The water budget of a coastal lagoon, Oikamanai Lagoon, Hokkaido, Japan, sporadically opening to the Pacific Ocean, is estimated by establishing a bathymetric map of high accuracy (0.3m depth interval), and by monitoring the meteorology, lagoon water level and river stage. The opening to the ocean is produced by incising the sand bar from the overflow and discharge of lagoon water at the lowest site of the sand bar. The overflow results from an increase of the lagoon water level basically by snowmelt or rainfall river runoffs. As a result, the drainage by the opening to the ocean caused the lagoon to decrease the water volume up to more than 96%. The estimate of the water budget at nearly constant water level under closed condition of the lagoon suggests that, as the net groundwater output from the lagoon, the confined groundwater outflow to the ocean across the sand bar prevails. The gravelly confined aquifer was inferred to be at the similar elevation with some thickness along the grave-sand bar about 2000 m long. Meanwhile, during the regrowth of the sand bar at the outlet after the opening, the gravelly sediment first deposited near the sea level.Hence, the spatial distribution of the gravelly aquifer along the sand bar suggests that the whole sand bar was simultaneously broken in the past and then was grown up again.
Oikamanai Lagoon, one of lagoons in the Tokachi coastal region of Hokkaido, Japan, is a few times per year opened to the ocean by water overflow across the sand bar, which is triggered by rainfall or snowmelt runoffs of inflowing rivers. The water budget calculation for the lagoon under closed condition indicates that the net groundwater output occurs as confined groundwater output, probably to the ocean through the sand bar. The net groundwater output is then balanced mainly by river inflow. For the heat budget of the lagoon, the heat flux by the net groundwater output was estimated as an unknown factor, which was balanced by the heat flux of river inflow and a heat storage change of the lagoon. The two net groundwater outputs from the water and heat budgets exhibit the linear relationship near to the one-to-one correspondence.Hence, two unknown factors, groundwater outflow, G out , and groundwater inflow, G in , were estimated separately. As values averaged over the budget period, G out and G in were estimated at 1.04 m 3 s -1 and 0.078 m 3 s -1 (7.5% of Q Gout ), respectively. Thus, the groundwater outflow to the ocean, occupying most of groundwater output, may control the water and material cycles and the water residence time, related to the ecosystem in or around the lagoon.
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