It is important to elucidate the quantitative features of hydrologic cycle in carrying forward the project of water utilization. In the present paper, the author discussed the residence time of lake water taking notice of the variation in concentration of dissolved constituents in a brackish lake.
Lake Suigetsu, having a surface area of 4.0 km
2 and a maximum depth of 34m, is affected by two different kinds of water; salt water supplied from Wakasa Bay and fresh water from River Hasu (Fig. 1). The results of the periodical investigation show that the thermal and chemical stratifications are well maintained throughout the year at Lake Suigetsu (Fig. 2, Fig. 3). There is a peak of the concentration of dissolved constituents be-tween August and September, and the minimum lies between February and March. There-fore, it is considered that effect of sea water appears sensitively on the physicochemical properties of lake water (Fig. 4). On the other hand, it seems that the quality of lake water is little influenced by that of River Hasu (Fig. 5, Fig. 6).
Judging from the above mentioned characteristics of Lake Suigetsu, chloride was chosen as an index in order to calculate the residence time of water. The data of seasonal variation of total chloride content in lake water obtained every fifth meter stratum reveal that the hypolimnion of the lake is only weakly affected by sea water (Fig. 7, Fig. 8). The discharge and the chloride flux determined at Urami Canal, which connects Lakes Suigetsu and Kugushi, fluctuate with time corresponding to the change of sea level (Fig. 9). Therefore, a strongest correlation is recognized between the chloride concentraion at Urami Canal and the mean sea level at Wakasa Bay (Fig. 10).
Dividing the amount of changes of total chloride content in the lake by the chloride concentration at Urami Canal, the amount of displaced water was calculated for every month. At Lake Suigetsu, it may be justifiable to consider that the sea water flows into an intermediate stratum of the lake in the form of a density current (Fig. 11). As is evident from Table 1, the residence time, the turnover time in the strict sense, of lake water at hypolimnion is 2-7 years.
The point which the author especially emphasizes is the applicability of this method. It appears to be the strongest proof that the value obtained by tritium analysis is almostt the same as the result calculated here. Within the limits of this investigation, these results suggest that it is necessary to reconsider the meromixis at Lake Suigetsu.
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