Using SWAT-LUD Model to Estimate the Influence of Water Exchange and Shallow Aquifer Denitrification on Water and Nitrate Flux

Numerous studies have pointed out the importance of groundwater and surface water interaction (SW-GW) in a river system. However; those functions have rarely been considered in large scale hydrological models. The SWAT-LUD model has been developed based on the Soil and Water Assessment Tool (SWAT) model; and it integrates a new type of subbasin; which is called subbasin-LU (SL); to represent the floodplain area. New modules representing SW-GW exchanges and shallow aquifer denitrification are developed in the SWAT-LUD model. In this study; the SWAT-LUD model was applied to the middle floodplain area of the Garonne catchment in France. The results showed that the SWAT-LUD model could represent the SW-GW exchange and shallow aquifer denitrification appropriately. An annual 44.1 x 10(7) m(3) of water flowed into the river from the study area; but the annual exchanged water volume was 6.4 x 10(7) m(3); which represented just 1% of the river discharge. A total of 384 tons of N-NO3- (0.023 t-ha(-1)) was consumed by denitrification in the floodplain shallow aquifer annually. The nitrate concentration (N-NO3-) decrease in the channel was 0.12 mg.L-1; but in the shallow aquifer it reached 11.40 mg-L-1; 8.05 mg.L-1; and 5.41 mg.L-1 in LU1; LU2; and LU3; respectively. Our study reveals that; in the Garonne floodplain; denitrification plays a significant role in the attenuation of nitrate associated with groundwater; but the impacts of denitrification on nitrate associated with river water is much less significant.