ESTIMATION OF GROUNDWATER RECHARGE IN NATIONAL CAPITAL TERRITORY, DELHI USING GROUNDWATER MODELING

Journal of Indian Water Resources Society, Vol 34, No.1, January, 2014 ESTIMATION OF GROUNDWATER RECHARGE IN NATIONAL CAPITAL TERRITORY, DELHI USING GROUNDWATER MODELING Vikrant Vishal 1, Sudhir Kumar 2 and D.C.Singhal 1 ABSTRACT National Capital Territory, Delhi in India is under the grip of extreme pressure to meet demand for its water resources due to urbanization, improvements in living standards, expanding population etc. Therefore, quantitative evaluation of spatial and temporal distribution of groundwater recharge is a pre-requisite for operating ground water resources system in an optimal manner and meet the demand. The main objective of this study was to predict groundwater recharge in NCT of Delhi. The methodology was achieved using numerical groundwater model (VISUAL MODFLOW, 4.3). MODFLOW model is derived from a combination of topology, soil type, land use, well location using geographic information systems (GIS). The model was calibrated and validated and then used to predict groundwater recharge. The output of the model was found to be in agreement with the earlier records. Moreover, the simulation results also show reasonable declination of water table elevations in the study area during the period of study. Keywords: NCT of Delhi, Groundwater recharge, Visual MODFLOW, Arc GIS, Simulation. INTRODUCTION Water is the most precious natural resource on the earth, the principal constituent of all living things. It is also a key factor in conditioning the earth for human existence and in influencing the civilization process. Groundwater recharge may be defined as the downward flow of water reaching the water table, forming an addition to the groundwater reservoir (Lerner et al., 1990). Groundwater is also the most important source of water supply to meet the requirements of National Capital Territory (NCT) of Delhi. Groundwater models are useful tools that allow for better understanding of groundwater recharge estimation, groundwater movement, contaminant transport and the interactions between groundwater and surface water. Groundwater flow models are used to calculate the rate and direction of movement of groundwater through aquifers (Owais et al, 2007). The outputs from model simulation are the hydraulic heads and groundwater flow rates which are in equilibrium with the specified hydrogeological conditions (i.e. hydrogeologic framework, hydrologic boundaries, initial and transient conditions, hydraulic properties and sources) defined for the modeled area. A simulation of groundwater flow that takes into consideration the parameters and properties variability of aquifer is only possible through mathematical modeling. Most models that are usually employed to simulate groundwater flow are based on the Partial Differential Equation (PDE) which can be solved numerically through Finite Difference (FD) or Finite Element (FE) techniques. These methods discretize the time and flow domains, and require computation of the hydraulic head in each cell by dividing the stress periods into smaller time steps. Several studies with the use of the FD and FE methods have been carried out by researchers (Wang and Chunmaio, 1998; Bakker, 1999; Gupta et al., 1984; Mazzia and Putti, 2002). The development of high speed computers may ease solving 1. Department of Hydrology, Indian Institute of Technology, Roorkee, Roorkee-247667, India Email:vikrant.vishal003@gmail.com or dcsinghal@rediffmail.com 2. National Institute of Hydrology, Roorkee, India Email: sudhir.nih@gmail.com Manuscript No.: 1332 the PDE in groundwater modeling using numerically-based models such as MODFLOW. In itself MODFLOW is a fullydistributed three dimensional groundwater model which uses a block-centred approach and a modular structure consisting of a main program and a series of subroutines that are grouped into packages (McDonald and Harbaugh, 1988). MODFLOW has been updated and it comprises different refinements such as the revised version in 1996 (Harbaugh and McDonald, 1996). The most widely used numerical groundwater flow and mass transport model is MODFLOW which is a three-dimensional model. It uses block-centred finite difference scheme for saturated zone. MODFLOW is widely used to either predict groundwater flow or head fluctuations (Pulido-Velazquez et al., 2007) or to verify other groundwater simulation methods, such as spreadsheet simulation model (Karahan and Ayvaz, 2005). Due to its capability, MODFLOW is widely used to simulate different types of groundwater problems in different geographical regions, such as the arid, semi-arid and tropical areas. It is a well known model in the field of groundwater. In this study, MODFLOW was applied to simulate groundwater recharge in the semi-arid region of Delhi territory in India. STUDY AREA AND CHARACTERISTICS OF AQUIFER The National Capital Territory (NCT) of Delhi is located in northern India between the latitudes of 28 24 17 and 28 53 00 North and longitudes of 76 50 24 and 77 20 37 East (Figure 1). It has an area of 1,483 km 2. Delhi, as the capital of India, has a specific status in the Indian political federalism. This territory has a pseudo-state status and is under the mixed control of the central government and of a local government similar to that of other Indian states. Delhi is under the grip of extreme pressure to meet demand for its water resources due to urbanization, improvements in living standards, expanding population etc. In Delhi the gap between water supply and demand has led to an uncontrolled groundwater abstraction and hence to a declining water table all over the city (CGWB 2006, Maria 2004). As per a recent NASA report and other government reports, the water table in some parts of Delhi is going down by an average 1m every year because of over extraction and less recharge to groundwater. In 2000, the Central Groundwater Authority had notified South, South-west districts and Yamuna Flood Plain in 15

Fig. 1: Location Map of NCT of Delhi Fig. 2: Geological Units of NCT of Delhi (Source: CGWB) 16

Delhi banning all groundwater extraction implying that no tube wells could be constructed in these areas without prior permission. In a subsequent order, the Governor of Delhi further notified all other districts of Delhi territory. It is, therefore, necessary to evaluate the existing trend and availability of ground water in the Delhi territory in time and space and its movement for proper planning in near future. The Delhi region is bounded by the Gangetic alluvial plain in the north and east, the Thar desert in the west and Aravalli hill ranges to the south (Kaul and Pandit, 2004). It is covered with alluvial deposits occasionally traversed by exposures of linearly elongated quartzite ridges (Figure 2). It is bounded by river Yamuna towards east. The alluvial deposits are major repository of groundwater in the area.wells are extensively constructed in the Yamuna flood plain which are comprised of Younger alluvial deposits, fine grained Older alluvial deposits in the Chattarpur basin, South Delhi and also the Older alluvial deposits in western and northern parts of NCT, Delhi. Yamuna active flood plain aquifer is about 35 km long along river Yamuna occupying an area of about 97 km 2. Delhi quartzite is the bed rock formation occurring at depths between 30 m and 203 mbgl. Older alluvium overlies the bedrock. The average slope of the Yamuna river bed is from north to south. There are numerous large and small drains entering river Yamuna in the stretch between Wazirabad barrage and Okhala barrage. A major drain in the study area is Najafgarh Drain which receives water from the Sahibi River catchment and is a tributary of river Yamuna. It flows from west to east direction. The study area also contains a major unlined canal called Western Yamuna Canal. Groundwater Flow Pattern Groundwater flow pattern in the NCT Delhi has been developed based on the water level monitoring data of CGWB. The location of the observation points used are given in Figure 3. Figure 4 shows the broad groundwater flow configuration in the area for January 2009. The flow seems to be largely influenced by the river Yamuna and other drains in the area. Towards east of Delhi ridge, the flow is towards southeast (upto river Yamuna) whereas towards west of the ridge, the groundwater flow is westwards generally following the ground slopes. The westward slope of the water table is terminated near the Najafgarh drain where a ground water depression is developed with a minimum elevation of 201 m AMSL in the west-central part of the area. These observations are also supported by the isotopic observations made by Kumar et al. Fig. 3: Location of head observation wells and rivers before converting the area to grids 17

Fig. 4: Water Table Contour map (January, 2009) (2011) which indicated the northward, westerly and southwesterly inflow of groundwater terminating in the Najafgarh depression zone. However, towards south, the Chhatarpur region forms a closed basin feature with its recharge occurring in the peripheral hill region. The spatial variation between the adjacent water table contours seems to reflect changes in the aquifer permeability of the area. Thus, in the vicinity of river Yamuna, the flow pattern seems to be towards southwest in the eastern Delhi around Shahdara. Due to varying permeability, the groundwater velocities are highly variable, being higher in the permeable zones within the Yamuna flood plain in the vicinity of river Yamuna and Western Yamuna Canal (WYC). The average groundwater velocities in the various aquifer layers are given as under: (From top) I layer: 0.0756 m/day II layer: 0.0423 m/day III layer: 0.0263 m/day IV layer: 0.0121 m/day MODEL SETUP The numerical finite-difference model (Visual MODFLOW4.3) was used to simulate groundwater recharge with initial and boundary conditions. The input data for the simulation model may be classified as spatial and temporal. The spatial input includes aquifer characteristics, such as water table (of unconfined aquifer), boundaries, hydraulic conductivity, specific yield, location of wells, recharge area, drainage area, whereas the temporal input includes time dependent data. The period of simulation is divided into a series of stress periods within which an overall water budget is calculated for specified stress. This controls the model output according to user s specification. As shown in Figure 5, the modeled area is replaced by a set of discrete nodes in a grid pattern covering the modeled area. The regional model consists of a finite difference mesh of 250 columns and 250 rows overlaid on (61.56 x 61.56 km) or about 3790 km 2 study area. The grid is regular with a cell size of 246.24 m by 246.24 m. The boundary conditions in the model are assigned as river boundaries along the i) Yamuna river ii) Western Yamuna canal and iii) Najafgarh drain. The aquifer system is schematized in 4 layers as shown in Figure 6. The height of conceptual model varies from about 120 to 300 m above mean sea level. In the model all layers are considered as horizontal layers except upper most layer (ground surface). 18

Fig. 5: Model Grid Fig. 6: Cross section along column showing Layers The uppermost layer shows variable thickness in space due to variable topography. The thickness of layers are 55 m, 10 m, 10 m and 13-77m from bottom upwards respectively. We have considered varying thickness for the top layer. Hydraulic property values are assigned in the model based upon geological condition of the study area and previous studies. In the model, the hydraulic values have been assumed constant for same type of strata. The hydraulic parameters values adopted in the groundwater model are chosen on the basis of published work. MODEL CALIBRATION Model calibration consists of successive refinement of model input parameters from the initial estimates to improve the fit between observed and model-predicted results. A solution of a steady-state groundwater flow problem requires following information: hydraulic conductivity, boundary conditions, and 19

Fig. 7: Discrepancy Values (Zone Wise) Fig. 8: Comparison of Observed and Calculated Heads at Selected Observation Wells the location and magnitude of applied sources/sinks such as system. Those parameters that are known or can be estimated/ wells. The calibration procedure typically begins with assumed are initially specified as part of the input data set. selective definition of parameters/inputs based on available Firstly, model is divided into three different zones and then, dataa and/or an initial conceptual model of the hydrogeologic steady state model was simulated using the different recharge values by trial and error method and try to overlap water table 20

contour with actual nature of water table contour in the NCT of Delhi (Figure 7) and also try to minimize percentage discrepancy in the interested zone. Percent discrepancy is plotted as the temporal changes in the Flow Mass Balance (total flow IN minus total flow OUT) expressed as a percentage of the total flow. It is important to note that true steady state conditions do not exist in the NCT of Delhi aquifer. Spatial and seasonal changes in groundwater fluxes, as well as rainfall, combine to make the aquifer a dynamic system. Therefore, the model was calibrated for transient state and attempts were made to match historical field data of water level in the study area with simulated water level for hydrological years 2005 to 2008(Jan- Dec) as shown in Figure 8. MODEL VALIDATION The parameters determined during model calibration are used for model verification. Here verification has been done with two sets of water table elevation data for year 2009, i.e., on 15 th May 09 and 15 th Aug 09. The verified model for pre-monsoon 2009 (485 days) shows simulated heads which are quite close to the observed heads giving correlation coefficient (R 2 ) as 0.995 (Figure 9). Also, the verified final model for all times Fig. 9: Computed vs. Observed Head for Transient State (485 Days) Fig 10: Computed vs. Observed Head (All Time Plot) for Transient State for all Observation Points 21

shows water table heads quite close to the observed heads giving correlation coefficient (R 2 ) as 0.987 (Figure 10). RECHARGE ESTIMATION Study area (NCT of Delhi) is divided into nine zones overlapping with the district boundaries to estimate the groundwater recharge (Table 1). Table1: Estimation of Groundwater Recharge DISTRICT RECHARGE (MCM) CENTRAL 2.22 EAST 4.19 NORTH 5.63 NEW DELHI 1.85 NORTH EAST 6.14 NORTH WEST 45.43 SOUTH 20.16 SOUTH WEST 36.65 WEST 5.97 RESULTS AND DISCUSSION A three-dimensional groundwater flow model was developed. The model was calibrated by adjusting model input parameters until a best fit was achieved between simulated and observed water levels. The NCT of Delhi is an area with political state boundaries, and no real aquifer boundaries. In hydrogeological terms, the boundary conditions in the model are assigned as river boundaries along the Yamuna River, Western Yamuna canal and Najafgadh drain. The goal of constructing hydrologic and hydrogeological models, through which we can understand the behaviour of the NCT of Delhi aquifer has been achieved and provides a good insight of the groundwater flow in the aquifer. A methodology has been presented with step-by-step procedure to estimate the groundwater recharge based upon groundwater modeling approach. The methodology incorporates the 3D groundwater flow equation. This methodology is expected to give accurate estimates of groundwater recharge. However, to improve the reliability of groundwater recharge estimates, we must monitor aquifer behavior on a continuous or periodic basis to ensure that adequate data are available. The groundwater flow models as well as all automated data (such as geological cross-sections, land-use map, rainfall, depth of water table and slope of the topography, etc,) will be very useful for further regional development. Findings and conclusions The results of the groundwater modeling in the National Capital region of Delhi indicate that steady state conditions do not exist in the aquifer regime of NCT region. So the model was calibrated for transient state and attempts were made to match the historical ground water level data of the study area with the simulated water levels for hydrological years 2005 to 2008 (Jan-Dec) as shown in Figure 8. The validation of the model was done with two sets of water table elevation data (for May. 2009 & August 2009). The simulated water level data is found to be quite close to observed heads with acceptable correlation coefficient. With such a validated model, the study area has been divided into nine zones for estimation of ground water recharge (as given in Table 1). The table indicates that the recharge is maximum (45.43 MCM) in the Northwest district followed by 36.65 MCM in the southwest district. However, it is minimum (1.85 MCM) in the New Delhi District followed by 2.22 MCM in the Central District. Such a wide variation seems to reflect the diversity in the geology of the study area as well as the ongoing urbanization which influence the overall recharge in the region. REFERENCES 1. Bakker, M., 1999. Simulating groundwater flow in multiaquifer systems with analytical and numerical Dupuit- Models. Journal of Hydrology, 222(1-4), 55-64. 2. CGWB, 2006. Ground Water Year Book 2005-2006, National Capital Territory, Delhi. Central Ground Water Board, Govt. of India, New Delhi 3. Gupta, S.K., Cole, C.R. and Pinder, G.F., 1984. A Finite Element Three-Dimensional Groundwater (FE3DGW) Model for a Multi-aquifer System. Journal of Water Resources Research, 20(5), 553-63. 4. Harbaugh, A.W. and McDonald, M.G., 1996. Programmer s Documentations for MODFLOW-96: An Update to the US Geological Survey Modular Finite- Difference Groundwater Flow Model, US Geological Survey Open-File. 5. 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