Kumar et al.,

The results indicated that all seven out of eight rain-

Magnitude of the trends

gauge stations showed positive correlation with

The magnitude of the trend in the annual and

annual rainfall, while winter and summer season

seasonal rainfall as determined using the Sen’s

showed negative correlation in most of the rain-

slope estimator is presented in Table 3. The analysis

gauge stations. The highest correlation coefficient

revealed a positive trend in annual and seasonal

(0.982) was observed during South West monsoon

rainfall in Aliyar Nagar, Attakatti, Chinnakallar and

whereas lowest correlation (0.010) was observed

Weaverly which is similar to calculate Mann-Kendal

during winter season of Aliyar Nagar station.

analysis. The Mann-Kendall statistical value of Z =

Significance of rainfall trends analysis

2.11, Sen’s slope estimate Q = 10.95 and the regression

slope coefficient a = 10.05 indicates a positive trend

The result of the Mann-Kendal analysis to deduct the

in annual rainfall for Aliyar station. The maximum

trend is presented in Table 2. All the eight rain-gauge

increase rainfall out of eight rain-gauge stations was

stations in the Aliyar sub basin showed different

experienced by Chinnakallar in annual rainfall (90.51

trends in annual and seasonal rainfall. Aliyar

mm/year) followed by South West monsoon (74.69

Nagar, Attakatti, Pollachi and Weaverly stations

mm/year). The maximum reduction of rainfall was

had shown an increasing the trend, whereas Topslip

found for Topslip (-8.60 mm/year) in South West

and Vettaikaranpudur showed the opposite trend in

monsoon followed by annual rainfall (-3.53 mm/

annual and South West monsoon rainfall. Seasonal

year).

analysis of rainfall trends indicated that South West

monsoon rainfall had increased in Chinnakallar,

Regression analysis

Attakatti, Weaverly, Aliyar Nagar and Anamalai

The slopes of the regression analysis (linear trend)

stations whereas a decreasing trend was observed in

for Aliyar sub basin is shown in Table 4. The linear

Topslip, Vettaikaranpudur and Pollachi. The rainfall

regression analysis of annual and seasonal rainfall

trend obtained during North East monsoon in winter

series have shown significant linear trend at few

and summer season was positive over all eight rain-

stations of the study area. Significant rising trend

gauge stations.

was noticed at Chinnakallar station for annual and

Negative trend was observed in Pollachi, Topslip

North East monsoon rainfall series (significance

and Vettaikaranpudur during South West monsoon

level P ≤ 0.05). Significant falling trend has been

and positive trend was recorded in Weaverly station

noticed during South West monsoon at Topslip. The

during North east monsoon at the 0.01 significance

presence of trend in annual and seasonal rainfall

level. Annual rainfall of Aliyar Nagar and Attakatti

series determined by Mann-Kendall Z statistics

station and North East monsoon rainfall of

and Sen’s Slope estimator reflected in the linear

Weaverly and Attakatti has shown increasing trend

regression analysis. A positive trend of 94.88 mm/

at the 0.01 significance level, whereas Pollachi,

year was recorded in Chinnakallar rain-gauge station

Vettaikaranpudur and Topslip has shown downward

for annual rainfall, which was significantly greater

trend at 0.1significance level.

than zero (significance level P ≤ 0.05).

42