Procedia
Engineering
Procedia Engineering 00 (2011) 000–000
www.elsevier.com/locate/procedia
I-SEEC2011
Numerical Investigation of the Groundwater Balance
in the Mae Sai Aquifer, Northern Thailand
Phatcharasak Arlaia*, Arun Lukjana and Manfred Kochb
a
Research Center of Sustainable Water Resource and Disaster Mitigation Management,
Nakhon Pathom Rajabhat University, Nakhon Pathom,73000, Thailand
b
Department of Geohydraulic and Engineering Hydrology,
Faculty of Civil Engineering,The University of Kassel, Kassel, Republic of Germany,
Elsevier use only: Received xx xx xxxx; Revised: xx xx xxxx; Accepted:xx xx xxxx
Abstract
Mae Sai is an important border district in the north of Thailand located in the area of the golden triangle delta.
Nowadays, Mae Sai is the major trade hub among Burma and Thailand. Consequently, the economy of Mae Sai has a
tendency to grow, with the consequence that more infra-structure for serving the future economic growth will be
needed. Water resources, namely, groundwater is one of the commodities that need to be developed further to fuel the
future economic growth in the area. Therefore, the Department of Groundwater Resources which is the key
authorized department for groundwater resources management in Thailand is currently investigating the
hydrogeology and groundwater resources of the aquifers in the Mae Sai aquifers system. Using first results of this
field investigation, this article aims to numerically determine the groundwater balance in the Mae Sai aquifers. The
Mae Sai aquifer is conceptualized as a multi-layer aquifer and groundwater flow is simulated by a fully 3D finite
difference model. During the analysis the groundwater model is calibrated in both steady-state and transient mode in
order to make sure that the model can correctly mimic the groundwater mechanisms in the Mae Sai aquifer system.
After successful model calibration, the model is applied to numerically calculate the groundwater balance in the
aquifer system. The numerical results show that (a) the top layer of the multi-aquifer system is the most productive
aquifer with groundwater yields of about 186,000 cubic meter per day, whereas the 4th aquifer is the least productive,
with a groundwater yield of only 14,000 cubic meter per day, (b) the rainfall recharge is the most influential inflow
into the groundwater system, whereas the inflow from the bordering river (simulated through a general head
boundary condition) plays only a small role, and (c) the most number of pumping wells are developed in the 3rd
aquifer (2,400 cubic meter per day).
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011
Keywords: Mae Sai aquifer; groundwater model, groundwater balance
2
Koch, Arlai and Lukjan/ Procedia Engineering 00 (2011) 000–000
1. Introduction
Mae Sai is an important border district in the north of Thailand located in the area of the golden triangle
delta. Nowadays, Mae Sai is the major trade hub among Burma and Thailand. Consequently, the economy
of Mae Sai has a tendency to grow. As a consequence, more infrastructures for serving the economic
growth in the future, i.e., water resources, need to be developed in Mae Sai. This includes also
groundwater which is the major source of water in the Mae Sai basin. To look for the future potential of
groundwater, the Department of Groundwater Resources, which is the key authorized department for
groundwater resources management in Thailand, is currently investigating the hydrogeology and
groundwater resources in the Mae Sai aquifers system.
Here we report on first results of the application of the 3D groundwater flow model – MODFLOW
(Harbaugh and McDonald, 2000) – to the Mae Sai aquifer system (see Fig. 1). After proper
conceptualization of the multi-layer aquifer system the fully 3D finite difference groundwater model is
set up. The model is calibrated in both steady-state and transient mode in order to make sure that in can
correctly simulate the important groundwater mechanisms in the Mae Sai aquifer system. After successful
calibration the groundwater balance in the aquifer system is computed. The results of this, yet
preliminary, modeling study may serve local water authorities as some guidelines for planning well ahead
for the future groundwater development in the basin.
2. Study area
Mae Sai Basin is located in the Mae Sai-, Chaing San- and Mae Jan district (Amphoe in Thai) which is
the northernmost district of Chaingrai in the northern Thailand. The westernmost part of the basin
contains several mountains. The northern basin is a flat area, while the areas in the southeastern and
western sections of the basin form high terraces, where most of the natural recharge into the aquifer
system occurs. The basin’s altitude varies from +140 to +1450 m. (MSL) (Fig.1). The geological
investigation of the Department of Groundwater Resources (DGR, 2009) indicates that the Mae Sai basin
is comprised of 4 aquifer layers which are intervened by 4 thin aquitard layers (Fig.1).
3. Set-up of the conceptual and numerical groundwater model
3.1. Set-up of the conceptual model
The conceptual 3D groundwater model represents the overall hydrogeological and physical
characteristics of the aquifer system. That is, the conceptual model is set up based on the geological data,
geological description, hydrogeology, hydrology, topography, groundwater extraction, soil conditions and
land uses, some of which has only recently been collected (DGR, 2009).
Fig. 2a shows that the 3D groundwater model of the Mae Sai basin has been conceptualized vertically
by four aquifers and four aquitards. The low or high altitude ridges of mountains located along the
western and eastern borders of the model are specified as no-flow boundary. In the north, the plain opens
towards Myanmar and, assuming a hydraulic connection between the basin with the area inside Myanmar,
a general head boundary condition is used here. Recharge boundary conditions are imposed in areas of the
old terraces at the basin’s flanks in the west and south-east, whereas river boundary conditions are set up
along the courses of the Mae Nam Kam and Mae Nam Khong rivers (Fig. 2b).
Koch, Arlai and Lukjan/ Procedia Engineering 00 (2011) 000–000
3
Fig. 1. The hydrogeological map of Mae Sai basin (bounded by solid blue line).
GHB
River
No Flow
(a) Conceptual model and grid discretization of Mae Sai
multilayered aquifers system
(b) Boundary Conditions
Fig. 2. Conceptual model (a) and boundary conditions used (b) for the Mae Sai multilayered aquifers system
4
Koch, Arlai and Lukjan/ Procedia Engineering 00 (2011) 000–000
3.2. Set-up of the numerical groundwater model and optimization of the grid size
Error of measure
In general, the grid design depends on the available data and the objectives of the simulation.
Normally, a width of a grid cell in one direction should not be greater or less than 1.5 times the width in
the other direction, in order avoid serious errors in the simulation (Domenico and Schwartz, 1998). In
addition, for getting reasonable computing times, the grid size should not be too small. Therefore the most
suitable grid size should be selected based on these two criteria, but also taking the hydrogeology of the
study area into account. Thus, before the detailed calibration of the model was endeavored, the size of the
optimal horizontal grid, i.e. the optimal number of horizontal grid points was determined. This was
achieved by repeated steady-state calibrations using coarse estimations of the important hydrological
parameters entering the model and checking the Root Mean Squared Error (RMSE) between the observed
and modeled hydraulic heads. Based on these simulations the grid curve as shown in Fig. 2a has been
constructed. It shows that the optimal the grid size of the Mae Sai groundwater model is 400x400 squared
meters. Fig. 2b shows the ensueing FD-grid used in the subsequent simulations.
30
20
10
0
800
700
600
500
400
300
200
Grid spacing
(a) RMSE (m) versus grid size for the Mae Sai aquifers
system. An “optimal” grid size of about 400 m is
obtained.
(b) FD grid of upper Chiang Rai aquifers
Fig.3 Optimization of the model grid (a) and corresponding FD–grid (b) for the Mae Sai groundwater model
4. Steady-state and transient model calibration
Groundwater model calibration is usually carried out to ensure that the model can reasonably well
mimic the groundwater flow system, namely, fit the observed piezometric heads. The latter were
measured at 48 monitoring wells that were installed by DGR during 2008. The measurement period
available in the present study is from January to June, 2009. The calibration was done in both steady and
transient state. The hydraulic conductivity Kxy in the 4 aquifer layers and the groundwater recharge turned
out to be the most sensitive calibration parameters and were adjusted in an iterative manner during the
calibration process. Some results of the calibrations in both steady-state and transient modes are shown in
Fig. 4. The set of calibrated horizontal hydraulic conductivity of aquifers varies between 7 and 83 m/day,
the storage coefficient between 1x10-4 to 1x10-6, and the recharge is ~ 9% of the average annual rainfall.
Koch, Arlai and Lukjan/ Procedia Engineering 00 (2011) 000–000
5
Row68
Row93
Column 30
Inactive Cell
Inactive Cell
(a) Scattered plot between observed- and computed head in the
steady state calibration
(b) Comparison of observed-versus computed head in the
transient calibration
Fig.4 Steady-state (a) and transient (b) calibration of the Mae Sai aquifers model
5. Groundwater balance in the Mae Sai aquifer
The groundwater balance is a tool to determine the potential groundwater resources in the basin. The
MODFLOW groundwater model is used to compute the groundwater balance in the steady state. The
most salient results of these groundwater balance computations are unveiled in Fig.5. Thus one notices
that the top layer is the most productive aquifer layer, with a potential storage of 1.86x105 cubic meters
per day, while the less productive aquifer is the 4 th aquifer layer where to the potential storage is only
0.14x105 m3/day. The main source of water of the aquifers system is recharge from rainfall to the top
aquifer layer, amounting to 0.87x10 5 m3/day. The maximum pumping rate that can be sustained under
these long-term steady-state conditions is about 0.036x105 m3/day.
6. Conclusions
We have shown that numerical groundwater balance computations can assist water management
authorities in the long-term development of the groundwater resources in a complicated aquifer system.
Thus, in the present case of the Mae Sai aquifer system the results indicate that a significant amount of
groundwater pumping can be sustained in steady conditions, provided that the pumps are installed in the
3rd layer of the aquifer system.
6
Koch, Arlai and Lukjan/ Procedia Engineering 00 (2011) 000–000
RCH=87,115 CMD
RIV=27,855 CMD
GHB= 3,505 CMD
PW= 354 CMD
RIV= 93,690 CMD
2to1=67,379 CMD
PW= 721 CMD
1to2=70,342 CMD
Aquifer 2 =112,091 CMD
2to3=43,991 CMD
3to2=41,749 CMD
PW= 2,400 CMD
Aquifer 3 =58,109 CMD
RCH=257 CMD
3to4=13,961 CMD
4to3=13,861CMD
PW= 100 CMD
GHB= 21,467 CMD
Aquifer 1 =185,853 CMD
Aquifer 4 =13,961 CMD
Remark RCH = Recharge Boundary
GHB= General Head Boundary
CMD = Cubic Meter per Day
RIV = River Boundary
PW = Pumping Well
Fig.5 Groundwater balance diagram of the Mae Sai aquifers system
Acknowledgements
We would like to express our special thanks to the Department of Groundwater Resource for funding
support as well as scientific cooperation by providing some of the indispensable hydrogeological data.
References
[1] Alley, W.M., T.E. Reilly and O.L. Franke (1999) Sustainability of Ground-Water Resources, U.S. Geological Survey Circular
1186, USGS, Reston, VI, US. http://pubs.usgs.gov/circ/circ1186/.
Koch, Arlai and Lukjan/ Procedia Engineering 00 (2011) 000–000
[2] Alley, W.M. and S.A. Leake (2004) The Journey from Safe Yield to Sustainability, Groundwater, 42, 12 – 16.
[3] Anderson, M.P and W.W. Woessner (1992) Applied Groundwater Modeling: Simulation of Flow and Advective Transport,
Elsevier, Amsterdam, The Netherlands.
[4] Arlai, P. (2007) Numerical Modeling of possible Saltwater Intrusion Mechanisms in the Multi-Layer Coastal Aquifer System
of the Gulf of Thailand, Ph.D.Thesis, Kassel University, Germany.
[5] Arlai, P., M. Koch and A. Lukjan (2010) Modeling Investigation of the sustainable Groundwater yield for the Wiang Pao
Aquifers System, Northern Thailand, Water Resources Manag. (submitted).
[6] Arlai, P., M. Koch and S. Koontanakulvong (2006) Modeling Flow and Transport for sustainable Yield Estimation of
Groundwater Resources in the Bangkok Aquifer System, EGU, Vienna.
[7] Department of Groundwater Resources (2009) Groundwater Resources Assessment in the Kok River Basin, Bangkok.
[8] Domenico, P.A. and F.W. Schwarz (1998) Physical and Chemical Hydrogeology, John Wiley & Sons, New York.
[9] Harbaugh, A.W. and M.G. McDonald (1996) Programmer's documentation for MODFLOW-96, an update to the U.S.
Geological Survey modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report, 96-486, 220 pp.
[10] Kalf, F. R.P. and D.R. Woolley (2005) Applicability and methodology of determining sustainable yield in groundwater
systems, Hydrogeol. Journal, 13, 295-312.
[11] Koch, M., (2008) Challenges for future sustainable water resources management in the face of climate change, In:
Proceedings of “The 1st NPRU Academic Conference”, Nakhon Pathom University, Thailand, October 23-24, 2008.
[12] Maimone, M. (2004) Defining and Managing Sustainable Yield, Groundwater, 42, 809-814.
7