationship between Tg and Ta for a given area for a given period if the Tg -Ta relationship becomes the linear relationship.