Rainfall characteristics of Coimbatore district in Tamilnadu

The Coimbatore district is situated in North Western part of the TamilNadu, the district covering an area of 4889 sq.km falls between 10°13'4" North to 11° 24'5" North latitude and 76° 39' 25" East to 77° 18' 26" East longitude. The temperature ranges between 18.32°C in the month of January and 36.42°C in the month of April. The area is bonded by Western Gates on West, Nilgiri hills in North West and Anaimalai and Palani hill in south. The district gradient of slope gradually decreases towards west to east, the district drain Bhavani, Noyal, Pallar, Amaravathi rivers. The study deals the rainfall characteristics of the Coimbatore District, which includes the spatial distribution and variability through different seasons, precipitation ratio and frequency occurrences. The study is based on 49 years of the monthly rainfall data for 33 rain gauge stations.

Winter Season

During the winter season the region heavily experiences low rainfall and it is driest among the all season which is maximum recorded at adjacent area of the region which Coonoor 86.82 mm in the month of February and lowest recorded at Coimbatore town which is 4.1mm in the month of February. The amount of rainfall decreases north west to north east and central part of region such as Annur, Sulur, Sultanpet, Krishnapurm and Podanur. Its rainfall average 20mm, this season contributes 2.07% to mean annual rainfall.

The variability rainfall is higher in this season while comparatively to other season, because low rainfall areas experiences greater fluctuation of variability. The lowest variability recorded adjacent area of region which is Coonoor 77.29% maximum at Palladam 232.2% and lowest variability recorded north western part which is Anaiktti 115.34% and southern part such as Upper Niradam (Chinnakallar), Solaiyar Nagar, Valaparai, Anaimalai experiences the low variability and central part eastern part gradually increase variability of rainfall, this fluctuations indicates the region experiences heavy low rainfall.

Summer Season

This is hot weather season the mount of rainfall gradually increases and however, the amount of rainfall occurrence in this season is largely due to convection effect, this season contributes 14.97% of mean annual rainfall this season average rainfall is 186.01mm and almost 80% area receives more than 186 mm accept north western and southern part of district. The highest amount of rainfall is recorded at the Upper Niradam (Chinnakallar) 224.14mm in the month of May and seasonal average is 370.7 mm and where the lowest rainfall is recorded in Krishnapuram 5.3mm and its seasonal average 69.8 mm.

The variability during this hot weather season maximum recorded at P.N. Palayam (86.36%), Attaikatti (79.93%), Negamam (78.34%), Sulur (77.86%), and Krishnapuram (76.07%) Coimbatore town (67.35%) Podanur (63.36%), Peelamedu (56.6%), and variability generally decreases from north to south and lowest variability experience adajecent area at Tthirumrthy Nagar 44.32%, and Solaiyar Nagar (49.07%), Sultanpet (52.03%), Vettaikarnpudur (54.93%), Mettupalayam (56.6%), and Pollchi (58.06%),.The variability of region highest experience in north and eastern part of region. This season will be low because west of influences by the pre monsoon winds.

South West Monsoon Season

Huge amount of rainfall variation noticed in this season, half of northern part of district receives low rain and half of southern part receives heaviest rainfall during season. The average rainfall of this season 573.20 mm and it is 46.13 % of mean annual rainfall. The highest seasonal rainfall recorded in Upper Niradam (Chinnakallar) (3655.5mm), and 1202.1 mm in the month of July, 996.25mm in June, 924.22 mm in August. Soliyar Nagar 2846.1mm, Anaimalais 2702.8mm, Valparai 2098.1mm, Topslip 772.4mm, lowest seasonal rainfall recorded in Krishnapurm 60.5mm, and 7.4 mm rainfall recorded in the month of August Sulur 113.9, Sultanpet 136.4 mm rainfall but southern parts of Districts receives more than 80% rainfall during this season. Most of south and west of Pollchi and Palaghat gap experiences highest rainfall, because of influence by leeward of Western Ghats. Upper Niradam, Soliyar Nagar, Anaimalais and Topslip are wettest places of district.

The south west monsoon rainfall variability experiences very less variability, comparatively to the other season, that is the 28.02% lowest at Topslip, Anaimalai, Valparai, Solaiyar Nagar and west of Pollachi and south of Chitrachavdi experiences very less variability due to heavy rainfall during this season. Where as very high variability experiences at adjacent area of region Poolankinar 89.99%, Sultanpet, Sulur, Krishnapurm, Podanur, and P.N.Palayam.

North East monsoon season

The North east monsoon season is important rainy season in the district, total rainfall in this season is low comparatively to the south west monsoon, but the eastern, north western, andcentral Upland region such as Annur, Sulur, Sultanpet, Negamam, Krishnapuram, Podanur Coimbatore town, and Peelamedu experiences maximum quantum of rainfall. This season contributes 36.82% of mean annual rainfall and average rainfall this season is 457.49 mm rainfall. The rainfall is gradually increase east to north west and south western part of the district, the highest rainfall recorded in adjacent area at Sundakapalayam 1713.3 mm, Upper Niradam 599.3, Anaikatty 589.64mm, Attakati 570.2, and where as lowest rainfall is recorded in Vettaikarnpdur 258.2mm, Annur 260.6mm and Krishnapuram 271.4mm, and Sultanpet 284.4mm, 80% of area receives well rain in this season, and most of agricultural activity takes places for showing crops.

The north east monsoon rainfall variability experiences higher than the south west monsoon due to lesser rain, the maximum variability recorded at adjacent area of such as Sundakaplayam 453.74% and lowest at Kethi 24.37% and north of Pollachi indicated higher variability and Anaimalai, Topslip, and Sultanpet experiences low rainfall variability. However rest of area experiences uniformity of variability.

Variability of Rainfall

“Variability defined as the deviation from mean” or “ratio of the standard deviation to the mean rainfall” and in other words variability of co-efficient of variation. Annual rainfall variability of region stretches between 19.69% to 387.91% at adjacent area of the region which is Coonoor and Sundakapalyam respectively, and 21.16% at Topslip and highest variability recorded in the central part of region that is Padanur (52.28%) and P.N.Palayam (51.81%) at northern part of region. Sultanpet experiences very low variability in eastern part district and in south western part Topslip and Vettaikarnpudur experiences low variability and Anaikatti, Coonoor experiences low variability in north western part of the region where as rest of district experiences homogenous variability in the region.

Coimbatore district bounded by Western Ghats,in the west and north of Nilagiri hills, in the south Anaimalai and Palani hills, and this district stretches in rain shadow region of Western Ghats. Normal rainfall of region during 1959­-2008 is 1242.49 mm and this district receives above normal rainfall of Tamil Nadu state. The maximum rainfall recorded at Upper Niradam (Chinnakallar) 1202.01mm in the month of July,and lowest of region recorded at Coimbatore town 4.1 mm in the month of February. The Coimabtore districts experiences driest in the month of January. The analysis of region heavy rainfall experiences towards the south and south western part and north western part in summer, south west monsoon and north east monsoon season, and huge amount of deficit in the east, north east and gradually in south east part and central part of district. The south west monsoon contributes the highest percentage of rainfall, which is 46.13% (573 mm), north east monsoon 36.82% (458 mm),summer 14.97% (186 mm) and winter season contribute lowest of which is 2.07 % (26 mm).

Source: Ishappa Muniyappa Rathod, Aruchamy.S, epartment of Geography, School of Geosciences,Bharathidasan University, Tiruchirappalli­.
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