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Groundwater modeling of unconfined aquifer system of crystalline area - a case study in Lapsiya watershed, Hazaribagh, India
Ashok Kumar Earth Resource Division Remote Sensing Application Centre IGSC- Planetarium, Patna - 800 001, India Tele# +91-612-689001, Mobile# +91-9875036588 Ashok_bcst@yahoo.com / groundwater@indiatimes.com Web: http://www/geocities.com/ashok_bcst
In the India considerably large geographical area comes under crystalline area. Groundwater occurrence and its management are the major task before the scientists and planner. These area experiences acute crisis of groundwater for drinking water and irrigation. In these areas due unconfined nature of aquifer system, the storage and retrieval of groundwater is major task before the scientists. The weathered materials are the principal aquifer system and ground water occurs under water table condition. Beneath the weathered horizon, fractures system within the basement surface is also supposed to be potential aquifer zone. But determination of fracture geometry is difficult task and these fractures zone have not been fully exploited. It has been established that aquifer geometry of the unconfined aquifer system is important parameters in understanding the groundwater storage, retrieval and recharge process in aquifer. The Digital Surface Terrain Modeling (DSTM) and Digital Basement Terrain Modeling (DBTM) exercise provides the upper and lower limit of the unconfined weathered aquifer system (Kumar et. al., 1997). This approach has been well tested in identifying the groundwater retrieval and storage sites in Chotanagpur region of India. But for complete understanding the complex mechanism of groundwater, this approach is not sufficient. The long term planning and management of groundwater needs understanding groundwater interaction with surface water, recharge, seepaze process, intake and rate of withdrawal in space and time and its long term effect on the aquifer system to achieve the sustainability. The entire exercise becomes complex process and it is outside preview of static modeling exercise such as DBTM approach. Several attempts have been made through computer modeling in alluvial plain of India but less stress has been made for the modeling of the aquifer in hard rock area. In present study, modeling exercise has been attempted in Lapasiya watershed, Hazaribagh, India. It has helped in understanding the behavior of unconfined aquifer system with various varying input parameters. The outcome of the model helped in identifying suitable area for groundwater augmentation on the long term. The present model also helped in optimization of rate of new wells. The model has simulated up to a level to the near real field condition. The present modeling exercise and its results has given enough scope for taking up such types exercise in other parts of hard rock of India. There is still possibility for further refinement of various parameters. The present modeling exercise is a parts of UNDP training programme and it may not been treated as final. Groundwater Modeling Modeling is an attempt to replicate the behaviors of natural groundwater or hydrologic system by defining the essential features of the system in some controlled physical or mathematical manner. Modeling plays an extremely important role in the management of hydrologic and groundwater system. Objective of Modeling in Case Study
Data required for the modeling and its source
Ground-Water Flow Equation The partial-differential equation of ground-water flow used in MODFLOW is (McDonald and Harbaugh,1988) 
where Kxx , K yy , and K zz are values of hydraulic conductivity along the x, y, and z coordinate axes, which are assumed to be parallel to the major axes of hydraulic conductivity (L/T); h is the potentiometric head (L); W is a volumetric flux per unit volume representing sources and/or sinks of water, with W<0.0 for flow out of the ground-water system, andW>0.0 for flow in (T-1); SS is the specific storage of the porous material (L-1); and t is time (T). 
Study Area
The Lapasiya watershed (AIS & LUS , 1988) is a part of Upper Hazaribagh plateau and forms the 500-600 (above m.s.l.) meters erosion surface. On the whole the plain is undulating with some minor ridges interrupting the level nature topography. The area may be termed as buried pediplain. The cover material is formed by coarse alluvium in the immediate valley of streams while rest of the pediplain has a gravely ferruginous soil. The porosity of soil does not permit wetting of the topsoil and the water rapidly percolates to the lower horizons. The present study area is a part of upper Hazribagh plateau. The watershed has total areal extent of 85 sq. km. Area on average receives 1322.41 mm of rainfall. Surface Water Resource Total 55 water bodies mostly ponds/ tanks have been identified in the watershed with the help of remotely sensed data. In which “Charwa” dam are the major water body and its areal extent are approximately 100 ha. The entire water bodies nearly harvest 8-10 % of the total annual rainfall (Kumar, 1997). Land Utilization Kharif (paddy crops) including current fallow, water body, settlements etc covers 67.43 percent of watershed whereas rabi crop covers 07.43 per cent of the watershed area. The areal extent of rabi crops is indicator of utilization status of surface and ground water (Kumar, 1997). Aquifer System Thick weathered material serves as potential aquifers. In the valley portion water table generally cuts the topographic surface and groundwater get lost as seepage (spring). Water table in the valley portion ranges between 2.00m to 3.0m b.g.l. and generally deep on the upland in the range of 4 to 10m b.g.l. (Kumar, 1997). It has been observed that in case of maximum thickness of saturated weathered horizon of phreatic aquifer about 12m, yield of the dug wells range from 1.0m3 to 2.5m3 / day for a draw down of 0.5m to 3.00 m and well recuperates within 2 to 24 hr. Specific capacity of the aquifer varies from 1.39 to 5.61 lpm/m. draw down for the hilly areas having thin mantle of weathered material and 3.12 to 8.54 lpm/m draw down to low lying areas underlain by thick weathered material and soil covers. It has been observed that 70 per cent of total groundwater reserves get lost as base flow in river (Bhattacharya , 1990 ).
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