The paper includes in the first stage an estimate of the runoff volume and sediment load, resulting from rainfall on a watershed of Alkhoser seasonal river, north-eastern Mosul city/Republic of Iraq, which has an area of up to 725 km2. There is a proposal for construction of a rainwater harvesting dam for the supplementary irrigation in the region which is exploited by planting wheat and barley crop widely. Digital elevation model (DEM) was adopted in order to describe the topography of the region, which is one of the main factors affecting the water flow direction and velocity. It also adopted the daily data of rainfall, maximum and minimum temperature, for the period 1989 to 2007 for Mosul to estimate the volume of runoff and sediments resulting from each effective rainstorm. The applied model includes the Geographical Information System (GIS).