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<p>The Cagayan River Basin (CRB), the largest river basin in the Philippines, is highly exposed to extreme rainfall and recurrent flooding. Despite this, basin-scale quantitative tools that can support both reservoir inflow forecasting and downstream flood assessment remain limited. This study assessed the use of the Rainfall-Runoff-Inundation (RRI) model to evaluate Magat Dam inflow, downstream water level, and flood inundation during major rainfall events in the CRB. The model was calibrated and tested using observed dam inflow and river water-level data from extreme events. It reproduced key hydrologic responses, particularly peak discharge and peak timing. During Typhoon Ulysses (Vamco), the model showed satisfactory performance for dam inflow (RSR 0.36, NSE 0.87, PBIAS 6.90, R² 0.88) and river water level (RSR 0.50, NSE 0.75, PBIAS − 0.39, R² 0.75). Validation during Typhoon Tisoy (Kammuri) and the December 2020 northeast monsoon further showed that the model reproduced the broad spatial pattern of observed flood depths in riverine communities, with inundation estimates ranging from 0 to 6&#xa0;m. During Typhoon Ulysses, subwatershed analysis showed that Cagayan Segment 1 contributed the largest simulated peak discharge to Buntun Bridge, while downstream flooding reflected cumulative inflows from multiple upstream subwatersheds rather than Magat inflow alone. Model performance was stronger for high-flow events than for low-flow conditions, suggesting that the approach is more suitable for extreme-event forecasting than for baseflow representation. These results indicate that the calibrated RRI model can support flood early warning and dam operation in the CRB.</p>

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Evaluating a Rainfall-Runoff-Inundation Model for Dam Inflow and Flood Inundation Forecasting in Extreme Rainfall Events

  • 期刊论文

  • 发表日期：

    2026年5月16日

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  • Arlen S. Alejandro,
  • Lanie A. Alejo,
  • Orlando F. Balderama,
  • Jeoffrey Lloyd R. Bareng,
  • Sameh A. Kantoush

摘要

The Cagayan River Basin (CRB), the largest river basin in the Philippines, is highly exposed to extreme rainfall and recurrent flooding. Despite this, basin-scale quantitative tools that can support both reservoir inflow forecasting and downstream flood assessment remain limited. This study assessed the use of the Rainfall-Runoff-Inundation (RRI) model to evaluate Magat Dam inflow, downstream water level, and flood inundation during major rainfall events in the CRB. The model was calibrated and tested using observed dam inflow and river water-level data from extreme events. It reproduced key hydrologic responses, particularly peak discharge and peak timing. During Typhoon Ulysses (Vamco), the model showed satisfactory performance for dam inflow (RSR 0.36, NSE 0.87, PBIAS 6.90, R² 0.88) and river water level (RSR 0.50, NSE 0.75, PBIAS − 0.39, R² 0.75). Validation during Typhoon Tisoy (Kammuri) and the December 2020 northeast monsoon further showed that the model reproduced the broad spatial pattern of observed flood depths in riverine communities, with inundation estimates ranging from 0 to 6 m. During Typhoon Ulysses, subwatershed analysis showed that Cagayan Segment 1 contributed the largest simulated peak discharge to Buntun Bridge, while downstream flooding reflected cumulative inflows from multiple upstream subwatersheds rather than Magat inflow alone. Model performance was stronger for high-flow events than for low-flow conditions, suggesting that the approach is more suitable for extreme-event forecasting than for baseflow representation. These results indicate that the calibrated RRI model can support flood early warning and dam operation in the CRB.