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Basement Topography / Depth of Weathering
Based on depth of basement obtained from anylysis of VES data, sub-surface topographic model/ basement topographic model for Lapasiya (fig. 6) has been generated. Average depth of weathering is approximately 15-20 m. 




Conceptual Model 

1. As discussed in the previous sections, topography is undulating and pediplain has developed over granite gneiss’s with high drainage network. The channel of 4th order drainage remains wet throughout the year due to seepage of groundwater. Therefore, wet channel may be assumed as constant head boundary for present modeling exercise. Otherwise, it will be difficult to do the modeling of the area. We may also assume, wet channel as drain boundary condition. For this purpose, data on base flow in the channel is essential besides the drain conductivity. In the present exercise constant head boundary condition has been taken into consideration.
   
   
2. Although, aquifer system in hard rock consists of weathered and fractured system. The modeling of fractures is beyond the scope of present study because it is complex and detailed field data on fracture geometry and geo-hydrological characteristics is required. In hard rock area, the weathered material serves as principal aquifer. This aquifer is unconfined in nature and groundwater occurs under water table condition. Therefore, top layer excluding fractures has been taken for modeling. This is single layer case (Fig. 1.1).
   
   
3. Other basic assumption has been made in delimiting the area i.e. watershed. In practical purposes, the major water divides i.e. Lapasiya watershed outer boundary has been taken as no-flow boundary in modeling (Fig. 1.2).
   

Software Used - Visual MODFLOW 2.8
Visual MODFLOW is a computer program based on USGS MODLOW code with pre and post processor. It simulates three-dimensional ground-water flow through a porous medium by using a finite-difference method. Groundwater flow within the aquifer is simulated using a block-centered finite-difference approach. Flow associated with external stresses, such as wells, areal recharge, evapo-transpiration, drains, and streams, can also is simulated. The finite-difference equations can be solved using different solvers.

Input to the Model

Upper Boundary  The Upper boundary of the aquifer has been taken from the Digital Surface Terrain Model (Kumar, 1997). The upper surface of aquifer has taken from the topographic elevation value available in the Survey of India topographical sheets. The upper surface of the aquifer can be further improved if the contour values available in 1:25000 scale of Survey of India will be taken into consideration. The model success very much depends on the simulation of the upper topographic surface (Fig. 1a).	Constant Head Boundary   As discussed earlier the wet drainage channel of 4th order have been taken as constant head boundary. The large tanks have also been taken as constant head boundary. In the present study same extent of channel has been taken for constant head boundary for the entire period of simulation. Length aspects of constant head boundary can be improved with the help of remotely sensed data of different time period (Fig. 1.3).	Evapo-transpiration   Its estimation needs information on soil physical characteristics, land cover types, atmospheric condition etc. In the present case study, evapo-transpiration value has been approximated from the data available for same agro-climatic zone. There is scope for further refinement.
		Observation Wells   Sites used for initial head have been taken as observation wells. This is required for testing the simulated results (calculated) with observed head (Kumar, 1997), Fig. 1.7.
	Hydraulic Conductivity   The hydraulic conductivity of weathered material is very difficult to estimate. Normal pumping test has serious limitations in hard rock area and obtained results are highly variable. Based on available data on the different parts of Chhotanag-pur plateau, it has approximated between as 0.5 to 1.0 m/day (Athawale, 1984 & Karnath, 1994), Fig. 1.5.
Lower Boundary  The lower boundary of the aquifer has been inputted from the earlier Digital Basement Topographic data (Kumar, 1997). This is also very important parameter, which is required to inputted in detailed due to erratic behavior of basement topography (Fig. 1b).
		Pumping Wells   In the present study area, there is three deep bore wells. Ground water is being mostly augmented by dug well. In Initial phase, total drinking water requirement of village has been taken as one pumping well into the system. Similarly groundwater draft for the irrigation purposes has been taken as separate well. Besides that the deep bore well sites identified in the earlier NRDMS project have also been taken into consideration (Kumar, 1997), Fig. 1.9.
	Recharge  The total estimated recharge into the system have been assumed for each month depending upon the amount of rainfall during the month. It has been distributed in between 2 per cent to 40 per cent. The recharge from monsoon rainfall have assigned as 270, 136 and 40 mm for the upland, midland and lowland respectively. Recharge from tank has been assumed as 0.5 m /day (Athawale,1984 & Karnath, 1994 ), Fig. 1.4
Initial Head   The data collected in the earlier NRDMS project (Kumar, 1997) has been taken into consideration and it has been inputted into the modeling environment. The water table data of Jan 1994 has been taken as initial head in this model in the model (Fig. 1.6).

Model Simulation

Steady State Simulation
First the model has been simulated in steady state for period of one day (Fig. 1.10 & 1.14). All data, such as constant head, recharge, evapo-transpiration have been inputted month wise so that transient state run may carried out month wise. The grid cells representing hill in the watershed became dry in the steady state run. Some other area also became dry and it has been re-adjusted by re-defining the basement geometry at the particular point. It has been corrected some time by adjusting the hydraulic conductivity. Steady state run of model has been carried out by using the various solvers (Preconditioned Conjugate Gradient Package (PCG2), Slice Successive Over-relaxation Package (SOR), Strong Implicit Procedure Package (SIP), WHS Solver for Visual MODFLOW) available within the visual MODFLOW. Many time default solver WHS has not converged whereas PCG2 has given good results.

Transient State Simulation
After the successful run in the steady state, model was run for one-year period at the stress period (Fig. 1.15) of one month. Initially model was simulated without pumping well and simulated results were compared. The model acted like the field situation i.e. rise of water table in the monsoon period, decrease in water table after the monsoon. This indicates conceptual model and initial parameters were ok. Input parameters can be further refined i.e. spatial variation of recharge at different macro/micro-landform and soil types (topographic and soil maps used), spatial variation in evapo-transpiration in different land use units (land use map used), Variation in hydraulic conductivity on different landform and weathered material (aquifer hydro-geophysical property used). After refinement of the model input, model was finally calibrated for the actual field condition.

Thereafter model was simulated with the pumping wells (only drinking water wells and irrigation dug wells). Many of the pumping well dried up in one year (Fig. 1.13 & 1.16). This was due to cumulative pumping rate for the entire village was taken at one point. This can be further improved if it will be distributed in different location within the village area instead of putting cumulative value at one point. Similar results were obtained for the irrigation well. These error indicates that model is behaving correctly with the parameters. Due to very less hydraulic conductivity, radius of influence of wells in the weathered aquifer system is very limited even not more than 100m. Due to non-availability of spatial distribution of irrigation and drinking water wells, further improvement was not carried out. Few wells have not gone dry which are pumping less amount of groundwater for irrigation and drinking water purposes.



Thereafter, earlier identified deep bore well sites have been added into the system with constant pumping rate starting from 200 m3/day. These wells have been active for the period of one year. Most of them have gone dry at end of the one year. This indicates that we can not take the water at this rate. Model was thereafter model has been simulated with the reduced pumping rate. In this way different conditions have been generated and deep bore well pumping rates have been optimized. After running model with irrigation well, drinking well, deep bore well, more wells with less pumping rate was inputted into the system, this has helped in the determining the suitable area where we can observe the less draw down.

Model has been also simulated for the 10 years to generate the scenario for long tern planning of ground water of the aquifer system. 

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