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Sultan Qaboos University (2019)

Estimation of Groundwater Renewal Rate in Al-Khoud Area Using MODFLOW Numerical Mode

Al-Mundhar Al-Nassri

Titre : Estimation of Groundwater Renewal Rate in Al-Khoud Area Using MODFLOW Numerical Mode

Auteur : Al-Mundhar Al-Nassri

Université de soutenance : Sultan Qaboos University

Grade : Master of Science (MS) 2019

Résumé
Groundwater resources are widely used to meet the water demand in Oman. The aim of this study is to understand the effect of pumping on different aquifer formations in the Al-Khoud lower catchment area, Muscat, Sultanate of Oman. Continuous water level observations and the available records from MRMWR were utilized in the study. Pumping rates in 60 pumping wells were collected from PAW. Steady-state and transient groundwater-flow models were developed using MODFLOW software considering three aquifer layers ; upper gravel, clayey gravel and cemented gravel. Residual errors of simulation in 11 observation wells during calibration in July 2016 were below 1 m with RMSE of 0.50 m. Sensitivity analysis indicates water-levels are more sensitive to the groundwater inflow, water abstraction and horizontal hydraulic conductivity. Direct groundwater recharge effect was found to be the least significant due to urbanization associated with imperviousness in the study area. Vertical hydraulic conductivity is less sensitive compared to horizontal hydraulic conductivity. Water level observations and simulated results reveal 0.6 m to 1.5 m head differences between three layers at two well locations. Model analysis found that groundwater pumping, hydraulic conductivity and vertical anisotropy were the governing parameters affecting the hydraulic head differences. Transient model was calibrated with daily pumping rates disaggregated from monthly dataset. Specific yield of the layers was found to be 0.15 at the upper gravel, 0.08 at clayey gravel and 0.05 at cemented gravel. Model shows that the smaller specific yield and higher pumping rate collectively contribute for a significant water-levels fluctuation. Sensitivity analysis indicates that the screen depths of pumping wells and discharge rates affects the water levels differences and ultimately changes the daily water levels. Effect of groundwater pumping in PAW well zone near to the Al-Khoud dam dissipate towards the coast. In contrast, local pumping from private wells found to have a significant effect on groundwater level variations in the downstream area. Projected water demand by PAW in 2020 and 2024 may have further effect on aquifer. Water level in some locations may deplete more than 1 m compared to 2016 estimations. Similarly, increase in water abstraction from irrigation wells will affect groundwater levels. Water level depletion was estimated 0.4 m over 111 days stress period in response to 10 % increase in pumping rate at a well in the downstream. Continuous observations in the downstream area indicate that the groundwater level is falling below the mean sea level. Increased pumping in this area may further enhanced the seawater intrusion into the aquifer. Therefore, it is recommended to have a comprehensive inventory of groundwater pumping wells in Al-Khoud lower catchment including the public and PAW well field zone. The inventory would assist sustainable long-term groundwater resource management in the study area.

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Page publiée le 4 mai 2021