Temporal variation of rainfall trends in parambikulam aliyar sub basin, Tamil Nadu

or

varies from 4750.25 mm for Chinnakallar to 741.54

mm for Vettaikaranpudur. Looking at the amount

, if N is even.

of rainfall in different seasons (Table 1), it is evident

To obtain an estimate of B in Equation f(t) the n

that all the stations receive the maximum rainfall in

values of differences xi – Qti are calculated. The

monsoon seasons and minimum rainfall in winter

median of these values gives an estimate of B. In

season followed by summer season.

this study the excel template application Makesens

The coefficient of variation (CV) of the annual rainfall

was used to facilitate the computation of the Man-

varies between 20 % (Anamalai) and 57 % (Weaverly)

Kendall statistics S, Sen’s slope Q and intercept B

indicating that there is significant variation in the

(Salmi et al. , 2002).

total amount of rainfall between the locations.

Weaverly station shows the maximum coefficient

Regression analysis

of variation during the summer season (71 %) and

One of the most useful parametric models used to

shows the minimum coefficient of variation during

develop functional relationships between variables

winter seasons.

is the “simple linear regression” model. The model

To test whether the annual and seasonal rainfall

for Y (e.g. rainfall) can be described by an equation

data follow a normal distribution, the skewness and

of the form

, where, X = time in years, m =

kurtosis were computed. Skewness is a measure of

slope coefficients and c = least square estimates of the

symmetry, or more precisely, the lack of symmetry.

intercept.

The data set is said to be symmetric if it looks the

The slope coefficient indicates the annual average

same to the left and right from the center point. The

rate of change in the rainfall characteristic. If the

skewness for a normal distribution is zero, and any

slope is statistically significantly different from zero,

symmetric data should have skewness near zero.

the interpretation is that, it is entirely reasonable

Negative values for the skewness (Annual rainfall

to interpret. There is a real change occurring over

of Aliyar Nagar and Topslip) indicate that data

time, as inferred from the data. The sign of the slope

are skewed to the left and positive values for the

defines the direction of the trend of the variable:

skewness indicate that data are skewed to the right.

increasing if the sign is positive and decreasing if the

The coefficient of skewness of monsoon seasons

sign is negative. We used the t test to determine if the

and annual rainfall is nearly zero indicating a near

linear trends were significantly different from zero at

normal distribution of rainfall in the sub basin.

the 5% significant level.

Rainfall during winter season is seen more skewed

when compared to the rainfall during monsoon

Results and Discussion

season.

Kurtosis is a measure of data peakeness or flatness

Statistical analysis of rainfall

relative to a normal distribution. That is, data sets

The graphical representation of annual and seasonal

with a high kurtosis tend to have a distinct peak near

rainfall series for the eight rain-gauge stations is

the mean, decline rather rapidly, and have heavy

given in Figure 2-9. Most of the rainfall events in

tails. Data sets with low kurtosis tend to have a flat

this area have a short duration but high intensity.

top near the mean rather than a sharp peak. The

In order to classify the annual rainfall amounts,

standard normal distribution has a kurtosis of zero.

the rainfall pattern (Schiettecatte et al. , 2005) has

Positive kurtosis indicates a peaked distribution and

been used. The statistical analysis of rainfall data is

negative kurtosis indicates a flat distribution.

presented in Table 1. From the table it can be seen

The correlation coefficients between rainfall and

that Chinnakallar received the highest mean annual

time for all eight stations are presented in Table 1.

and seasonal rainfall. The mean annual rainfall

41