This research aims to determine the relationship between precipitation and runoff in the basin of river Acharashir and study the various factors that affect them by building a mathematical model of the basin using GIS and WMS programs. The importance of such models is in the ease of changing the data without destroying the entire model. We can design a model to simulate different scenarios of possible changes in land use change or soil type in the area of the basin or even climatic changes taking place in our region recently. Such changes affect the size of the flows caused by rainstorms and therefore some of the engineering structures maybe go out of the service because they have been designed on smaller flows. This research is important for the future design of any engineering facility in the area of the basin, and in the optimal management of water surface and groundwater.