Abstarct: Tar Lake is a mountainous lake which is located at 14 km east of Damavand city. The lake has been created due to a landslide in north edge of the Zarinkuh. The area of this lake and its catchment is equal to 29/2 and 1091 hectares, respectively. The basin of this lake is mainly composed of formations Jirood, Mobarak, Shemshak, Delichay, Lar and Karaj Green Tuffs. To determine the annual recharge percentage in Tar basin, the effective parameters including precipitation (value, type and concentration), lithology, percentage of north and north-east hillsides, important karst features, depressions and epikarst were considered. By providing and overlapping the information laxyers of mentioned parameters the annual recharge of Tar Lake was assessed about 42%. The surface runoff of the basin is mainly due to snow melting and annually it is equal to 2.826 Mm3. The mean annual rainfall in the Tar basin calculated equal to 848 mm using Elevation – precipitation graph. In the lake basin, there are no surface inflow and outflow, no subsurface inflow, no pumping and no change in storage. Therefore, the annual subsurface outflow of this basin is estimated to be 48.6 Mm3. For Tar Lake in a water year, the surface and subsurface inputs are respectively 826.2 and 135.0 Mm3, direct rainfall is 20.07 Mm3 and evaporation is 339.0 Mm3. Since there is no surface outflow, and the change in lake storage is negligible, the annual volume of subsurface outflow from this lake is estimated to be about 2.829 Mm3. The water of Tar Lake contains very little dissolved solids such that its electrical conductivity is about 160 mhos/cm. This is because the rainfall in the lake basin is relatively high and the length of the basin is short. The predominant water type and facies in this basin is calcium bicarbonate.