Bowen's ratio represents the ratio of sensible heat (H) to latent heat (LE) between the air layer and the underlying water or ground surface, which is calculated as follows:
r=H/LE≅{0.64θw-θa/e(θw)-ea (°C, mb) 0.5θw-θa/e(θw)-ea (°C, mmHg)
It gives the useful method to the heat or water budget studies of the earth's surface, and the evaluation of this ratio for the oceans had been made by Jacobs or Sverdrup, but exact knowledges of the values for inland waters had not been obtained because of the lack of basic hydrological data. In this paper, the author intends to make clear the seasonal and regional variations of Bowen's ratio for the four kinds of water body (1. Lake and reservoir, 2. Middle and lower reaches of river, 3. Coastal sea, 4. Ground water), by making use of the miscellaneous water temperature data. The data were collected from the publications of Meteorological Agency, Agricultural Ministery, Yoshimura's and Torii's reports and others.
Calculations of the ratio were made by using the monthly mean air and water temperatures. Characteristics in the distribution of Bowen's ratio may be summerized as follows:
1. Lake and Reservoir (Fig. 2 a-b)
Bowen's ratio shows the clear annual variation with large in the winter and small value in the summer season. The range of the annual variation is wider in the northern part than in the southwest part of Japan. In Hokkaido, it takes negative value in early summer, corresponding to the relations of θw<θa and e(θw)>ea. During mid-summer the ratio lies between 0.15 and 0.2 in all districts of Japan.
2. River (Fig. 2 c-h)
The values for the river water temperature show larger annual variations than the previous one, especially they are remarkable in the northern Japan and the coast of Japan Sea. Because these regions are covered by deep snows in winter and their rivers discharge the snow-melting water in spring season, the temperature of the rivers deos not rise so high as on the Pacific Coast. They vary greatly from place to place and time to time, so it is hard to explain them in brief.
3. Coastal Sea (Fig. 2 i-n)
The ratio for the coastal sea water shows nearly the same magnitude as those of lakes and reservoirs discussed above. In winter they indicate the values between 0.5 and 1.0 and in summer 0.1 and 0.15. In the northern part of Pacific coast (East coast of Hokkaido and off the coast of Sanriku) negative values are indicated in spring and early summer.
4. Ground water
In this report, ground water temperature of a place is considered to be nearly equal to the mean annual air temperature of the place. The range of the annual variation of the ratio is wider in the northern Japan. In early summer, negative values are shown all over Japan.
In the practical application of Bowen's ratio, it is desirable to express it in easily observable terms such as θw, θa or Δθ. As shown in Fig. 4. a relation similar to the saturated vapor pressure-temperature curve is obtained between vapor pressure and temperature of air. It is expected from the relation shown in Fig. 4 that Bowen's ratio depends closely upon the difference between water and air temperatures. This relation is shown in Fig. 5, and by using the the monthly mean water and air temperatures Bowen's ratio for each kind of water body at each region in Japan is evaluated from the graph.
