Title of the
Study: Variation of Air Temperature and Rainfall during Yala and
Maha Agricultural Seasons
Investigator(s): B.R.S.B. Basnayake and T.K. Fernando
Scientific background and
scope/objectives of the study:
From
about the 1980’s, the unpredictable climate changes were considered as an issue
of great concern in Sri Lanka. Within this period extreme events of weather
were noticed. One of these events of grate significance is the drought that
affected Hambantota recently, (in 2001) and other areas. Flooding and the likely
implications of the El-Niňo conditions are dominant.
Since fewer studies have been carried out in this vast field of science it is better to gain some knowledge about the local climatic changes at first. Because, Sri Lanka is an isolated country and may have variability rather than global trend in change of climate. Therefore, in the Sri Lankan context it is important to distinguish between past and present climate conditions and study the fluctuations of mean maximum air temperature, mean minimum air temperature, mean air temperature and rainfall during the Yala and Maha agricultural seasons. This can be implemented by analyzing past climatic data in a scientific view and manner. These analyses will certainly be helpful for planners and policy makers who are engaged in fields such as agriculture, engineering and water resource management etc.
Experimental Method:
Fourteen
of the 20 meteorological stations maintained by the Department of Meteorology
were considered in this study. These cover all of the agro-climatic zones.
Their elevations range from 2.1m (Puttalam) to Nuwara-Eliya (1895m) above mean
sea level. Stations at Colombo, Galle, Kandy, Nuwara-Eliya and Ratnapura are in
the wet zone and those at Badulla and Kurunegala are in the intermediate zone
while the remaining stations at Anuradapura, Hambantota, Trincomalee, Vauniya,
Manar, Puttalam and Batticoloa are in the dry zone.
Time
series analyses of mean maximum air temperature, mean minimum air temperature,
mean air temperature and rainfall were performed for Yala (April to August) and
Maha (September to March) agricultural seasons from 1931 to 2000.
Trend
analyses were done by filtering the different series using the Gaussian
Low-Pass filter with the nine weights 0.01, 0.05, 0.12, 0.20, 0.24, 0.20, 0.12,
0.05, and 0.01. Mathematically the filter is represented by the equation;
Where 
is the filtered value of the series corresponding to the tth
term thereof, and wi, is the weight by which the value of the series
i units removed from t is multiplied. The length of the filter defined in this
way is (2n+1) time units.
Results Obtained:
Even though Fourteen (14) meteorological stations were considered for the present study, Mannar and Vauniya meteorological stations were not taken for analysis due to lack of sufficient data.
Time
series analysis of Temperature:
Trend analysis for mean maximum air temperatures have shown increasing trends for both Yala and Maha seasons over the period of 1931-2000 at most of the stations except at Nuwara-Eliya where there is not significant increase in the maximum air temperature. Mean minimum air temperatures for both Yala and Maha seasons have shown increasing trends except Kurunegala where a decreasing trend has been indicated. Trend analyses for mean air temperatures have shown increasing trends at all stations both during the Yala and Maha seasons.
Time
series analysis of rainfall:
In the case of rainfall, decreasing trends have been indicated during both Yala and Maha seasons except at Puttalam during the Yala season where an increasing trend has been shown.
Linear
Regression Analysis:
Linear
regression analyses have been carried out for mean maximum air temperature,
mean minimum air temperature, mean air temperature and for rainfall.
The highest mean maximum air temperature trend has been recorded as +0.03250C per year at Puttalam during the Yala season and the lowest rate of increase of 0.00130C per year has been at Nuwara-Eliya during the Maha season The coefficient of determination (R2 ) is very small at Nuwara-Eliya for mean maximum air temperature both during the Yala and Maha seasons. For the mean minimum air temperatures, the highest temperature trend has been +0.03280C per year, (at Nuwara-Eliya during Maha season) and the lowest has been –0.00130C per year at Kurunegala in the same season. The rate of variation of the mean minimum air temperatures during both Yala and Maha were negative at Kurunegala and the R2 has been very small. The highest mean air temperature trend has been observed during the Maha season at Anuradhapura, which is about +0.03120C per year and the lowest, is about +0.00140C per year at Kandy during Yala. The highest decrease of rainfall has been observed at Trincomalee, about 5.24mm per year, during the Yala season. The lowest rainfall decrease has been at Batticaloa during the Yala season (about 0.39mm per year, where the R2 is also very small). Positive trend (+0.99mm per year) has been observed at the Puttalam meteorological station during the Yala season with a R2 value of about 0.10.
Significance
Test:
Significance test has been carried out, at the significance level of 5%, to evaluate whether there is significance or not. It has been reveled that there is no significance either in temperature or rainfall trends.
Conclusions:
Surface mean air temperatures have shown increasing trends at all most all-12 stations both during the Yala and Maha agricultural seasons. This could probably be due to both enhanced greenhouse effect and urbanization effect. Rainfall have shown decreasing trends at all stations both during the Yala and Maha seasons except at Puttalam, where it has shown an increasing trend during Yala season about 1 mm per year.