Introduction The primary source of agricultural production for most of the country is rainfall. Efficient Utilization of rainfall improves the crop growth and development. About 80% of the world and 60% of Indian Agriculture is rain dependent. Changing rainfall pattern effect is directly felt on cropping pattern. Considering the importance and issues of rainfed farming, the study was undertaken in Coimbatore region of Tamil Nadu for determining the rainfall pattern which will be useful in crop planning. The two and three parameter distribution Normal, log-Normal, Weibull, Gamma, Logistic, Exponential, GEV (Generalized extreme values) were used here to study the best fit. The rainfall seasons of Tamil Nadu are Kharif (June to September), Rabi (October to January) and Zaid or summer (February to May). The selection of best fitted distribution is the key in determining rain-fall pattern. International Journal of Current Microbiology and Applied Sciences ISSN: 2319-7706 Volume 7 Number 04 (2018) Journal homepage: http://www.ijcmas.com Rainfall plays an important role in agricultural production system. Studying the rainfall distribution through the statistical models is also essential in agricultural field. In this paper, an attempt has been made to show the pattern of rainfall in TNAU1 Station Coimbatore through the Statistical distributions. In the study, the different statistical distributions are fitted for seasonal rainfall data and the best fit is determined. The historical rainfall data were collected for 34 years (1982 -2015) from the Meteorological Observatory AC & RI Coimbatore. The statistical distributions like Normal, Log-normal, Weibull, Gamma, Logistic, Exponential and Generalized Extreme value (GEV) are used for three different seasons such as Kharif, Rabi and summer. For Parameter estimation the Method of Maximum Likelihood are used here. The Comparisons of best distributions by goodness of fit-tests such as Kolmogorov–Smirnov, Anderson Darling test and Chi square test are also made in this paper. The trend test such as Mann Kendall trend test and Sen’s slope estimation were performed for studying the rainfall pattern. The Maximum rainfall occurred in the Rabi season with 9.028 (mm) and minimum rainfall in the summer with 0.371 (mm). K e y w o r d s Seasonal rainfall, Statistical distributions, Goodness of fit–test, Mann Kendall trend test, Sen’s slope estimator Accepted: 23 March 2018 Available Online: 10 April 2018 Article Info Int.J.Curr.Microbiol.App.Sci (2018) 7(4): 3053-3062 3054 Probability and frequency analysis of rainfall data enables us to determine the expected rainfall at various chances (Bhakar et al., 2008). The rainfall pattern decides the cultivation of crops, their varieties, adoption of cultural operations and harvesting of excess rain water of any region (Sinhabab, 1977; Budhar et al., 1987; Thalur, 1998, Kar et al., 2004). Kumar et al., (2007) conducted a study to identify best fit trend of weekly rainfall data of 45 years (1955-1999) for the western Uttar Pradesh at various probability levels and expected rainfall frequency for crop planning and management to work out the irrigation period required for the crops and found 70% probability level was useful for planning of kharif crops and to decide proper time for various agriculture operations. Rajendran et al., (2016) conducted a study to analyze the nature of distribution and frequency of rainfall of 32 years (1982-2013) for Dharmapuri district. Rainfall frequency analysis was done using Weibull’s method and resulted that annual average rainfall of 938.1mm can be expected to occur once in 2.5 years at a probability of 40%. Fisher (1924) studied the influence of rainfall on the yield of wheat in Rothamasted. He showed that it is the distribution of rainfall during a season rather than its total amount which influence the crop yield. Materials and Methods Coimbatore region comprise of 10 Taluks with Latitudes N 11° 0' and 16.4016' and Longitudes E 76° 57' and 41.8752' (Fig. 1). The total geographical area of the district is 642.12 km2 (net area sown is 6.0%). The normal annual rainfall of North East Monsoon is 328.9mm and South West Monsoon 189.8mm. Thus the major part of rainfall comes from the North-east Monsoon. The Yearly rainfall (mm) data of 34 years (19822015) were collected from Meteorological Observatory AC and RI, Coimbatore and it was converted by arithmetical ways to seasonal rainfall data. The best fit probability distribution was evaluated by using the goodness of fit test such as Kolmogorov Smirnov test, Anderson darling test and chisquare test at 5% level of significance. Statistical distributions Probability distributions are widely used for understanding the rainfall distribution and computation of assumed rainfall (Rajendran et al., (2016). In this study the distributions such as normal, log-normal, Weibull, gamma, logistic, Exponential, extreme value distributions are considered for selection of appropriate distributions. These are the class of continuous probability distributions.
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