A modified geographic information system (GIS) model incorporating both the riparian function model (RFM) and the groundwater routing model (GRM) was used to study the effectiveness of riparian ecosystems to denitrify nitrogen (N) in groundwater. Literature values were used to develop the RFM and to determine relevant parameters, and the GRM was developed by analyzing regional geographical features to distinguish groundwater paths. The modified GIS model was evaluated by simulating N concentrations in nine rivers around Lake Kasumigaura, Japan, where groundwater is contaminated by N originating from agricultural activities and livestock. The N load distribution data for the model were precisely estimated for an agricultural unit to simulate river N concentrations during the non-irrigation season (October-March). The following results were obtained: (1) The strength of N reduction in the riparian area differed between locations depending on the toposequence and soil type. (2) Groundwater seeped from the sides of deeper valleys around river mouths where the valleys had eroded below the impermeable layer 3.0 m below ground level, but no seepage was observed in shallower valleys in the middle and upper parts of streams. (3) With reasonable parameters for the RFM and GRM, the modified GIS model adequately reproduced N concentrations in rivers (root-mean-square error = 0.85 mg-NL^<-1>). We conclude that the modified GIS model is suitable for simulating N concentrations in rivers in the region around Lake Kasumigaura.

View full abstract