Water and heat budgets in a coastal lagoon controlled by groundwater outflow to the ocean

Oikamanai Lagoon in the Tokachi coastal region of Hokkaido, Japan, opens to the ocean several times a year due to water overflow across the sand bar triggered by rainfall or snowmelt runoffs of inflowing rivers. The water-budget calculation for the lagoon under closed conditions indicates that the net groundwater output occurs as confined groundwater, probably flowing into the ocean through the sand bar. The net groundwater output is then balanced mainly by river inflow. For the lagoon’s heat budget, heat flux caused by net groundwater output was an unknown factor balanced by heat flux of river inflow and a change in the lagoon’s heat storage. The two net groundwater outputs from water and heat budgets exhibit the linear relationship close to the one-to-one correspondence. Hence, two unknown factors—groundwater outflow (Gout) and groundwater inflow (Gin)—were estimated separately. Gout and Gin values averaged over the budget period were estimated to be 1.04 m3 s−1 and 0.078 m3 s−1 (7.5 % of Gout), respectively. Thus, the groundwater outflow to the ocean, representing most of the groundwater output, may control water residence time and water and material cycles related to the ecosystem in or around the lagoon.