How much of a basin's drainage network should flood models capture?
As climate change is expected to increase the risks associated with extreme rainfall and flooding, managing floods requires approaches beyond conventional infrastructure such as dams and reservoirs.
Recognizing this, Japan's Ministry of Land, Infrastructure, Transport and Tourism introduced the "River Basin Disaster Resilience and Sustainability by All" (RBDRSA) project, which takes a comprehensive approach to flood management across the entire river basin.
Implementing this basin-wide approach requires accurate modeling of water movement through the watershed. Small drainage channels can carry rainwater toward larger rivers, yet their role in mountainous and forested areas remains poorly understood. In these regions, drainage networks can extend far upstream, making it important to determine how far upstream the network needs to be represented for reliable predictions.
Accurately representing these networks could improve flood simulations and contribute to more reliable flood forecasts, providing useful information to support evacuation decisions. However, the present study did not directly evaluate early warning systems or evacuation decisions.
Testing upstream drainage networks
To answer this question, researchers led by professor Yasuo Nihei from the Department of Civil Engineering at Tokyo University of Science (TUS), Japan, used data from a real flood in the Ichinomiya River Basin, where drainage channels extend into mountainous areas. The team included Ryutaro Otsuka, a TUS alumnus who completed his master's degree in 2025, and Kengo Ohno, a second-year master's student.
They simulated rainfall runoff, river flow and flooding under different drainage channel configurations and assessed how these configurations affected water levels, discharge and the basin's overall water balance.
This paper was published in Journal of Hydrology: Regional Studies.
"In typical large-scale flood analyses, rivers are primarily modeled explicitly, while the influence of smaller channels and drainage pathways upstream is often represented indirectly through runoff parameters. However, since channels play a crucial role in draining rainwater into rivers, we believed that directly reflecting them in the model would allow us to reproduce water movement within the basin more closely to real-world conditions while reducing reliance on parameter adjustments," says Nihei.
The Ichinomiya River Basin is a 203 km² (78 square miles) basin in Chiba Prefecture, Japan, that has experienced typhoon-related flooding. Following severe flooding caused by Typhoon Bualoi in 2019, flood-management measures were developed for the basin under the RBDRSA project. In 2023, the basin experienced another flood caused by Typhoon Yun-yeung while these measures were still being implemented.
The researchers modeled the 2023 flood to investigate how far upstream drainage channels need to be represented in a flood model. They divided the watershed into various cells representing the land surface, small drainage channels and larger rivers to simulate how rainwater moved through the basin.
They defined the drainage channels using a minimum upstream drainage area (Amin), a threshold that determines which parts of the watershed are represented as drainage channels in the model. They ran the model with seven different drainage network configurations, using Amin values ranging from 1.00 to 0.025 km², with lower values progressively adding smaller drainage channels to the model.
Small channels shift flood timing
The drainage network had a strong effect on water movement in the basin. Including more small channels (lower Amin) provided additional pathways for rainwater to reach rivers, reducing water retained on slopes and inundation depth in flooded areas. It also increased water volume in the drainage channels and rivers and caused river water levels and discharge to rise earlier during the initial stage of the flood.
These results highlight the importance of drainage channel configurations in determining not only the magnitude of simulated flooding but also the timing of river water-level rises.
A basin-specific threshold needs wider testing
Of the seven configurations, an Amin of 0.1 km² was sufficient to reproduce the main drainage channels, while values between 0.1 and 0.2 km² provided good accuracy for reproducing river water levels. This threshold range was identified for the Ichinomiya River Basin and the September 2023 flood event; appropriate values may differ depending on watershed topography, drainage channel characteristics and modeling conditions. Including even smaller channels did not always improve the model and could reduce its accuracy by representing channels that do not actually exist.
According to the researchers, the threshold upstream drainage area may be a more appropriate measure than conventional drainage density for determining how far upstream channels should be represented, particularly when the topographical characteristics of the watershed are important. However, the appropriate value depends on the characteristics of the watershed, and further validation across different river basins and flood events is necessary.
"The Amin method for designing waterway networks presented in this study can be used to improve models that comprehensively evaluate rainfall runoff, river flow and flooding across an entire watershed," says Nihei.
These findings could help the RBDRSA project improve the evaluation of flood-management measures. More accurate representation of drainage networks could contribute to more reliable flood risk assessments and support the quantitative evaluation of basin-wide flood-management measures.
Further validation in other watersheds and under different flood conditions could support more quantitative assessments of their effectiveness under future climate change. However, this study did not directly simulate future climate change scenarios.