Disaster Advances Vol. 7 (12) December 2014 Regional groundwater flow modeling in Lower Bhavani River basin, Tamil Nadu, India Anandakumar S.1 and Subramani T.2* 1. Department of Civil Engineering, Kongu Engineering College, Erode, Tamil Nadu, INDIA 2. Department of Mining Engineering, CEG, Anna University, Chennai-600025, Tamil Nadu, INDIA *[email protected]; [email protected] test hypotheses regarding the behavior of particular facets of groundwater systems.20 Groundwater model describes various parameters such as groundwater flow, solute or material transport on the basis of various simplifying assumptions. Computer simulation has been widely used to understand the responses of the aquifer system to changing hydrological stresses.13 A number of groundwater modeling studies have been carried out around the world for effective groundwater management.9,12,13,23,24,26 The most widely used numerical groundwater flow model is MODFLOW which is a three-dimensional model originally developed by the U.S. Geological Survey.19 Abstract Groundwater flow models are beneficial for the management of groundwater resources as they give an approximate estimate about the various hydrogeological parameters. They also help in illustrating a clear picture of the flow pattern in an aquifer. Such a numerical three-dimensional groundwater modeling study was attempted in Lower Bhavani River basin, South India with the main objectives of simulating the regional groundwater flow and identification of the distribution of heads for improved understanding of the natural flow system. Bhavani River is one of the important tributaries of Cauvery River and originates in the Silent Valley range of Kerala State, India. In India, as in many parts of the world, considerable research has been carried out to understand the aquifer system and groundwater flow. In the study area, the depth of groundwater is very shallow in the central part. However, depth of occurrence of groundwater and its fluctuation are considerably high in the north-eastern and south-western parts of the basin. Some of the previous studies carried out in this region are: (i) Groundwater resources and development potentials in Erode District, Tamil Nadu, India, by the Central Ground Water Board6, Government of India, (ii) Groundwater level monitoring, rainfall recording, pump test analysis and estimation of concentrations of major ions in groundwater by the PWD22, Tamil Nadu, (iii) A case study on groundwater quality of hard rock aquifers of Erode District by Chidambaram et al8, (iv) Understanding of major ion groundwater chemistry by Anandkumar et al1,3 (v) Seasonal behavior of rainfall pattern by Anandkumar et al2 and (vi) Identification of hydrogeochemical processes using Netpath modeling by Subramani et al.27 The model simulates groundwater flow over an area of 2,475 km2 with 55 rows and 45 columns, with a single vertical layer. The model was simulated in transient state condition using three-dimensional partial differential equation of groundwater flow from 1995-2006. The model was calibrated for steady and transient state conditions. There was a reasonable match between the computed and observed heads. The transient model will run until the year 2015 to forecast the dynamic groundwater flow under various scenarios of over pumping and less recharge. The model predicts the behavior of this aquifer system under various hydrological stress conditions. The results indicate that the aquifer system is stable under the present conditions. The model also predicts the changes in groundwater head with changes in hydrological conditions like drought occurring once in three years and a normal run for another 8 years without any major changes. Study area Physiography and land use: Bhavani River is one of the important tributaries of Cauvery River and originates in the Silent Valley range of Kerala State, India (Figure 1). The Lower Bhavani River Basin lies between 11° 15‟ N and 11° 45‟ N latitudes and 77° 00‟ E and 77° 40‟ E longitudes. Bhavani, Gobichettipalyam, Satyamangalam and Andiyur are the major settlements in this region.17 The study area includes reserve forest, built-up lands, agricultural fields and barren lands. Tanks in the north east and south west part of the study area are mainly rain-fed and remain dry throughout the year except during rainy seasons. The Bhavani River flows from west to east in the study area and confluences with the Cauvery River at Bhavani Town. Keywords: Groundwater flow, Groundwater head, Flow modeling, Lower Bhavani River basin, South India. Introduction Groundwater has become a highly dependent source of water. Low risk of contamination and its wide distribution make it more preferable than surface water. Groundwater modeling has emerged as an important tool for collecting various databases of an aquifer. Hence, groundwater modeling studies have enabled researchers to develop a better understanding of the functioning of aquifers and to 41 Disaster Advances Vol. 7 (12) December 2014 The area is comprised of hilly regions and plain terrain with maximum and minimum altitudes of 1,487 m and 215 m above mean sea level (MSL) respectively. The terrain slopes towards south-east. Topography, in general, plays a vital role in groundwater management for understanding the slope of the terrain and surface runoff. The topography of this region mainly controls the occurrence of groundwater, land use and drainage pattern. Scattered hillocks of moderate elevation occur within the uplands. The plains area is characterized by gentle undulations with a general gradient due east and south-east. landform and bedrock type and also suggest soil characteristics and site drainage condition. In addition, the stream pattern is a reflection of the rate at which precipitation infiltrates in comparison with the surface runoff. The infiltration/runoff relationship is controlled largely by permeability, which is, in turn, a function of the type and fracturing of the underlying rock or surface bedrock. When comparing two terrain types, the one that contains the greatest drainage density is usually less permeable.10 A well-developed dendritic to sub-dendritic drainage system is generally noticed in the basin which indicates the occurrence of rocks of uniform resistance.29 The map also illustrates the major drainages and man-made canals in the basin. The basin area is drained by the Bhavani River and its tributaries. The Bhavani River, which has its origin in the Silent Valley range of Kerala State, enters the study area about 30 km west of Bhavanisagar Reservoir and flows more or less in an easterly direction and confluences with the Cauvery River at Bhavani Town. A number of streams have their origin in the hill ranges of the Eastern Ghats and have their flow direction towards south. The drainage pattern in the area is also controlled by structural features. Amongst the different drainage patterns recognized in the study area, the dendritic, sub-dendritic, radial and parallel patterns are noteworthy. The major crops are paddy, banana, groundnut and sugarcane. The term “land use” relates to the human activity or economic function associated with a specific piece of land. The land use pattern of the study area mainly depends on topography, land form and soil cover. Thick vegetation is seen in the hill ranges in the north-western and northern parts of the study area. The areas adjacent to Bhavani River are intensively irrigated. Sparsely irrigated areas are noticed along some of the tributaries of Bhavani River where the groundwater is effectively utilized. More cultivable, fertile agricultural lands are noticed along the canals. Banana and sugarcane are the common wet crops. Paddy is also cultivated in some places during the monsoon season. Dry crops are sowed in many places. Climate and Rainfall: The climate of the study area is dry except during the monsoon season. The area experiences dry climate with maximum temperature of 40° C during April and May and minimum temperature of 22° C during November and December. The first two months of the year are pleasant. During March, the sky is clear and the mercury gains an upward trend which persists till the end of May. Highest temperature is normally recorded during May. During pre-monsoon period, the mercury reverses its trend and by September the sky gets overcast. In spite of the heavy overcast sky, the rains are meager during September.22 The north-east monsoon gets vigorous only during October or November. Geological setting: The Archean basement of the study area mainly consists of fissile hornblende-biotite gneiss (mainly in plains) and charnockite (mainly in hills).14 The study area also comprises of garnetiferousquartzofeldspathic gneiss, hornblende-biotite gneiss, quartzite, pyroxene granulite, ferruginous quartzite, talctremolite schist, amphibolite, gabbro/anorthosite, pink migmatite, dolerite dykes and granite intrusions. Garnetiferous-quartzofeldspathic gneiss and hornblende biotite gneiss are the other major rock types in this region. Soil characteristics of a terrain are important since they meet the basic needs of all agricultural production. Different soils that occur are derived from a wide range of geological materials. Knowledge about the types of soils, their extent and occurrence is of primary importance. Basement rocks of the basin are covered by various types of soils namely red non-calcareous soil, red calcareous soil, brown soil and black soil. Red soils of calcareous and noncalcareous varieties occupy most of the basin area. Brown and black soils occur as pockets in some places.3 The average annual rainfall of the basin is 617.7 mm. The Palghat gap in the Western Ghats which has a soothing effect in the climate of Coimbatore district, does not render much help in bringing down the dry climate in this area.18 The basin receives more rainfall during Northeast monsoon season period. The average annual contribution of this monsoon is about 338 mm which is nearly 45% of the total rainfall of the basin. Highest intensity of rainfall occurred during this monsoon is 692 mm recorded in the year 2005. The average annual rainfall received during the southwest monsoon is 210 mm.2 Hydrogeomorphology: It is possible to delineate various hydrogeomorphic units from satellite imageries through visual interpretation.21 Groundwater recharge, transmission and discharge of the basin are controlled by the basin geomorphology, geology and structural patterns.15 Hydrogeomorphological maps help to identify the various geomorphic units and groundwater occurrence in each unit. Sreedevi et al25 used remote sensing and GIS techniques to Drainage system: Drainage pattern is one of the most important indicators of hydrogeological features since it is controlled by underlying lithology.7 According to Lillesand and Keifer17, the drainage pattern and texture seen on aerial photographs/satellite imageries are the indicators of 42 Disaster Advances Vol. 7 (12) December 2014 study the occurrence of groundwater in various geomorphic units of the Pageru River Basin, Cuddapah District, India. Hydrogeomorphological map of the study area was prepared from the IRS-1D-LISS III satellite imageries. The various geomorphic units as identified from the studies are structural hills, residual hills, valley fills, uplands, pediments and bajadas. It is seen that structural and denudational process controls influence the fluvial processes in this region. Boundary Conditions: The boundary conditions modeled were as per the watershed boundary (Figure 3). The northern and southern parts of the area had negligible flow and hence considered as “no-flow boundaries”. Along the western boundary, the existing hydrogeological boundary, the Bhavanisagar Reservoir, was considered as the “general head boundary”. The Bhavani River divides the basin into two halves; it was considered as the “river boundary”. The eastern part of the study area is also a part of the watershed boundary with one outlet falling under the river boundary itself. The aquifer top and bottom were derived mainly based on the lithology of boreholes and by intensive field surveys. The single, unconfined layer is comprised of the top soil and weathered rocks. The thickness of the unconfined aquifer varies from 6 to 21 m. Hydrogeological setting: Generally, the entire area of study is traversed by metamorphosed gneisisic rocks of Archean age. The occurrence and movement of groundwater in hard rock formations are restricted to open systems of fracture like fissures and joints in nonweathered portions and also in the porous zone of weathered formations. In hard rock regions, the weathered thickness is discontinuous both in space and depth. Hence, the recharge of groundwater in hard rock formation is influenced by the intensity of weathering. The subsurface condition was analyzed from borehole lithology and pumps test data. Borehole lithology revealed that the thickness of the aquifer in the study area is highly erratic and varies between 6 m and 20 m below ground level. Grid design: The geographic boundaries of the model grid covering 2,475 km2 of the study area were determined using the map module. The map was projected using the metric coordinates in the map module and then imported into the Visual MODFLOW v.4.0. The finite-difference grid superimposed on the study area was constructed based on the conceptual model representing the physical properties of the groundwater system. The grid network had a constant spacing of 1 km by 1 km. The model grid was discretized into 2,475 cells with 55 rows and 45 columns with one vertical layer (Figure 3). The length of model cell was 1 km each along the east-west and northsouth directions of the study area. Inter-granular porosity is essentially dependent upon the intensity and degree of weathering and fracture development in the bedrock. Deep weathering has been noticed in the gneissic formation and moderate weathering in the charnockite. Pump test data revealed that permeability varies between 0.0031 and 11.4642 m/day and transmissivity between 0.1142 and 100.3536 m2/day. Spatial variation map of transmissivity prepared from the pump test data of 19 bore wells indicates that the values are relatively higher in the central part of the basin. Increasing trends of transmissivity (Figure 2) are also seen in the south-eastern and south-western part. Input parameters Initial groundwater head: The initial groundwater head of the study area is shown in fig. 4. After detailed analysis of the hydrographs (rainfall and water level fluctuation studies), it was decided that the groundwater head data of January 1995 represented the initial groundwater head distribution of the study area. The duration of the rainfall was normal and the groundwater fluctuation was also representative of the normal year. Results and Discussion A numerical three-dimensional groundwater flow model was developed for the Lower Bhavani River Basin with the objectives: (i) To simulate the regional groundwater flow and (ii) To identify the distribution of heads for improved understanding of the natural flow system in the study area. Few scenarios were also developed for proper understanding of the aquifer system. Aquifer characteristics: Aquifer properties such as hydraulic conductivity, horizontal transmissivity and specific capacity used in the model were derived from the pumping test results available at 19 sites and are listed in table 1. Groundwater abstraction: The groundwater of the study area is abstracted for irrigation and domestic purposes. Agricultural activity in this area is mainly dependent on surface water resources as there is a good canal network in this region. The land use pattern, drainage pattern and period of canal flow show that groundwater is used only during the summer period (one season). The domestic and drinking water requirement of the study area were calculated based on population statistics.28 Model conceptualization: The conceptual model of the system was arrived at from the detailed studies of geology, borehole lithology and water level fluctuations in wells. Groundwater of the study area was found to occur in weathered rocks. The groundwater head in wells penetrating only up to the weathered formation as well as in wells penetrating up to the hard rock formation was more or less the same. Hence, the top soil and the lower weathered and fractured rocks could be considered as a single, unconfined aquifer. Groundwater recharge: The recharge varies considerably due to differences in land use pattern, soil type, geology, 43 Disaster Advances Vol. 7 (12) December 2014 topography and relief. The recharge to the aquifer system is from rainfall, irrigation and inflow from the river and storage tanks. Rainfall is the principal source of groundwater recharge in this region. A comparison between the monthly rainfall values and consequent variations in groundwater levels over a span of 30 years revealed that groundwater was replenished whenever the monthly rainfall exceeded 60 mm. The aquifer gets recharged and the groundwater level shoots up immediately after rainfall of above 60 mm. The major portion of the study area is geologically covered by fissile hornblende-biotite gneiss. The infiltration capacity of rocks in their weathered portion ranges from 08–12%. The recharge values range from 8 to 11% of the rainfall.28 observation wells distributed throughout the aquifer, the transient models were considered to be calibrated satisfactorily. The sensitivity of the model to input parameters was tested by varying only the parameter of interest over a range of values and monitoring the response of the model by determining the root mean square error of the simulated heads compared to the measured heads. Simulation results: The model was simulated in transient condition for a period of 12 years from 1995 to 2006. There was fairly good agreement between the computed and observed heads (Figure 5). A study of the simulated potentiometric surface of the aquifer indicated that the highest heads are found on the western side of the study area which is a general reflection of the topography. The regional groundwater flow direction is towards the center. Groundwater flows from the north towards the center of the Bhavani River. The rate of leakage between the river and aquifer was estimated using the difference between the river and groundwater heads. The Bhavanisagar Reservoir is situated in the western part of the study area. Its contribution to groundwater recharge was calculated based on the difference between the head in the adjoining wells and the reservoir head. The reservoir level data was also inputted in the model. The simulated and the observed regional heads for December 2005 are shown in figure 6. The computed and observed groundwater heads of well nos. 1 and 19 of the study area are shown in figure 7. The computed head values mimic the observed head values. At present the aquifer is stable. There is a very gradual decline of groundwater head over 10 years in the northern and southern parts of the study area; this might be mainly attributed to the flow towards the river while in the central part near the river, the groundwater level increases nearer to the surface. Model calibration: The calibration strategy was to initially vary the best known parameters as little as possible and vary the poorly known or unknown values most to achieve the best overall agreement between simulated and observed results. Steady state model calibration was carried out to minimize the difference between the computed and field water level conditions. Steady state calibration was carried out with the water level data of January 1995 in 25 wells distributed over the basin. Of all the input parameters, the hydraulic conductivity value was the only “poorly known” parameter as only 12 pumping tests had been carried out in this area. Lithological variations in the area and borehole lithology of existing large diameter wells were studied. Model forecast: The aquifer response for different input and output fluxes was studied in order to sustainably manage the Lower Bhavani River Basin aquifer system. The model was run for a further period of 8 years from 2007 to 2015. Before commencement of this simulation, the data pertaining to average rainfall, abstraction, tank water, river flow and recharge were provided to the model up to 2015. Based on these, it was decided to vary the hydraulic conductivity values up to 10% of the pumping test results for the aquifer in order to get a good match of the computed and observed heads (Figure 5). Figure 5 indicates that there is very good match between the calculated and observed water heads in most of the wells. Root mean square error and mean error were minimized through numerous trial runs. Two prediction runs were planned to evolve optimal management schemes: i) for normal rainfall condition and ii) for drought (once in three years) condition. (i) Normal rainfall condition: The model was run to predict the regional groundwater head in this area until the year 2015. For these runs, the monthly average rainfall calculated from 60 years rainfall data was used. The present level of groundwater abstraction was considered for this simulation. The simulated regional groundwater head for September 2015 is shown in figure 8. There is not much increase or decrease in groundwater level. Such observation is made in most of the wells. In the years when the flow in Bhavani River was considered, there is an increase in groundwater level by about 0.5 m in the wells located near the rivers. Transient state simulation was carried out for a period of 12 years from January 1995 to December 2006 with monthly stress periods and 24 hour time step. The trial and error process by which calibration of transient model was achieved comprised of several trials until a good match between computed and observed heads was obtained over space and time. The hydraulic conductivity values incorporated in the transient model were modified slightly from those calibrated by the steady state model. Based on the close agreement between the measured and computed heads from January 1995 to December 2006 at 25 44 Disaster Advances Vol. 7 (12) December 2014 (ii) Drought (once in three years) condition: Analysis of the past 10 years rainfall data indicates that in three years, the rainfall was less than the average of 617 mm/year. The average of these low rainfall years (drought period) was found to be 573.43 mm/year. In order to study the effect of drought years in this area, the model was run by assuming deficit rainfall once in three years until 2015. The monthly average of deficit rainfall years was calculated and used for this purpose. The groundwater level decline is by about 0.3 to 0.73 m during the assumed drought years (Figure 9). However, the groundwater level recovers to the level observed during normal rainfall within the next year. However, due to the contribution of the reservoir and the river, the deficit in rainfall subdues to normal within one to two seasons. The contribution of the reservoir to the aquifer system maintains the system in a stable condition. Figure 1: Location map of the study area 45 Disaster Advances Vol. 7 (12) December 2014 Figure 2: Spatial variation of transmissivity No flow boundary No flow boundary Figure 3: Discretization of the study area 46 Disaster Advances Vol. 7 (12) December 2014 Figure 4: Initial groundwater head of the study area during January 1995 Figure 5: Comparison of computed and observed groundwater heads under steady state condition 47 Disaster Advances Vol. 7 (12) December 2014 Computed-December 2005 Observed-December 2005 Figure 6: Computed and observed groundwater heads of December 2005 48 Vol. 7 (12) December 2014 Groundwater head (m) Disaster Advances Groundwater head (m) Time Time Figure 7: Simulated and observed groundwater heads of well 1 to 19 Table 1 Pumping test results Well No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Accepted Hydraulic Conductivity (m/day) 4.3636 0.0651 11.4642 0.6818 0.0455 0.0271 0.0448 0.3968 0.0647 0.1093 0.2565 0.1415 1.2222 1.4462 0.0105 0.0031 0.0501 0.0522 0.8165 Accepted Transmissivity (m2/day) 100.3536 3.8904 39.2517 22.4922 3.1238 1.8649 3.0628 26.2760 2.0707 6.3823 10.6838 5.8945 3.3956 59.9761 0.6871 0.2460 3.3705 2.0884 0.1142 49 Specific Capacity (m2/sec) 0.0001266 0.0001847 0.0086319 0.0001167 0.0001391 0.0000113 0.0000144 0.0001426 0.0000612 0.0007762 0.0000790 0.0001557 0.0000903 0.0001683 0.0001035 0.0000140 0.0001943 0.0000232 0.0000103 Vol. 7 (12) December 2014 Groundwater head (m) Disaster Advances Time Groundwater head (m) Figure 8: Computed groundwater head of well number 19 until 2015 under normal rainfall conditions Time Figure 9: Computed groundwater head of well number 19 until 2015 under drought condition The simulated results indicate that this aquifer system is stable under the present condition. The spatial groundwater head follows the topography and the groundwater water flows from the northern part towards the central portion. The reservoir and the river contribute to the stable maintenance of the aquifer system. The model predicts changes in groundwater head with changes in hydrological conditions like drought occurring once in three years and a normal run for another 8 years without any major changes. The aquifer system is stable with few of concern areas near the river and canal in the eastern part of the basin with increasing water level. Thus, an integrated approach may be necessitated in this region to avoid water logging conditions in the future. Conclusion Detailed modeling study was attempted in Lower Bhavani River basin, Tamil Nadu, India to understand the groundwater flow mechanism. The topography of the basin mainly controls the occurrence of groundwater, land use and drainage pattern. The Archean basement of the region mainly consists of fissile hornblende-biotite gneiss and charnockite. The occurrence and movement of groundwater in the basin are restricted to open systems of fracture like fissures and joints in non-weathered portions and also in the porous zone of weathered formations. The thickness of the aquifer is highly erratic and ranges between 6 and 20 m. Inter-granular porosity is essentially dependent upon the intensity and degree of weathering and fracture development in the bedrock. Deep weathering is observed in the gneissic formation and moderate weathering in the charnockite. A single-layered, finite-difference flow model was used to simulate the groundwater head in the Lower Bhavani River basin for a period of 12 years (1995-2006) for better understanding of the aquifer system. Acknowledgement The corresponding author thanks the Department of Science and Technology (DST), Government of India for providing necessary funds under „Young Scientist‟ Scheme to carry out the work. 50 Disaster Advances Vol. 7 (12) December 2014 14. GSI., Geological and Mineral map of Tamil Nadu and Pondicherry. Published by the Director General Geological Survey of India on 1: 500,000 scale (1995) References 1. Anandakumar S., Subramani T. and Elango L., Spatial variation of groundwater quality and inter elemental correlation studies in Lower Bhavani River Basin, Tamil Nadu, India, Journal of Nature Environment and Pollution Technology, 6, 235-239 (2007) 15. Krishna Rao P. R., Hydrometeorological aspects of estimating groundwater potential, Seminar on Groundwater Potential of Hard Rock Areas of India, Bangalore, 1-2 (1971) 2. 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