Observations of the IPCC's 5th Assessment Report (WGI AR5) have showed a general increase in heavy precipitation at the global scale. Extreme precipitation is expected to increase with warming. Analysis of observed annual maximum 1 day precipitation over global land areas with sufficient data samples indicates a significant increase in extreme precipitation globally, with a median increase about 7% °C. It has also been reported that on various aspects of the global hydrological cycle including increasing atmospheric moisture content is directly relevant to extreme precipitation changes.
In the “Climate Profile of India” (2010) of Indian Meteorological Department (IMD), Govt. of India reported that the average annual rainfall is less than 13 cm over the western Rajasthan, while at Mausiram in the Meghalaya has as much as 1141 cm during 1901-2003. The rainfall pattern roughly reflects the different climate regimes of the country, which vary from humid in the northeast (about 180 days rainfall in a year), to arid in Rajasthan (20 days rainfall in a year). Based on a homogeneous data series for the period 1901-2003, IMD has reported that mean monthly rainfall for the country as whole during July is highest and contributes about 24.2% of annual rainfall. The mean south-west monsoon (June, July, August & September) rainfall contributes 74.2% of annual rainfall (1182.8 mm). The decades 1961-70, 1971-80 and 1981-90 were dry periods. However, during the winter season, rainfall is decreasing in almost all the subdivisions except for the sub-divisions Himachal Pradesh, Jharkhand and Nagaland, Manipur, Mizoram & Tripura.
Based on the amount of rainfall in a day, IMD has classified into six categories and regrouped into three broad categories viz. (i) light to rather heavy rainfall (0 < R ≤ 64.4 mm), (ii) heavy rainfall (64.4 < R ≤ 124.4 mm) and (iii) very heavy to exceptionally heavy rainfall (R > 124.4 mm ), which is also referred as extreme rainfall events. The frequency of extreme rainfall event shows increasing trend over the Indian monsoon region during the southwest monsoon season from June to September (JJAS) of 1901-2003.
Daily rainfall data set for the last 58 years (1954 –2012) of the state shows that the rainfall of the state varied from 703 – 2217 mm in the state. Majority of the districts of Manipur experienced a decrease in precipitation in the past 100 years, despite a considerable increase in precipitation is observed in the northern districts of Manipur i.e. Senapati and Ukhrul. During the same period, a marginal decrease in precipitation is observed over the central and southern Manipur districts such as Imphal (east & west), Bishnupur, Thoubal, Tamenglong, Jiribam, Chandel, Churchandpur. The decadal assessment of the monthly total rainfall reveals that the period of raining days in Manipur has expanded from three months i.e. June-August to seven months i.e. April-October. Manipur has experienced for increasing of heavy rainfall event (80 -100 mm per day) trends during the last 50 years (1960 – 2010) i.e. 99.2 mm (3 May 1991), 80.9 mm (22 April 1993), 86.6 mm (14 June 1994), 90.6 mm (18 July 1995), 90.3 mm (13 August 2002), 88.8 mm (15 August 2009), etc.
The Indian Network for Climate Change Assessment (INCCA) has projected to receive increased precipitation to the entire state of Manipur by 2030s. The northern parts of the state are projected to experience an increase of ≥19% rainfall. This roughly correlates with observed trends over the last 30 years. The districts of Tamenglong and Senapati are projected to experience an increase in precipitation of ≥ 21%. Further, an increase in the number of extreme rainfall (100 mm/day) conditions is also projected for the state in the near future.