SUPPLEMENTARY DOCUMENT
Geomorphology, Rainfall and Runoff in Chao Phraya Upper Plain
Applied Water Science Journal
Vitor Vieira Vasconcelos, Stockholm Environment Institute. E-mail: vitor.v.v@gmail.com,
Phone: 66 (0) 948681838,15th Floor, Witthyakit Building, 254 Chulalongkorn University,
Chulalongkorn Soi 64, Phayathai Road, Pathumwan, Bangkok, 10330, Thailand
(corresponding author)
Sucharit Koontanakulvong, Faculty of Engineering, Chulalongkorn University, Phayathai Road,
n. 254, Pathumwan, Bangkok, 10330, Thailand
Chokchai Suthidhummajit, Faculty of Engineering, Chulalongkorn University, Phayathai Road,
n. 254, Pathumwan, Bangkok, 10330, Thailand
Paulo Pereira Martins Junior, Mining School – Federal University of Ouro Preto, Campus Morro
do Cruzeiro, Ouro Preto, Minas Gerais State, Brazil, CEP: 35.400-000
Renato Moreira Hadad, Spatial Treatment Department, Pontifical Catholic University of Minas
Gerais State, Av. Coração Eucarístico de Jesus, n. 500, Belo Horizonte, Minas Gerais, Brazil,
CEP: 30535-901
1.
Geomorphology
The entire area can be classified as a piedmont plain, broadly covered by soils with a
predominantly clay texture. A detailed inspection reveals various geomorphological systems
and subsystems, as shown in the proposed geomorphological map.
The mean values and standard deviations of the geomorphic indexes (Table 1) indicate
clear flooding patterns in each geomorphological system. There are frequent floods in the
floodplain, which is lower and flatter than the other systems. There are occasional scattered
floods from local rain in the tributary sub-basins in the fans-terraces complex, which has slightly
more undulatory topography than the floodplain but is still near the river level. Finally, there are
no floods in the geomorphic threshold system, which has an elevation higher than the level of
the rivers and a slightly more rugged topography than the fans-terraces complex.
Table 1 Hydrogeomorphic indexes of each geomorphological system
Geomorphological
System
Floodplain
Fans-Terraces
Geomorphic Threshold
Vertical distance to Rivers
Base Level (meters)
Mean
Std
0.950
1.576
1.336
3.110
3.547
5.847
Slope
(degrees)
Mean
Std
0.422 0.411
0.483 0.745
0.688 1.304
Terrain
Ruggedness Index
Mean
Std
0.811
0.593
0.899
0.976
1.139
1.860
On the western, northern and eastern borders of the aquifer, it is possible to identify the
patterns of the fans, which reflect the original process that deposited the sediments of this
aquifer. These gentle, dissected, conic fans have a radially divergent hydrographic pattern. The
surfaces of the fans are gently undulatory following the same radial pattern and generate zones
of well and poorly drained soils in the higher and lower areas, respectively. The alluvial fan
complexes are characterized by intermittent, switching and diverging stream channels due to
active erosion and aggradation of the outwash sediments (Takaya 1971, p. 392; Murata and
Matsumoto 1974, p. 283). These sediments are relatively porous, and many channels are
seasonal or suddenly vanish along their courses. Thus, the flooding patterns in these areas
were an ever-changing mosaic of relatively small local drainages and flooding due to local
rainfall.
In some parts of the fan systems, the stabilization of the erosional and depositional
processes has allowed the development of terraces, in which the clay has experienced
laterization. These areas have more embedded rivers, and the terraces are sufficiently high or
far from the river to never be flooded.
In the eastern and northern parts of the aquifer, following the valleys of the Nan and Yom
Rivers, and in the lower part of the Ping River, there is a classic floodplain. In this floodplain, it is
common to find banks (natural flood embankments, denominated as levees) approximately 6 m
high that follow the river channel. Behind these levees, there are extensive flat areas of back
marshes (also called back swamps) that are on average 4 to 5 meters lower than the levee
(Murata and Matsumoto 1974; Haruyama 1993). This geomorphological configuration reflects
the deposition by annual floods. The heavier sediments, such as silt and sand, are deposited on
the levee near the river, forming the banks, whereas the clays are deposited on the floodplain
(back marshes) (Huggett 2007, p. 233). In the transition zone between the alluvial fans and
floodplain, the channels of the three main rivers display a decrease in the cross-sectional area
of their embankments (Sriariyawat et al. 2013) and an increase in the sinuosity of their
meanders, in accordance with their propensity to overflow the levees and flood the adjacent
areas. In a few areas of the floodplain, the coalescence of the levees of the current and former
river channels forms local high areas that are safer from floods or even block the spread of
floods to adjacent areas.
At the southern edge of the aquifer, rocky outcrops (mostly carbonate and volcanic rocks
with a few granitic and metamorphic rocks) are present and act as a geomorphic threshold
between the higher central plain and the delta of the Chao Phraya River downstream. These
stratigraphic structures, due to their elevation, have sequestered the sediments that constitute
the aquifer in the higher central plain and have prevented their transport to the delta during the
Quaternary Period. Due to the relatively more undulatory relief, the weathering of the rocky
outcrops and the thinner aquifer in this area, the hydrogeological processes are expected to be
different here than in the rest of the aquifer. Because this area is more elevated, it is never
flooded and also acts as a bottleneck for the floodplain at the confluence of the three main
rivers. This bottleneck forces floods on the floodplain to spread along the boundary of the
geomorphic threshold.
2.
Rainfall and Runoff Analysis
The time series of rainfall on the plain as well as the streamflow during the wet season and
dry season are presented in Figure 1. The three data series are generally consistent with each
other, particularly in terms of the peaks in rainfall and river runoff in the years 1970, 1981 and
1995 and in the limited amount of rainfall and streamflow during the dry period between 1990
and 1993. However, these peak rainfall periods and droughts vary in magnitude between the
three series, generating distinct subsets in relation to the proposed thresholds (Table 2). It is
worth noting that the average annual rainfall during the period 1993-2004 is slightly higher (50
mm) than that during the period 1968-1992, primarily because of the heavy rainfall in 1995.
Tebakari (2004) found that the streamflow in the Chao Phraya basin during the dry season
is mostly controlled by releases from the Bhumibol and Sirikit dams rather than by natural flow.
Thus, the distribution of streamflow between the dry and wet seasons will be primarily a matter
of dam management, which depends heavily on whether the rainfall matches the weather
forecast, particularly at the end of the wet season. This dam management is particularly
important in conjunctive water use management, as Koontanakulvong (2006) and Bejnaronda et
al. (2011) estimated that the streamflow in the rivers during the dry season is the primary factor
controlling the inter-annual change in the amount of pumped groundwater in the region.
Rainfall (mm/year)
Rainfall Threshold
3
Streamflow in Wet Season (million m /month)
3
Streamflow in Dry Season (million m /month)
Streamflow in Wet Season - Threshold
Streamflow in Dry Season - Threshold
1800
Rainfall (mm/year)
3
Streamflow (million m /month)
1600
1400
1200
1000
800
600
400
200
0
Years
Fig. 1 Average annual rainfall and streamflow time series for the study period on the Younger
Terrace Aquifer
Table 2 Coincidence in the classification of water table depth in well measurement data with
regard to rain and streamflow during the dry season and streamflow during the wet season
Compared subsets
Coincidence in wet/dry classification (%)
Rain - streamflow during the dry season
60.39
Rain - streamflow during the wet season
43.83
Streamflow during the dry and wet seasons
66.56
The graph in Figure 2 shows the monthly rainfall and runoff averages for the Younger
Terrace Aquifer. The average rainfall is 1,186 mm/year, of which 88.5% falls during the wet
season and 11.5% during the dry season. The average monthly streamflow follows the general
pattern of the rainfall, with an accentuated peak from August to September related to the main
orographic storms at the head of the basin, upstream of the aquifer. Because there is almost no
rainfall, the remaining streamflow in the dry season primarily consists of water released from
dams (Tebakari, 2004) and base flow.
900
Average Rainfall (mm/month)
800
Rainfall (mm/month)
Streamflow (million cubic metters/month)
700
Average Streamflow (million cubic
meters/month)
600
500
400
300
200
100
0
May Jun
Jul
Aug
Sep
Oct
Nov Dec
Jan
Feb Mar Apr Month
Fig. 2 Average monthly rainfall and average monthly streamflow on the Younger Terrace
Aquifer
References
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Water in Northern Chao Phraya Basin Final Report. Department of Groundwater Resources.
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2013