Assessment of groundwater recharge from the dam of Wadi Al-Jizzi, Sultanate of Oman
Water is the most precious resource in arid and semi-arid countries including the Sultanate of Oman. In such countries, groundwater is regarded a major source of water. However, the long-term yield of aquifers should be evaluated. Annual recharge of groundwater resources, in such areas, is generally small but groundwater levels may also recover dramatically during wet years.
Due to the increase in population and the associated increase in agricultural and industrial activities groundwater abstraction from the different aquifers in Oman has increased tremendously since the mid-1970s. This over exploitation of the groundwater resources has led to a considerable decline in the subsurface water levels and hence affected the water balance in the interrelated hydrogeological systems.
Although surface water is not always a major source of water it is sometimes feasible to build dams across the main Wadis to provide water for domestic and/or irrigation purposes. The collected surface water at dams can also be used to recharge the depleted aquifers. This is quite true in arid and semi-arid regions. The Sultanate of Oman is no exception.
Efficient management of groundwater resources is not only needed but also necessary to provide a sustainable groundwater supply. Artificial recharge of aquifers, application of water conservation plans, water and wastewater reuse, and improved irrigation methods are some of the techniques which could be used to increase the availability and sustainability of water resources. To that end, eighteen dams have been built in the Sultanate of Oman since the mid-1986. These dams are mainly built to recharge the groundwater systems and minimize the seawater intrusion in the coastal areas.
The basic idea of any recharge dam is to hold the surface water runoff that might develop from heavy rainfall events in a particular Wadi at a location where the recharge to the groundwater is desired. The water is either infiltrated from the reservoir in the upstream side of the dam to the underlying aquifer or released through culverts to infiltrate downstream through the Wadi channel. The efficiency and the performance of the constructed dams as an effective tool for groundwater recharge needs to be investigated and reassessed.
The main objective of this research is to study the efficiency of the recharge dam and simulate the groundwater conditions at Wadi Al-Jizzi. The same methodology can thus be considered in the assessment of the performance of the other dams of similar hydrogeological and physical setting. To that end, all previous studies and reports related to Wadi Al-Jizzi have been reviewed. Physical, geological, hydrological and hydrogeological settings of the area have been identified. All available records for groundwater levels and quality have been assessed and presented in different forms of analysis. The trends of groundwater levels during the last two decades have also been identified.
The USGS finite difference groundwater model “MODFLOW” has been employed to simulate the groundwater conditions in Wadi Al-Jizzi. The study domain (30x30 km) was discretized into a total number of 22500 regular cells. Each cells have dimensions of 200x200 meters. Various types of boundary conditions were applied to accurately simulate the field conditions. Several assumptions were made. The aquifer was assumed homogeneous, the hydrodynamic dispersion effects were neglected, the flow in the aquifer was assumed fully horizontal, the flow in the aquitard was assumed vertical and the saline groundwater was assumed at rest.
The model parameters were calibrated for the period 1985 to 1994 until a good match between the simulated and observed groundwater levels was obtained. The model was then validated, without changing the calibrated parameters, for the period 1995 to 2002. Predictions were made for the groundwater levels till the year 2020 assuming no change in groundwater abstractions and same climatic conditions.
Pumping of groundwater resources in Wadi Al-Jizzi area should be reduced and controlled as much as possible. A database regarding the geological and hydrogeological setting should be established. Further development of the present numerical model might be essential to better simulate the groundwater levels and study various scenarios for groundwater pumping and management. This would help achieve the conservation of groundwater resources in Wadi Al-Jizzi catchment. The proposed scenarios should take into account the political, economic and social impacts. The Water Resources National Master Plan for integrated management of all available water resources should be updated and implemented. This master plan should account for dry and wet conditions.