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INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES

Volume 2, No 1, 2011

© Copyright 2010 All rights reserved Integrated Publishing services

Research article ISSN 0976 – 4380

Submitted on July 2011 published on September 2011 219

Rainfall Rhythm of Suruli AR Watershed, Theni District, Tamil Nadu – A

GIS Approach Umamathi. S1, Aruchamy. S2

1- Research Scholar, Department of Geography, School of Geosciences, Bharathidasan University, Tiruchirappalli – 620 024

2- Professor, Department of Geography, School of Geosciences, Bharathidasan University, Tiruchirappalli – 620 024

[email protected]

ABSTRACT

The rainfall is primary source for water and it is characterized by its amount, intensity and distribution in time. The study could form a basis for planning the agricultural activity of the study area. In order to understand the spatial pattern of annual and seasonal rainfall, their variability, precipitation ratio and frequency occurrence of rainfall have been analysed through GIS environs. The study has used the long term (30 years) mean monthly rainfall data for eleven rain gauge stations, located in and around Suruli Ar watershed to analyse its various facets. Accordingly, the mean annual rainfall is about 870 mm which exceeds 1426 mm and drops as much as 638 mm. The mean annual rainfall variability of the watershed is 28.3%. The area in and around Suruli Ar watershed experiences high abnormality as the precipitation ratio is 185% whereas the lowest is about 44.4% at Sothuparai. The entire watershed is subjected to larger temporal fluctuations rather than the spatial distribution of rainfall.

Keywords: Rainfall, Variability, Abnormality, Precipitation Ratio, Frequency Occurrence and GIS.

1. Introduction

Rainfall is one of the most governing factors in planning the agricultural program for any area. Monsoon depressions and cyclonic storms are the most important synoptic scale disturbances which play a vital role in the space–time distribution of rainfall over India (Sikka, 1977). To prepare a proper crop and water management plan and to design irrigation drainage, erosion control and flood control structure, the knowledge of rainfall pattern, total rainfall, its distribution and daily/monthly or annual maximum and minimum rainfall are essential. Hence, the present study has been taken to understand the rainfall rhythm of Suruli Ar Watershed, which includes the spatial distribution and variability through different seasons, precipitation ratio and dependability. Geographical Information System (GIS) plays a vital role in interpolating and displaying various attributes of rainfall.

2. The Study Area

The present study area, Suruli Ar (Ar – River in Vernacular) watershed covers a major portion of Kambam valley, lies between the Cardamom hills in the west, the highwavy mountain range in the southeast, Theni-Allinagaram in the east and the Palani hills in the north. It comprises Bodinayyakkanur Taluk and major portion of Uthamapalayam and Theni taluks of Theni District. It is located between 9°34’ N and 10°9’ N latitudes and 77°10’ E and 77°30’ E longitudes with an area of 1296 km2. It consists of 3 Taluks, 5 blocks with 50

Rainfall Rhythm of Suruli AR Watershed, Theni District, Tamil Nadu – A GIS Approach

Umamathi. S, Aruchamy. S

International Journal of Geomatics and Geosciences

Volume 2 Issue 1, 2011 220

villages (Figure 1). The temperature ranges between 16o C in January and 40.5o C in May. The mean annual rainfall is about 870 mm. The major rivers are Suruli Ar, Theni Ar and Suthagangai Odai which form the Suruli Ar watershed. The elevation of the watershed ranges from 300 m to 2617 m. In general, the slope runs roughly from SW-NE direction and ends at the junction of the Suruli and the Vaigai rivers. The soils are river alluvium, forest humic soil, sandy loam, sandy clay loam and silty clay. About 28 types of crops are being cultivated of which paddy, maize, groundnut, chilly, vegetables and coconut are the major crops. The Population of the watershed is about 6,50,000 (2001) of which 70% are rural and the rest are urban. The population density is 502 persons/ km2.

Figure 1: Study Area

3. Database and Methodology

The monthly rainfall data for 30 years (1980 – 2009) have been collected for 11 stations which are located in (6 stations) and around (5 stations) Suruli Ar watershed (Figure 2). The long term mean monthly, seasonal and annual rainfall has been calculated. The coefficient of variability has been worked out. Simple cartographic and GIS interpolation techniques are employed to map spatio-temporal variation rainfall of the watershed.

4. Results and Discussion

4.1 Mean Annual Rainfall

The long term mean annual rainfall of the watershed is about 870 mm of which the winter, summer, southwest and northeast monsoon contribute 3.7, 24, 23.9 and 48.4% of the annual rainfall respectively. The study area is characterised by sharp areal differences in the quantum of rainfall due to the varying nature of topography and the influence due to orientation and configuration of Western Ghats. It is interesting to note that among 11 rainfall stations the





maximum of 1425.6 mm records at Sothuparai (outskirts of the watershed) whereas the minimum of 637.5 mm of rainfall at Bodinayyakkanur, where the stations are located in the northern part of the study area (Figure 3). Hence, the spatial difference in the rainfall distribution within a shorter distance (about 20 km) is too large which is as much as equal to mean annual rainfall of the study area due to the sharp descent in elevation from Sothuparai. It shows the influence of orographic effect in rainfall distribution.

Figure 2: Rain gauge Stations Figure 3: Mean Annual Rainfall

Table 1: The long-term (1980 - 2009) mean seasonal and annual rainfall (in mm)

Rainfall in mm.

S.N

o

Rain Gauge

Station

Win

ter

Su

mm

er

SW

monso

on

NE

monso

on

Annual

1 Andipatti 24.6 188.4 274.6 405.1 892.6 2 Aranmanaipudur 12.6 229.2 183.7 383.7 809.1 3 Bodinayyakkanur 42.1 168.5 114.1 312.9 637.5 4 Gudalur 27.6 207.4 301.1 355.8 891.9 5 Kambam 23.0 162.3 257.9 316.1 759.2 6 Mayiladumparai 47.5 238.0 105.2 474.9 865.5 7 Periyakulam 53.2 268.9 230.4 504.5 1057.0 8 Sothuparai 25.5 348.4 295.9 755.8 1425.6 9 Uthamapalayam 29.1 160.6 183.0 364.3 736.9

10 Vaigai Dam 33.3 172.6 206.9 438.2 851.0 11 Veerapandi 33.2 161.5 132.6 323.8 651.1

Mean 32.0 209.6 207.8 421.4 870.7

Percentage 3.7 24.1 23.9 48.4 100





While analyzing the spatial distribution of mean annual rainfall and annual rainfall of various stations (Table 1), the southern portion receives the high rainfall as indicated in Gudalur station which is about 891 mm. From here the rainfall decreases spatially towards northeast successively as elevation decreases. For example, Kambam 759.2 mm, Uthamapalayam 736.9 mm, Veerapandi 651.1 mm. It clearly indicates that the impact of orographic rainfall. The station Bodinayyakkanur located in the foot hills of Palani Hills which receives 637.5 mm rainfall. From here, there is a sudden change in increasing rainfall towards northeast and spell about 1426 mm at Sothuparai within the short range of distance. This areal difference also testimony for the orographic effect of rainfall as indicated in the previous statement. Further, it can be substantiated with the rainfall as well as elevation of other stations. From Sothuparai, the elevation decreases towards southeast successively, where Periyakulam records 1057 mm, Vaigai Dam is the lowest part in the region receives 851 mm and further increases towards southeast where Andipatti receives 892 mm as elevation increases. By keeping this, the entire Suruli Ar watershed falls under the following rainfall zones in its spatial pattern:

1. Very low rainfall zone (below 700 mm): the central part of the study area bordering Bodinayyakkanur in the north, Veerapandi in the east, Uthamapalayam in the south and Tevaram in the west.

2. Low rainfall zone (700-800 mm): the areas between Kambam and Uthamapalayam and a stretch from Kottagudi to Theni.

3. Normal rainfall zone (800-900 mm): the areas in and around Andipatti and Gudalur.

4. Moderate rainfall zone (900 – 1000 mm): tracts from Bodi North Hills to Agamalai in the northern part and from Mel Gudalur to Highwavys township.

5. High rainfall zone (above 1000 mm): the northern and southern hilly tracts of Western.

The very low rainfall zone covers an area of 494 sq.km which accounts for 38.1 % to the total area of the watershed followed by low rainfall zone 23.2 %, normal rainfall zone 17.9 %, moderate rainfall zone 12.2 % and high rainfall zone 8.6 % respectively. It should be noted that a major portion of cultivable lands fall largely within the zones of low and very low region. It accounts for 61.3 % that itself the testimony for semi-arid condition.

4.2 Mean Seasonal Rainfall

4.2.1 Winter

According to the rhythmical changes in the climatic element, the year is divided into four well-marked seasons. They are winter (Jan - Feb), summer (Mar - May), southwest monsoon (Jun - Sep) and northeast monsoon (Oct - Dec). The southwest and northeast monsoons are the two main seasons for agricultural activities. However, the northeast monsoon accounts for largest share in the total annual rainfall within short spell (Figure 4). The winter rainfall in the months of January and February is only 3.7 %. During winter season the station Bodinayyakkanur receives the highest rainfall of 42.1 mm within the watershed and decreases following the order of moderate, normal, low and very low rainfall regimes. The lowest rainfall of 12.6 mm is recorded at Aranmanaipudur, which is located in the opening area of the Kambam valley. The spatial pattern of winter rainfall is unique and totally different from other seasons.





4.2.2 Summer

Summer receives more rainfall than the southwest monsoon because the watershed is under rain-shadow region which is located in the leeward side of Western Ghats. Since the rainfall is confined to summer convection the variations in rainfall depend on purely local phenomena and the summer rainfall is associated with thunderstorm showers. When compared with other parts of Tamil Nadu the summer rainfall is high in the study area. The summer rainfall contributes 24% of annual rainfall of the watershed. A trough of high rainfall zone moves to the northeast in and around Sothuparai, which receives 348.4 mm of rainfall. From here, the rainfall decreases towards the central parts of the study area. The trend follows the mean annual rainfall distribution. On contrast to winter, Bodinayyakkanur receives the low summer rainfall of 168.5 mm. The lowest amount of summer rainfall is recorded in Uthamapalayam and Kambam receiving 160.6 mm and 162.3 mm respectively. There is a much variation in the rainfall amount during summer and winter.

4.2.3 Southwest Monsoon

The study area receives 23.9% of rainfall during this season. It is mainly associated with southwest monsoon rain bearing winds and elevation of topography. During this season the rainfall of different stations within the watershed recorded the lowest of 114 mm at Bodinayyakkanur, 132 mm Veerapandi and 183 mm Uthamapalayam successively from north to south and they form very low and low trough. On either side of this trough, the rainfall increases succeedingly following the normal, moderate and high rainfall zones, notably in the southern part whereas the northern rim of the watershed ends with normal rainfall category of 150 mm – 200 mm. while analyzing the stations in and around the study area the rainfall fluctuates from 105.2 mm at Mayiladumparai to the highest of 301.1 mm at Gudalur.

The pattern of rainfall reflects the characteristics of summer. However, in the north and northeastern part of the area, the rainfall recedes in their areal extent from high summer rain to normal southwest monsoon whereas in the southwest drastically increases. Accordingly, the area under the lowest rainfall is also successively decreasing in the mid-portion of the study area. In this season the areal extent of very low and low category is as much as normal, moderate and high rainfall zones. Again it substantiates the earlier statement as the testimony for the area falls under semi-arid condition.

4.2.4 Northeast Monsoon

The watershed receives 48.4% of the total annual rainfall during this season. The northeast monsoon followed almost the same trend of mean annual rainfall which exemplifies the dominance and intensity of northeast monsoon over the study area. The highest rainfall zone during this season concentrated over Agamalai and Periyakulam in the vicinity of northern boundary of the watershed, which receives more than 500 mm of rainfall. However, the adjacent station Sothuparai receives the highest amount of rain for 755.8 mm. From the northern rim, which is the hilly area towards south till Bodinayyakkanur (312.9 mm), the rainfall descents as the elevation decreases drastically. Beyond this a larger portion of the study area upto Gudalur forms a bigger trough with very low rainfall zone of less than 350 mm. From Gudalur, the rainfall increases towards east and extreme south of the watershed area comes under low and normal rainfall category.





From foregoing analysis on the spatial pattern of rainfall during annual, winter, summer, southwest monsoon and northeast monsoon, it is noted that Bodinayyakkanur is the breaking point station from where the rainfall increases or decreases on either side as the case may be. In general, as the watershed is bordered by hills except the north-east opening of the valley portion, it receives the highest rainfall in the northern and southern boundary of the study area. In contrast, the interior part receives relatively low – very low amount of rainfall as the area lies in the rain-shadow region of the valley floor. It is inferred that during all seasons except winter, the spatial distribution of rainfall pattern is controlled by elevation of the terrain.

Figure 4: Seasonal Rainfall

4.3 Mean Annual Variability

According to Trewartha, the variability of rainfall may be defined as the deviation from the mean. The coefficient of variability has been worked out by using standard deviation method for the series of years. The variability gives the stability of rainfall in the district.

Coefficient of Variability = SD/Mean x 100

Where, SD = Standard Deviation = √∑d2/n





The coefficient of variability of rainfall is the standard deviation from the mean expressed as percent of the mean annual rainfall. The isolines have been drawn on the basis of values compiled for 11 stations in order to bring out the spatial pattern (Table 2).

Table 2: The long-term (1980 - 2009) mean annual precipitation ratio and variability

Variability in %

S.N

o

Rain Gauge

Station

Annual

Pre

cip

itati

on R

ati

o

(%)

Win

ter

Sum

mer

SW

mon

soon

NE

monso

on

Annual

Vari

abilit

y

1 Andipatti 91.0 188.8 44.6 34.0 46.3 24.9 2 Aranmanaipudur 46.8 41.9 53.5 32.8 26.1 19.8 3 Bodinayyakkanur 136.2 135.4 42.8 56.1 49.1 33.8 4 Gudalur 184.9 122.6 60.5 54.8 46.4 37.8 5 Kambam 157.5 98.6 46.0 48.5 44.5 30.4 6 Mayiladumparai 66.8 55.5 55.6 53.6 37.9 31.8 7 Periyakulam 119.5 135.9 40.8 50.1 50.5 28.4 8 Sothuparai 44.4 156.4 33.2 15.2 28.3 18.0 9 Uthamapalayam 124.4 106.7 59.1 35.0 40.9 27.2

10 Vaigai Dam 108.3 111.1 43.9 38.2 31.6 24.3 11 Veerapandi 131.0 101.5 57.4 53.4 51.4 34.6

Percentage 46.2 19.8 17.4 16.7

The annual variability varies from 18 to 38% in the watershed (Figure 5). The lowest variability of 18% occurs at Sothuparai where the maximum rainfall is found in this watershed (1425 mm). The areas lie in the openings of the valley namely, Aranmanaipudur, Vaigai Dam and Andipatti receives low variability of 19.8, 24.3 and 24.9% respectively which shows high dependability. The stations located at the foot of the hills/vicinity of hills are under moderate-high variability. For example, Bodinayyakkanur, Veerapandi, Mayiladumparai and Gudalur have the variability of 33.8, 34.6, 31.8 and 37.8% respectively whereas the stations located at the lower elevations adjacent to above experience normal variability in and around Uthamapalayam (27%). From here, the variability steadily increases towards southwest and northeast till beyond Veerapandi again which steadily decreases till the end of the mouth of the valley portion. Accordingly, the dependability also follows the same trend as stated above.

From foregoing analysis based on Table 2 and Figure 5, it could be observed that 704.4 sq.km falls under high variability (>30-50%) which accounts for 54% variability followed by the area under moderate variability (20-30%) 581 sq.km and 45% and low variability is meager for 11 sq.km which accounts for 1%. However it should be noted that the entire watershed is fall within 18 to 38% of variability. Therefore, dependability also changes accordingly, lower the variability higher the dependability and vice versa.





Figure 5: Mean Annual Rainfall Variability

4.3.1 Winter Variability

Among all seasons (Figure 6), the winter variability of rainfall is high which ranges between 49 – 188%. During this season, the variability is triple fold than summer, southwest and northeast monsoon. Accordingly, the dependability of rainfall is also relatively very less. From the spatial distribution of winter variability, the eastern portion notably at the mouth of the valley and southeastern margin has very low and low variability fall under less than 50 and 50-80% variability classes. The southern portion covering Uthamapalayam, Kambam and Gudalur areas fall under normal variability (80-110%) which increases to moderate (110-140%) and high (>140%) extend on either side towards north as well as southeast. As the variability ranges between 50 to more than 140% indicates that the rainfall is scanty (> 40%) and hence the dependability is unreliable. However, cultivation practices like short-term crops are possible in and around Uthamapalayam, Kambam, Gudalur and Aranmanaipudur.

4.3.2 Summer Variability

During the summer season, the pattern of rainfall variability is from NW to SE in the order of very low (< 40%), low (40-45%), normal (45-50%), moderate (50-55%) and high (>55%) which increases succeedingly. The very low and high variability occupy almost equal. The rest of low, normal and moderate are equally distributed orienting SW-NE which passes middle of the watershed. The variability during this season ranges between 33.2-60.5%. While comparing winter and summer the lowest variability class does not have much variation whereas the highest variability class is three fold less than winter which shows that there is a large variation in spatial distribution as well as trend.





4.3.3 Southwest Monsoon Variability

Among the 11 stations the variability is relatively low in Sothuparai (15.2%) which is located about 5 km away from northeastern rim of the watershed whereas the high is at Bodinayyakkanur (56.2%), located at the foot of the northern hills (Palani hills). As far as the distribution of variability is concerned the watershed is divided into two halves, viz. northern and southern parts, which are separated by a line connecting Kottur and southern margin of Bodi west hills. In the northern portion, the low variability (20-30%) starts from Aranmanaipudur which increases towards west as much as above 50% that runs along the western margin till end of the southern portion of the watershed. Another stretch of low variability class starts from Erasakkanaickanur hills and increases towards west till margin of the Kambam valley west reserved forest.

4.3.4 Northeast Monsoon Variability

The rainfall variability of northeast monsoon resembles almost the characteristics and the general pattern of mean annual rainfall variability. Owing to this nature, the annual rainfall variability reflects characteristics of NW monsoon season rainfall variability. This is the season gets more rain which is favorable for agricultural operation as the continuation of southwest monsoon rainfall. A band of area under moderate variability class passes through middle portion of the watershed from northern margin to southern margin. From this zone, the variability increases towards southwest and as well as northeast. From here, the variability decreases towards east along the lower valley portion until touches Vaigai Dam. During this season, the rainfall variability lies within 26.1-51.4% which indicates less in its fluctuation.

Figure 6: Seasonal Rainfall Variability





From the foregoing facts and figures, it is inferred that the variability fluctuation is less which ranges between 15.2-51.4% during both monsoon seasons, though summer touches the maximum of 60.5%. However summer rain is useful for preparation of land for agricultural activities and summer ploughing before sowing and seedling. In continuation of summer rainfall, the southwest and northeast monsoon rains are main causes for agricultural activities as the fluctuation of variability is less than interior part of Tamil Nadu. Therefore, the dependability for agricultural activities is relatively large during southwest and northeast monsoon seasons.

4.4 Precipitation Ratio

The abnormalities of rainfall at any location may be brought by a simple ratio of precipitation. It is the difference between the maximum and minimum rainfall of over the series of years expressed in the terms of mean. (Px-Pn/Pm x 100, where Px and Pn represent the maximum and minimum of rainfall over the serious of years and Pm is the mean annual rainfall). This ratio may give the stability of rainfall with special relationship. Higher the ratio is higher in abnormality in rainfall and vice versa.

The lowest abnormality of 44 % is found at Sothuparai for a smaller extend and also at Aranmanaipudur (46.8%) at the mouth of Kambam valley (Figure 7). From here, the abnormality increases towards west margin of the watershed and take diversion towards southeast for a larger extent upto Erasakkanaickanur hills. Therefore, about 50% of the area falls under 110-140%. The same pattern reflects in northeast monsoon variability pattern. Hence, these areas are subjected to low stability and reliability of rainfall. Further, the areal extent of high precipitation ratio (>140%) occurs in the southwestern portion and in and around Veerapandi. A major portion of the watershed is under higher abnormality which explains that agriculture is gamble.

Figure 7: Precipitation Ratio Figure 8: Frequency Occurrence of Rainfall





4.5 Frequency Occurrence of Rainfall

The frequency of rainfall amount of each station has been tabulated and i.e., mode frequency table (Table 3). They are brought under four classes, ie. less than 500, 500 -700, 700-900, 900-1100 and above 1100 mm which represent 9, 31, 32, 14 and 14% respectively (Figure 8). About 32% of the occurrences are under normal rainfall years (less than mean annual rainfall), and about 28% of rainfall years fall under above normal and as well as excess. About 31% is well below normal (500-700 mm) and 9% below 500 mm which defines deficit. Hence, altogether about 72% are below normal ie. less than long term mean annual rainfall of the watershed which indicates low stability and dependability. Further, above mean is only 28%. Therefore, this also substantiates that the area under agriculture is under uncertainty and gamble.

Table 3: Frequency Occurrences of Rainfall for Suruli Ar Watershed

5. References

1. Banukumar, K., and Aruchamy, S. (2007), “Climatic Types of Tamil Nadu, India”, Journal of Spatial Science, 1 & 2, pp 1- 8.

2. Gangai, P., Aruchamy, S., and Selvam, K. (2008), “Rainfall Characteristics and Drought Prone Area of Pudukkottai District, Tamil Nadu”, Indian National Geographer, 23(1&2), pp 59-70.

3. Kathiyar, V.S. (1990), “Indian Monsoon and its Frontiers”, Inter India Publications, New Delhi, India.

4. Ramamurthy, K. (1943), “A Study of Rainfall Regime at Vellore”, Indian Geographical Journal, 18, pp 197 – 203.

5. Subrahamanyan, V.P. (1983), “General Climatology”, Heitage Publishers, New Delhi, India.

Frequency Occurrence in %

S.No Rain Gauge

Station < 500 500 - 700 700 - 900 900 - 1100 > 1100

1 Andipatti 3 24 34 28 10 2 Aranmanaipudur 0 20 80 0 0 3 Bodinayyakkanur 31 41 17 7 3 4 Gudalur 3 41 28 7 21 5 Kambam 10 41 31 14 3 6 Mayiladumparai 0 40 20 20 20 7 Periyakulam 0 10 38 21 31 8 Sothuparai 0 0 0 40 60 9 Uthamapalayam 10 34 41 10 3 10 Vaigai Dam 14 45 31 7 0 11 Veerapandi 31 31 34 3 0

Mean 9 31 32 14 14





6. Samuel Selvaraj, R., and Tamil Selvi, S. (2010), “Stochastic Modeling of Daily Rainfall at Aduthurai”, International Journal of Advanced Computer and Mathematical Sciences, 1(1), pp 52-57.

7. Sikka, (1977), “Some Aspects of the Life History, Structure and Movement of Monsoon Depression”, Pageoph, 115, pp 1501–1529.

8. Trewartha, G.T. (1954), “An Introduction to Climate”, 3rd edition, McGraw-Hill, New York.

9. Thornthwaite, C.W. (1948), “An Approach toward a Rational Classification of Climate”, Geographical Review, 38(1), pp 55-94.

10. Zamil, A. (1995). “Statistical Geography”, Rawat Publication, New Delhi, India.

rainfall rhythm of suruli ar watershed, theni …rainfall rhythm of suruli ar watershed, theni...

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