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Groundwater modeling of unconfined aquifer system of crystalline area - a case study in Lapsiya watershed, Hazaribagh, India
Model Calibration
The observation well used in the model has been used for calibration of the model. The model calculated heads and observed heads have been analyzed. Majority of the heads falls in the 90 per cent confidence level (Fig. 1.18). The 95 per cent confidence level is supposed to be optimal. Therefore there is scope to refine the various parameters taking in-homogeneity in the aquifer system. Same exercise has been carried out in transient simulation. The calculated and observed heads have been plotted for the all the stress period. It has been found that heads are behaving with seasonal change in the water table.
Results Inspection of model output has indicated that a place where basement depth is more, failure well is less. This means that well success is hard rock area depends on the thickness of the aquifer material. The largest water body in the watershed "charwa dam" effects on the surrounding ground water movement has been noticed. It has been observed that the up stream drainage area of the dam drains the groundwater to the dam. But much lateral control on groundwater movement has been noticed. The flow lines are coming to the dam area and it is moving towards down streamside. The volumetric calculation of total available utilizable groundwater within aquifer has been made using output generated in the steady state. Total volume is 230.050x106 m3. This clearly indicates that availability of resource is not a problem. The model simulation has indicated that this type of aquifer can be pumped with slow rate (most appropriately at the rate of 100 m3/day) due to high draw down. Similarly, well can not be pumped for long duration at one stretch. In the entire watershed putting huge number of dug wells can augment groundwater and shallow tube wells energized with 2 H.P. pumps. In middle portion and mid-north-east corner of the watershed, we can pump the water even at high rate i.e. up to 200 m3/day. Because simulation results are stable. This area gets ground water recharge from the upper reaches of watershed and recharge guided by the main river channel. Another observation has been made regarding seepage loss of groundwater in drainage (presently it is a constant head boundary). It is decreasing with time due to continuous pumping. The seepage loss of groundwater can be optimized through the modeling simulation.
Regional flow pattern of Groundwater The flow direction and velocity vector obtained for different period indicates (Fig. 1.11) that majority of the flow direction is in NE direction. This is shortest route of the groundwater movement from the upper reaches to lower reaches. It has been also observed that micro water divides are also controlling the flow patterns. Few heads of the observation sites located near the constant head boundary i.e. drainage channel has not shown any change with time. This is because no seasonal variation has been taken into account in assigning constant head boundary for the whole simulation period. Flow Budget from the model output Results of flow budget (Fig. 1.17) indicate that an amount of 9712.80 m3 per day has been pumped on the 1st Jan. from the 18702-m3 available effective storage of the aquifer. After end of 31st Jan., all the pumping wells are not able to pump more than 4361.5 m3 per day. This indicates that some of wells have gone dry. Total storage available in the system also comes down to 7843.50 m3. The result indicates decrease in pumping volume till the month of June-July. The in out to the system is also decreases till the month of June-July. After start of monsoon i.e. June- July, situation reversed after increase in recharge to the system. Inspection of draw down of the individual pumping wells indicated that radius of influence wells are very limited and rarely interfering the other wells. Further wells are going dry only where depth of basement is shallow and pumping rate is high. It has been found that 50 m3/day upping rate is optimum. Even in some places, groundwater may be pumped with the rate of 100 m3/day - 200 m3/day Conclusions The modeling exercise of unconfined aquifer system of hard rock area in Indian condition is possible and model can be simulated to near real field condition. Based on present modeling exercise following points emerged out
Acknowledgement Groundwater modeling of Lapasiya watershed, Siwane sub-basin, Hazaribagh, India was part of UNDP-DST training programme on GIS based Groundwater Modeling at Centre for Groundwater Studies, CSIRO, Wembley, Western Australia. Author is thankful to Dr. Chris Barber, Director, CGS, Western Australia, Dr. Kumar A. Narayan, Principal Research Officer; Dr. Ramsis Salama, Research Group Leader; Mr. Tonny Barr and Dr. Raiyast Ali, Scientists, Land and Water, CSIRO, Wembley, Western Australia, and Dr. Prabhakar Clement, Centre for Water Research, University of Western Australia, Perth, Australia for providing the training in the Visual MODFLOW and GMS package of groundwater modeling. References
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