We measured heat and soil moisture conditions of growing periods at Nakasatsunai upland field where monthly average temperatures of winter are below the freezing point. Using following equation and Bowen ratio, heat balance (net radiation(Rn), soil heat flux(G), latent heat flux(lE) and sensible heat flux(H)) during growing period from 1983 to 1992 was calculated.

     Rn = H + lE + G

Soil temperatures at the depths of 1, 5, 10, 20 and 50 cm, and soil water suctions at the depths of 5, 15, 25, 35 and 45 cm were also measured. Evapotranspiration(ET) was calculated from lE and vaporization heat. Rainfall data(R) at Obihiro meteorological observation site, which located near the Nakasatsunai field, was used.

1. Monthly averages for ten years

   1. Rn, lE and ET showed the maximum in July. H and G almost declined from late May in which measurement had started in each year. G became minus value from September.
   2. ET was larger than R in late May and June, and almost equal in July, and smaller from August to early October.
   3. The percentages of days on which the pF value of soil layers of 5cm and 25cm was above 2.5 were about 50% in late May and June. The percentages were about 25% in July and August.
   4. Results of 2) and 3) indicate that soil moisture was deficient for crop growth in late May and June.
   5. From late May to August, soil temperatures at the depths of 5cm, 20cm and 50cm rose respectively, and the deeper the depth, the lower the soil temperature. From September, soil temperature at each depth changed to decline. In early October, the deeper the depth, the higher the soil temperature.

2. Daily changes of heat balance and soil temperature at wet and dry soil conditions on sunny days of early July

   1. On sunny days of which Rn of daytime was about 500 ly/day, at dry soil (pF values were about 2.8), about 40% of Rn was distributed to lE, 50% to H, and G was about 30 ly/daytime. At wet soil (pF values were about 1.8), 80% of Rn distributed to lE, and 10% to H, and G was about 60 ly/daytime.
   2. The differences between maximum and minimum of air temperature and soil temperature at the depth of 1cm were larger at dry soil than those at wet soil. The differences of maximum and minimum soil temperatures at the depths of 5cm and 20cm were larger at wet soil than those at dry soil. Dry surface soil seemed to function as a heat isolating material.
   3. The rises of daily average soil temperature at depths of 5cm, 10cm and 20cm at wet soil were larger than those at dry soil.

3. G and the soil temperature difference between 1cm and 50cm depth

   1. Ten-day averages of G are significantly correlated with the differences between ten-day averages of soil temperatures at 1cm and 50cm depth.

Dr. Teruo ISHIWATA
Civil Engineering Research Institute
Hokkaido Development Bureau
1-3 Hiragishi
Toyohira-ku
C.Sapporo, JAPAN    062
Telephone: 011-841-1111 EXT 371   Japan       
Fax:       011-824-1226  Japan
E-mail: ishiwata@ceri.go.jp