NORMAL ANNUAL RAINFALL

  • Normal annual rainfall varies in a large range from 556.7mm (Sulur, Coimbatore dist.) to 3818.7mm (Devala, Nilgiri dist.).

  • Normal annual rainfall is low (600-800mm) in districts of Erode, Namakkal, Dindigul, Madurai, Virudunagar, Tuticorin, Ramanathapuram & Karur.

  • Probability of receiving normal rainfall is below 50% (40-50%) over major portion of study area.

  • Probability of receiving normal rainfall is 50-60% over some randomly distributed areas.

VARIABILITY OF ANNUAL RAINFALL

  • Variability of annual rainfall over study area is rather high.

  • Coefficient of variation (CV) of annual rainfall varies between 20 and 43%.

  • CV of annual rainfall is 20-30% over major part of study area.

  • CV of annual rainfall is high over major portion of coastal areas (30-40%).

  • CV of annual rainfall is highest (>40%) over small area in southeastern part of Tuticorin district.

  • Some small and many large areas can be classified as “drought areas”.

  • Almost all “drought areas” coincide with areas that experienced high CV.


DROUGHT FREQUENCY

 

  • Frequency of occurrence of droughts is high (5-10 years per drought) over major part of study area.

  • Frequency of occurrence of drought is very high (<5 years per drought) over small areas of Nilgiri, Erode, Coimbatore, Dindigul, Theni, Pudukkottai, Sivaganga, Dharmapuri, Thanjavur & Nagapattinam districts & Karaikal Region

TREND OF ANNUAL RAINFALL

 

  • Major portion of study area experienced declining trend in annual rainfall (0 to 5 mm/year).

  • Significantly, almost entire district of Nilgiri experienced greatest declining trend in annual rainfall i.e. in the range of –5 to –10 mm/year during study period.

VARIABILITY OF RAINFALL OVER COIMBATORE DISTRICT, TAMIL NADU

ABSTRACT

Study of the variability of rainfall over the district of Coimbatore, Tamil Nadu, was made during of AAP 2005-2006. Under this, analysed long-term rainfall data (1901-2000) with a view to study the variability of annual rainfall and seasonal rainfall (SW monsoon and NE monsoon), incidence, intensity and periodicity of droughts. For this studies, annual/seasonal rainfall data for six rain gauge stations spread over the district have been statistically analysed and the results are presented in the form of various maps and tables. For each station, the 100-year normal annual/seasonal rainfall has been computed and the probability of occurrence of normal rainfall has been studied. The departure of each year’s annual/seasonal rainfall from the normal has been computed. Based on this departure, the category of drought, if any, has been determined and “drought areas” have been demarcated. The frequency of occurrence of drought at each station has also been estimated in terms of number of years per drought. The trend of annual rainfall at each rain gauge station over the study period has been determined. The variation in long-term normals of rainfall (50 years Vs 100 years) at each station has also been studied.

The salient features are as follows:

Annual Rainfall

  1. The normal annual rainfall over the district varies over a wide range i.e. from 560.3 mm to 886.6 mm (Plate-I);
  2. The chances of receiving normal annual rainfall vary from 40% to 50% over the district;
  3. The probability of receiving excess rainfall varies from 8% to 19% over the district;
  4. On the whole, the coefficient of variation of annual rainfall from the normal is rather high all over the district ranging from 24% to 35%; it is the highest over a major part of the district in the central, northern, eastern and southeastern parts around Coimbatore, Mettupalayam, Palladam, Sulur and Udumalpet;
  5. The total drought years over the district ranged from 11% to 22% over the district; an area in the extreme northern, northeastern, eastern and southeastern parts of the district around Mettupalayam, Palladam and Udumalpet experienced drought conditions for more than 20% of the study period; Hence, this area of the district comes under the category “drought area”.
  6. The frequency of occurrence of drought is rather high (5 years or less, per drought) over a small portion of the district in the northern northeastern, eastern and southeastern parts around Mettupalayam, Palladam and Udumalpet; over the remaining major part of the district the frequency is less and is in the range of 5-10 years per drought;
  7. When the 50-year and 100-year normals of annual rainfall were compared, it is observed that only a small area in the central part around Sulur experienced a rise (0 to 5%) in the normal annual rainfall; the remaining major part experienced a decline (0 to -10%) in the normal annual rainfall;
  8. Almost the entire district experienced a declining trend in annual rainfall except a small area around Sulur, which alone experienced a rising trend; the trend of annual rainfall ranged from –2.56262 mm/year to 1.08489 mm/year.

 Seasonal Rainfall (SW Monsoon)

  1. The normal annual rainfall over the district varies over a range of 109.3 mm to 390.9 mm (Plate-II);
  2. The chances of receiving normal annual rainfall vary from 41% to 47% over the district;
  3. The probability of receiving excess rainfall varies from 16% to 32% over the district;
  4. The coefficient of variation of annual rainfall from the normal is very high all over the district ranging from 31% to 67%; it is the highest in the northern, eastern and southeastern parts around Mettupalayam, Sulur, Palladam and Udumalpet (60-70%);
  5. The total drought years over the district ranged from 20% to 42% over the district; the entire district experienced drought conditions for more than 20% of the study period during this season; Hence, the entire district comes under the category “drought area” during this season;
  6. The frequency of occurrence of drought is very high with the entire district experiencing one drought in 5 years or less during the season;
  7. When the 50-year and 100-year normals of annual rainfall were compared, it is observed that northwestern half of the district around Mettupalayam and Coimbatore experienced a decline in the seasonal normal rainfall (0 to 15%). The other (southeastern) half of the district experienced a rise in the seasonal normal rainfall (0 to –15%);
  8. The trend of seasonal rainfall ranged from –0.3918 mm/year to 0.4103 mm/year. Almost entire northwestern half of the district around Mettupalayam, Coimbatore and Pollachi experienced a declining trend in seasonal rainfall in the range (0 to –1 mm/year). The remaining southeastern half around Sulur, Palladam and Udumalpet experienced a rising trend in the seasonal rainfall in the range 0 to 1 mm /year

Seasonal Rainfall (NE Monsoon)

The normal annual rainfall over the district varies over a range of 285.1 mm to 421.7 mm (Plate-III);

  • The chances of receiving normal annual rainfall vary from 40% to 54% over the district;
  • The probability of receiving excess rainfall varies from 23% to 39% over the district;
  • The coefficient of variation of annual rainfall from the normal is very high all over the district ranging from 46% to 57%; it is the maximum in a large area around Sulur, Palladam and Udumalpet in the northeastern eastern and southeastern parts;
  • The total drought years over the district ranged from 23% to 39% over the district; the entire district experienced drought conditions for more than 20% of the study period during the NE monsoon season; Hence, the entire district comes under the category “drought area” during this season;
  • The frequency of occurrence of drought is very high with almost the entire district experiencing one drought in 4 years or less during the season; only two small areas around Coimbatore and Palladam respectively experienced alone drought in a negligible fraction over 4 years;
  • When the 50-year and 100-year normals of annual rainfall were compared, it is observed that the entire district recorded a decline in seasonal normal rainfall during the NE monsoon season, the decline being in the range of 0 to –15%; this decline is the maximum around Palladam in the eastern part (-10 to –15%);
  • The trend of seasonal rainfall ranged from –1.7710 mm/year to 0.1464 mm/year; almost the entire district experienced a declining trend in the seasonal rainfall; a rising trend was observed only over a small area in the central part.