Submarine Groundwater Discharge Estimation from Kikuchi River Basin to the Ariake Bay, Japan

Abstract: Submarine groundwater discharge (SGD) affects environmental conditions in semi-enclosed sea areas. Semi-enclosed areas often experience eutrophication caused by excess nutrient transport from agricultural areas. However, it is often unclear how much the nutrients that are transported through the groundwater discharge. Therefore, it is necessary to quantify groundwater discharge to the sea. For this quantification, a methodology to estimate SGD is presented. A distributed groundwater recharge model (GRM) and a groundwater flow model (GFM) were developed and coupled to estimate SGD from Kikuchi River basin into the Ariake Bay, Kumamoto, Kyushu, Japan. The river basin located in Kumamoto prefecture was studied as a representative area for the entire Ariake Bay catchment. The GRM separates rainfall into direct runoff, evapotranspiration, and groundwater recharge. Parameters are set based on land use. Groundwater level was calculated by the GFM to estimate subsurface water runoff, base flow to the river, and evaporation from the shallow groundwater table. The SGD was calculated using a water balance approach. The result of calculation shows that SGD accounts for 47 mm year-1, corresponding to 3.6 % of the river discharge and 2.3% of the average annual rainfall. The results are in the same range as earlier reported in the research literature and thus appear reasonable.