The temperature of irrigation water is very important for the growth of rice-plants, but it is sometimes too cold in the earlier stage of irrigation season in Hokkaido. Whereas the annual average of ground-water temperature is usually higher than that of river water in the district concerns. Ground-water may be thereby possiblly more profittable than surface water for irrigation in some period. Then the author investigated the temperatures of river waters and ground-waters to know when temperature of river water overruns that of ground-water in the district concerns in 1950. As the results, the followings were learned.
Generally speaking, water temperatures of river seem to be 27°C in maximum, and are higher in lower reach than in upper reach, in main stream than in tributary at confluence, snd also in rivers flowing southward than those flowing northward. And they vary diurnally and annually, but their diurnal variations are less than 7°C and smaller by far than those of atmospheric temperature. And the maximum and the minimum appear from 1 to 3 hours later than those of air, and the temperature is the lowest during from 4 to 6 o'clock a.m, and the highest during from 3 to 4 o'-clock p. m. However the variation is very small in such rivers as Tomakomai river, having springs in source. The temperature falls down as a rule for a while in the middle or the last decade of May in Hokkaido. If a river water is taken in tunnel, it sometimes gets warm due to the temperature of the ground, but sometimes becomes cool, because it is not heated by sun.
On the other hand, temperatures of ground-waters are related with those of air and river, and they are in general higher in lower reach, in southern regions, and in the districts, where temperatures of air and river are high, and furthermore, they seem to be high in the area descending southward. The temperatures of free or shallow ground-water are ranged from 5°C to 14°C, and their annual averages are usually 1°C or 2°C higher than those of atmospheric temperature in the district concerns. While temperatures of deep-seated confined ground-waters are in common from 8.5°C to 16°C, and show the geothermal gradients of from 25m. to 40m. for 1°C. The gradients seem to be larger in Pleistocene or Pliocene series than in Alluvial series.
Comparing temperatures of river waters with those of ground-waters, river waters mostly overrun ground-water in temperature in the first or the middle decade of May. Then it seems not to deserve to use ground-water specially for irrigation. However temperatures of ground-waters are extraordinarily high in the lower reach of Ishikari river and in Takikawa Town, and they are higher than surface waters even in the middle of June. The ground-water temperature in Sapporo City is higher than that of Toyohira river before the middle of June. The waters of Tomakomai river and Horonai river become warmer than the ground-water in the district concerns in July. And the temperature of Chubetsu river is lower than the ground-water in the district even in the middle of June, so far as those of night concerns. Accordingly further study seems to be necessary for utilization of ground-water in these districts.
