Document Type : Research Article
Authors
Shahrood University of Technology
Abstract
Introduction: Recharge estimation is one of the major issues in management of groundwater resources. Many methods have been applied to calculate the groundwater recharge, among which the water table fluctuation, chloride mass balance and water balance methods have been widely used. In this study the recharge quantity into alluvial unconfined aquifer of Aleshtar in Lorestan province with an area of about 128 km2 was estimated using three methods of water table fluctuation, chloride mass balance and water balance. The aquifer is more important, as it supplies the water for agricultural consumptions. The aquifer is discharged by 322 pumping wells. It is also drained by the gaining river of Aleshtar which crosses the plain in a general trend of the north to the south.
Materials and Methods: Three methods of water table fluctuation (WTF), chloride mass balance (CMB) and water balance were used to calculate the recharge to Aleshtar aquifer in Lorestan province.
In water table fluctuation (WTF) method, water table data from 18 piezometers installed in Aleshtar aquifer during an 11-year period (2003-2014) were collected and analyzed. The values of groundwater rise () which is equal to the difference between the peak of the rise and low point of the extrapolated antecedent recession curve at the time of the peak were calculated and then multiplied by the specific yield to determine the value of recharge based on the following equation:
Δh/Δt (1)
In which R is recharge, Sy is the specific yield and Δt stands for the time.
Recharge value was also calculated by chloride mass balance (CMB) method. In this regard chloride concentrations were measured in 33 groundwater samples and 5 rainfall samples and then recharge was calculated by the following equation:
(2)
Where R is annual groundwater recharge (mm), P is annual precipitation (mm), is mean chloride concentration in rainfall (mg/l) and is average chloride concentration of groundwater (mg/l).
Recharge estimates were also performed by the water balance method based on the following equation:
(3)
In which R denotes groundwater recharge, is groundwater outflow, is groundwater inflow, is groundwater drainage, is evapotranspiration from the groundwater table, is groundwater pumping and is change in groundwater volume storage.
Results and Discussion: Investigating 11-year groundwater hydrograph of Aleshtar aquifer shows a decreasing trend against time. In the current situation, the annual rate of water table decline is about one meter. In order to estimate recharge value using water-table fluctuation method, the value of 0.05 was considered for specific yield based on dominant soil texture in drilling logs and the value of annual recharge into the aquifer was estimated at 28.3 million cubic meters. Temporal variations of recharge showed an increasing trend with time. This is probably related to capacity increase of the aquifer to receive recharging water due to the decline in water table. It was further confirmed by investigating the upstream and downstream hydrographs of the Aleshtar River which showed a decreasing trend in contribution of the groundwater (base flow) at the river discharge with the time. The average concentration of chloride ion in groundwater and rainfall samples were measured as 40.23 and 6.4 mg/l, respectively. Then, recharge value was calculated about 10 million cubic meters using chloride mass balance method. The annual water balance of the Aleshtar aquifer was investigated considering the main components of groundwater inflows (32.46 million cubic meters), groundwater outflows (6.25 million cubic meters), groundwater drainage by the Aleshtar river (15.76 million cubic meters), discharge by pumping wells (49.22 million cubic meters) and change in aquifer storage (-6.41 million cubic meters). The evapotranspiration was not considered as the depth to water table is more than 5 meters, anywhere. Then, the amount of annual recharge using water balance method was estimated about 32.4 million cubic meters.
Conclusion: The similarity of the recharge values calculated by water table fluctuation and water balance methods confirm the accuracy of the calculated total recharge by the both rainfall and irrigation return flows to the Aleshtar aquifer. By subtracting the irrigation return flows, the annual rainfall recharge is estimated at 18.5 and 22.6 million cubic meters by the water table fluctuation and water balance methods, respectively. Due to the uncertainties in recharge estimation by different methods, rainfall recharge to the aquifer was determined in the range of 10 to 22 million cubic meters per year and the rainfall recharge coefficient of 28% was introduced for Aleshtar aquifer.
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