This paper introduces the Spatiai Anaiysis of surface Runoff in ArcGis (SARA vi.O) interface to estimate curve number and runoff voiume for hydroiogic evaiuations. This version runs in a vector platform as with other extensions of ArcGIS, introducing changes to the originai Naturai Resources Conservation Services-Curve Number method, an(j ailowing adaptation to iocal conditions. The programming syntax was deveioped in Visuai Basic 6.0 as an ActiveX Dynamic Lini< Library (DLL) to interact with ArcMap, and it is reieased as Free and Open Source Software (FOSS) that can be modified or upgraded to other programming ianguages.