Numerical Groundwater Modelling Of Nanka Aquifer Flow System In Anambra State
Keywords:
Groundwater flow, simulation, calibration, model-gridAbstract
Numerical groundwater flow models of the Nanka aquifer domain within the Anambra State part of the Anambra Basin were developed to better understand the aquifer system and to determine the long term availability of groundwater by simulating groundwater conditions resulting from historical pumpage for the period 1979–1999 and simulating possible future conditions resulting from ground-water pumpage.
The models were developed using assumptions and approximations to simplify the actual aquifer system. The model idealized the complex hydrogeologic relations of the actual system based upon the data and the assumptions used to develop it. The Nanka ground-water flow system was numerically defined by discretizing the aquifer system into finite difference grids, determining the boundary conditions for the aquifer, estimating the rate and distribution of recharge and discharge, and estimating the aquifer properties within the model. To simulate historical conditions, steady-state (predevelopment) and transient-state (postdevelopment) models were formulated and calibrated. Results of the steady-state simulation were used as initial conditions for the transient-state model.
The simulated hydraulic heads for steady state conditions generally were within 0.94 to 25.42m of the measured water levels for 1979-83, of nine observation wells. The Root Mean Square Error (RMSE) for steady state calibration was 15.53m. The Mean Error (ME) was 3.62m, which indicated a model bias toward under-estimating head values. After calibration, for transient state conditions, the simulated heads were within about 4.90 to 26.21m of measured water levels for the entire domain. For the transient verification, the RMSE was 12.86m, and the ME was –7.07m.
The calibrated Nanka groundwater flow transient model could be used to simulate the potential effect of water-extraction plans on hydraulic heads and ground-water movement.
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