Adapting an Event-based, Distributed and Conceptual Model to the Hydrological Response of Small Catchments in the Island of Tahiti

The tropical volcanic Island of Tahiti is prone to heavy rainfalls and flash floods which can generate very severe damages. Hydrological response here appears highly variable from one catchment to the other, in terms of runoff production as well as peak flow amplitude. There is a strong need for an operational method able to estimate well the design flood metrics on any Tahitian catchment, taking into account the variability of their response. This study deals with five small catchments (surface range between 0.6 and 1.4 km²). It aims at adjusting the equations of the production function of the Soil Conservation Service – Lag and Route (SCS-LR) model in order to improve its flexibility until it is able to reproduce the hydrological responses. In practice for each catchment, a few dozens to a hundred of rainfall-runoff events were extracted from available records and then event cumulated rainfall and runoff were calculated and plotted to characterize the asymptotic behavior. The theoretical equation of the SCS production function was successively adjusted for the five catchments. Original SCS equations well fitted rainfall-runoff events for one particular catchment (linear determination coefficient R² of 0.85) while weak fits were obtained for the other catchments. We applied modified equations of SCS inspired by Mishra and Singh (with introduction of parameter M in SCS equations) and better regressions with R² ranging between 0.52 and 0.87 were estimated. However when using this modified model, compensation of cumulated rainfall by the optimization of the initial condition parameter S decreased values of correlation coefficient. Further exploitation of asymptotic behaviors must be undertaken to modify and render SCS equations more adapted to Tahitian hydrological variability.