Hydrological analysis still a very dominant part of the analysis and require careful handling and calculation of top debit is a very important element in flood control. Rainfall, catchment area, as well as geographic information such as slope and land cover is a parameter for estimating debit by using rational method. The purpose of this study is Estimating thedebit flow based on the rainfall data in the Poleang-RorayaRiver Region using open-source software-based Geographic Information System (GIS). In this study calculated rainfall plans to return period of 2 years, 5 years, 10 years, and 25 years with the method of Log Pearson Type III distribution. By using the ArcGIS softwaredebit flow obtained was 377.05 m3 / sec on a return period of 2 years; 448.17 m3 / sec on a return period of 5 years; 482.00 m3 / sec on a return period of 10 years; and 512.49 m3 / sec on a return period of 25 years..