Estimation of spatial variation in submarine groundwater discharges from a coastal grid 50 m square, using a topographic model: An example of the application to the middle of the Seto Inland Sea

To estimate the spatial variation in submarine groundwater discharge from a 50 m square coastal grid over a distance of more than 100 km, we applied a topographic model to the coastal area of the middle of the Seto Inland Sea. Topographic gradients between the coastal grid and the points were automatically calculated using GIS and an altitude dataset with 50 m square grid. Based on those calculations, the hydraulic gradients and cross-sectional areas were estimated, with submarine groundwater discharges being calculated by a Darcy equation. Our results were summarized as follows; 1) the coastal areas of Kagawa Prefecture on the southern part of the Mid Seto Inland Sea has lower slopes than those in Okayama Prefecture. In particular, the areas with flat surfaces such as reclaimed lands with a slope of less than 0.002 were larger in Kagawa Prefecture than Okayama Prefecture. 2) In the estimated annual SGDs exceeded 0.2 x 10⁵ m³ year-1 at most of that were the grids in Okayama Prefecture. On the other hand, the SGDs on reclaimed lands that were distributed partly along the coastal grid of Kagawa Prefecture were estimated to be almost 0. 3) Cumulative annual SGDs estimated by this method were 78.4 mm in Okayama Prefecture and 50.5 mm in Kagawa Prefecture. These values accounted for around 5% of the total precipitation in both prefectures. The SGD estimated by the water balance model approximately corresponds to percentage.