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
(Source: Ministry of Environment and Forests, Government of India)
2
Annual groundwater draft in comparison with net annual availability
in Ganga basin
100
90
80
70
60
50
40
Domestic and Industrial uses
Irrigation
30
20
Net annual Groundwater
availability (BCM/year)
10
0
(Source: Ministry of Environment and Forests, Government of India)
3
Annual replenishable groundwater in comparison with annual draft
in Ganga basin
90
80
76.35
70
60
50
37.19
40
29.19
30
9.31
9.45
0
30.36
Annual Groundwater Draft
(BCM per Year)
48.78
20
10
Annual Replenishable
Groundwater (BCM per Year)
11.56
0.43
5.58
17.12
2.27
11.65 12.99
1.39 10.77 1.06
0.12
(Source: Ministry of Environment and Forests, Government of India)
0.3
0.48
Schematic illustration for evaluating stream-aquifer interaction
Evaporation
Reach inflow
Groundwater
inflow
Change in storage
Stream inflow
Rainfall
Inflow or leakage
to/from
groundwater
Recharge to
groundwater
Reach Outflow
Stream Reach
Evapotranspiration
Groundwater
outflow
Stream outflow
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
Soil
Moisture
Ground water
7
Movement of water through the hydrologic cycle
(Source: usgs.gov)
8
Effluent and influent streams
Gaining stream
Losing stream with shallow watertable
Base flow
Losing stream with deep watertable
9
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
ET
Overland Flow
Pr
Seepage
Pumping well
Ir
Boundary
Per
Cap
Boundary
Qper
Sgrw
Qdr
Watertable
Qlsi
Qup
Qlso
Qdo
Clay
11
Groundwater Balance Equation
Considering the various inflow and outflow components in a given study area,
the groundwater balance equation can be written as:
Rr + Rc + Ri + Rt + Si + Ig = Et + Tp + Se + Og + S
where,
Rr =
Rc =
Ri =
Rt =
Si =
Ig =
Et =
Tp =
Se =
Og =
S =
recharge from rainfall
recharge from canal seepage
recharge from field irrigation
recharge from tanks
influent seepage from rivers
inflow from other basins
evapotranspiration from groundwater
draft from groundwater
effluent seepage to rivers
outflow to other basins; and
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

h

h

h
( Kx ) 
( Ky )  ( Kz )  0
x
x
y
y
z
z
with a source/sink term

h

h

h
( Kx ) 
( Ky )  ( Kz )   R *
x
x
y
y
z
z
18
Allows for multiple
chemical species
Dispersion
Change in concentration
with time
Advection
Chemical
Reactions
Source/sink term
 is porosity
D is dispersion coefficient
v is velocity
19
Model Grids
Finite Difference Grid
Finite Element Grid
20
Modelling Process
Conceptual
Model
Update Model
Unsatisfactory Results
Mathematical
Model
Computation
Satisfactory Results
Compare Model
and Field
Poor Fit
Calibrate
Model
Conclude study
(Decisions & Recommendations)
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
22
maximum basin-level efficiency.
23
24
• 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
350,000
300,000
294,364
250,000
200,000
150,000
198,962
143,961
112,490
100,000
71,485
34,341
50,000
4,317
0
1,484
Drainage Area (SqKm)
(Source: Status paper on river Ganga, NRCD, MoEF, 2009)
27
Soil types in Ganga basin
1% 2%
5%
4%
Mountain Soils
11%
Submontane Soils
Alluvial Soils
4%
Red Soils
3%
Red and Yellow Soils
Mixed Red and Black Soils
6%
Deep Black Soils
52%
12%
Medium Black Soils
Shallow Black Soils
Laterite and Lateritic Soils
(Source: Central Pollution Control Board, National River Conservation Directorate (MoEF) (2009))
28
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
Groundwater Modelling
• 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)
31
Model Design
• Conceptual Model
• Selection of Computer Code
• Model Geometry
• Grid
• Boundary array
• Model Parameters
• Boundary Conditions
• Initial Conditions
• Stresses
32
Modelling Process
Conceptual
Model
Update Model
Unsatisfactory Results
Mathematical
Model
Computation
Satisfactory Results
Compare Model
and Field
Poor Fit
Calibrate
Model
Conclude study
(Decisions & Recommendations)
33
General governing equation for transient, heterogeneous,
and anisotropic conditions

h

h

h
h
( Kx ) 
( Ky ) 
( K z )  Ss
 R*
x
x
y
y
z
z
t
Kx, Ky, Kz are components
of the hydraulic conductivity
Specific Storage
Ss = V / (x y z h)
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
Managed Aquifer Recharge
37
• 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
39
Methods for groundwater recharge
40
 Management of Excess Rainwater
 Mismatch between water supply and demand
Average value of rainfall and
water requirement (mm)
Rainfall (mm)
Water requirement (mm)
95
excess
90
85
excess
excess
80
75
70
deficit
65
deficit
60
55
50
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
Week number
 Possible solutions
 Rainwater conservation and recycling
 Multiple use of harvested water
 Managed aquifer recharge
 Management of stagnant water in lowland areas
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
44
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
47
48
49
50
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
52
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: 861404 km2) 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
(Source: physicalgeography.net)
57
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%.
90
80
76.35
70
60
50
40
Annual Replenishable
Groundwater (BCM per Year)
37.19
30.36
29.19
30
Annual Groundwater Draft
(BCM per Year)
48.78
20
10
9.45
0
11.56
9.31
2.27
1.39
5.58
10.77
1.06
11.65
12.99
17.12
0.43
0.12
0.3
0.48
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 1.025 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
Solute Transport Model
Flow Equation
Advection-Dispersion Equation
Distribution of Head
Velocity Field
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
Before extensive pumping
After extensive pumping by many wells
Pumping causes a cone of depression and draws the salt water upwards into the
well.
71
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
(Source: Status paper on river Ganga, NRCD, MoEF, 2009)
77
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)
v = q/n = K I / n
• Solute transport – requires
information on flow (velocities)
calculate concentrations
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)
General 3D equation

h

h

h
h
( Kx ) 
( Ky )  ( Kz )  Ss
 R*
x
x
y
y
z
z
t
2D confined:
2D unconfined:

h

h
h
(Tx ) 
(Ty )  S
R
x
x
y
y
t

h

h
h
( hKx ) 
( hKy )  S
R
x
x
y
y
t
Storage coefficient (S) is either storativity or specific yield.
S = Ss b & T = K b
Groundwater flow is described by Darcy’s law.
This type of flow is known as advection.
Linear flow paths
assumed in Darcy’s law
True flow paths
The deviation of flow paths from
the linear Darcy paths is known
as dispersion.
Figures from Hornberger et al. (1998)
In addition to advection, we need to consider
two other processes in transport problems.
• Dispersion
• Chemical reactions
Advection-dispersion equation
with chemical reaction terms.
advection-dispersion equation
groundwater flow equation
h

h

h

h
Ss

( Kx ) 
( Ky ) 
( Kz )  W *
t
x
x
y
y
z
z
advection-dispersion equation
groundwater flow equation
h

h

h

h
Ss

( Kx ) 
( Ky ) 
( Kz )  W *
t
x
x
y
y
z
z
Flow Equation:
 2h
h
T 2  S
t
x
1D, transient flow; homogeneous, isotropic,
confined aquifer; no sink/source term
Transport Equation:
 2c
c
c
D 2 v
 R
x
t
x
Uniform 1D flow; longitudinal dispersion;
No sink/source term; retardation
Flow Equation:
 2h
h
T 2  S
t
x
1D, transient flow; homogeneous, isotropic,
confined aquifer; no sink/source term
Transport Equation:
 2c
c
c
D 2 v
 R
x
t
x
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