Abstract:
The time series analysis was performed with Seasonal
Autoregressive Integrated Moving Average (SARIMA) and Sama
circular model (SCM) for the rainfall of Ampan, Karaveddi, and
Puloly regions of Jaffna to understand the behaviour of rainfall
and forecast it with a suitable model. Minitab 17 software was
used to run the model with the available monthly data from
2013 to 2019. Time series plots were used for pattern
recognition, the independence of the residuals was checked
using autocorrelation function (ACF), and Lijung-Box Q
statistics (LBQ). The normality of residuals was checked using
probability plot. The model with the lowest predicting errors
was selected to forecast the future values. The monthly rainfall
fluctuates around the mean of 41.6, 71.9, and 35.3 mm for
Ampan, Karaveddi, and Puloly respectively. The models
SARIMA (0,0,0) (0,1,1)6, SARIMA (1,2,1) (0,1,1)6, and SARIMA
(1,1,0) (0,1,1)6 were found as most appropriate for Ampan,
Karaveddi, and Puloly respectively and 𝑌𝑡 = 𝑌𝑡−1 − 0.18 +
23.5 sin 2𝜔𝑡 + 28.5 cos 1.5𝜔𝑡 + 20.10 cos 2𝜔𝑡 −
26.47 cos 5.5𝜔𝑡 , 𝑌𝑡 = 𝑌𝑡−1 −2𝑌𝑡−2 −5.9+ 73.5 sin 4.5𝜔𝑡 and
𝑌𝑡 = 𝑌𝑡−1 − 2𝑌𝑡−2 + 0.69 + 23.17 cos 5.5𝜔𝑡 were found as most
appropriate SCM for Ampan, Karaveddi, and Puloly
respectively. Among these models, SCM predicts reliable data
with minimum error and it finds the seasonal and cyclic pattern
of the rainfall. A five-month seasonal pattern and cyclic
behaviour at 13 - months interval was noted in Ampan.
Similarly, 10 - months seasonal pattern was observed in
Karaveddi. The Puloly region expressed the cyclic pattern of
rainfall only at 13-month interval. AD value is 0.40, 0.63 and
0.68 for Ampan, Karaveddi and Puloly respectively. The
decreasing trend of estimated rainfall in Ampan (0.21
mm/year) and increasing trend in Puloly (1.15 mm/year) and
Karaveddi (0.61 mm/year) is an alarming sign to the
agriculture sector.
