Integrated Water Resources Management (Proceedings of a symposium held at Davis. California. April 2000). IAHS Publ. no. 272. 2001. 221 Application of a physically-based erosion model for a large river basin in Japan CELSO A. G. SANTOS Department of Civil Engineering, Potiguar University, Rna Hernani Hugo Gomes 90, Natal-RN, 59082-270 Brazil e-mail: celso.santos@unp.com.br MASAHIRO WATANABE & KOICHI SUZUKI Department of Civil and Environmental Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan Abstract Physically-based models have proved to be vety useful for modelling the runoff-erosion process for small basins. However, the application of such models to large river basins could be compromised, especially when they do not consider groundwater contributions. Thus, a distributed physically-based runoff-erosion model was developed and applied in order to simulate the runoff and erosion for the Ishite River basin in Japan. The model also computes the runoff discharge of the river, and sediment yield during and between rainfall events, so that it considers the groundwater. Since the tested model is physically-based, it can in principle overcome many of the deficiencies of empirical models, therefore making the model a promising tool for further simulations in larger river basins where groundwater should be considered. Key words groundwater flow; Horton equation; infiltration; Japan; kinematic model; large basin; physically-based model; runoff-erosion model INTRODUCTION Many analytical models, of various complexity, that evaluate sediment yield have been developed over recent years to obtain a useful tool for estimating soil erosion, and this soil erosion modelling has moved from empirically-based and simple mathematical models towards physically-based and mathematically much more complicated models. Basically, there are three categories of soil erosion model: empirical, conceptual, and physically-based models. The empirical models are based on data from field observ- ations, and mostly standard runoff plots on uniform slopes. The main limitation of this type of model is their limited applicability outside the range of conditions for which they have been developed. Their adaptation to a new environment requires a major investment of resources and time to develop the database required to drive the model. The physical basis of the physically-based models can, in principle, overcome many of the deficiencies of empirical models. Therefore, a distributed physically-based runoff- erosion model was developed in order to simulate the runoff and erosion for a large river basin due to continuous rainfall events; i.e. it computes the runoff discharge of the river and also sediment yield between rainfalls, by taking into account the groundwater contribution. The following sections describe the basic equations of the model developed, and present the simulation results for the rainy season of 1997 in the Ishite River basin in Japan.