Hydraulic Simulation of Groundwater Flow

Transcription

1 10 Hydraulic Simulation of Groundwater Flow V. Ramani Bai, Sumiani Yusoff & S. Mohan INTRODUCTION The most formidable groundwater challenge is to attain the sustainable use and management of groundwater in vast and growing regions where the resource is under threat. The over-depletion of groundwater is becoming a major problem in the North China plains, in western and peninsular India and in urban areas across Asia. Over 25 per cent of India s harvest is in areas where groundwater is being seriously overexploited. Here the challenge is to sustainable development and management this potential, for the benefit of the poor people. The objective of this research work is to assess the groundwater potential in the Vaigai basin, to estimate the aquifer parameters and to compare the estimation of groundwater balance by different methods and to use most reliable result for the development of the model. These objectives will be helpful particularly in creating a detailed picture of groundwater situations and use patterns. This also includes collection, synthesizing past and current scientific work to re-examine important technical and behavioral relationships related to groundwater. To formulate a realistic assessment of opportunities and threats. The Vaigai basin shown in Figure 10.1 (indicating Theissen polygons) in Tamil Nadu is chosen for this study. It has an area of 7000 sq.km where current (and projected) supplies of surface and groundwater are not deemed sufficient to meet current (and projected) needs. In 1985 a tunnel diverted waters from the Periyar river in Kerala under a

2 Figure 10.1: Theissen polygon for estimation of groundwater recharge. 128 Land-Use and Environmental Resources: Methods and Management

3 Hydraulic Simulation of Groundwater Ramani Bai et al. 129 contentious 999-year agreement between Kerala and Tamil Nadu. The groundwater elevation data from observation wells of the basin are taken from 120 wells data available at the government department and the data used for the study are tabulated in Annexure 1. The western and northwestern parts of the basin receive heavy rainfall during the monsoons, with an average rainfall of 850mm over the basin. Hundreds of works have been carried out in Vaigai basin and most recent and important works are described here. The underlying geology of the basin is moderately complex; forming a combination of unconfined, confined, semi- confined and fully confined aquifers with significant spatial variation in productivity. Earlier Works of the Vaigai System Various organizations in India has carried out the study of water resources of Vaigai river basins and the differences between the present work and the earlier works are represented in Table Some of them are described in detail in the following section. Groundwater Division of Public Works Department (PWD, 1976) has conducted a groundwater balance study in Vaigai basin between Vaigai dam and Peranai. They have also conducted water quality studies on groundwater and shown that there is a general improvement in the quality of water in the few cross-sections of the basin. The rugged upland topography with the bedrock devoid of fissures and fractures are the reason for the insignificant addition to the groundwater in the upper reaches. The insignificant extraction of groundwater in the reaches I to III is due to the unfavorable sub-surface formations. Table 10.1: Comparison with earlier studies on Vaigai Sl.No. Earlier Studies Current Study 1. Demands are for sub-basin wise only Demands are for command area wise 2. Groundwater modeling for estimation Groundwater modeling for future pumping policy of aquifer parameters 3. Water balance for entire basin Water balance zone wise 4. Yearly estimation of dynamic reserve Monthly estimation of dynamic reserve of of Groundwater Groundwater Institute for Water Studies (IWS, 1985) had carried out assessment, planning, development, and management of water resources of the Vaigai River basin. The MITSIM MIT simulation model using IBM 370/155 computer was adopted for simulation studies. Historical data available for the years ( ) were used for the simulation runs of the model. The water management strategies and institutional framework required for effective management of available water resources have been evolved for implementation. The remedial measures were suggested to control the erosion in the Vaigai basin area and siltation in the Peranai Regulator by active afforestation, development of orchards, plantation, gully plugging and by construction of check dam.

4 130 Land-Use and Environmental Resources: Methods and Management The National Water Policy (NWP, 1987) has laid great stress on the need for optimal water resources development. They have recommended many basins for study including Vaigai basin. Water and Power Consultancy Services (India) Ltd. (1996) carried out a Comprehensive study of Vaigai river basin. This encompasses assessment of surface and groundwater resources, their present level of use, the projected demand for the year 2015 AD for irrigation, municipal and industrial use, integrated operation of the reservoirs, macro planning of numerous tanks, conjunctive use etc., A simulation model to optimize the use of available resource of water in the Vaigai basin. Number of recommendations such as restoring Periyar operative FRL to 152 Ft., sharing of shortages in the ratio 2:5:4 for the zones III, IV and V, changes in gate operation schedule/cropping pattern, investigation of a storage site at identified location have been suggested. They have derived general conclusions on opening of gates for irrigation in June for the I crop of double crop areas of Cumbum and PMC and September 2 nd fortnight for the II crop of double crop area of Cumbum and PMC, PMC main, PMC extension and for TMC main and extension. Madras Institute of Development Studies (MIDS, 1997) has studied the condition and characteristics of groundwater utilized for irrigation in the basin. This study has examined the important features of groundwater irrigation such as statistical data, extent of water table decline, socio-economic and environmental implications, assessment of well irrigation investments and characterizes and functioning of groundwater markets in the basin. World Bank (WB, 1999) developed a Decision Support System model and the Tools for the Holistic Analysis of Natural Network Information (THANNI) have been applied in Vaigai River Basin. The intricate network of thousands of traditional storage tanks needs structural and siltation rehabilitation. This DSS has two major components: an Information System to help organizing basic hydrologic, agricultural, urban and other information about the basin; and an optimization model that maximizes the benefits from water use subject to a variety of hydrological, economic, legal and policy constraints. Both the information system and the model have been made in a flexible manner to allow for consideration of different scenarios, data updates, model reformulation and further analysis. The THANNI DSS used a variety of popular software, such as EXCEL spreadsheets and the GAMS optimization package to create an interactive graphical interface for ease of input and output. The optimisation model helps give greater insight into the physical and conceptual interconnections in the basin and the intersectoral and inter-regional tradeoffs involved in the consideration of various hardware (infrastructure) and software (policy) options for basin management. Currently, the model as formulated is a monthly model for the chosen planning year. The decision variables are the inter-sectoral water allocation, cropping pattern, flows in the system, reservoir operation, etc. The objectives are to maximize water use benefits subject to constraints on area available, meeting crop water requirements, meeting in-stream water use requirements, etc. The interactive model can be optimized for each scenario considered. THANNI gives decision-makers an additional tool for policy and scenario analysis and gives stakeholders a focal point for discussions. These should be augmented with expert opinion, views, policies, and other tools such as Geographic

5 Hydraulic Simulation of Groundwater Ramani Bai et al. 131 Information Systems, and simulation, water quality, stochastic models, etc. as appropriate. Institute for Water Studies (IWS, 1999) has carried out a micro level study of Water Resources plan for Vaigai river basin. They have done micro level study on assessment of annual surface water potential for75 per cent dependable year using SCS (Soil Conservation Services) method dividing the Vaigai basin into ten sub-basins. The annual groundwater assessment is made for all the ten sub-basins. The groundwater quality and suitability for drinking and irrigation purposes have been assessed by chemical analysis of samples from 84 observation wells established in Vaigai basin for ten years from The general laws, regulations and institutional elements, which are applicable to this basin, are also drawn in this study. Office of Chief Engineer, State and Surface water resources data center (CE, 2000) has identified Vaigai basin as first one for the value addition through the inputs from hydrology project. They developed a comprehensive and reliable hydrological data stations, covering all aspects of hydrology, meteorology, hydrometeorology, hydrogeology and hydrochemistry. They have also established the Hydrological Information System (HIS) to document the data collected through various discipline. Center for Water Resource (CWR, 2000), Anna University carried out base line survey of irrigation commands including Lower Vaigai basin. The study envisages assessment of the adequacy of available water to service the planned command area. They have also conducted socio-economic survey and studied the environment and public health aspects of the basin. The present study is aimed at assessing resources both surface water and groundwater and problems like flood, drought, waterlogging, sedimentation, environmental issues etc., including collection of necessary data; stating the developmental objectives in terms of goals, socio economic priorities etc., to identify technical alternatives and feasibility and developing an integrated system model to suggest policy guidelines. Overall, this is a venture on using advanced modeling tools and upto date data for developing a micro-system modeling as against macro-system modeling and to integrate with the macro-model. Groundwater Modelling Strategy With the increase in demand of water for competing uses, it is difficult to meet the entire demand from a single source and is a challenge to plan and optimally utilize the available water resources for proper management. The same situation exists for almost all irrigation systems in the world. It also exists in Vaigai basin as perceived from Figure Further the excessive development of groundwater in deltas of coastal areas may lead to salt water ingress in the freshwater aquifer and may contaminate it if unchecked. This may happen when sufficient surface water supplies do not reach the tail end of command areas and in the presence of overdrawal of groundwater. Table 10.2 shows a comparison between the earlier and present works carried out in the water resources of Vaigai basin. Monthly available groundwater as dynamic

6 132 Land-Use and Environmental Resources: Methods and Management Figure 10.2: Surface and groundwater resources of Vaigai Basin. reserve at each node in the basin is also estimated considering their ayacut as influencing area for respective nodes. An unusual rainfall on February 17 during , which is the 75 per cent dependable year for groundwater, has caused a rise in groundwater in this month. The present work focuses on micro-level studies on the entire system by considering all outlets of the system from reservoir to the field channel. In addition the present work has employed recent computer softwares in all the segments of the study to achieve more accurate and quick results by using available database. Table 10.2: Comparison of recharge estimation in Vaigai Basin Sl.No. Year Organization Qty of G.W in Mcm in a Year By current study Madras Institute of Development Studies Water and Power Consultancy Services (India) Ltd Groundwater Dept., PWD,Tamil Nadu 993 Groundwater Models Modeling groundwater systems involve both water quantity (flow) and water quality considerations. Groundwater models incorporate mathematical representations of some or all of the following processes: movement of water and fluids through saturated or unsaturated porous media or fractured rock, transport of water soluble constituents, transformation of contaminants by chemical, biological

7 Hydraulic Simulation of Groundwater Ramani Bai et al. 133 and physical processes and heat transport and associated effects of temperature variations on groundwater flow and pollutant transport and fate of contaminants. Classification of Groundwater Models Groundwater models may be functionally characterized as (1) descriptive prediction models, (2) prescriptive management models and (3) parameter identification models. Most groundwater models are prediction models, which use input data characterizing the natural groundwater system and human-induced development to predict the hydraulic, and/or water quality conditions expected for specified management scenarios. The commonly used descriptive groundwater models simulate flow, with or without solute transport, in continuous porous media. Other more complex models simulate flow in fractured media, multiphase flow, or complex chemistry (geochemical models). This model provides descriptive predictions of the system response to a specified management plan. Prescriptive management models, on the other hand, are oriented towards determining optimal management plans for meeting specified objective. These models involve linking an optimization algorithm with a flow and/or solute transport model. Parameter identification or calibration models address the inverse problem of determining values for the parameters, which characterize the groundwater system. There is a continuous development in the field of computer softwares applicable for groundwater studies. Some of them are Flowpath, Flownet, Airflow, Prince, Aquifertest, Visual Groundwater, Visual Help etc. Groundwater Hydraulic Simulation by Visual Modflow Model Development Data Used The groundwater annual recharge potential of Vaigai basin has been assessed using 28 yrs ( ) of water level data. The observation well data collected from Groundwater Division are used. The data such as bore hole vertical section, map of observation wells, administrative boundary map and basin location map are used to develop the model. In order to construct the numerical model, geological borehole descriptions for pumping and observation wells, aquifer types and their properties are gathered to obtain an idea about the geological setting of the study area. Aquifer Properties: Most of aquifer area in the basin is found to be confined and semi-confined in which water table varies in undulating form depending on aquifer properties, areas of recharge, discharge and pumping from wells. Geology The main rock types are found to be Charnockites, Khondalites, Granites, limestones, and calcareous sand stones of archaean age. They are exposed in the plains of Melur taluk with acid intrusive pink granite and gneiss. Minor pegmatite and quartz veins are seen throughout.

8 134 Land-Use and Environmental Resources: Methods and Management Water Quality The geochemicals such as calcium bicarbonate, calcium chloride, magnesium bicarbonate, magnesium chloride, sodium bicarbonate, and sodium chloride are present and thus a calcareous nature of soil is shown. Generation of the Model The model domain is created using the base map of Vaigai river basin is shown in Figure 10.3 with its well locations, solid boundaries etc. using world co-ordinates and model co-ordinates for better visualization. Model grids are smaller and more homogeneous than would be used in practice. The model is defined within the georeferencing point as (225 km, 124km) fixing the boundary of the study area. The basin boundary is superimposed over the finite difference lattice. The generated model domain is shown in Figure Period of Simulation: Simulation of the model is run for pre-monsoon, monsoon and post-monsoon seasons of the river basin and water levels at each grid point are estimated. The simulation is performed with a groundwater modeling software Visual MODFLOW. Validation of the Model The properties of the aquifer such as hydraulic conductivity and storage coefficient of the basin are plotted in regular contours. These provide an idea about the Figure 10.3: Vaigai Basin Model Domain with Wells and Elevation Contours

9 Hydraulic Simulation of Groundwater Ramani Bai et al. 135 possible sources of water in the aquifers in the basin. The contours of head equipotential lines and water table contours are shown in Figures 10.4 and Groundwater model, which is developed under static head, is validated with observed head Vs calculated head plots. The direction of velocity of flow in plain are obtained as output from Visual Modflow. Figure 10.4: Head equipotential lines. Results of Simulation The computed groundwater level in the grid points has been taken and the average groundwater level for the basin is assessed. Flow associated with external stresses such as wells, areal-recharge, drains, and streams has been simulated. The study is done for visualizing water table contours, direction of groundwater flow, the flow regimes on draw down contours, head equipotential, water table contours, recharge potential, layer thickness, flux between layers and head difference between layers of the aquifer. A color-coded plots of steady state and transient pathlines with time markers and velocity vectors are studied. Conclustion The most effective form of flow model results when the data are fitted to the model systematically. For this the simulation of the model is done for 63 wells and is compared with the observations made in the remaining 22 wells. Results indicated

10 136 Land-Use and Environmental Resources: Methods and Management Figure 10.5: Water table contours. that the groundwater levels would generally decline between 8 to 10metres within ten years, showing groundwater depletion in the basin. The calibrated model can be implemented as a guiding tool for the future groundwater management in the basin. References Environmental status report of vaigai river basin (prepared by the environmental cell of CE/P and D//PWD). Flow data (daily readings) vol.8 in Palanichetty patty anicut(the last anicut in Cumbum valley form 6/74 to 6/94. Flow data (daily readings) vol.9 in Palanichetty patty anicut (the last anicut in cumbum valley) from 7/94 to 12/99 Govt, of Tamil Nadu PWD, Periyar - Vaigai project Madurai. Comprehensive System Study of Vaigai River Basin (WAPCOS) Data Annexure Vol. 3/96. India water resources management sector review, working paper 6, River Basin Modelling. Samani, N., Kompani-Zare, M., Barry, D. A. (2004). MODFLOW Equipped with a New Method for the Accurate Simulation of Axisymmetric Flow. Advances in Water Resources, 27:

11 Hydraulic Simulation of Groundwater Ramani Bai et al. 137 Senthilkumar, M., Elango, L. (2004). Three-dimensional Mathematical Model to Simulate Groundwater Flow in the Lower Palar River Basin, Southern India. Hydrogeology Journal State framework water resources plan, Annexture -10, Vaigai river basin, Institute of water studies, Taramani, Chennai.

12 138 Land-Use and Environmental Resources: Methods and Management ANNEXURE 1 Groundwater observation wells in Vaiqai basin Sl.No. Available Well Wells having Data Useful data WK WK WK WK A WK WK WK WK WK A WK C WK WK WK WK WK A WK WK WK WK A WK A WK A WK A WK A WK A WK WK WK WK WK A WK WK WK WK A WK A WK WK WK WK WK A-1 WK WK & 45B B WK WK WK WK WK WK WK A-1 WK A-1 WK B-1 WK WK WK1 Sl.No. Available Well Wells having Data Useful data A WK A WK WK WK WK WK B WK B WK A WK A WK WK WK WK WK A WK B WK B WK WK WK A WK A WK A WK B WK WK WK WK WK WK AB WK A WK B WK B WK AB WK A WK B WK A WK A WK A WK A WK A WK A WK A WK A WK WK WK WK A WK A WK WK WK B WK B WK WK WK A WK A WK A WK A WK WK WK WK WK1 Contd...

13 Hydraulic Simulation of Groundwater Ramani Bai et al. 139 Annexure 1 Contd... Sl.No. Available Well Wells having Data Useful data WK WK WK WK WK WK B WK B WK WK WK WK WK WK WK WK WK WK WK WK WK WK WK A WK A WK WK WK WK WK WK WK WK WK WK WK A WK A WK WK WK A WK A WK WK WK WK WK1 Sl.No. Available Well Wells having Data Useful data WK WK A WK A WK WK WK A WK A WK WK WK WK WK WK WK A WK WK WK A WK A WK WK WK A WK A WK WK WK WK WK WK WK A WK A WK WK WK A WK A WK WK WK WK WK A-1 WK WK WK1

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