(1) In the previous report on the thermal structure of the Tagokura Reservoir (1963), the author reported on the distribution of temperature and velocity of water in it. In the present report, he wishes to describe about seasonal variations of them and discuss the mechanism which governes the formation of water temperature, from the view point of heat budget of the reservoir. Observations of temperature and current of water in the reservoir had been carried bi-monthly from spring of 1962, and it ended on November of 1963. Water temperature was measured by thermistor thermometer at each one meter from the surface at several sounding positions in the reservoir. Results of observations of water level, discharge and climate that had been made by Tagokura Power Station and Tadami Hydro-meteorological Station were collected and arranged for the calculation of heat budget. At the same time, river water temperature at Otori (inflow site of the reservoir) was measured in summer months, as the basis of estimation of inflowing water temperature.
(2) General features of temperature distribution in the reservoir are illustrated by Fig. 2. In spring and early summer, thermal stratification in the reservoir is not strong. Because of this weak thermal stratification, strong heating by forced convection intrudes into the deep layer. In mid-summer, the existence of two layers of thermoclines becomes evident. As the inflow water-mass sufferes thermal modification by turbulent exchange, its temperature increases as it flows towards the downstream. Combining these characteristics, estimation of the movement of water is tried. Seasonal change of trajectories of general circulation in the reservoir is shown in Fig. 6.
(3) Flow and general circulation in the reservoir were measured in summer observation periods. Velocity of the inflowing water-mass was measured by specially designed current meters. Temperature of inflow water was cold in 1962 and 1963 comparing with the preceding years, because the Okutadami Reservoir drainaged cold water. General circulation in the reservoir was measured by current-cross. Two kinds of the crosses were tested in this survey. One of which was made of alminium sheet of 0.8mm thickness, having pericyles of 40×25cm. The other was made of plastic (Takiron) and the dimension of it was 25×50cm. The cross was connected with the wood float by a nylon thread, and the position of the float was observed from land or the cruising boat. The flow in the reservoir consist small clrculatlons in each small lake basin.
(4) The equation of heat balance is expressed as follows.
Rn+
H+
LE+
P*+_??_=ΣQ
Rn denotes net-radiation on water surface,
H is sensible heat transfer,
LE is latent heat transfer and
P* is heat supply by precipitation. Right side of the equation represents the total change of heat storage in the reservoir. Each term is calculated basing on ten day's mean value of climatic and hydrological data. Each term of the above equation is obtatined by the following ways.
_??_
γ: Bowen ratio
ΣQ: change of total heat storage
_??_: advection
First step of the calculation is to arrange the water balance in the reservoir, which is given by Eq. (4). In this equation, subscript “I” means inflow water, “OT” is Otori, “OUT” is drainage from Tagokura Power Station and “T” means discharge of branches flowing into the reservoir. Symbol “PS” denotes total precipitation on the reservoir surface. Change of total heat storage is calculated by Eq. (9). In the application of this method, the reservoir surface is devided into four sections. Separation of sensible heat flux (
H) and latent heat flux (
LE) is made by use of Bowen ratio.
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