43 •
Identification of the sources of most useful information for developing strategy for facing future challenges on climate change impacts
• Collect information on relevant studies carried out previously by national or international
agencies
5.1 Change of precipitation pattern
5.1.1 Chiang Rai
5.1.1.1 Change
in annual mean temperatures and annual rainfall
From the 29 year monthly mean temperature and precipitation data, it is found that the annual mean temperature and annual total rainfall over Chiang Rai shows increasing trend Figure
5.1. The annual normal mean temperature is found to be 25 C during this period and annual
normal mean rainfall is found to be 169.4 cm with a standard deviation of 13.8. With respect to these values the number of cases with more less than normal mean temperature
and normal rainfall during first 14 years and second 15 years are given in Table 5.1. The lowest rainfall recorded during the El Nino year of 1987 and highest rainfall during 2001.
100 125
150 175
200 225
250
1 980
1 982
1 984
1 986
1 988
1 990
1 992
1 994
1 996
1 998
2 000
2 002
2 004
2 006
2 008
Year T
o ta
l R a
in fa
ll c
m
24 25
26
M ea
n T
em p
er at
u re
d eg
C
Annual Total Rain Annual Mean T
Linear Annual Total Rain Linear Annual Mean T
Figure 5.1. Annual mean temperature yearly total rainfall over Chiang Rai Source: Thailand Meteorological Dept. and Analysis
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Table 5.1. Annual mean temperature and rainfall comparison during two periods over Chiang Rai; 1980- 1993 and 1994-2008 Source: Thailand Meteorological Dept. and Analysis
Periods Annual mean temperature
Annual total rainfall
More than mean 25
C Less than
mean 25 C
More than Mean 169.4 cm
Less than mean 169.4 cm
1980-1993 14 2
12 5
9 1994-2008 15
12 3
8 7
Table 1 indicates that the rapid increasing trend of the number of years with more than annual mean temperature and a similar rapid decrease of years with less than annual mean.
Regarding the rain fall, no significant difference is noted between higher and lower rainfall years in the period 1994 to 2008.
5.1.1.2 Changes
in rainfall intensity and rainy days
From the amount of daily rainfall data different type of rainfall days are identified viz., rainy days if rain amount 0.1 mm, light rain days if the amount is from 0.1 to 10 mm,
moderate rain days if the amount is from 10.1 to 35 mm and heavy if the amount is from 35.1 to 90 mm and very heavy rain days if the amount is 90 mm. This is the same
classification used by Thailand Meteorological Department TMD. The time series of seasonal frequency April-May, Oct-Nov and June-September; JJAS of these data during
past 29 years over Chiang Rai is shown inFigure 5.2, Figure 5.3, and Figure 5.4 respectively.
It is seen from Figure 5.2 that slight increasing trend of rainy days Figure 5.2a and days with heavy or very heavy rainfall days Rain 35 mmday are reported during April-May
Figure 5.2d Increasing frequency of heavy or very heavy rainfall events can cause more flooding and also more threat to disaster. During October-November except heavy to very
heavy rainfall events Figure 5.3d all other shows decreasing trend, whereas during October- November it shows slight increasing trend.
During the monsoon season from June to September, the frequency of heavy to very heavy rainfall event is increasing with peak value reported during 2006. It may be mentioned that
2006, flood condition was very worst during August Figure 5.4d.
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Figure 5.2. Frequency of rainy days different rainy days over Chiang Rai in Apr-May Source: Thailand Meteorological Dept. and Analysis
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Figure 5.3. Frequency of rainy days different rainy days over Chiang Rai in Oct-Nov Source: Thailand Meteorological Dept. and Analysis
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Figure 5.4. Frequency of rainy days different rainy days over Chiang Rai in JJAS Source: Thailand Meteorological Dept. and Analysis
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5.1.2 Udon Thani
