Numerical groundwater models as powerful tools for sustainable groundwater resources management in the Philippines

This paper is aimed at demonstrating the applicability of a numerical groundwater model as a potential tool for sustainable groundwater resources management in the Philippines. A three-dimensional numerical groundwater model was developed for a typical aquifer system in a typical shallow aquifer area in Southern Luzon based on USGS-MODFLOW employing finite difference solution scheme. The model was calibrated under transient conditions using independent sets of two-well pumping and recovery test data obtained at the site. Other model parameters such as the aquifer hydraulic properties were based on single-well pumping test performed at the site and on secondary data gathered from various agencies. Model calibration yielded satisfactory results as depicted by a close match between simulated and observed values of head during pumping and recovery. The mean error, mean absolute error and root mean square error values during model calibration ranged from -0.19061 to -0.11226 m, 0.11226 to 0.19061 and 0.11556 to 0.22604 m,respectively indicating satisfactory fit between simulated and observed head values. Model validation using two independent sets of two-well pumping and recovery test data obtained, at the site also showed reasonable match between simulated and observed head values.Model validation using April 2003 data yielded mean error values of -0.0216 to 0.4447 m, mean absolute error of 0.0216 to 0.4447 m and root mean square error of 0.0216 to 0.4447 m. Further model validation using additional data yielded mean error, mean absolute error, and root mean square error ranging from 0.0078 to0.2453 m, 0.1106 to 0.2453 m, 0.11085 to 0.25733 m, respectively. All these indicate that the performance of the calibrated model is practically satisfactory. Model simulations were consequently performed under various scenarios namely: 1) two units of 6-m deep shallow tubewells pumping simultaneously at the same discharge and at varying well spacing, 2) one unit of 6-m deep shallow tubewell and one unit 12-m deep well pumping simultaneously at the same discharge and at varying well spacing, and 3) several units of 6-m deep shallow tubewell pumping simultaneously at the same discharge and at a fixed spacing. Simulation results showed that closer spacing between 6-m deep shallow tubewells results to significant well interference. At a typical pumping discharge of 1000 cu m/day, the steady state drawdown increases by 4.4 cm for every 10 m decrease in well spacing of less than 100 m. Model simulations under cases where one unit of 12-m deep well is installed and pumped closer to a 6-m deep shallow tubewell did not yield significant difference with the simulation results under the two 6-m deep shallow tubewell simultaneous pumping scenario. However, increasing the discharge and well screen in the deeper well resulted to significant increase in drawdown in the shallower well indicating significant well interference at greater pumping discharge rates and longer screen lengths. Simulations under a case where numerous units of 6-m deep shallow tubewells pumping simultaneously showed that increasing the number of units of 6-m deep shallow tubewells around a given 6-m deep shallow tubewell significantly increases well interference. Model simulation showed that the final or steady state drawdown is increased from 0.75 m to 1.0 m, 1.21 m, 1.48 m, and 1.74 m under rwo-, three-, four-, and five-well pumping scenarios, respectively, when the wells are installed within 200 m from a given well and pumped simultaneously at a rate of 1000 cu m/day. Many other potential groundwater utilization scenarios may be simulated by the numerical grounwater model developed in this study. While the performance of the model could be further improved with recalibration, using additional multiple pumping test data and by taking into account the inherent spatial variability of aquifer hydraulic properties, the groundwater model developed may be used as a tool for sustainable shallow aquifer management in regions with similar stratification. Its application in other aquifer regions in the Philippines could be extended with the necessary model adjustments and model validation. Consequently, simulation results could serve as basis for developing sustainable well installation and pumping strategies and for sound policy formulation on groundwater utilization in the Philippines. This study has demonstrated the power of computer-aided numerical groudwater models for sustainable groundwater resources management in the Philippines.