LONGITUDINAL EVALUATION OF ALTERED FLOW REGIMES IN THE OMARU RIVER NETWORK USING A DISTRIBUTED HYDROLOGICAL MODEL

Abstract

 To date few studies have tackled flow alterations by withdrawal for hydropower generation and flood control at a river network scale. Here we applied a distributed hydrological model to the Omaru River catchment in southwest Japan to assess longitudinal patterns of altered flow regimes by dams and weirs, and mitigation by inflow of discharges used for hydropower generations and tributaries. Runoff analyses were performed either for the cases where the boundary conditions of dams and weirs (e.g., dam/weir outflow) was included or not in the model. Subsequently, the extents of the alternations were quantified by the ratio of the flow regime metrics between them. Runoff analyses were found to be highly predictive throughout the three years (NS = 0.909-0.962). Although the discharge used for hydropower generation well mitigated the alternaions under higher flow condition; 25 percentile of annual flow (Q25), the mitigation effects were insufficient under median (Q50) to 97 percentile low flow (Q97) conditions. The mitigation effects by tributaries tended to increase in proportion to the increase in inflow, specifically in case of the flow regime metrics with lower flow rate. This suggests that the mitigation effect is more dependent on discharge than on spatial location of the inflow.