Generally, Northeast monsoon is the principal rainy season for Tamilnadu. Particularly, it is the dominant contributor for the coastal and southern parts of the state. Kodaikanal, Ooty, which are very popular Hill resort stations of Tamil Nadu also get their major share of rainfall only during northeast monsoon. However, the Northwestern parts of the state receive an equal amount of rainfall during the southwest monsoon.
Valparai in Coimbatore district of Tamil Nadu has been identified as the rainiest station of the state during southwest monsoon by Sridharan and Mutuchami1. Situated to the south of Palghat gap, Valparai, the pollution free heavenly land over one kilometer asl is on the Anamalai mountain ranges of western Ghats. The unique grass forest, grass hills are located near Valparai. The hill sports grasses, elephants, cheetahs’ bears. It is Anaimudi the highest peak, scaling 2695mts in Anamalai range. Chinakallar, in Valparai Taluk is geographically and meteorologically an important place, always found wet and misty. It is also being felt by many in the region as the Chirapunji of south India. Such a mention is also found in tourist brochures2.
In this paper an attempt is made to study in detail the rainfall aspects within Valparai taluk which may be of considerable interest for matters relating to tourism. It is also observed that there are marked variations in rainfall both in intensity and duration. This aspect is further used to obtain the relation with the seasonal rainfall of the other subdivisions of South India and neighborhoods where the Arabian Sea branch of monsoon current is dominant.
Data: Daily rainfall data of the stations in the taluq, viz, Valparai, Sholayar Chinakallar and Upper Nirar Dam for a period of 27 years commencing from 1980 has been obtained from the Department of Statistics, Government of Tamil Nadu. Monthly data for a period of 65 years from 1940 also has been utilized. The locations of the stations are as under:
Valparai 10 20’ N 76 55’ E [Height: 1077 m ]
Chinakallar 10 15’ N 77 11’ E [Height:1250m]
Sholaiyar 10 18’N 76 53’E [Height:888m]
The mean southwest monsoon rainfall of Valparai, Sholayar, Chinakallar and Upper Nirar Dam are 2667, 3005, 2975 and 3672mm respectively with standard deviations 790, 969, 1091 and 1018. The cumulative rainfall for all the months of the season is depicted in Figure 2. Temporal variation of rainfall is studied on Chinese week basis with each week comprising ten days dividing each month into three weeks and the season into 12 weeks. The mean weekly rainfall for all the four stations is given in Figure 3. July and August had never been dry in any of the place. These are the peak rainfall months. During July and August there has never been any a dry spell exceeding five days.
Figures 3 to 5 show the probabilities of very high rainfall (vhrf), heavy rainfall (hrf) and rainy days during the season over the four stations. IMD criterion of 115, 65 and 25 mm have been taken as a day of very heavy rainfall, heavy rainfall and a rainy day respectively. The probability of occurrence of vhrf has two maxima. The principal part will be during the last part of July and the secondary part during the last part of June. It is the highest in Upper Nirar dam throughout the season. It is followed by Sholayar up to second part of July and then by Chinakallar.
Heavy rainfall extending for a period of the order of 5 to ten days is not an uncommon feature in the region. In August 1986 vhrf on all the 9 days at a stretch in Valparai resulted in an accumulated rainfall of 1370 mm. Such a highest single quantum in Chinnakallar was in June 2004 with 140 cm in 9 days, in Upper Nirar in August 1986 with 163 cm in 9 days and in Sholayar in June 1985 with 95 cm in 5 days.
Heavy Rain Fall [HRF] also has a higher probability mostly over Upper Nirar Dam. However, such a probability is found to exceed over Sholayar and Chinakallar, during the last week of June and then during the last week of the season. It is consistently lowest over Valparai.
Tables 1 to 3 show these probabilities of very heavy rainfall, heavy rain fall and rainy days for spells of two days, three days etc.
The number of rainy days is also highest in Upper Nirar Dam site. Though the amount of rainfall during each week is higher during each week in Chinnakallar as compared to Valparai, the number of rainy days in each week is higher over Valparai. Amidst the rain, some amount of dry spell also is required for tourists to go around these places. Fig 4 shows the probability of occurrence of a dry spell of three days during each week throughout the season.
It has also been observed that the rainfall of July over this region has a good positive relation with the entire southwest monsoon rainfall of the subdivisions of south India and the adjoining Maharashtra and Vidarbha. The June rainfall of all the stations was negatively correlated with the SW monsoon rainfall of Rayalaseema and Marathwada of which the coefficients of Upper Nirar Dam and Valparai are significant. The correlation coefficients were 0.5193 significant at 1% level and 0.4053 significant at 5% level respectively.
The anomalies of the July rainfall over the stations in the taluq have been worked out and compared with those of the other subdivisions pertaining to the entire southwest monsoon season. Many of them were found to be positively correlated with varying coefficients. The significant among them have been presented in Table 4. None of the negative correlations were significant.
Summary and Conclusions: 1.Upper Nirar Dam is the Rainiest place in the region and Valparai is the place with comparatively lowest rainfall. Though the rainfall is the least in Valparai, the number of rainy days is more in comparison with Chinakallar.
2. For tourism, September and June are relatively more comfortable with a reasonable blend of dry spells.
3. The southwest monsoon rainfall of Rayalaseema is negatively correlated with the June rainfall of Valparai and Upper Nirar Dam.
4. July rainfall of different stations can be taken as indicators of the entire seasonal rainfall over the other subdivisions which can be of use for rescheduling or reorganizing the plans related to water management.
Valparai in Coimbatore district of Tamil Nadu has been identified as the rainiest station of the state during southwest monsoon by Sridharan and Mutuchami1. Situated to the south of Palghat gap, Valparai, the pollution free heavenly land over one kilometer asl is on the Anamalai mountain ranges of western Ghats. The unique grass forest, grass hills are located near Valparai. The hill sports grasses, elephants, cheetahs’ bears. It is Anaimudi the highest peak, scaling 2695mts in Anamalai range. Chinakallar, in Valparai Taluk is geographically and meteorologically an important place, always found wet and misty. It is also being felt by many in the region as the Chirapunji of south India. Such a mention is also found in tourist brochures2.
In this paper an attempt is made to study in detail the rainfall aspects within Valparai taluk which may be of considerable interest for matters relating to tourism. It is also observed that there are marked variations in rainfall both in intensity and duration. This aspect is further used to obtain the relation with the seasonal rainfall of the other subdivisions of South India and neighborhoods where the Arabian Sea branch of monsoon current is dominant.
Data: Daily rainfall data of the stations in the taluq, viz, Valparai, Sholayar Chinakallar and Upper Nirar Dam for a period of 27 years commencing from 1980 has been obtained from the Department of Statistics, Government of Tamil Nadu. Monthly data for a period of 65 years from 1940 also has been utilized. The locations of the stations are as under:
Valparai 10 20’ N 76 55’ E [Height: 1077 m ]
Chinakallar 10 15’ N 77 11’ E [Height:1250m]
Sholaiyar 10 18’N 76 53’E [Height:888m]
(Upper NirarDam 10 17’ N 77 01’E [Height: 1116m] )
Discussion: The stations Chinakallar and Upper Nirar dam are situated about 20 km southeast of Valparai. They are separated by a distance of a couple of kilometers and are situated in such a way that they lie on the western slopes of the Western Ghats with the river Nirar flowing in the valley formed by the western ghat ranges in SW - NE direction. Sholayar is situated at a distance of 20 km southwest of Valparai. Sholayar dam is the second largest Rock dam in Asia. The locations of these stations in altitude are proximate to the dominant moisture incursion layers. This region may be considered as the gateway of the peninsular region to the monsoon currents. The strong cross-equatorial low level jet stream (LLJ) with its core around 850 hPa of the Asian summer monsoon (June–September) is found to have large impact over the rainfall here. During the onset of monsoon over Kerala, which is just a few km west of this locations and when the convective heating of the atmosphere is over the low latitudes of the Indian Ocean, the axis of the LLJ is oriented southeastward over the eastern Arabian Sea and it flows east ward between Sri Lanka and the equator. In contrast, during active monsoon periods when there is an east–west band of strong convective heating in the latitudes 10°–20°N from about longitude 70° to about 120°E, the LLJ axis passes from the central Arabian sea. (Dr.P V Joseph et al, 2006) Further, the Orography of the stations is in such a way that there are considerable variations within the Taluk which leave the different signatures of the orientation of the influx as well as the flux density. For example, Upper Nirar Dam is located in Chinnakallar itself. But the raingauges under consideration are separated by a distance of slightly over two kilometers. Yet there is a discernable variation of rainfall both in intensity and duration. Some common ingredients for heavy Orographic rainfall are (1) conditionally or potentially unstable airstream impinging on the mountains, (2) the presence of a very moist LLJ, (3) the presence of steep Orography to help release instability, and (4) the presence of a quasi-stationary synoptic scale system [if any] is required to impede or slow the progress of the topographically forced convective system over the threat area.The mean southwest monsoon rainfall of Valparai, Sholayar, Chinakallar and Upper Nirar Dam are 2667, 3005, 2975 and 3672mm respectively with standard deviations 790, 969, 1091 and 1018. The cumulative rainfall for all the months of the season is depicted in Figure 2. Temporal variation of rainfall is studied on Chinese week basis with each week comprising ten days dividing each month into three weeks and the season into 12 weeks. The mean weekly rainfall for all the four stations is given in Figure 3. July and August had never been dry in any of the place. These are the peak rainfall months. During July and August there has never been any a dry spell exceeding five days.
Figures 3 to 5 show the probabilities of very high rainfall (vhrf), heavy rainfall (hrf) and rainy days during the season over the four stations. IMD criterion of 115, 65 and 25 mm have been taken as a day of very heavy rainfall, heavy rainfall and a rainy day respectively. The probability of occurrence of vhrf has two maxima. The principal part will be during the last part of July and the secondary part during the last part of June. It is the highest in Upper Nirar dam throughout the season. It is followed by Sholayar up to second part of July and then by Chinakallar.
Heavy rainfall extending for a period of the order of 5 to ten days is not an uncommon feature in the region. In August 1986 vhrf on all the 9 days at a stretch in Valparai resulted in an accumulated rainfall of 1370 mm. Such a highest single quantum in Chinnakallar was in June 2004 with 140 cm in 9 days, in Upper Nirar in August 1986 with 163 cm in 9 days and in Sholayar in June 1985 with 95 cm in 5 days.
Heavy Rain Fall [HRF] also has a higher probability mostly over Upper Nirar Dam. However, such a probability is found to exceed over Sholayar and Chinakallar, during the last week of June and then during the last week of the season. It is consistently lowest over Valparai.
Tables 1 to 3 show these probabilities of very heavy rainfall, heavy rain fall and rainy days for spells of two days, three days etc.
The number of rainy days is also highest in Upper Nirar Dam site. Though the amount of rainfall during each week is higher during each week in Chinnakallar as compared to Valparai, the number of rainy days in each week is higher over Valparai. Amidst the rain, some amount of dry spell also is required for tourists to go around these places. Fig 4 shows the probability of occurrence of a dry spell of three days during each week throughout the season.
It has also been observed that the rainfall of July over this region has a good positive relation with the entire southwest monsoon rainfall of the subdivisions of south India and the adjoining Maharashtra and Vidarbha. The June rainfall of all the stations was negatively correlated with the SW monsoon rainfall of Rayalaseema and Marathwada of which the coefficients of Upper Nirar Dam and Valparai are significant. The correlation coefficients were 0.5193 significant at 1% level and 0.4053 significant at 5% level respectively.
The anomalies of the July rainfall over the stations in the taluq have been worked out and compared with those of the other subdivisions pertaining to the entire southwest monsoon season. Many of them were found to be positively correlated with varying coefficients. The significant among them have been presented in Table 4. None of the negative correlations were significant.
Summary and Conclusions: 1.Upper Nirar Dam is the Rainiest place in the region and Valparai is the place with comparatively lowest rainfall. Though the rainfall is the least in Valparai, the number of rainy days is more in comparison with Chinakallar.
2. For tourism, September and June are relatively more comfortable with a reasonable blend of dry spells.
3. The southwest monsoon rainfall of Rayalaseema is negatively correlated with the June rainfall of Valparai and Upper Nirar Dam.
4. July rainfall of different stations can be taken as indicators of the entire seasonal rainfall over the other subdivisions which can be of use for rescheduling or reorganizing the plans related to water management.
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