A pseudo-reservoir concept in SWAT model for the simulation of an alluvial floodplain in a complex tropical river system

Study region: The new formulation was applied in a complex tropical river system of Luanginga sub-basin, western Zambia. Study focus: We propose a novel pseudo-reservoir concept (hereinafter PSRC) in the framework of the Soil and Water Assessment Tool (SWAT), to mimick and model the behavior of an alluvial floodplain. Outflows from PSRC are predicated on a conceptual storage-outflow-stage relationship, and decomposed into overflow and baseflow by considering floodplain hydrogeomorphic attributes and their dynamics in relation to critical bankful height. The model was tested for its skill to improve simulation of discharge. It`s performance was evaluated against SWAT targeted release reservoir and a configuration without a reservoir. New hydrological insights: We demonstrated that PSRC via its parameters was dynamically and significantly (p-value < 0.05) sensitive to discharge. It improved SWAT discharge predictions by four to over 100 percent compared to two model control set-ups. Results were influenced by sensitivities of parameters to flow regime and objective functions. The SWAT configured with PSRC (SWAT(psrc)) was the only model which accounted for uncertainties in the ninety-five percent prediction uncertainty band (95PPU) very well. We found that SWAT(psrc) simulated average values (i.e 95PPU) replicated discharge measured using state of the art acoustic doppler current profiler (ADCP) with a high degree of accuracy (Nash-Sutcliffe 0.86; R-2 0.96; PBIAS 10.7). All in all, results indicated that PSRC is a robust model. SWAT(psrc) together with its uncertainty bands has potential for improving understanding of water resources dynamics in typical environments as the present study area. The model can also potentially be useful for applied studies such as restoration designs and impact assessment of modified floodplains.