Heat balance at the earth's surface is a base for understanding the nature of the earth surface as Voeikov, Thornthwaite and others had suggested. The study of heat balance hac close relations not only with climatology, but also hydrology, glaciology, geomorphology and biogeography. Regimes of heat and water are two fundamental elements controlling physical features on the earth, and distribution and variation of these elements are explained through the analysis of heat balance. It may be said that heat balance study gives the methodology in physical geography which aims to explain many complex phemomena occurring near the earth's surface.
This special volume of the Review is one of the results of activities of “Research Group of Heat Balance Climatology” in our Association.
General explanation on heat balance of the earth is illustrated in the figure. Study of heat balance of the earth includes wide fields of the science ranging from satellite climatology to groundwater hydrology, limnology and oceanography. But the papers presented in this volume are mainly concerned with boundary layer climatology and thermal characteristics of the ground surface. These problems are especially important in understand ing the nature of the earth's surface, and they must be studied by geographers who know the complex system of the earth's surface.
Recent progress in heat balance studies made by Japanese geographers are briefly reviewed in this article. Papers may be classified into four categories: (1) boundary layer physics; (2) local climatology and urban climatology, (3) hydrology, soil physics and glaciology; and (4) radiation balance. All the papers on heat balance presented in the Review, important papers published in other journals and books on this problem are listed in the bibliography for the last 15 years. The contributions of geographers on this field of the study mainly concentrate to urban climatology, radiation balance and hydrology. As seen from the bibliography, many papers were published in 1970's, and it seems that the heat balance method is getting a foundation in physical geography. For the future development of this method, we have several problems to be solved, for example, curricurum of geography in universities, research fund and application of the method.

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Net radiation Rn, incoming total short-wave radiation S_t, soil heat flux G, air temperature T_a, wind speed U, sensible heat flux H and actual evapotranspiration E_a by the use of weighing lysimeter were measured from December 1981 to November 1982. Hourly data of them were analyzed in order to make clear the characteristics of heat balance terms of grass land at the experimental field of Environmental Research Center, University of Tsukuba.
The results are summarized as follows:
1. Daily mean values of Bowen ratio were calculated using the data of H and E_a which were directly measured by sonic anemometer-thermometer and weighing lysimeter, respectively. The mean values of Bowen ratio 0.17 in summer, 0.22 in autumn, 1.92 in winter and 0.64 in spring were found. These variations were associated with water content near the soil surface besides the change of albedo.
2. The regression formula for the dependence of net radiation on incoming total shortwave radiation was obtained as
R_n=-52.8+0.716S_t (W m^-2)
However, it had a noteworthy hysteresis, i.e., annual cycle. Monthly means of daily totals of net radiation balance expressed as percentage to total short-wave radiation, varied from a maximum of 59% to minimum of 8%. In annual total, ∑(R_n)_y/∑(S_t)_y, was 39.4%.
3. The result of heat balance calculations showed the large residua (ΔM) mainly caused by advection prevailing in winter (December to Febuary) and April (Table 1). However, from May to October except August, the values of ΔM were negligibly small in comparison with the values of (R_n-G).
4. Hourly mean values of equilibrium evaporation, LE_e, obtained by using Eq. (11) coincided with the values of actual evapotranspiration LE_a except winter season.
5. Daytime (06:00_??_18:00 JST) and nighttime (18:00_??_06:00 JST) net evapotranspiration were estimated (Fig. 10). It was found that the ‘distillation’ of water vapour from soil surface to grass stand occurred in comparatively calm night was much greater than dew-fall, especially in summer season.

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When a heavy rain of more than several tens millimeters occurred on the surface soil with a large temperature gradient, rapid changes in soil temperature at the 1m depth were observed in the observation field of heat and water balance, Environmental Research Center, University of Tsukuba (Fig. 1). Soil temperature at the 1 m depth rose to 1.4°C in a day after the beginning of the rain amounting to 68 mm on May 19, 1978. This change of soil temperature began at the same time when water table began to rise at 8 hours after the beginning of the rain (Fig. 2). It was clear that soil water flux estimated by the changes of soil temperature, using Eq. (3) solved from Eq. (1) under the condition of Eq. (2), agrees with the rainfall intensity (Fig. 3).
An experiment of artificial rainfall-infiltration was carried out in the bare soil in the neighbourhood of the observation field, E. R. C. on September 1, 1978. The amount of arti-ficial rainfall using sprinkler was about 350mm for 20 hours. In this experiment, the observations of soil temperature and water content were made (Fig. 4). Soil temperature and water content from the ground surface to the 10 cm depth changed at 1 hour after the beginning of the rain. When these changes reached the 50cm depth at 2 hours, the changes of soil temperature below 75cm occurred instantaneously (Fig. 5). Soil water movement converted an initial large temperature gradient into a new small one at 6 hours. Subsequently, the soil was cooled from the surface to the water table (Fig. 6).
From these observation results, it becomes clear that the response of soil temperature to a rain is reflected on the difference of soil water movement between the suspended and the capillary water zones pointed out by Sakura and Taniguchi (1983).
From the data mentioned above, the following conclusions are obtained.
1. In the suspended water zone, the wetting front made by rainwater moves downward through the surface of soil particles. Therefore, the change of soil temperature is transferred downward with the wetting front. After the beginning of the rain and before the wetting front is made, the heat exchange occurs between rainwater and soil particles. In addition, this zone is influenced by diurnal variation of air temperature.
2. In the capillary water zone, the vertical distribution of water content is built on the pressure equilibrium between capillary and gravitational forces. When the wetting front reaches the upper boundary of the capillary water zone, this equilibrium is destroyed and soil water begins to move. If a large temperature gradient is formed, the soil temperature profile will be maintaining the initial gradient and moving downward corresponding to the rapid movement of soil water. Therefore, the change of soil temperature occurs quickly and largely. In condition of a small temperature gradient in soil, it will be a little.
3. In case of extreme dry soil in the surface layer, rainwater is used for an increase of soil moisture to field capacity in the suspended water zone. Until soil water content reaches field capacity and wetting front is formed, the change of soil temperature occurs in the surface soil zone.
4. In the experiment of artificial rain amounting to about 350mm for 20 hours, after an initial gradient of soil temperature was converted to a new temperature gradient at 6 hours after the beginning of the rain, soil temperature profile maintained the new gradient and cooled from the surface to the water table. This phenomenon can be explained by the fact that rainwater with a low temperature could move through large pores selectively because soil water became the condition of a saturation and a positive pressure. Soil water in the unsaturated condition can move uniformly rather than in the saturated condition.

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The change of thermal properties by the replacement of natural surface by artificial materials has been considered as one of the causes of urban heat island. However, there are very few reports about the thermal properties of such urban surface covering. In simulation analyses of urban heat balance, many parameters including thermal properties were given arbitrarily for the lack of data. In this paper, asphalt pavement was chosen as one of the typical land uses in urban area, then its thermal properties as well as the feature of heat balance were investigated. Instead of real road surface, some experimental apparatus were made from asphalt blocks (30cm square and 5cm thickness, each) in which the heat flux plates and thermistor thermometers were set at every contact boundary. The observations were performed on two setlings of the apparatus; (a) buried in soil (paddy field and bare land) and (b) laid on roof top (Fig. 1).
The thermal conductivity (A) was caluculated from the relations between heat flux and temperature gradient at each 5, 10, 15 and 20cm depth (Fig. 2). In order to estimate the heat capacity (cρ), the heat storage of every 5cm layer was derived from heat flux differ-ence between upper and lower boundary, and its relation to time-difference of temperature was shown in Fig. 3. These results are summarized in Table 1. While the thermal conductivity of asphalt pavement is not so different from that of soil, its heat capacity is apparently small in comparison with that of soil (Table 2). In this connection, the thermal diffusivity (λ/cρ) of asphaat pavement is larger than soil.
The result above is inconsistent with the widely accepted view that because of its large heat capacity the nocturnal surface temperature of asphalt pavement keeps higher than of rural area. As seen in Fig. 6, however, despite of larger cooling rate of asphalt layer than soil, in summer the large difference of surface temperature between them in the afternoon is hold through the night. Moreover, such an enormous temperature range of asphalt layer makes it possible to store great ammount of heat in spite of its small heat capacity.
As it is evident from the Fig. 4, at the surface of asphalt pavement solar energy is redistributed to sensible heat flux (H) and conductive heat flux into asphalt layer (G) approximately at the ratio 2:1. If examined in detail, the ratio of G/R_n (R_n: net radiation) has a peak in the morning and decreases gradually in the afternoon (Fig. 5). According to previous observation, the time of maximum heat island intensity occurred after sunset, it is in accord with the peak of heat release from asphalt layer. It suggests that the heat storage of asphalt pavement is one of the important factors of nocturnal heat island.
The heat flux in asphalt layer is larger than that in soil at each depth (Fig. 7), and nocturnal upperward flux dose not compensate for daytime downward flux in asphalt layer. It means that covering of asphalt pavement results in increase of soil temperature beneath it in warm season.
In addition to its high surface temperature, the dew point temperature near the asphalt surface is lower than that on soil surface due to the lack of latentt heat flux, or evapotranspiration (Fig. 8). Then less possibility of dew condensation at asphalt surface at night may be inferred.

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Evapotranspiration plays an important role in determining dry and wet conditions of the earth's surface. Many factors consist in the evapotranspiration process and they combine one another in complex ways. Therefore, previous studies of evapotranspiration have been done over uniform surfaces, that is, open water, uniform vegetation, or bare soil surfaces. As advances in the understanding of evapotranspiration process have occurred, it becomes possible to calculate evapotranspiration by adding the soil surface and plant surface com-ponents. A separation of evapotranspiration into soil water evaporation and plant canopy tran-spiration was carried out in this study. Micrometeorological observation was conducted over a pasture field in the Environmental Research Center, University of Tsukuba, during the summer of 1978. The method proposed by Deardorff (1978) was used in computation. Hourly variations of albedo, atmospheric stability and excess resistance were considered in the calculation. The results obtained in this study are summarized as follows.
1. The albedo of pasture and that of bare soil showed hourly variations depending on solar elevation. The hourly albedo values ranged from 0.16 to 0.26. This fact indicates that the hourly variations of albedo should be considered in the analysis of radiation balance for short periods.
2. Soil water evaporation proceeded in the nighttime, even though condensation occurred on pasture leaves. This was caused by the mulching effect of pasture canopy, which pre-vents soil surface from extreme radiative cooling.
3. The soil water evaporation amounted to 25.9% of the total evapotranspiration during the observation period. The proportion of soil water evaporation from a field of pasture was greater than those obtained from other crops.

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In the periglacial environment of the Japanese Alps, temperature distributions in surface soil layer are important for the formation of micro topography. However, the lack of soil temperature data in alpine zone of the Japanese Alps are serious. The purposes of this paper are to make clear the diurnal and annual variations of soil temperature on the summit area of the Mt. Tateyama and the Ontake Volcanoes, and the lapse rates along the mountain slopes. Field observations were carried ont in the cooling phase of the year from mid summer to early winter. As we failed to get the data in mid winter, we utilized the data of annual soil temperature change at Tokuyama Village, heavy snow fall area west of the Ontake, combining with the data of the field observations, to obtain empirical equa-tions of annual change of the soil temperature on the several slopes of the Ontake Volcanoes.
The lapse rates and the diurnal changes of soil temperature during the field observations are shown in Figs. 4 and 5 and Table 1. Annual change of soil temperature in alpine zone (T_z) is expressed by the Ingersoll's equation as follows;
T_z=T_M+A₀exp (-az)sin(wt-az-γ)
where T_M mean soil temperature A amplitude of soil surface temperature α;_??_ρ; density of soil, c; specific heat of soil, pk_g; Austausch oefficient, ω=22π/p, p; period of temperature cycle, z; depth of soil. Some of the results of the field observations and the estimations by the use of the above equation are shown in Figs. 8_??_10.

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