When modeling by IFAS it becomes necessary not only to obtain the model parameters but also to define the cell size, which influences both the tank model and the kinematic wave model. Since PWRI-DH model, the model in which is based IFAS, is a distributed model, the cell size defines the discretization of the computational domain. On the other hand, PWRI-DH model use altitude data such as GTOPO30 and Hydro1k, both with resolution of 1 km and IFAS is restricted to a minimum cell size of 100 m. Because of the restriction on cell sizes, an error on the predicted discharge is obtained, since this size of cell is not small enough to capture any details. As results, it is necessary to find a cell size able to predict discharge correctly, and more important, to quantify the error produce by the model and taking it into account in the analysis of the results. In this paper, an analysis of the influence of cell size on the IFAS predicted discharge is performed. The effect of cell size on the delimitation of the river as well on the definition of the land use, on the definition of the vegetation cover, on the definition of the topographic of the river basin is evaluated in detail. From the results of this study, the authors have been able to improve the accuracy of the PWRI-DH model and therefore to predict discharge using IFAS, more accurately. Finally, conclusions of this study are presented.

1. [1] D. W. Knight and A. Y. Shamseldin, “River Basin Modelling for Flood Risk Mitigation,” Taylor & Francis, pp. 1-17, 2005.
2. [2] G. Blöschl, M. Sivapalan, T. Wagener, A. Viglione, and H. Savenije, “Runoff Prediction in Ungauged Basins,” Cambridge University Press, pp. 11-52, 2013.
3. [3] A. Sugiura, S. Fujioka, S. Nabesaka, T. Sayama, Y. Iwami, K. Fukami, S. Tanaka, and K. Takeuchi, “Challenges on Modelling a Large River Basin with Scarce Data: A Case Study of the Indus Upper Catchment,” 20th Int. Congress on Modelling and Simulation, pp. 2346-2352, 2013.
4. [4] Y. Iwami, “AWCS Flood Session: Towards IFI-AP-ICHARM 10th Anniversary,” Int. Centre for Water Hazard and Risk Management under the Auspices of UNESCO (ICHARM), http://www.icharm.pwri.go.jp/publication/pdf/2016/4337.pdf, 2016. [accessed December 22, 2017]
5. [5] A. Aziz, “Rainfall-Runoff Modelling of the Trans-boundary Kabul River Basin using Integrated Flood Analysis System (IFAS),” Pakistan J. of Meteorology, Vol.10, No.20, pp. 75-81, 2014.
6. [6] M. Masoudian, “The topographical impact on effectiveness of flood protection measures,” Kassel University Press, pp. 15-45, 2009.
7. [7] B. Bhakta, T. Okazumi, M. Miyamoto, S. Nabesaka, S. Tanaka, and A. Sugiura, “Fundamental Analysis for Flood Risk Management in the Selected River Basins of Southeast Asia,” J. Disaster Res., Vol.9, No.5, pp. 858-869, 2014.
8. [8] B. Bhakta, H. Sawano, M. Ohara, and N. Nagumo, “Improvement in Flood Disaster Damage Assessment Using Highly Accurate IfSAR DEM,” J. Disaster Res., Vol.11, No.6, pp. 1137-1149, 2016.
9. [9] K. Hashimoto and M. Hasegawa, “Runoff Model to Evaluate the Land Uses Changes,” Civil Engineering Technic Magazine, p. 13, 1977 (in Japanese).
10. [10] Int. Centre for Water Hazard and Risk Management under the Auspices of UNESCO (ICHARM), “IFAS ver.2.0 technical manual,” 2017.