Abstract
Abstract: Al Kufrah Basin, which is a part of the huge trans-boundary Nubian sandstone Aquifer system (NSAS) shared by Libya, Egypt, Sudan and Chad, is a major water resource in North Africa. The NSAS consists of a number of aquifers laterally and /or vertically interconnected, extending over more than 2X106 km2. In recent years, the demand in some areas has resulted in withdrawals that substantially exceed replenishment of the aquifers. A 3-D groundwater flow model was constructed and calibrated to simulate the sub-regional groundwater flow during the pre- and post-development periods in response to stresses within Al Kufrah Region.The developed model utilized PMWIN® Software, a fully integrated modeling platform that uses the USGS Modular MODFLOW2000®. The entire simulated thickness of 300 m was modeled as three hydraulically connected horizontal layers; each layer consists of 181 rows and 181 columns with grid spacing of 500 m by 500 m to cover a total area of 8190 km2. General Head Boundaries (GHB) were assigned to the southern and eastern nodes to simulate the groundwater inflow and outflow. Evapotranspiration was assigned to the upper model layer, with a maximum rate of 2750 mm/yr and an extinction depth of 2 m below the ground surface. Groundwater flow parameters (principally hydraulic conductivities, boundary conditions and recharge) were set during the steady state calibration process. Calibration proceeded by varying these input parameters until the model results most closely matched field measurements. In addition, the simulated hydraulic heads were consistent within the model domain with the actual field measurements (before the year 1968). Five stress periods - transient state flow model, was constructed and calibrated using all available data from 1968 to 2010. Specific storage values range from 10-5 to 10-2 for the middle and the lower layers and specific yield range between 2x10-3 and 2x10-1 for the upper layer were initially assigned based on the previous hydrogeological studies of Al Kufrah Basin, these values were repeatedly adjusted for the three layers during the calibration of the first period until satisfactorily calibrated hydraulic heads were reached. A value of about 10% of the total applied water was assigned to layer 1 as a surface recharge. Analysis of the residual statistics and spatial distribution of residuals as well as the visual comparison between simulated and observed hydrographs and potentiometric surfaces were used to analyze the ability of the calibrated model to simulate aquifer conditions within acceptable error. The root of mean square errors range from 0.8 m to 2.34 m for the 33 observation wells. Calibrated model output includes a 43-years estimate of the water budget by the end of each stress period at the shallow and deep layers. The historical calibration of this transient state model indicated a very close matching between the calculated and the observed drawdown values that have exceeded 40 m during some periods. Keywords: Nubian Aquifer System, Al Kufrah region, Transient state flow model, Stress Periods.