Six years intensive rainfall observation in Mae Chaem Watershed,Northern Thailand*KURAJI Koichiro1, KOWIT Punyatrong2, ISSARA Sirisaiyard3(1: University Forest in Aichi, The University of Tokyo,
2: Royal Initiatives and Security Projects Division, National Park Wildlife and Plant Conservation Department,
3: The 7 th Watershed Development Office, National Park Wildlife and Plant Conservation Department)* University Forest in Aichi, The University of Tokyo, 489-0031, Goizuka, Seto, Aichi, Japane-mail: kuraji@uf.a.u-tokyo.ac.jp
Key words: Northern Thailand, rainfall, mountainous area, Mae Chaem watershed1. Introduction
In the tropical monsoon Asia, rainy and dry season isdistinct, but the intra-seasonal transition in rainfall during
the rainy season was not studied well especially in themountainous area.
GAME-T established a rain gauge network in themountainous watershed in northern Thailand andobservations have continued since September 1997. Fromthese observations, Dairaku et al. (2000) reported thealtitudinal increase in rainfall in this watershed using datafrom 1 June until 23 November 1998. Kuraji et al. (2001)
found that the altitudinal increase in rainfall was obvious in
the two wet seasons in 1998 and 1999 with the incrementin 1999 being more than 2.5 times greater than that in 1998.
In the two wet seasons, the mean rainfall intensity has no
altitudinal dependence, whereas the proportion of hours
with rain has a clear tendency of altitudinal increase. The
relationship between the spatial scale of the rain event andthe altitudinal increase was also examined. In these papers,
however, the intra-seasonal transition of rainfall, duration,intensity, spatial scale and altitudinal increase in rainfall
were not examined.
The objective of this research is to understandintra-seasonal transition of rainfall characteristics in a
mountainous watershed under tropical monsoon climate in
Thailand using the data set obtained by GAME-Tobservation network. One of the advantage of this study isthat we use 6 years continuous data from 1998 until 2003
because only 2 years data were used in our previous studies(e.g. Kuraji et al, 2001).
To analyze intra-seasonal transition in rainfall, following
two topics were examined in this paper:
1. Definition of the regimes in the rainy season,
2. Seasonal transition of the altitudinal increase inrainfall.

2.

sub-catchment of the Ping River basin. The Ping River, 740
km long with a catchment area of 33,898 km2, flows
southward to join the Nan River at Nakhon Sawan
Province where the Chao Phraya River is formed. The
highest point of the Mae Chaem watershed is the Doi
Inthanon (Mt. Inthanon) summit, 2,565 m above sea level,
which is the highest peak of Thailand. The lowest point of
the watershed is 282 m (No.16 in Fig. 2).
Automatic tipping bucket rain gauges (No.34-T: OtaKeiki Co., 20 cm orifice diameter and 0.5 mm per tip) withpulse-count time-recording data loggers (KADEC-PLS:KONA System Co., one second time resolution) wereinstalled at 15 sites (No.1 - 15 in Fig.2) throughout thecatchment. The sites were located in front, or at the back ofeach field station of the Royal Forestry Department (RFD)
for greater security. To avoid effects of under-catch due tosurrounding forests and exposed ridges, carefulconsideration was given to the location of the rain gauges.
The rainfall amount in any time period (e.g. season, month,day, hour) is calculated by totaling the number of tips ineach period, multiplied by 0.5mm per tip and the data until
2001 were available through GAME-T data center.

Mae ChaemWatershed

25°20°15°Thailand

Study site and methods

GAME-T established a rain gauge network in the Mae
Chaem watershed to determine rainfall amount in themountainous area in the upper Chao Phraya river basin.
The Mae Chaem (Chaem River) watershed is located at
18°06’ - 19°10’ N and 98°04’ - 98°34’ E (Fig.1) with acatchment area of 3,853km2 at the river flow gauging
station operated by the Royal Irrigation Department (RID).
The name of this station is P.14, and the location is shownin Fig. 2 as No.16. The figure "P" stands for the Ping Rivershowing that the gauging station is located in a10°

5°N
95°E
100°
105°
110°
Fig.1: Location of the Mae Chaem watershed3. Results and discussions
3-1. Seasonal variation and altitudinal increasein rainfall
Rainfall in the region is characterized by large seasonaland inter-annual variations, as well as altitudinal increase.
Figure 3 shows the seasonal variation of rainfall at DoiInthanon (No.7 in Fig.2), Bo Kaeo (No.2 in Fig.2) andPOU (No.10 in Fig.2). From Fig. 3, rainfall in these sites
had two seasons: wet season (May-October) and dry
season (November-April) with a relatively small amount ofrainfall in July. The inter-annual fluctuation in rainfall wasmainly caused by the fluctuation of rainfall in May, June,
August and September and not in July. The rainfall in Doi
Inthanon, the highest point in this watershed, had thelargest amount of rainfall whereas POU, which locates inthe low elevation part of this watershed, had the smallest
rainfall throughout the 6 years.

990

1400
650
1180

1470
1630
2565

1000

1540

800

490
1450
1330

1250

Matsumoto (1997) studied the onset and withdrawal of
the wet season over the Indochina Peninsula using 5-day
averaged rainfall data and showed the distribution maps ofthe date of onset and withdrawal of the wet season.
According to his study, the average date of onset andwithdrawal of the wet season in the Upper Chao Phraya
river basin is around 28 April and 1 October, respectively.
Figure 4 shows the seasonal change of the coefficient ofvariation of the 5-day rainfall (CV5) in the Mae Chaemwatershed. Here, CV 5 was calculated as dividing thestandard deviation of 5-day rainfall in the 8 sites (No.2,3,4,7,9,10,11) by the average of them. The CV5 value is a
useful indicator of spatial scale of rainfall (Kuraji et al.
2001). From Fig. 4, intra-seasonal change of CV5 in therainy season was appeared. Before the early May, thenumber of non-zero 5-day rainfall sites in a pentad wasfrequently equal to zero or one or the range of CV5 values
was between 0.5 and 2.5 even almost all pentads haverainfall at all sites. From the early May to the early June,
the CV 5 value was clearly decreased to the range between
0.2 and 0.7, but returned back to the range between 0.5 and
1.0 after the early June until late July. After the late July,
the CV 5 value was changed again to the range between 0.2
and 0.7 until late September. After that, the CV 5 value
gradually increased until late October, the starting point of
the next dry season. This seasonal transition pattern of
rainfall in the Mae Chaem watershed in the rainy season
may be universal, because the phases of seasonal transitionof the CV5 value synchronized for 4 years. From thishypothesis, we define three intra-seasonal regime in therainy season from the 4 years CV 5 data, as shown in Fig.4
by the vertical broken line. We call these three
intra-seasonal regime as early rainy regime (5/11-6/9, 30days), middle rainy regime (6/10-7/24, 45 days) and laterainy regime (7/25-9/22, 60 days) hereafter.

380

Monthly Rainfall (mm)1100

3-2. Definition of the intra-seasonal regime:
seasonal transition of CV5600

4002000

1998

8101999

2000

Doi Inthanon (2565m)

2001

2002

2003

Bo Kaeo (1400m)

POU (490m)

Fig.2: GAME-T rain gauge sites (No.1-15) and the RID
river runoff gauging station (No.16). Elevation of
all rain gauge stations are also shown.

Fig.3: Seasonal variation in rainfall in Mae Chaem
watershed, 1998-2003.

3-3. Altitudinal increase in rainfall in theintra-seasonal regime
Dairaku et al. (2000) pointed out that the altitudinalincrease in rainfall is distinct in the 1998 rainy season inthe Mae Chaem watershed. This tendency was also
observed in 1999 rainy season and 1998-99 dry season(Kuraji et al. 2001).
Fig. 5 shows the relationship between mean daily
rainfall and elevation of the 15 sites in the three regimes in
the rainy season. Altitudinal increase in rainfall wasdistinct for all regimes for the 4 years. The relationshipbetween mean daily rainfall and elevation for the early
rainy regime was similar with that for the late rainy regime.
In the middle rainy season, however, the smaller rainfall in
the low elevation sites than these for the early and laterainy regime, corresponding with the “break” of monsoon(Matsumoto, 1997). The mean daily rainfall in the highelevation site was larger than that in the early and late rainy
regime in 1999 ands 2001 but smaller than that in 2000 and2002. As a result, the ”break” of monsoon in the highelevation sites was obvious in 2000 and 2002 but not in1999 and 2001. The inter-annual variation in rainfall wasthe highest in late rainy regime in the high elevation area,
but the highest in early rainy regime in the low elevation
area. In the middle rainy regime, the inter-annual variationin rainfall was not distinct in both low and high elevation
area.

4.

Conclusions

This paper focused on definition of the 3 regimes in therainy season and seasonal transition of the altitudinalincrease in rainfall in the mountainous Mae Chaem

watershed, Northern Thailand.
It was found that the coefficient of variation in 5-day
rainfall among 8 sites was a useful indicator to understandseasonal transition in rainfall pattern in the Mae Chaem
watershed. Using this indicator, rainy season was divided
into early rainy, middle rainy, late rainy regime. The
seasonal transition of the altitudinal increase in rainfall
were analyzed using these regimes. It was found that allregimes have their original pattern of the altitudinalincrease in rainfall.
The definition of regimes in this study might correspondto the larger scale monsoon activity. This fact showed thatthe monsoon strongly affect the rainfall pattern in themountainous area. The prevailed mechanism of rainfall in
one regime might be different from that in the otherregimes.

References
Dairaku K., Kuraji K., Suzuki M., Nipon T., Warin J. and Kowit
P., The effect of rainfall duration and intensity on
orographic rainfall enhancement in a mountainous area: acase study in the Mae Chaem watershed, Thailand. J.
Japan Soc. Hydrol. & Wat. Resour. 13, 57-68, 2000.
Kuraji K., Kowit P. and Suzuki M., Altitudial increase in rainfall
in Mae Chaem watershed, Thailand. J. Meteorol. Soc.
Japan, 79, 1B, 353-363, 2001.
Kuraji K., Otaki S., Kowit P. and Suzuki M., Intra-seasonalchange in rainfall in Mae Chaem Watershed, Thailand.
Proc. 5th Inter. Study Conf. GEWEX in Asia and GAME,Nagoya, Japan, 285-290, 2001.
Matsumoto, J., Seasonal transition of summer rainy season over
Indochina and adjacent monsoon region. Adv. Atmos. Sci.,
14, 231-245, 1997.

32.5CV 5

2
1.5
125

Early Rainy

2000

20
2002
2001
15

1999

10

5

0

0.5

1000

2000

3000

Elevation (m)

1999

2000

2001

2002

Fig.4: Seasonal variation of the coefficient of variationfor 5-day rainfall between 8 sites (CV5) (1999-2002).
The vertical broken lines show the intra-seasonal
regime shown in Table. If the number of non-zero
5-day rainfall sites in a pentad was equal to zero, theCV5 cannot be calculated. If the number of non-zero
5-day rainfall sites in a pentad was equal to one, theCV5 value (about 2.65 regardless of the rainfallamount) in that pentad was not shown for brevity.

Mean Daily Rainfall (mm)25Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Middle Rainy
202001
2000
2002
1999

15

105000Mean Daily Rainfall (mm)Mean Daily Rainfall (mm)2501000

2000

3000

Elevation (m)
2002
1999

Late Rainy
2020002001

15105001000

2000

3000

Elevation (m)Fig.5: Altitudinal increase in rainfall in early rainy,
middle rainy and late rainy season in Mae Chaem
watershed for 4 years (1999-2001).