Presentation on theme: "Groundwater Modelling of Ganga Basin – Opportunities and Challenges Prof. S. N. Panda Head, School of Water Resources 1."— Presentation transcript:
Groundwater Modelling of Ganga Basin – Opportunities and Challenges Prof. S. N. Panda Head, School of Water Resources 1
Physiography and groundwater flow of Ganga basin 2 (Source: Ministry of Environment and Forests, Government of India)
Annual groundwater draft in comparison with net annual availability in Ganga basin 3 (Source: Ministry of Environment and Forests, Government of India)
Annual replenishable groundwater in comparison with annual draft in Ganga basin (Source: Ministry of Environment and Forests, Government of India)
Inflow or leakage to/from groundwater Change in storage Reach inflow Reach Outflow Stream Reach Evaporation Groundwater inflow Groundwater outflow Stream inflow Stream outflow Recharge to groundwater Evapotranspiration Rainfall Schematic illustration for evaluating stream-aquifer interaction 5
Problems with groundwater in the Ganga Basin Imbalance in groundwater draft Waterlogging and salinity in canal commands Groundwater pollution 6
Types of Terrestrial Water Ground water Soil Moisture Surface Water 7
Movement of water through the hydrologic cycle 8 (Source: usgs.gov)
Base flow 9 Gaining streamLosing stream with shallow watertable Losing stream with deep watertable Effluent and influent streams
Water Balance Concept The basic concept of groundwater balance is: Input to the system ‑ outflow from the system = change in storage of the system (over a period of time) 10
Flow components for assessing groundwater balance 11 Boundary Pr ET PerCap Q dr Q per Q lsi Q do Q up Q lso Clay Watertable S grw Overland Flow Ir Seepage Pumping well
Groundwater Balance Equation Considering the various inflow and outflow components in a given study area, the groundwater balance equation can be written as: R r + R c + R i + R t + S i + I g = E t + T p + S e + O g + S where, R r = recharge from rainfall R c = recharge from canal seepage R i = recharge from field irrigation R t = recharge from tanks S i = influent seepage from rivers I g = inflow from other basins E t = evapotranspiration from groundwater T p = draft from groundwater S e = effluent seepage to rivers O g = outflow to other basins; and S = change in groundwater storage 12
Groundwater Survey and Investigation Water table contour map Water table contour map showing a local mound and depression in water table and direction of groundwater flow 13
Flow net Flow net technique for estimation of subsurface horizontal flow 14
Depth-to-Water Table Map or Isobath Map 15
Groundwater Quality Map 16
Components of a Mathematical Model Governing Equation (Darcy’s law + water balance equation) with head (h) as the dependent variable Boundary Conditions Initial conditions (for transient problems) 17
General governing equation for steady-state, heterogeneous, anisotropic conditions, without a source/sink term with a source/sink term 18
Dispersion Advection Chemical Reactions Source/sink term Change in concentration with time is porosity D is dispersion coefficient v is velocity Allows for multiple chemical species 19
Model Grids 20 Finite Element GridFinite Difference Grid
Conceptual Model Update Model Calibrate Model Compare Model and Field Mathematical Model Computation Conclude study (Decisions & Recommendations) Conclude study (Decisions & Recommendations) Satisfactory Results Poor Fit Unsatisfactory Results Modelling Process 21
Opportunities and Challenges in the Ganga Basin Wide variation in climate from semi-arid to sub-humid/sub-tropical regions Large-scale spatial variation in – Soil texture and land-use – Type of aquifers and its properties Spatio-temporal variation in - meteorological parameters associated with uncertainties - groundwater recharge and discharge components Groundwater level monitoring is not being done regularly and intensively Setting up/optimising monitoring networks and setting up groundwater protection zones Groundwater resources too need to be planned and managed for maximum basin-level efficiency. 22
Diversified geological climatological and topographic set-up, giving rise to divergent ground water situations Excessive use of our rivers, are causing downstream problems, of water quality and ecological stress. Climate change impacts directly on the availability of water resources both in space and time. The precarious balance between growing demands and supplies brings forth the importance of maintaining quality of both surface and ground water. 25
Application of existing groundwater models include water balance (in terms of water quantity) gaining knowledge about the quantitative aspects of the unsaturated zone simulating of water flow and chemical migration in the saturated zone including river-groundwater relations assessing the impact of changes of the groundwater regime on the environment 26
State-wise distribution of the drainage area of Ganga river 27 (Source: Status paper on river Ganga, NRCD, MoEF, 2009)
Soil types in Ganga basin 28 (Source: Central Pollution Control Board, National River Conservation Directorate (MoEF) (2009))
Data requirement for groundwater balance study over a given time period: Precipitation River Canal Tank Water table Groundwater draft Aquifer parameters Land use and cropping patterns 29
Management of a groundwater system, means making such decisions as: The total volume that may be withdrawn annually from the aquifer. The location of pumping and artificial recharge wells, and their rates. Decisions related to groundwater quality. Groundwater contamination by: Hazardous industrial wastes Leachate from landfills Agricultural activities such as the use of fertilizers and pesticides 30
The only effective way to test effects of groundwater management strategies Conceptual model Steady state model Transient model Processes Groundwater flow (calculate both heads and flow) Solute transport – requires information on flow (calculate concentrations) Groundwater Modelling 31
Model Design Conceptual Model Selection of Computer Code Model Geometry Grid Boundary array Model Parameters Boundary Conditions Initial Conditions Stresses 32
Conceptual Model Update Model Calibrate Model Compare Model and Field Mathematical Model Computation Conclude study (Decisions & Recommendations) Conclude study (Decisions & Recommendations) Satisfactory Results Poor Fit Unsatisfactory Results Modelling Process 33
General governing equation for transient, heterogeneous, and anisotropic conditions Specific Storage S s = V / ( x y z h) K x, K y, K z are components of the hydraulic conductivity 34
Types of Solutions of Mathematical Models Analytical Solutions: h= f(x, y, z, t) Numerical Solutions Finite difference methods Finite element methods 35
Model Design Conceptual Model Selection of Computer Code Model Geometry Grid Boundary array Model Parameters Boundary Conditions Initial Conditions Stresses 36
37 Managed Aquifer Recharge
Suitability of groundwater in increasing dry season productivity in the coastal region of the Ganga basin How the recharge mechanisms can be used to reduce salinity. Climate change impact on groundwater. 38
Methods for groundwater recharge 40
Management of Excess Rainwater Mismatch between water supply and demand Rainwater conservation and recycling Multiple use of harvested water Managed aquifer recharge Management of stagnant water in lowland areas Possible solutions 41
Rainwater Conservation a. Storage of rainwater on surface reservoir b. Recharge to ground water -Pits -Trenches -Dug wells -Hand pumps -Recharge wells -Recharge shafts -Lateral shafts with bore wells -Spreading techniques 42
Methods of Rainwater Storage Infiltration Injection 43
Benefits Ideal solution to water problems in water stress areas Capture and storage of water in monsoon when rainwater is abundant More water will be available for summer use Rise in groundwater level - Improves declining aquifers May increase base flow to streams Mitigates the effects of drought Reduces the runoff which chokes the storm water drains Flooding of roads and low land areas are reduced Quality of water improves Soil erosion will be reduced Saving of energy per well for lifting of ground water – 1 m rise in water level saves about 0.40 KWH of electricity 45
What is Managed Aquifer Recharge (MAR)? Managed Aquifer Recharge is: – The infiltration or injection of water into an aquifer – Water can be withdrawn at a later date but also left in the aquifer (e.g. to benefit the environment) Why Consider MAR? Allows storage of water in wet seasons Improvement in groundwater quality Allows increased use of groundwater from other parts of the aquifer systems To stop seawater intrusion in coastal areas To maintain or increase available water supplies for use in agriculture, drinking water supply, and industry 46
The point of origin of the Ganga, known as the Gangotri (left) and Devprayag, the point of confluence of the Alaknanda (from right) and Bhagirathi (from left) to form the Ganga (right). 51
Ganga River Basin, India The river systems in India are grouped into four broad categories: The Himalayan rivers The Peninsular rivers The Coastal rivers The Inland rivers The Ganga River (length: 2525 km long; catchment area: km 2 ) is fed by runoff from Vast land area bounded Himalaya in the north. Peninsular highlands and the Vindhya Range in the south. The states of Haryana, Rajasthan, Uttar Pradesh and West Bengal, comprising 50% of the basin area. The basin spreads over four countries: India, Nepal, Bangladesh and China. 53
Soil and rainfall (isohyetal) map of Ganga Basin (Source: Ministry of Environment and Forests, Government of India)
Vegetation Types of Ganga Basin (Source: Ministry of Environment and Forests, Government of India)
Groundwater An important component of water resource systems and source of clean water. More abundant than Surface Water Extracted from aquifers through pumping wells and supplied for domestic use, industry and agriculture. With increased withdrawal of groundwater, the quality of groundwater has been continuously deteriorating. Linked to Surface Water systems and sustains flows in streams
Groundwater in Hydrologic Cycle 57 (Source: physicalgeography.net)
Dynamic Groundwater Resources of India Total replenishable groundwater in the country = 433 BCM 5,723 units (blocks, talukas, mandals, districts) assessed – 15% over-exploited 4% critical 10% semi-critical Delhi, Haryana, Punjab, Rajasthan are overusing their groundwater resources. Andhra Pradesh has the highest number of over-exploited units. The agricultural (tube-well dependent) state of Punjab has developed (usage compared to availability) its groundwater upto 145%. Delhi is mining 170% of its groundwater. Countrywide percentage of groundwater development is 58%.
Annual replenishable groundwater in comparison with annual draft in Ganga basin
Ground Water and Surface Water Interaction Ground water and surface water contained in the hydrological system are closely interrelated The studies examines the processes of ground water flow generation and estimation of ground water discharge including ground water discharge to rivers (base flow) In a ground water basin, it is common to identify several aquifers separated either by less permeable or impermeable layers
the upper aquifer is recharged through the bed and banks of the river. The lower aquifer is recharged through the intervening aquitard finite difference equations describes the response of the aquifer system to applied stresses quasi three-dimensional model simulates a ground water system having any number of aquifers
The studies on the ground water/surface water interrelationship made it possible to solve a number of important scientific and practical problems : to estimate base flow and, therefore, sustained low river discharges of different probabilities to estimate the ground water contribution to total water resources and the water balance of regions to evaluate quantitatively the natural ground water resources for determining the prospects of their use within large areas and as a component of the safe ground water yield
The methods for estimating the ground water discharge of the upper hydrodynamic zone are fairly well developed as compared to deep artesian aquifers and their contribution to surface runoff
Seawater Intrusion A natural process that occurs in virtually all coastal aquifers. Defined as movement of seawater inland into fresh groundwater aquifers, as a result of higher seawater density than freshwater groundwater withdrawal in coastal areas
Sea Water Intrusion In the coastal margins of ground water basin, the lowering of water level or potentiometric head results in the intrusion of sea water Inland gradient for saline intrusion result from pumping at rate higher than the recharge to the ground water basin wedge-shaped intrusion occurs as sea water is approximately times heavier than fresh water
Field surveys (geophysical and geochemical studies) can only reveal the present state of seawater intrusion but can not make impact assessment and prediction into the future Mathematical models are needed for these purposes Ghyben-Herzberg relation is a highly simplified model Dynamic movement of groundwater flow and solute transport needs to be considered A density-dependent solute transport model including advection and dispersion is needed for the modelling
Flow Equation Advection-Dispersion Equation Distribution of Head Velocity Field Solute Transport Model Concentration distribution in time and space
Ground Water Pollution Restoration to the original, non-polluted state of polluted ground water is more difficult than surface water Geologic and hydrogeologic setting along with magnitude of the pollution hazard for a specific incident must be evaluated. Movement of contaminants and its control largely depends on the hydrogeologic environment Processes of migration and alterations present in ground water are also present in the unsaturated zone
Remedial action can be classified into three broad categories Physical containment measures, including slurry trench cutoff walls, grout curtains, sheet piling, and hydrodynamic control Aquifer rehabilitation, including withdrawal, treatment, reinjection (or recharge), and in-situ treatment such as chemical neutralization and biological neutralization Withdrawal, treatment and use
use of models provide more appropriate and rigorous method for integrating all the available data together It evaluates the response of the aquifer system to a contamination event The models are derived from the expression of the flow and transport processes in terms of mathematical equations Equations are solved by incorporating appropriate parameter values and boundary conditions
Seawater Intrusion Pumping causes a cone of depression and draws the salt water upwards into the well. 71 Before extensive pumping After extensive pumping by many wells
Groundwater An important component of water resource systems. Extracted from aquifers through pumping wells and supplied for domestic use, industry and agriculture. With increased withdrawal of groundwater, the quality of groundwater has been continuously deteriorating. Water can be injected into aquifers for storage and/or quality control purposes.
MANAGEMENT means making decisions to achieve goals without violating specified constraints. Once contamination has been detected in the saturated or unsaturated zones, requires the prediction of the path and the fate of the contaminants, in response to the planned activities. Any monitoring or observation network must be based on the anticipated behavior of the system. The tool for understanding the system and its behavior and for predicting the response is the model. Usually, the model takes the form of a set of mathematical equations, involving one or more partial differential equations. We refer to such model as a mathematical model. The preferred method of solution is the analytical solution.
For most practical problems we transform the mathematical model into a numerical one, solving it by means of computer programs.
What is a “model”? Any “device” that represents approximation to field system – Physical Models – Mathematical Models (Analytical and Numerical) Modeling begins with formulation of a concept of a hydrologic system and continues with application of, for example, Darcy's Law to the problem, and may culminate in a complex numerical simulation.
TYPES OF MODELS CONCEPTUAL MODEL MATHEMATICAL MODEL ANALOG MODEL PHYSICAL MODEL
Line diagram of the Ganga with major tributaries 77 (Source: Status paper on river Ganga, NRCD, MoEF, 2009)
Importance of ground water flow models Construct representations and helps understanding the interrelationships between elements of hydrogeological systems Efficiently develop a sound mathematical representation Make reasonable assumptions and simplifications Understand the limitations of the mathematical representation and interpretation of the results
Groundwater models can be used : To predict or forecast expected artificial or natural changes in the system. To describe the system in order to analyse various assumptions To generate a hypothetical system that will be used to study principles of groundwater flow associated with various general or specific problems.
Processes to model 1. Groundwater flow 2. Transport (a) Particle tracking: requires velocities and a particle tracking code. calculate path lines (b) Full solute transport: requires velocites and a solute transport model. calculate concentrations
Processes we need to model Groundwater flow calculate both heads and flows (q) Solute transport – requires information on flow (velocities) calculate concentrations v = q/n = K I / n Requires a flow model and a solute transport model.
Modelling Process Establish the Purpose of the Model Develop Conceptual Model of the System Select Governing Equations and Computer Code Model Design Calibration Calibration Sensitivity Analysis Model Verification Prediction Predictive Sensitivity Analysis Presentation of Modeling Design and Results Post Audit Model Redesign
Mathematical model: Simulates ground-water flow and/or solute fate and transport indirectly by means of a set of governing equations thought to represent the physical processes that occur in the system. (Anderson and Woessner, 1992)
2D confined: 2D unconfined : Storage coefficient (S) is either storativity or specific yield. S = S s b & T = K b General 3D equation
Groundwater flow is described by Darcy’s law. This type of flow is known as advection. True flow paths Linear flow paths assumed in Darcy’s law Figures from Hornberger et al. (1998) The deviation of flow paths from the linear Darcy paths is known as dispersion.
In addition to advection, we need to consider two other processes in transport problems. Dispersion Chemical reactions Advection-dispersion equation with chemical reaction terms.
1D, transient flow; homogeneous, isotropic, confined aquifer; no sink/source term Flow Equation: Transport Equation: Uniform 1D flow; longitudinal dispersion; No sink/source term; retardation
1D, transient flow; homogeneous, isotropic, confined aquifer; no sink/source term Flow Equation: Transport Equation: Uniform 1D flow; longitudinal dispersion; No sink/source term; retardation
Conceptual Model A descriptive representation of a groundwater system that incorporates an interpretation of the geological & hydrological conditions. Selection of Computer Code Depends largely on the type of problem(Flow, solute, heat, density dependent etc. along with 1D, 2D, 3D) Model geometry It defines the size and the shape of the model. It consists of model boundaries, both external and internal, and model grid. Grid In Finite Difference model, the grid is formed by two sets of parallel lines that are orthogonal. In the centre of each cell is the node
Boundaries Physical boundaries are well defined geologic and hydrologic features that permanently influence the pattern of groundwater flow (faults, geologic units, contact with surface water etc.) Hydraulic boundaries are derived from the groundwater flow net and therefore “artificial” boundaries set by the model designer. They can be no flow boundaries or boundaries with known hydraulic head.
Model Parameters Time, Space (layer top and bottom), Hydrogeologic characteristics (hydraulic conductivity, transmissivity, storage parameters and effective porosity) Initial Conditions Values of the hydraulic head for each active and constant-head cell in the model.
Calibration and Validation Calibration parameters are uncertain parameters whose values are adjusted during model calibration. Typical calibration parameters include hydraulic conductivity and recharge rate. Model validation is to determine how well the mathematical representation of the processes describes the actual system behavior.
Groundwater Flow Models MODFLOW (Three-Dimensional Finite-Difference Ground-Water Flow Model) When properly applied, MODFLOW is the recognized standard model. Ground-water flow within the aquifer is simulated in MODFLOW using a block-centered finite-difference approach. Layers can be simulated as confined, unconfined, or a combination of both. Flows from external stresses such as flow to wells, areal recharge, evapotranspiration, flow to drains, and flow through riverbeds can also be simulated.
Other Models MT3D (A Modular 3D Solute Transport Model) FEFLOW (Finite Element Subsurface Flow System) HST3D (3-D Heat and Solute Transport Model) SEAWAT (Three-Dimensional Variable-Density Ground-Water Flow) SUTRA (2-D Saturated/Unsaturated Transport Model) SWIM (Soil water infiltration and movement model) VISUAL HELP(Modeling Environment for Evaluating and Optimizing Landfill Designs) Visual MODFLOW (Integrated Modeling Environment for MODFLOW and MT3D)
Several methods to control saline intrusion Reduction of ground water extraction Artificial recharge by spreading Physical barrier Mathematical modelling of unsteady flow of saline and fresh water in aquifer