Groundwater is the major source for water supply in the Samail lower catchment (Al Khawd fan) in the Sultanate of Oman. The main aquifer system appears in the alluvial deposits, which contains three units of gravel. In fact it is hardly to differentiate among them; the average saturated thickness ranges between 55 m and 140 m where it reaches to 300 m in the central part. A groundwater flow model was calibrated for the alluvial aquifer system under steady-state conditions and transient-conditions. The objective was to determine estimates and spatial distribution of the hydraulic conductivities of the aquifer system, the distribution of recharge from precipitation, wadi flow, in addition to through flow from the upper catchment in the south and ultimately estimate the groundwater balance of the system in the fan. Data on the stable isotopes of oxygen and hydrogen from the study area suggest that groundwater recharge in the area is from recent water. The calibrated steady-state model shows that aquifer hydraulic conductivities in the study area range from 0.01 to 35 m/day. The calibrated recharge was divided into three components, which represents 12%, 24% and 30% of the precipitation in the form of rainfall, wadi flow and dam reserve respectively. The water level elevations show a consecutive decline and rise indicating that groundwater level correlate well with the rainfall pattern in the area. Advectiondispersion solute transport process was simulated over the area. Zone concept was used to provide TDS values as the measurement of salinity. The simulation results show an increasing of TDS close to the coast reaches to 12,000 ppm. The TDS hydrographs exhibit a low TDS range downstream of the dam reflecting the effect of recharge from the dam reserve. Evaluation of predictive scenarios was applied until the year 2020. It shows a slow release of storage resulting in a slow decline of groundwater level. It also shows aninsignificant amount of salt water intrusion to the aquifer. Keywords Al Khawd fan – Groundwater – MODFLOW – Calibration – Transport

Groundwater is the major source for water supply in the Samail lower catchment (Al Khawd fan) in the Sultanate of Oman. The main aquifer system appears in the alluvial deposits, which contains three units of gravel. In fact it is hardly to differentiate among them; the average saturated thickness...