Annual Average Rainfall (1990 - 2019)
In the study area, many regions failed to receive regular monsoon rainfall, and abnormal intense rainfall was reported during the last decade due to climate change and global warming. The study area’s annual rainfall has varied from 646 to 1389mm for 102 years (1901-2002) (Rangarajan et al., 2019). During 2000 - 2010, rainfall varied from 550 to 789mm, and the average annual rainfall was 680mm. It is less than the state’s average rainfall (Selvam 2012). During the study period also less annual average rainfall than the Tamil Nadu, the annual average rainfall (960mm) for 30 years (1990 – 2019) except in the year 2008 (1002mm) (Fig.2). The minimum amount of rainfall (< 250mm) was noticed in the years 1990, 1995, 1999, 2006, and 2016, and the maximum rainfall (482mm – 795mm) was reported in 1993, 1997, 2004, 2005, 2010, 2015, and 2019. The intensity of rainfall varies from year to year, and the rainfall variation has been increasing since 2006. It leads to extreme floods or drought. It affects the environmental livelihood and ecosystem. Olayide and Alabi, (2018) also state that the dynamics of extreme weather condition causes flooding and drought, which leads to economic losses to the nation.
Seasonal Variation of Precipitation (1990 - 2019)
Rangarajan et al., (2019) exposed that in the Thoothukudi district, some significant irregularities were noted in all the monsoon seasons. In NE monsoon contributes 43%, and the least contributes 6% during the winter season for 102 years (1901 - 2002). From the year 2000 - 2010, the NE monsoon contributed 65.4%, and the SW monsoon contributed 8.06% (Selvam 2012). The irregularities of rainfall have been enduring in all the seasons in the Thoothukudi district for 30 years (1990 - 2019). The rainfall was much less from 1990 to 2012 compared to the seasonal average rainfall in Tamil Nadu (Fig.3). While the seasonal rainfall has risen to the extreme from 2013 onwards. The high intensity of rainfall was noticed in all the seasons expect SW monsoon from 2013 - 2015, as shown in Fig.3 (A, B, D). The unpredictable rainfall has affected the environment as well as a livelihood due to the flood. In the winter season, extreme rainfall was noted in 2013 and 2014 (Fig.3A). In the summer season, the rainfall was high in 2013, 2014, and 2018 (Fig.4B). In 2014, 2015, 2019, the high amount of rainfall was absorbed in the NE monsoon (Fig.3D). Whereas, in the SW monsoon, the amount of rainfall was significantly less from 1990 to 2013. Afterwards the rainfall sightly increased in the SW monsoon (Fig.3C). The physiographic settings has been significantly affecting the average winter rainfall (Charabi and Al-Hatrushi, 2010). The seasonal rainfall pattern might have changed depending on summer rainfall. Meehl and Washington (1993), Douville et al. (2000), Hu et al. (2000), Ueda et al. (2006), Annamalai et al. (2007), Kripalani et al. (2007), Turner et al. (2007), Stowasser et al. (2009) and Alessandri et al. (2010) also specified that the weakening of summer monsoon affects the frequency of monsoon rainfall. The Indian summer monsoon might have a severe effect due to reduced meridional gradient flow due to warming in the Asian region (Ueda et al. 2006). The Indian monsoon system is a prominent cycle of global climate circulation, and the dynamics of the Indian summer monsoon in very peculiar because the Indian summer monsoon has significant implications for EI Nino and ENSO events. The rainfall changes affect ocean and land surface temperatures.
Decadal Monthly variation of precipitation
Rainfall change is one of the phenomena of climate change, and its leads to drought. The regional drought has been depending on monthly rainfall. The intensity of monthly rainfall in the regional area leads to seasonal monsoon variation, whether it is high or low. In the study area, monthly average rainfall was mostly high in Oct and Nov among the 12 months for 30 years. The coefficient of variation (CV) in the monthly rainfall in the Thoothukudi district for three decades reveals that, during the I decade, the monthly rainfall CV is less (<100%) in Apr, May, Aug, Sep, Oct, and Nov (Fig.4A), due to SD close to the mean rainfall (Table 2). Whereas the rainfall CV is high (>100%) in Jan, Feb, Mar, Jun, Jul, and Dec, due to the SD spread out from the mean rainfall in every month of the decade. In the II decade, Apr, May, and Jul to Dec, the rainfall variation has less than 100% of CV due to the minimum SD of mean rainfall, and in the remaining month of Jan, Feb, Mar, and Jun rainfall CV is >100% (Fig.4B). It means monthly rainfall variation is high in every year. In the III-decade high intensity of rainfall was reported, so a high variation of SD has been noted in the months (Table 2 and Fig.4C). While, the month of Nov, rainfall CV has noticed <100% and remaining all the month the rainfall CV is >100% (Table 2). It means a month of Nov, rainfall SD is minimum to the mean of the decade, and the remaining months of rainfall SD are spread out from the mean (Fig.4C). Overall, the results show that Apr, May, Aug, Sep, Oct and Nov, the intensity and frequency of rainfall was reported as a significantly consistent variation. While Jan, Feb, Mar, and Jun, the rainfall variation is dispersion for I and II decades. This rainfall pattern has changed in III decade, all the month of rainfall variation is dispersion except Nov. In the month of Jul and Dec, the rainfall variation has been changing dynamically in every decade, either dispersion or consistent variation. Generally, the Thoothukudi district received a good amount of rainfall in Oct and Nov. During the study period, the I and II decades of the rainfall intensity has low than in the III decade.
Drought condition of Thoothukudi District
Drought is a deficiency occurring in the natural climatic phenomenon of precipitation and is difficult to monitor due to minor changes (Murthy et al., 2016). Min et al., (2011) state that rainfall discrepancies are one of the high climatic factors. It strongly influenced the intensity and frequency of climate extremes, when the impact of climate change was observed using rainfall data. Besides, greenhouse gases in the atmosphere also contribute to climate change. These gas concentrations help to magnify higher rainfall events. Whereas low intense rainfall events lead to earth warming in many parts of the world, even in small areas (Semenov and Bengtsson, 2002; Wilby and Wigley, 2002). Alessandri et al. (2010) addressed the changes in the Indian summer monsoon from the modeling studies, emphasizing the expected rise in greenhouse gas emissions. Numerous studies suggest that Indian rainfall has decreased significantly in most places over the past 50 years (Parthasarathy et al., 1993; Naidu et al., 1999; Dash et al., 2009; Kumar et al., 2009; Turner, 2010; Rana et al., 2012; Rana et al., 2014). The decreasing rainfall is an indicator of drought. It is essential for drought monitoring (Smakhtin and Hughes, 2007). Moreover, the decadal monthly rainfall variation leads to drought conditions, it is scanty or excess. The drought condition varied for every regional area at the same time of precipitation. Spatially, the study area has 20 rain-gauge stations. Its drought condition varied for each station due to the variation of monthly precipitation. During I decade, most of the study area was affected by scanty drought in all seasons due to the meager amount of rainfall in these regions, and the Excess rainfall was noted only in the winter season. Whereas, Normal and Deficient drought conditions have been noticed in a few more areas (Table 3 and Fig.5). Spatially, Surangudi, Vaippar, Vedanatham, Arasadi, Ettayapuram, Kadambur, Kalampatti, Kazhugumalai, and Maniyachi have to affect by scanty drought in all the seasons (Fig.8). Tuticorin, Tiruchendur, Srivaigundam, and Kovilpatti have an excess rainfall absorbed, and the normal rainfall was reported at Ottapidaram and Sathankulam regions, and the remaining stations reported from deficient to scanty rainfall in the winter season. In summer, Kayathar and Kovilpatti have normal rainfall, and Ottapidaram, Srivaigundam, and Sathankulam have reported deficient drought conditions (Fig.8). In SW monsoon, Ottapidaram and Kovilpatti have deficient, whereas the remaining stations are reported as scanty. In NE monsoon, Tiruchendur, Srivaigudam, Sathankulam, and Kovilpatti regions have normal drought conditions, and the rest of the stations are reported as scanty/ deficient (Table.3).
In the II decade (2000 – 2009), excess to normal rainfall was ruled in the winter and summer seasons, whereas deficient to scanty drought was dominated in SW and NE monsoon (Table 4 and Fig.6). Surangudi, Vaippar, Vedanatham, and Arasadi have scanty drought in all seasons (Fig.9). Ottapidaram has a deficient drought in all the season except SW Monsoon is scanty drought. In the winter season, Kadambur, Kalampatti, and Kazhugumalai have reported deficient drought. Kadalkudi and Maniyachi have Normal and Scanty droughts, respectively. Excess rainfall was reported in the remaining stations. In the Summer, Excess rainfall was recorded in Sathankulam, Kadambur, Kayathar, and Kovilpatti, and the remaining stations have a normal drought. In SW-Monsoon, Kayathar and Kovilpatti have deficient, and the rest of the stations reported scanty drought. During the NE-Monsoon, Tiruchendur, Kulasekarapattinam, and Sathankulam have normal drought and the rest of the stations recorded deficient rainfall (Fig.9).
During the III-Decade (2010 - 2019), scanty was reported all the stations in SW-Monsoon (Table.5 and Fig.7). In NE-Monsoon, Arasadi was only affected by scanty drought and the rest of the stations recorded as Excess to Deficient (Fig.10). Kulasekarapattinam has Excess rainfall in the winter season, and Tuticorin, Kayalpattinam, Tiruchendur, and Sathankulam have reported as Normal rainfall. Scanty to Deficient rainfall was reported in the rest of the stations. In the summer season, Sathankulam, Kalampatti, and Kazhugumalai have normal rainfall and the rest of the stations are scanty to deficient rainfall (Fig.10). The changes in the rainfall patterns affect agriculture and scheduled irrigation plans, which are dependent on the monsoons (Rangarajan et al., 2019). Moreover, seawater intrusion is also caused by the low and erratic rainfall in the coastal areas (Todd, 1959; Rajmohan et al., 2003; Adepelumi et al., 2009; Selvam, 2012a, Selvam, 2012b; Venkatramanan et al. 2015). Thoothukudi district has major industrializations like Harbour, Thermal power plants, Sterlite copper industry, Petrochemical industry, Alkali Chemicals and fertilizers, SPIC, spinning mills, etc., which are also leads to climate change. It affects the weather conditions such as participation pattern, surface temperature, humidity, etc.