Catchment Runoff and Estuarine Water Exchange Model for the Richmond River Northern NSW, Australia

Quantification of catchment runoff using a direct approach is difficult when the outlet is subject to tidal influence. Further, if the catchment is ungauged, runoff can only be determined by correlation between the gauged and ungauged portions of the catchment The Richmond River catchment (6.900 km2) in northern NSW has three subcatchments, the Richmond and Wilsons Rivers and Bungawalbim Creek. These subcatchments are only partly gauged (37%) and their outlets are controlled by tidal influence except during moderate to high flows. Runoff for the 1994-95 hydrological years for the whole catchment has been calculated using a monthly runoff coefficient from the gauged part of the catchment and an estuarine water exchange model has been developed and verified to estimate the exchange of water through the estuary mouth. The Wilsons River has the smallest catchment area (1420 km2), but contributes the largest percentage (59.5%) of total yearly basin runoff (628x106 m3). The Richmond River (2688 km3) and Bungawalbii Creek (1686 km3) sub-catchments contribute 40.28% and 0.22% of the yearly runoff respectively. The estuarine water exchange model showed a very good agreement (r = 0.83) with the directly measured volume of water exchanged through the estuary mouth. The average yearly inflowing and outflowing water fluxes through the Richmond River estuary mouth at Ballina were estimated to be 15.04 and 16.01X106 m3 per tide during the study period.