1

Surface and groundwater dynamic interactions in the Upper Great Chao

Phraya Plain of Thailand: semi-coupling of SWAT and MODFLOW

[1]Werapol BEJRANONDA, [1] Sucharit KOONTANAKULVONG, [2]Manfred KOCH

[1]

Department of Water Resources Engineering, Faculty of Engineering, Chulalongkorn University, Thailand,

sun2child@yahoo.com [2]

Department of Geohydraulics and Engineering Hydrology, University of Kassel, Germany

kochm@uni-kassel.de

Abstract Surface and groundwater dynamic interaction in the Upper Great Chaophraya Plain of Thailand is

explored. The surface-soil water model SWAT and the groundwater model MODFLOW are semi-coupled to

determine the various hydrological components and their relevant flow behaviour. The coupled models are

executed by running the simulations individually using the monthly river-groundwater interactions and the

groundwater recharge to generate the surface-groundwater dynamic interactions. The calibrated results from

coupled simulations show that the coupling-method improves both the streamflow and the groundwater

simulations. From the water balance analysis a quantitative picture of the local surface and subsurface water

interaction is obtained. The semi-coupled models result in streamflow- and groundwater level estimates that are

improved by 12% and 2.3%, respectively, especially in the dry season. The dynamic simulations reveal that the

interaction is seasonally dependent The improvement of the water balance analysis suggest that coupled

modelling is a key to clarify the relationship between surface- and subsurface water, as well as their interaction.

Keywords: Surface-groundwater interactions, semi-coupled model, SWAT, MODFLOW, Thailand

1. Introduction In spite of the tremendous steps made in recent years towards becoming an industrialised country,

Thailand economically still defines itself as an agricultural country, owing to the fact that the export

of agricultural products is still bringing in a large portion of its national revenue. Boosting up rice

production and the, often precarious, living conditions of the rice farmer has thus been an active policy

of the Thai government in recent years and has led it to develop agricultural price-subsidized schemes

and many irrigation projects in the Great Chao Phraya Plain to support the local farmers

(Koontanakulvong et al., 2005). At the same time both surface water and groundwater resources have

been developed to respond to the increased water consumption. Since it is not possible to provide

sufficient surface water for irrigation, conjunctive water uses are to be explored as part of a major

national effort to optimize the water resources for the various stakeholders involved.

Providing irrigation water through irrigation canals sipping adjacent streams has been the classical

approach method in so-called irrigation projects in Thailand to augment the agriculturally available

water there during a dry season or drought periods, the latter becoming more frequent in recent years,

due to climate change. As the canal irrigation water has to be increasingly complemented by pumped

groundwater, complex surface-groundwater interactions occur which need to be understood for the

setup of a comprehensive water balance analysis. This is the objective of the present conjunctive water

use study wherein a new semi-coupled surface- groundwater model will be employed.

Historically the coupling of surface- and subsurface flow models started with the setup of relationships

between river stages and groundwater storage (Pinder and Sauer, 1971). One widely used

computational model to solve for the complex flow between surface and subsurface has been the

stream-aquifer interaction program MODBRANCH (Swain, 1996), which couples the groundwater

flow model MODFLOW (McDonald and Harbaugh, 1998) and the river network program BRANCH.

Orhan and Mustafa (2004) coupled a one-dimensional channel network flow model that uses the St.

Venant equation with a 2D groundwater flow model. Smits and Hemker (2004) modeled the

interaction of surface-water and groundwater flow by linking Duflow to Microflow. Ellingson and

Schwartzman. (2004) integrated both an unsaturated zone flow model and a groundwater model into

the regional HSPF Model. Coupling of the surface-water model SWAT (Arnold and Fohrer, 2005;

2

Arnold et al., 1998) with the groundwater flow model MODFLOW was endeavoured by Il-Moon

Chung (2006). However the full coupling of surface- and subsurface water model is still complicated

for practical applications. Semi-coupling appears to be a more viable approach for practical

simulation. Here the surface-water model SWAT and the groundwater flow model MODFLOW are

semi-coupled. The dynamic interaction is achieved by running the two models individually, though

with an intermediate interchange of information between the surface- and subsurface compartments

using hydraulic functions. This semi-coupled model is applied to the Upper Great Chao Phraya Basin.

2. Study area

The Upper Great Chao Phraya Plain of Thailand (Fig.1) covers an area of ~38,000 km2, extending

over 8 provinces, with a population of 4 million people. The main land-use is 63% agricultural, out of

which 21% is irrigated, and 24% forest. More than 90,000 groundwater wells exist in the region

(Faculty of Engineering, 2007). The main groundwater basin is dissected by five major rivers that flow

from north to south and which, geologically, form a depositional flood plain. The basin is surrounded

in the east and west by mountains of volcanic rocks. The average elevation of the basin is 40-60

m.MSL. The basin drains into the lower basin in the south, though the free discharge is partially

obstructed by crystalline rocks there. The 900 - 1,450 mm annual rainfall within the study region is

apportioned to 81 % in the wet (Apr.-Sep.) and to 19 % in the dry season (Oct.-Mar.).

3. Theoretical basis of the coupled surface- and groundwater flow model

3.1 Coupling-process development The development of the new coupled model can be divided into three major steps: 1) study and design,

2) coupling and testing and, 3) application & evaluation (see Fig. 2).

Study and design: The watershed model SWAT-2005 and the groundwater flow model MODFLOW-

2003 are selected to simulate the surface water- and groundwater balances. The original versions of

SWAT and MODFLOW are set up for a simple test case and used to explore the relationships between

surface- and subsurface water. The study of the coupling process is designed to cover three major

subjects, namely, coupling components, areal coupling, and temporal coupling,.

Coupling and testing: The designed coupling processes are applied to a basic case to test the coupling

techniques by comparing their output with results of a simple analytical CN rainfall-runoff equation.

Once the coupled simulations match the analytical results and the theoretical hydrological processes

involved, the coupled model will be applied to the real field data.

Application and evaluation: The original SWAT and MODFLOW models are applied to the Upper

Great Chao Phraya Plain where they are also calibrated and verified. The theoretical coupling-process

is also included in the modeling procedure. Through comparison with the observed data, the

uncoupled and coupled simulations are assessed for the ongoing coupling-process development. The

coupling-process is then evaluated by measuring the quantitative improvements in the model accuracy,

defined as the difference of the calibration errors between the uncoupled and coupled models.

3.2 Coupling methodology

3.2.1 General aspects The interactions between the surface- and groundwater-modeled groundwater recharge and the river-

aquifer interactions are used to couple the models. The percolation, i.e. the infiltration of soil water to

the shallow aquifers, as included in SWAT, and the river-groundwater interaction, i.e. the exchange of

water between the river body and the groundwater as included in MODFLOW, are selected for the use

in the coupling process and for the interaction of the components. While the model is operating in the

“coupling process”-mode, SWAT and MODFLOW are connected through some selected components

that exhibit substituted duplicated functions. For example, in the coupled model the groundwater

recharge is replaced by the percolation, as computed by SWAT, and the baseflow is substituted by the

river-groundwater interaction, as computed by MODFLOW (cf. Figs. 3 to 5). The coupled model is

executed by a VISUAL BASIC interface program which has been written to operate and transmit each

of the two model’s information to the coupling process.

3

Fig. 1. Study area showing river networks and alluvial aquifers.

Fig. 2. Study scheme of the coupled model.

3.2.2 Areal coupling The space dimensions of both the surface- and the subsurface water model are calculated in different

ways. SWAT, a distributed hydrological model, is set up with polygon-basins and a river-network,

whereas the FD-model MODFLOW is constructed using grid-cells and river-nodes. These differences

in the spatial distributions of the two models require the coupling process to be accounted for by

interactions of (1) the land- and (2) the river phase. The land phase describes the one-way! recharge

from SWAT polygons to MODFLOW grid-cells, and the river phase the two-way! interaction of the

SWAT stream polyline with the MODFLOW river nodes (see Fig.4). The river recharge/baseflow in

the various sub-basins is calculated with the river-groundwater interaction function as programmed in

MODFLOW’s RIVER-modul. In order to define the interaction for the entire study area, the surface-

water model boundary is extended sufficiently outwards so that the subsurface boundary is included.

GIS assists in the coupling process by joining SWAT sub-basin-polygons to MODFLOW grid-cells.

Southeast Asia

Thailand

Ping river

Yom river Nan river

Chao Phraya river

Sakae Krang river

study area

alluvial

aquifers

Y.14

N.12A

P.7A

C.2

4

SURFACE WATER SYSTEM

GROUNDWATER SYSTEM

SURFACE AQUIFER

PRECIPITATION

1283

DIRECT

RUNOFF

342

E/T

768

SOIL PROFILE

187

PERCOLATION1

74(28.9*)

RIVER-GW

INTERACTION

5.4

LEAKAGE

0.14

PUMPAGE

12.4

LATERAL FLOW

0.02

STREAMFLOW

(OUT)

444(#42)

GROUNDWATER SYSTEM

LOWER AQUIFER

STREAMFLOW (IN)

403

WATER USE

63

STREAM

-FLOW

LATERAL

FLOW

96

SOIL

STORAGE

16.6

AQUIFER STORAGE

13.0

PUMPAGE

2.4

AQUIFER STORAGE

11.7

RIVER-GW

INTERACTION

0.55

LATERAL FLOW

0.77

PERCOLATION2

(15.8*)

SURFACE

ABSTRACTION

512

**Coupled

component Unit : mm. – depth of water

Fig. 3. Surface water and groundwater hydrological components with water balance analysis results.

Fig. 4. Conceptual coupling of the surface- and the groundwater flow model.

6

13

16

4

19

3

20

9

1

2

8

10

12

5

14

18

15

7

21

22

11

17

6

13

16

4

19

3

20

9

1

2

8

10

12

5

14

18

15

7

21

22

11

17

SWAT MODFLOW

land phase

coupling

river phase

coupling

SWAT

Polygons

MODFLOW

Grid-cells

model overlay

5

3.2.3 Temporal coupling As groundwater movement is usually very slow when compared to river discharge, different time-

scales of the surface- and groundwater simulations are needed. Thus, whereas the time-step chosen for

the groundwater model MODFLOW is one month, that of the surface water model SWAT is one day,

though the output for the latter is checked usually once a week. Therefore, during the coupling process

model input/output data is exchanged at specific coupling intervals to link the models’ different time

dimensions, namely, one month in the present situation. Although other exchange time intervals could

be selected, in the case of which an interpolation is required if the MODFLOW time-step is not an

integer multiplier of the SWAT time step, the one-month step turned out to be optimal, as this is also

the sampling time of the pumping data.

4. Model application to the Upper Great Chao Phraya river basin First the traditional SWAT and MODFLOW models are set up and run individually. Subsequently,

they are coupled together. The conceptual build-up of the coupled model is endeavored using

information on the topography and the hydrogeology (aquifers’ layouts and parameters, recharge

behavior, etc.). The surface- and subsurface boundaries of the coupled-model are defined identical to

those of the individual models SWAT and MODFLOW. The river network and the watershed

boundaries are used to describe the coupling interaction at the interface of specific coupling zones.

During the coupling development, the surface- and subsurface compartments of the Upper Great Chao

Phraya Plain are connected to each other through 5 river course and 22 watersheds (cf. Fig. 5).

4.1 Surface water model The major hydrological processes conceptualized in the SWAT surface-water model are rainfall,

streamflow and soil-water. Scattered rain falling on areas of the earth’s surface of various land-use

and soil type causes a different behavior of the overall water movement, whereby some rainfall is

converted to runoff, flowing out of the basin, and some infiltrates into the ground to become soil-

water, before recharging the shallow aquifer and/or adjacent streams.

The watershed area is used to define the model’s boundary. The P.7A, Y.14, N.12A stream gauges are

defined as vertices of the flow-in boundary, whereas the stream gauge C.2 at the Chao Phraya river in

the south is assigned the major basin’s outlet. Using a DEM of the total basin, 22 additional sub-basins

are defined, covering the Ping, Yom, Nan Sakae Krang and Chao Phraya river (Fig.1). Infiltration

(areal recharge) is derived from rainfall, land-use characterization soil properties.

4.2 Groundwater model The MODFLOW groundwater conceptual model, namely the aquifers and their confining boundaries,

are defined using the concept of hydrostratigraphic units. The aquifer system in this study is set up as a

two-layer aquifer, whereby the thickness of the upper, semi-confined layer varies between 40 and 100

m and that of the lower, confined layer between 100 and 300 m. The 3-D block-centred grid model

representing the groundwater basin has a grid-size of 10 km x 10 km (Fig.4), resulting in 320 elements

in the upper and 346 elements in the lower layer. The western, eastern and northern borders of the

model are assumed to be an impermeable body of consolidated rock and are defined as specific inflow

boundaries, using information from the available head distribution there. The southern boundary

which is partially blocked by impermeable rocks and forms a narrow trough between the mountains in

the east and west is set up as an outflow boundary. The hydraulic properties of the aquifer, namely,

hydraulic conductivity, transmissivity and specific storage have been estimated by means of pumping

test data. The river-aquifer interaction of the five main rivers which are mainly responsible for the

monthly recharge are derived from the hydraulic properties of the rivers’ bed materials, cross-

sections, river stages, and the computed, seasonally varying groundwater table. The recharge, the

river stages, as well as the surface- and groundwater use, especially for rice farm irrigation, has been

adapted in response to the seasonal climatic conditions, namely, in terms of the available amount of

rainfall and reservoir storage. As for the possibility of return flow of irrigated water into the canals, it

is considered to be insignificant in the study area since, during the dry seasons, the drainage canals are

nearly drying out and the irrigation area covers only 13% of the effective recharge area of entire model

(Bejranonda et al., 2006).

6

Fig. 5. The schematics of the coupled surface-water and groundwater model in the study area.

4.3 Coupled model The sensitivity study of the groundwater model (Bejranonda et al., 2006) indicated that the most

significant parameters affecting groundwater levels are the areal groundwater recharge and the degree

of river-aquifer interaction. Therefore, in the present study, the percolation (areal recharge) computed

by the SWAT surface-water model is applied on the top layer and on the outcropping sections of the

groundwater model, and the river-aquifer interaction (channel recharge) module of the MODFLOW

groundwater model is linked to the streamflow network of the SWAT model. The conceptual set-up of

the coupled model is based on the original SWAT and MODFLOW models in the study area, with

particular consideration of the connection of the rivers, aquifers and the soil water.

During the operation of the coupled model, the major rivers contribute water to the aquifers and vice-

versa – as calculated by the river-aquifer interaction function - and 22 sub-basins provide recharge to

the groundwater - regulated by the infiltration function - (Fig.5). Both surface- and groundwater

models are bonded through the groundwater recharge and the river-aquifer interaction, using a

VISUAL BASIC interface program (Bejranonda, 2007) to exchange input and output parameter

between the two models. The coupled parameters are transferred back and forth two times, 1) after the

original SWAT and MODFLOW have been run individually, the time-series output of the SWAT

modeled percolation is sent as input to the groundwater model, 2) then the groundwater model is run

again with these new input parameters and the model-given river-groundwater interaction terms to

recalculate the streamflow discharge computed earlier with the surface water model. The final output

of the coupled model represents then the results of the individual hydrological components as

incorporated in the original SWAT and MODFLOW model as well as the dynamic interaction at their

interface zones (Fig. 5).

4.4 Calibration and verification The calibration and verification of the SWAT surface-water model is performed using monthly

streamflow discharge obtained from the stream gauge at the basin outlet. The SCS CN-numbers of

three different land-uses, forest, rice-growing and agriculture, as well as the available water capacity

(AWC) of 5 different soil-types are calibrated in the time-interval 1993-1998. The subsequent

verification of the surface-water model has then been performed with 1998-2003 historical river

discharge data and results in relative seasonally and monthly errors of 53%/36% and 77%/59%,

respectively, in the wet/dry season. As these results represent the ability of the uncoupled surface

water model to simulate the basin discharge, they will subsequently be compared with those of the

coupled model.

The MODFLOW groundwater-model calibration and verification is performed in steady state as well

as in transient state. Following the seasonal crop pattern, the seasonal stress periods are used in the

calibration of the historical groundwater levels, recorded in two-week intervals. Since during 2001-

2003, due to an invariant situation for the surface water, the groundwater use was almost stable, the

precipitation

Surface water model

Groundwater model

surface water use

streamflow

(out)

lateral flow

(out)

Ping, Yom,

Nan,Sakae Krang,

Chao Phraya river

22 sub-basins River-aquifer

interaction

infiltration

streamflow (in)

lateral flow

(in)

groundwater

use

7

average water levels during the dry season of 2003 are selected to be the representative steady-state

water levels in the calibration. 13 zonal groups of the hydraulic conductivity are adjusted during the

steady-state calibration process. The transient-state calibration is carried out, using the 1993-2003

historical water levels, whereby in addition groups of the specific storage parameter are calibrated.

The transient simulation is initialised from an average wet-season water level and the pumping rates

weights are fine-tuned during the process, as these are often prone to errors.

As a final result of the calibration process the root mean square errors (RMS) of the hydraulics heads

are found to be 3.70 m in steady-state and 5.67 and 4.79 m for the wet and dry season, respectively, in

transient mode. An a posteriori transient-state verification/forecast, using two years of groundwater

level data (2004-2005) and water level data from 50 extra observation wells collected during the study

period (2005) has additionally been carried out, resulting in an RMS of 5.95 m. Again, these results of

the uncoupled groundwater model are the reference for the test of the following coupled model.

Using the new coupled model composed of the calibrated SWAT and MODFLOW models, the entire

historical data of both surface- and groundwater used above is re-simulated. The coupled model

results in model-fit errors for the streamflow discharge of 42%/27% and 70%/45% for the seasons and

the months, respectively, for the wet/dry seasons, whereas the RMS of the groundwater heads are

5.62 and 4.69 m for the wet and dry season, respectively. Comparing these results with those obtained

for the individual stand-alone models above, clearly indicates a simulation improvement of the

coupled model, especially as far as streamflow is concerned

4.5 Results of the coupled model

4.5.1 Surface- and groundwater flow results The simulation of both surface water and groundwater in the study region is performed with both the

uncoupled and coupled models, thus allowing to evaluate the beneficial effects of the latter. The

calibrated results of the coupled simulations for the 1993-2003 period provide monthly water flow

parameters within both the surface and the sub-surface compartment of the hydrologic cycle,

including its interaction. The simulation results are analyzed relative to the average yearly water

situation, the latter being categorized, respectively, as drought, dry, normal and wet.

Compared with the uncoupled surface-water model, the coupled surface water simulation shows the

calculated streamflow discharge at the basin outlet to be more accurate, especially, during low flow of

the dry season. The overall simulated hydrograph pattern is more accordant to the observed

streamflow, particularly in normal and wet years. The streamflow calculation of the uncoupled and the

coupled models is significantly modified. The shadowed area in Fig.6 shows the differences in the

calculated discharge of two model approaches which sum up to a total average of 240 million

m3/month. The streamflow hydrograph (Fig.6) is composed of low-flow and high-flow periods. The

model coupling increases the lower baseflow into the river during the low-flow period, but reduces

excessive baseflow during flood-events in within high-flow periods. According to the calculated

streamflow hydrograph, the coupling process reduces the discharge errors enormously in low-flow

and, somewhat less, in high-flow times.

The results of the coupled model shows that the seasonally groundwater levels are, on average, about 4

m below ground surface (GS) in the wet season, but drop to 6-9 m below GS in the dry season (Fig.7).

Significant head drops of 2.5-7 m are observed between the wet and the dry season in one year, mainly

in the dry season of a drought year. Benefitting from the coupling process, the accuracy of the

groundwater level simulations are improved considerably in the dry season, though somewhat less in

the wet season. Compared with the uncoupled simulations, there is a only a little bit of difference in

the groundwater levels of the order of 0.03-0.12 m. The coupled model produces a reduction of the

groundwater recharge, resulting in a groundwater level decrease of 0.11 and 0.12 m for the wet and

dry season, respectively. Because the groundwater recharge is dominated by surface water recharge

(Fig.8), the river-aquifer interaction has less influence on the groundwater levels. Nevertheless, for

the groundwater level close to the main rivers, where usually high hydraulic conductivities are

encountered, Fig.9. illustrates that the changes are rather insignificant.

8

4.5.2 Quantitative evaluation of the benefits of coupling Both uncoupled and coupled simulations are performed for the purpose to prove and to evaluate the

advancements achieved by using the coupling methodology. The results of the coupled simulations

provide monthly streamflow discharge and two-week’s groundwater levels. The residual errors in the

simulations represent the uncoupled and coupled models’ ability in better mimicking the coupled

surface- groundwater flow. A more quantitative analysis is provided by means of Eqs. (1) and (2)

which evaluate the differences of the errors for the two model approaches for, namely the streamflow

discharge and the average absolute groundwater levels, respectively. The results are listed in Table 1.

One notes the considerable improvement of the coupled model over the compartmental ones,

particularly, as far as streamflow is concerned, and more so for the dry than for the wet season.

−

−

−

= ∑∑== Coupled

n

i obs

iobsel

Uncoupled

n

i obs

iobsel

Q

QQ

Q

QQ

n 1

mod

1

mod

SW

)()(1Evaluation (1)

Uncoupled

n

i

iobsel

Coupled

n

i

iobsel

Uncoupled

n

i

iobsel hhn

hhhhn

∑∑∑===

−÷

−−−=

1

mod

1

mod

1

modGW )(1

)()(1

Evaluation (2)

-

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

1993-04 1994-04 1995-04 1996-04 1997-04 1998-04 1999-04 2000-04 2001-04 2002-04

stre

amfl

ow

(m

illi

on

m3/m

on

th)

different area of Uncoupled - Coupled

different area

coupled

uncoupled

Fig. 6. Comparing streamflow simulation between uncoupling and coupling models.

30

31

32

33

34

35

36

37

38

39

40

Apr-93 Apr-94 Apr-95 Apr-96 Apr-97 Apr-98 Apr-99 Apr-00 Apr-01 Apr-02 Apr-03

Gro

un

dw

ate

r le

vel

(m.M

SL

.)

Uncoupled (MODFLOW)

Coupled (MODFLOW + SWAT)

Observed well

55.5

23.9

9.2

3.4

-10.5

-15.6-20

-10

0

10

20

30

40

50

60

Wet Dry

rech

arg

e (m

m. -

dep

th o

f w

ate

r)

Surface Recharge

River to Groundwater

Groundwater to River

Fig. 7. Groundwater level simulations of uncoupled Fig. 8. Interaction of surface and groundwater

and coupled models. for the coupled models.

9

Fig. 9. Obvious different groundwater level at dry season of 2003 due to coupled models.

Table 1. Model-accuracy improvement due to the coupling process. improvement due to coupling process

Model Average Wet season dry season

Surface water model (evaluationSW) 11.5% 7.5% 16.7%

Groundwater model (evaluationGW) 2.3% 0.55% 2.9%

Table 2. Improvement of river-aquifer interaction in focus years.

change of abs. error in streamflow calculation after coupling

monthly streamflow seasonal streamflow water situation

whole year Wet dry Wet dry

1997 (wet water-year) -13% -7% -18% -10% -15%

2000 (normal water-year) -11% -5% -17% -8% -14%

A more detailed picture can be grasped from Table 2 where the same analysis is carried out for the two

focus years 1997 and 2000. One clearly notes the improvement in the streamflow fit in the presence of

the coupled river-aquifer interaction, mainly in the dry season and, more so, for the wet year 1997

than for the normal year 2000.

4.5.3 Dynamic interactions The coupling process generates a synchronous interaction between the surface water and the

groundwater and so allows to evaluate the unmatched interactions of the surface and subsurface water

models (Table 3). The water balance of the coupled models (Fig.3) illustrates that the interaction of

surface and sub-surface water amount to 5% of the total flow-in of the study area. The seasonal

interaction in Fig. 9 indicates that the aquifer contributes only an average of 3% of the annual aquifer-

recharge into the rivers during the wet season, but is recharged from the rivers with 77% of the total

recharge during the dry season. Furthermore, Fig. 9 shows that over recent times, while the

groundwater use has been increasing, and the surface water supply decreasing, the river-aquifer

interaction has also been declining. The surface recharge, on the other hand, is augmented both during

the wet and the dry season, owing to an adapted infiltration rate and temporary soil-water storage that

releases water during the dry season. Although the model coupling increases the groundwater

Uncoupled model Coupled model

darker the shade,

higher the hydraulic

conductivity

10

Table 3. Average annual flow of coupled components.

before coupling After coupling Parameters

SWAT MODFLOW coupled model

Surface recharge (mm.) - 17.2 47.4

River-gw interaction (mm.) 71 9 .0 13.5

River recharge-80

-70

-60

-50

-40

-30

-20

-10

0

10

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002

Sea

son

al

rech

arg

e w

et/d

ry (

mm

.)

Coupled SWAT+MODFLOWMODFLOWSWAT

positive(+) rivers contribute to aquifers

negative(-) aquifers contribute to rivers

Surface recharge

0

5

10

15

20

25

30

35

40

45

50

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

Wet

Dry

1993 1994 1995 1996 1997 1998 1999 2000 2001 2002S

easo

na

l re

cha

rge

wet

/dry

(m

m.)

Coupled SWAT+MODFLOW

MODFLOW

Fig. 9. Seasonally river recharge. Fig. 10. Seasonally surface recharge.

recharge significantly (Table 3), the excessive recharge areas are located mostly along the boundary

line of the groundwater model, where mostly forests and mountains are located. Thus groundwater

levels within the model are barely affected.

5. Conclusions Semi-coupling is another option for a simple and easy methodology to improve on coupled surface-

and sub-surface hydrological modeling. In the present study the selected components of the surface

water model SWAT and the groundwater model MODFLOW have been connected to each other, so

that dynamic interactions with monthly hydrological components are generated. The comparison of

the uncoupled and coupled models illustrates that the semi-coupling method improves the simulation

of streamflow and groundwater significantly. The water balance analysis is able to describe the local

interaction of surface- and subsurface water. The streamflow and groundwater level calculations are

enhanced in the coupled model by 12% and 2.3%, respectively, especially during the dry season of an

otherwise wet year. For the wet season, the groundwater level changes are barely affected by the

coupling process. The study results clearly show that the semi-coupling methodology proposed here is

a key to better simulate surface- and groundwater interactions, in agreement with the natural

hydrological processes involved.

Acknowledgements The authors wish to thank the staff at the Department of Water Resources

Engineering, Chulalongkorn University assisting with valuable information, as well as the Department

of Geohydraulics and Engineering Hydrology, University of Kassel, for the opportunity of several

research visits. We also acknowledge the assistance of the Royal Irrigation Department for providing

useful information on the study area. The paper could not be finished without the financial support of

the Department of Groundwater Resources, Ministry of Natural Resources, for which we are very

grateful.

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