Impact of climate change on monsoon onset over Tamil Nadu

“Timely sowing results in half the production” is an important quote of rainfed agriculture and preparedness for that depends on how accurately we predict the monsoon. Rainfall is highly variable in both temporal and spatial dimension. The dependency of rainfall forecast can be increased by using higher resolution rainfall data in both space and time. The existing pre-monsoon sowing week of Tamil Nadu was calculated long back at Agro Climatic Zone level and which is only seven for entire Tamil Nadu state. In order to ascertain the impact of changing climate on the onset of monsoon, an attempt has been made at Tamil Nadu Agricultural University under UGC sponsored scheme “Revalidating premon soon sowing week with higher resolution for changing climate of Tamil Nadu”. To identify the shift in the rainy season and for fixing the pre-monsoon sowing week at block level, observed rainfall over the period from 1951 to 2010 has been utilized. Daily rainfall observed at 18 TNAU research stations, 47 locations from State Ground and Surface Water Data Centre and 700 locations from district level State Documentation department were utilized in this study. As these observed records are available only for the period from 1970-2011, the APHRODITE's gridded data sets of daily rainfall was utilized to fill the gaps from 1951 and also used the PET data sets from CRU-TS 3.1 monthly climatology for the present analysis. The agro-climatic variable viz., the length of growing period and pre-monsoon sowing week were estimated using Jeevananda Reddy’s 14-week moving average technique. To predict the shift over the 60 years period, four period analyses were made for each of the 30 years from 1951 leaving a decade behind for the subsequent analysis. The differences in the onset have been observed over these periods in Tamil Nadu and are more pronounced by an earlier onset in southern districts and delayed in coastal part of Tamil Nadu where North East Monsoon is the case.