Streamflow simulation using the event-based SCS-MS/LR model in the Hitiaa catchment, French Polynesia

Abstract

The volcanic tropical Island of Tahiti is prone to heavy rainfalls and flash floods which regularly generate severe damages. Hydrological response on the island is highly variable from one catchment to the other, in terms of runoff production as well as peak flow amplitude. Infrastructures designers there require an operational method able to calculate the design flood metrics on any Tahitian catchment, taking into account the variability of their response. This study applies a parsimonious distributed event-based conceptual model to the observed rainfall-runoff events of 9 small catchments (<6 km2). A modified version of the Mishra and Singh Soil Conservation Service (SCS-MS) runoff model, associated with the Lag-and-Route (LR) model was applied to series of 9 to 176 rainfall-runoff events for each catchment that occurred between 2004 and 2021. Two dominant parameters are to be considered for the SCS-MS model: the size of the soil reservoir (Si) and the initial water content (M) in this soil reservoir. SCS-MS was found to perform better than SCS-CN and to fit better the asymptotic behaviour of the rainfall-runoff relationship. Si could be fixed to 2500 mm for all catchments except Nymphea where Si had to be lowered to 80 mm, because of the high density of urbanization over this catchment. M parameter was set variable from one event to the other as the initial condition of the model. The Nash criterion was calculated for each event and their median value ranged from 0.39 to 0.79 across the catchments, proving the flexibility of the model. A clear relationship could be fitted between the median M and Antecedent Precipitation Index (API), both calculated for each catchment. Further testing of this model on similar tropical volcanic context should be undertaken to verify its flexibility and to consolidate the calibration of its parameters.