Abstract
1. Vertical temperature structure of the natural lake differs according to the climate where the lake locates and dimensions of the lake basin. In the low latitude, thermal stratification remains weak throughout the year. In the high and middle latitudes, intense thermal stratification is created in summer, but nearly isothermal profile is observed in winter. Depth and intensity of the thermocline vary in proportion to fetch size (l) or surface area (S) of the lake (Arai, 1964, 1966).
Characteristics of temperature structure and heat balance of the lake have not been analyzed and explained from the climatological viewpoint. This study aims to analyze the macroscale characteristics of thermal structure of the lake on the basis of heat balance theories. Problems to be solved for this purpose are as follows.
(1) Determination of surface water temperature.
(2) Determination of annual variation of water temperature in deep layer of the lake.
Names and dimensions of lakes utilized in the analysis are listed in Table 1.
2. World distribution of equilibrium water temperature was decided for the analysis of the first problem. Equilibrium water temperature (θ∞) is a temperature when thermal equilibrium at water surface is reached (Eq. 1). Its numerical value is calculated by the following form which is derived from Eq. 1.
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In this, θa is air temperature, h is heat transfer coefficient, Rn is net-radiation on water sur-face, ΔE is saturation deficit and (de/dθ)θa is tangential gradient on saturation-vapor-pressure curve at the air temperature. Calculations were made by use of monthly mean values of climatic data at about one hundred stations over the world. Global distributions of annual mean, annual maximum and annual minimum equilibrium temperatures are summarized in Fig. 1 as latitudinal mean for the each continent. Two cycles appear in annual variation of the temperature in the equatorial zone, although a single cycle is observed in the middle and high latitudes.
Temperate lakes are possible to exist in the zone where annual minimum equilibrium temperature decreases below 4°C, and the possibility of freezing may be also inferred from Fig. 1 (C). Thermal (climatic) classification of the lake had been described by Yoshimura, but the author intends to introduce “equatorial lake” in the classification. This classification is expressed in Table 2.
Annual range of equilibrium temperature indicates a limit of the range of surface water temperature of lakes. The range of equilibrium temperature increases in the high latitude (Fig. 2), but the possible annual range is reduced to values in Fig. 3 because the water temperature does not fall below the freezing point. The maximum annual range occurres at about 40°N. For the verification of this result, the range of surface water temperature of lakes is summarized in Fig. 4 as latitudinal mean for the wholee world, which shows good agreement with the preceding figure.
3. In deep and wide lakes in the temperate climate, the surface temperature (θs) does not coincide with equilibrium temperature as shown in Fig. 5. This difference is caused by vertical heat transfer into the deep layer during the warming period, and heat supply from the deep layer during the cooling period. Heat balance at the water surface is expressed by the following equation,
Rn+H+LE=λ(θ∞-θs) (Eq. 3)
where, H denotes sensible heat flux at water surface, LE is latent heat flux and λ is a proportional coefficient (cal/cm2•sec•°C). As the total amount of surface heat flux equals to the change of heat content in the water column, Eq. 5 is obtained.
