The Gilan aquifer is located in the southwest of Kermanshah, west of Iran. This aqifer is discharged by the Gilan spring with an average discharge of 938 l/s. The catchment area of the Gilan spring was determined using hydrogeological approaches. Electrical conductivity (EC), water temperature, PH and major ions were measured in two weekly intervals from Sep. 2000 to Sep. 2002. EC, water temperature and discharge (stage) were performed two daily in wet season. Calcite, dolomite and gypsum saturation indexes and partial pressure of CO2 were calculated using WATEQF hydrochemical model. Time series variations of physicochemical parameters were depicted and evaluated. The hydrochemistry data, base flow separation method and recession coefficient imply that the type of flow is mainly diffuse in the Gilan aquifer. The EC time Seri of the Gilan spring is in contradiction with the conventional method. It is denoted that a high percentage of the spring water is supplied by conduit flow. The peak discharge occurred when the parts near the spring provide the Gilan spring discharge when the conduit recharged water from the remote area reach the spring during recession period. The contribution of conduit flow during the dry period reduces the variations of physico-chemical parameters and presents a low value of recession coefficient. This kind of behavior occurs in the aquifer with long and narrow catchment area, rain duration is significantly less than the lag time period and the EC decreases during the dry period.
