91

CHAPTER 4

CONCEPTUAL MODEL OF THE AQUIFER SYSTEM

4.1 INTRODUCTION

A conceptual model for a groundwater system, is a working description or

representation of the hydrogeological units and the flow system of groundwater.

Development of the simulation model of seawater intrusion into the coastal aquifers

demands careful conceptualization of the groundwater system, encompassing the

hydrologic processes taking place in the targeted groundwater system. The model

development requires data on hydrologic properties such as transmissivity,

conductivity, storativity and specific yield of the individual aquifer unit. The purpose

of building conceptual model is to simplify the field problem and organize the

associated data so that the system can be analyzed more readily (Anderson and

Woessner, 1992). Development of conceptual groundwater flow model serves as a

prerequisite to develop a numerical model and hence is a foundation to

mathematically model the AG aquifer system to seawater intrusion. Thus, model

conceptualization is, perhaps, the most critical step in developing a groundwater

model (Kresic, 1997).

Preparation of conceptual model involves identification of the study area,

deciding appropriate boundary conditions, creation of usually three dimensional

model of hydrogeological system and estimation of sources and sinks. In theory, the

closer the conceptual view approximates the real-world conditions, the more accurate

the groundwater modeling results (Maidment et al., 2005). At the same time, the

conceptual view should be as simple as possible, as long as it remains adequate to

reproduce the system’s behavior (Anderson and Woessner, 1992; Hill, 1998; Hill,

2006). The aim of this chapter is to present a conceptual groundwater framework of

the study area based on hydrological properties of the coastal aquifers.

4.2 HYDROLOGIC SETTING

Hydrogelogic data is an important component of groundwater modeling

(Maidment et al., 2005) hence hydrogeological properties were studied. Hydrological

92

properties of the aquifer sediments and groundwater level data were collected,

analyzed to characterize the aquifer system. Aquifer properties include a variety of

data such as borehole records, well pumping rates, groundwater levels, rainfall, and

river networks.

4.2.1 Aquifer System of the Study Area

AGP is covered with alluvial sediments of Quaternary age and consists of

bedded fluvial – deltaic sediments of Aleshroud, Haraz, Garmroud, Babolroud, Talar

and Siahroud river systems. Log data of exploratory drill holes and bore wells drilled

to a maximum depth of ~350 m provide reliable details on the subsurface geological

features of the study area. The aquifer system was constructed based on the

exploratory wells and is comprised of four layers of hydrogeological units – an

unconfined aquifer, an aquitard, a semi-confined aquifer and a horizon with brine

water.

Two typical cross sections of the subsurface from North to South and from

west to east directions are shown in Figures 2.7 and 2.8. As can be made out from the

cross sections, the thickness of the aquifers in layer 1 and layer 3 vary from 10 m to

60 m and from 50 m to 200 m respectively. As detailed in Chapters 1 and 3, the

studies carried out in the study area to identify the salinized areas indicated that the

northeastern and northern tips of the area either do not have operational wells and

wells are abandoned because of high salinity (Cl > 250 mg/L). The region between

two major drainages is also characterized by high incidence of chloride resulting in

the restriction of their usages.

Two types of saline water occur within the aquifers of the study area recent

and palaeo seawater. Recent seawater is present in the fine grained sediments in the

unconfined aquifer of the NE region. Old seawater in the form of brine is present as

the bottom most layer and most of these wells are either abandoned or grout sealed.

4.2.2 Stratigraphy and Cross-Sections

Numerical modelling of groundwater flow systems requires correct prediction

of the role of geological structures in controlling the groundwater flow (e.g. Spottke

et al., 2005); this requires sufficient information to develop an acceptable

hydrogeological framework (Rudorfer, 2009).

93

The subsurface materials need to be defined as distinct model layers, here

called hydrostratigraphic units with similar hydrogeologic properties characterized by

their porosity and permeability (Sanchez et al., 1999). This may require a combination

of geological formations into one hydrostratigraphic layer, or conversely subdividing

a single geological formation into aquifers and confining units (Anderson and

Woessner, 2001).

In this study, for the preparation of hydrostratigraphic units, initially 99 well

logs of the study area were assessed and finally 66 well logs were selected to create

cross sections and solid (Figure 4.1 & Figure 4.2). For the interpretation of well logs

data, GMS 8.0 software was used. When locations for currently existing bores were

available in a single coordinate system (WGS84), the GMS software was then used to

create numerous cross-sections (Figure 4.3), as well as a proposed solid model of the

alluvial plain. The initial stratigraphic analysis considers a locally complex

sedimentary geology that includes three major alluvial units together with several

other layers that are not laterally extensive. The side elevation view of the bores has

been used to interpolate the stratigraphy across this study area.

GMS was used to integrate the borehole lithology picks for conceptual

understanding of the hydrological framework of the study region. Hydrogeologic

units (HGUs) was defined on boreholes. HGUs are typically a simplified

representation of the soil layers (Table 4.1) from the borehole field data.

After preparation of all well logs diagrams and defined relation between them,

interesting cross-sections of study area were created. A borehole cross section is a set

of polylines and polygons that define the stratigraphy between two boreholes. A

borehole cross section can be created manually or automatically. Figure 4.3 shows

the procedure carried out to connect wells together to create cross sections. Cross

sections were created manually using the Boreholes menu command. This uses a

triangulation process to determine the most likely connections between boreholes

Figure 4.4.

94

Figure 4.1 Distribution of bores that were considered in the stratigraphy analysis of

the study area

Figure 4.2 Display of well logs used to create cross sections and solid

95

Table 4.1 Borehole Hydrogeologic Units used in the creation of Conceptual model

MATERIALS LEGEND

H1 Sand, gravel and silt Layer one with fresh water

H2 Clay, silty clay and sandy clay Impervious layer

H3 Silt, gravel and fine to medium sands Layer three with fresh water

H4 Marine sand and silt with bivalves Bedrock with brine water

Figure 4.3 An example of a cross section created by GMS software

4.2.3 Three-dimensional model of hydrostratigraphy

Although the GMS has an automatic function to create cross-sections but more

satisfactory and detailed output is obtained when the modeller uses prior knowledge

of the depositional environment and interpolates between the bores so the manual

tools were used to draw cross sections. The diagram (Figure 4.4) shows the

interpretive cross-section interpolation between the bores that it is used for

preparation a fence diagram of study area (Figure 4.5).

Figure 4.6 shows a three dimensional (3-D) conceptual Model of the study

area with the third layer made up of silt, gravel and fine to medium sands, containing

fresh water as a base. The surfaces exhibit some broad patterns, such as the general

northward dip of the top of the AG coastal plain, allluvial plains all along the major

river courses.

-320

-270

-220

-170

-120

-70

-20

30

80

130

0 5 10 15 20 25 30

Ele

vati

on

(m

)

Distance (Km)

96

Figure 4.4 Sketch of a profile line linking selected well logs to create 3D fence

diagram

Figure 4.5 Fence diagram showing the vertical relations between the hydrogeological

units of the study area

N

97

Figure 4.6 3D conceptual model (SOLID) of the study area

Figure 4.7 GMS Borehole Cross section created from solid in N - S direction

-320

-270

-220

-170

-120

-70

-20

30

80

0 4 8 12 16 20 24 28 32

Ele

vati

on

(m

)

Distance (Km)

N S

N

S N

SW

NE E

W

98

Figure 4.8 GMS Borehole Cross section created from solid in E – W direction

Figure 4.9 GMS Borehole Cross section created from solid in NE – SW direction

The three borehole cross sections (Figure 4.7 to Figure 4.9) on this model

illustrate the following hydrogeologic features:

- The horizontal and vertical extent of the hydrogeologic units (aquifers –

unconfined and semi-confined and aquitard).

- The hydrogeogologic units dip from south to north.

- These cross sections pass through most of the wells sampled for chemistry in

this study.

- Both unconfined and semi-confined aquifers are hydraulically connected with

each other in the areas of recharge i.e., in the southern part of the study area

and in the northern part thereby forming a unified aquifer system.

-320

-270

-220

-170

-120

-70

-20

0 10 20 30 40 50 60

Ele

vati

on

(m

)

Distance (Km)

E W

-120

-70

-20

30

80

130

180

0 10 20 30 40 50 60 70 E

levati

on

(m

) Distance (Km)

NE SW

99

- The thickness and extent of the impermeable layer between these two aquifers

are variable and hydraulic connectivity may be there between the aquifers in

certain parts of the study region.

Solid and cross sections were used for the site characterization and

visualization. Most importantly, the solid created in this study was used to define

layer elevation data for MODFLOW models using the Solids → MODFLOW

command .

4.2.4 Hydraulic property

Initial estimates of the hydraulic properties of the AG aquifer were made from

the MRWA who had created a data set by regularly carrying out the pumping test of

the aquifer system. From the pumped aquifer transmissivity, hydraulic conductivity

(horizontal and vertical) and storativity (storage coefficient) were determined. In

layered systems, one also uses pumping tests to estimate the properties of aquitards

(vertical hydraulic conductivity and specific storage). These estimates were used as

input data in a digital groundwater flow model and calibration of the model led to

refined estimates for some hydrogeologic units. The estimated values of the aquifer

parameters were used to draw time draw down curve for individual test and compared

with the observed time-drawdown/recovery data. The values of the aquifer

parameters were calibrated till a best match is obtained.

4.2.4.1 Hydraulic Conductivity: The hydraulic conductivity of the geologic material

in an aquifer or confining unit is a measure of the ease with which water can move

through the material. It is a function of properties of both the matrix and fluid. Water

in the regional flow system is assumed to have a uniform density and viscosity, and

thus the hydraulic conductivity only varies as the grain size, shape, sorting and

packing vary (Freeze and Cherry, 1979).

Hydraulic conductivity values were assigned/estimated for the hydrogeologic

units of the study area in the model on the basis of hydrogeological investigations

including previous studies, available maps and bore well lithological data.

Horizontal hydraulic conductivity was computed by dividing transmissivity

values by values of aquifer thickness. Hydrogeological map provide information on

thickness of aquifer. The data was gridded to find out cell-by-cell values of thickness

of aquifer. The same procedure was adopted as for computing transmissivity values

100

for each cell. Thickness of aquifer in each cell is given in Figure 2.14. Horizontal

hydraulic conductivity map generated from the aquifer thickness and aquifer

transmissivity show that horizontal hydraulic conductivity ranges from 0.22 to 58.6

m/day with an average of 4.87 m/day (Figure 4.10). Southern part of the study area

shows maximum hydraulic conductivity values witnessed in the area and the value

steadily decreases towards the northern part. High conductivity values correspond to

the areas of intense recharge regions in the area.

Figure 4.10 Map showing spatial variation in the values of horizontal hydraulic

conductivity of the study area

Vertical hydraulic conductivity was estimated for the hydrogeologic units of

the study area on the basis of published and well log data. Vertical hydraulic

conductivity was assumed 20% of horizontal hydraulic conductivity for upper layer

and 6.6% of horizontal hydraulic conductivity for impervious layer and lower layer.

In the field also horizontal hydraulic conductivities generally are greater than vertical

101

hydraulic conductivities, as a result of the depositional history of the sediments. On

the basis of hydraulic conductivity values, the aquifers of the AG area can be

considered as anisotrophic and heterogeneous.

During the calibration of the model, hydraulic conductivity of the upper layer

in the study area was estimated at 1.9 -10 m/day with average of 6.22 m/day and 0.1

m/day for the impervious layer and for lower previous layer it was assumed to be 1.9

- 10 m/day with average of 6.22 m/day.

4.2.4.2 Specific Yield: Specific yield (SY) is the factor that is used to convert

saturated thickness to the actual volume of groundwater available. Thus, Specific

yield (Sy) is an important factor in water availability and indicates the amount of water

released due to drainage from lowering the water table in an unconfined aquifer. Data

on specific yield of the AG aquifer could not be obtained due to non-availability.

Hence, the sediments obtained from the exploratory borehole logs were studied and

Specific yield was estimated using average values of sediments given in the literature

as shown below (table 3) and the data was slightly modified during the calibration of

the model. The specific yield map generated for the AGP indicates that the aquifer is

highly productive excepting in a small zone in the northern part of the area wherein

the estimated value is around 8 % (Figure 4.11).

Table 4.2 Selected Values of Porosity and Specific Yield of the sediments (Sources:

Freeze and Cherry, 1979; Todd, 1980; Driscoll, 1986)

Material Porosity (%) Specific yield (%)

Clay 45-55 1-5

Silt 35-50 5-10

Sand 25-45 10-30

Gravel 25-35 15-30

Sand and gravel 20-30 10-20

Glacial till 20-30 5-15

102

Figure 4.11 Map showing specific yield zones of the study area

4.2.4.3 Specific Storage and Storage Coefficient: Model requires data of specific

storage and the value of specific storage is multiplied with layer thickness to calculate

storage coefficient. Specific storage (Ss) and its depth-integrated equivalent,

storativity (S=Ssb), are indirect aquifer properties which cannot be measured directly

and they indicate the amount of groundwater released from storage due to a unit

depressurization of a confined aquifer. The stroage coefficient of unconfined aquifers

is virtually equal to the specific yield, which means that nearly all of the water is

released from storage by gravity drainage. The storage coefficient for various Layers,

which are being treated as confined or compressive units, can be calculated by

multiplying specific storage with their thicknesses. They are fractions between 0 and

1. The storage coefficient of most confined aquifers ranges from about 10-5

to 10-3

(Heath, 1983).

The size of the storage coefficient is dependent whether the aquifer is

unconfined or confined. In a confined aquifer setting, the load on top of an aquifer is

103

supported by the solid rock skeleton and the hydraulic pressure exerted by water (the

hydraulic pressure acts as a support mechanism). Conversely, in an unconfined

aquifer setting, the predominant source of water is from gravity drainage and the

expansion of water and compaction of the rock skeleton is negligible. Thus, the

storage coefficient is approximate to value of specific yield and ranges from 0.1 to

about 0.3.

The following table (table 4.3) provides representative values of specific

storage for various geologic materials. An initial value of 4.5 ×10-5

to 1×10-4

was

assumed of the storage coefficient for the confined aquifer and permeable zones by

considering the geologic material of aquifer.

Hydraulic properties like conductivity and storage coefficient are assigned for

each geologic formation using parameters. For transient simulations, it calculates the

effective specific storage and specific yield.

Table 4.3 Values of specific storage for various geologic materials (Domenico and

Mifflin, 1965 as reported in Batu, 1998)

Material Ss (ft-1

)

Plastic clay 7.8x10-4

to 6.2x10-3

Stiff clay 3.9x10-4

to 7.8x10-4

Medium hard clay 2.8x10-4

to 3.9x10-4

Loose sand 1.5x10-4

to 3.1x10-4

Dense sand 3.9x10-5

to 6.2x10-5

Dense sandy gravel 1.5x10-5

to 3.1x10-5

Rock, fissured 1x10-6

to 2.1x10-5

Rock, sound < 1x10-6

4.2.5 WATER BUDGET

Development of conceptual model normally involves preparation of water

budgeting for estimating groundwater recharge and discharge. Water budget

preparation is a complex exercise which involves collection of data related to rainfall

for number of years, identification of groundwater extraction sources and estimation

of gross extraction from each source once the data is collected recharge is estimated

by applying water balance equation. In the following sections, groundwater discharge

and recharge components of the AG aquifer has been discussed followed by the water

balance of the study region.

104

4.2.5.1 Groundwater Discharge: Groundwater discharge is defined as the volumetric

flow rate of groundwater through an aquifer or discharge is simply the movement of

water out of an area of saturated soil. Discharge from the ground-water reservoir takes

place by one or more of these methods: Evaporation, plant transpiration, seepage

into streams, as discharge from springs, or by pumping from wells. Recharge and

discharge processes are more discernible for direct observation in unconfined aquifers

than in semi-confined or confined aquifers. The amounts of lateral inflow and outflow

are subject to change from one year to another due to different hydrogeological

parameters (rainfall, pumping, etc).

According to MRWA there are about 80200 wells (Table 1.1) of varying

depths in AGP. Groundwater use of the study area is steadily increasing from time to

time as the population is increased. The groundwater discharge from the two aquifer

units of the area is mostly by withdrawals from wells People have been using

groundwater for thousands of years and continue to use it today, largely for domestic

and irrigation purposes. The estimated abstractions from wells viz., municipal,

agricultural, settlement abstractions, etc., constitute nearing 84% of the total discharge

of groundwater from the study area (Table 4.4).

Subsurface drainage is the second most important form of discharge (6%) of

groundwater from the aquifers of the study area. Groundwater is one of the

contributors to flow in many streams and rivers, specially in lean periods. Discharge

into the streams occurs mainly through seeps in and along the stream channels. In the

downstream regions of the study area i.e., at the mouths of rivers, where the rivers are

almost in equalibrium with the water table seepage enters the rivers.

Groundwater discharges from the system by evapotranspiration, where the

water table is shallow, is the another agent of discharge.

A spring is an easily visible point of groundwater discharge, but there are

many other places that act as discharge points. Groundwater discharge occurs when

water seeps from aquifers into rivers, streams, and lakes. Water may also seep out of

the ground into wetlands and marshes.

In areas where the water table is close to the top of the ground, groundwater

may be mostly discharged through the actions of growing plants as they draw water

out of aquifers, and release it into the air as moisture.

105

Table 4.4 Discharge components during the run of the model for the periods 2008-

2011

Discharge

Components Water year

2008-9 2009-10 2010-11 avg. %

Lateral outflow 49462 41366 39285 43371 3

Rivers 74059 57115 58064 63079 5

Drains 114563 70620 60846 82010 6

Wells 1134870 1136984 1162307 1144720 84

Evapotranspiration 26354 26896 28210 27153 2

Sum 1399307 1332980 1348712 1360333 100

4.2.5.2 Groundwater Recharge: Groundwater recharge is defined in general sense as

the downward flow of water reaching the water table, forming an addition to

groundwater reservoir. A clear distinction should thus be made, both conceptually and

for any modelling purposes between the potential amount of water available for

recharge from the soil zone and the actual recharge. Quantification of the rate of

natural groundwater recharge is a basic prerequisite for efficient groundwater

resources management (Lerner et al., 1990).

Groundwater recharge cannot be measured directly and is difficult to

accurately estimate. Indirect recharge estimate can be subjected to large errors and

therefore, when possible, more than one method should be used to verify the

estimates. A common indirect method for hydrologic system under equilibrium

condition is to equate groundwater recharge to groundwater discharge (Belay, 2006).

The Recharge Package of MODFLOW was used to simulate the spatial distribution of

the net recharge (rain, return flow) to the study area.

Groundwater recharge is the replenishment of an aquifer with water. Aquifer

recharge rates vary from year to year in response to changes in annual precipitation.

Various types of input have been identified in groundwater recharge eg., rain water,

return flows. For the study area, recharge from river (27.9 %) and lateral inflow

(27.5) are the main sources of recharge. Return flows from irrigation field and

domestic use account for nearing 40 % of the recharge whereas the rain contributes

only 4.7 % to the recharge of the aquifers (Table 4.5).

106

Table 4.5 Recharge during the run of the model for the periods 2008-2011

Recharge

Components Water year

2008-9 2009-10 2010-11 avg. %

Rivers 382244 353720 337855 357940 27.9

Lateral inflow 344936 355692 358168 352932 27.5

Rain 74570 65846 41870 60762 4.7

Irrigation Return flow 270366 289846 316298 292170 22.8

Waste water return flow 212216 216876 222410 217167 17.0

Sum 1284332 1281979 1276601 1280971 100.0

4.2.5.3 Water Balance: A water balance involves the calculation of the volume of

water both entering and leaving the aquifer system being studied and incorporates the

elements shown in Table 4.4 and 4.5. This is an important factor in all groundwater

assessments as it defines the resources available in any given region. Where

numerical models are being developed, a water balance may be used to gauge the

accuracy of the model in replicating the hydrogeological processes involved

(Brassington and Younger, 2009).

Conceptually, in this present study, water enters the aquifer as recharge by

rainfall, return water from drinking water and excess irrigation through the soil,

laterally from up-gradient parts of the aquifer, laterally from Caspian sea (seawater

intrusion), and through the riverbed sediments when the river water elevation is

greater than the water table elevation. Water exits the aquifer as discharge to the sea,

through water bores, discharge through drains, as river baseflow and via

evapotranspiration. Groundwater storage changes by the difference between the

inflows and outflows. Based on the estimates of all water inputs and outputs to AG

aquifer, a water balance can be developed as shown in table 4-6 for the years 2008-

2011.

107

Table 4.6 Water balance of the AG aquifer from 2008-9 to 2010-11

Water Budget

Inflow(m3) Out flow(m

3)

Components Water year

Components Water year

2008-9 2009-10 2010-11 2008-9 2009-10 2010-11

Rivers 382244 353720 337855 Lateral outflow 49462 41366 39285

Lateral inflow 344936 355692 358168 Rivers 74059 57115 58064

Rain 74570 65846 41870 Drains 114563 70620 60846

Return water

from irrigation 270366 289846 316298

Wells 1134870 1136984 1162307

Return from

drinking water 212216 216876 222410

Evapo-

transpiration 26354 26896 28210

Sum 1284332 1281979 1276601 Sum 1399307 1332980 1348712

4.3 NUMERICAL SOLUTION METHODS

The modeling process involves representation of physical reality by a

mathematical model. The numerical solution for the mathematical method is

described by partial differential equations (PDE). The three classical choices for the

numerical solution of PDEs are the finite difference method (FDM), the finite element

method (FEM) and the finite volume method (FVM) (Peir’o and Sherwin, 2005).

Finite difference and finite element methods are most popular to solve the

groundwater flow equations by discretizing the entire targeted aquifer system into

grids. However, the present author preferred to adopt FDM, as it provides graphical

user interfaces to its input file, and typically incorporating pre- and post-processing of

user data. Further, FDM is easy to handle for numerical computations. MODFLOW

is a well-known example of a general finite difference groundwater flow model.

4.4 BOUNDARY CONDITION

The groundwater system of the study area is to be delineated from the

surrounding groundwater systems as a first step in modelling. Consequently, a model

boundary needs to be defined before taking up any groundwater modeling exercise.

Model boundary is the interface between the study area and the surrounding

environment (Spitz and Moreno, 1996). Anderson and Woessner (1992) have defined

boundary conditions as mathematical statements specifying the dependent variable

(head) or the derivative of the dependent variable (flux) at the boundaries of the

108

problem domain. They have also mentioned that regional groundwater divides are

typically found near topographic high and may form beneath partially penetrating

surface water bodies. It is common for large scale models to be bounded by well-

defined physical features such as rivers, bedrock outcrops or groundwater divides

(Gurwin and Lubczynski, 2005; El Idrysy and De Smedt, 2006).

The study area AGP is characterized by well defined physical boundary

conditions. Figure 4.12 shows the boundary conditions. In the south, specified flow

pattern was taken as the boundary and the geographical boundaries were used to

conceptualize the other sides of the model. The eastern and western boundaries

coincide with existing main river courses of Aleshroud and Siahroud respectively.

The northern boundary coincides with the south Caspian coastal line.

Figure 4.12 Satellite imagery of the study area showing the boundary conditions fixed

in the model

In the study area, the general direction of groundwater flow is from south to

north i.e., from piedmonts of Alborz mountains to Caspian Sea. Groundwater as a

base flow enters in the active model area at its southern boundary and outflows at its

northern boundary. For modelling purpose, Aleshroud, and Siahroud rivers have been

109

treated as line sources of recharge and/or sinks. The upgradient (south) boundaries are

being simulated using specified flow package. The inflow in each layer has been

distributed among the active cells along the southern boundary. The down gradient

(north) Caspian sea Time-Variant Specified Head value was assigned as -26.62 m

equal to the sea level during of the running time of the conceptual model for steady

state.

The upper boundary of the model is the water table, which is a free surface

boundary that receives spatially variable recharge from precipitation, river,

agricultural and wastewater return flows. The lower boundary of the freshwater flow

system is the transition between freshwater and saltwater (brine water). The arbitrary

bottom altitude was specified as a no-flow boundary for this analysis.

4.5 SPATIAL DISCRETIZATION

Finite-difference methods require the discretization of space into grid

consisting of rows, columns, and layers of model cells. Hence, the aquifer system of

the study area was spatially discretized into square grid encompassing 112 rows and

150 columns. Vertically the aquifer is treated as a 3 layer system. Each model cell is

500 by 500 m. The 500 m × 500 m cell size was chosen based on the large size of the

area. The discretized model area is shown in Figure 4.13.

The model comprises about 2434 km2

(active area). The model area was

discretized into 16805 cells (Table 4.6). The areas lying outside the boundary of the

study area (about 1767 sq. Km) constitute inactive cells i.e., the cells existing outside

of the Aleshroud-Haraz and Talar-Siahrud basins i.e. left side of Aleshroud river and

right side of Siahroud river have been treated as inactive cells. The remaining cells

were treated as active cells. The model grid is aligned 15 degrees towards west so as

to align the model grid with dominant direction of the existing coastline.

As mentioned above, in the vertical dimension, the model is a three-layered

and the top most surface of the model represents the land surface of the study area.

Second and third layers of the model were visualized from the field data obtained

from the exploratory wells and also from the vertical electrical sounding.

While developing a groundwater model, not only spatial domain is discretized

into cells but temporal domain is also discretized into time steps. Typically a year is

digitized into 2 to 4 periods based on seasons with the requirement that during one

time period, various stresses and parameters exhibit unique relationship or remain

110

constant. In the present study, time is discretized into two periods in a year, that is,

non-growing season and growing season. Non-growing season is considered from the

end of September to the end of February. Most of the rainfall in the study area occurs

during this period owing to which the requirement of fresh water for agriculture is

low. The remaining period is considered as growing season where there is practically

less recharge from rainfall and groundwater extraction is more in comparison to the

non-growing period.

Figure 4.13 Model configuration of AG aquifer

Table 4.7 Specification of spatial discretization of model domain

Description No of Cells Area (Km2)

Inactive cells 7068 1767

Active cells 9737 2434

Sum 16805 4201

N

Active area

Inactive area