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Rainfall

Rainfall

Bihar experiences rainfall mainly during Southwest monsoon season (June to September). Figure 1 shows the annual departure of rainfall in different districts of Bihar. The anomalies were computed based on the LPA period of 1961-2010. Out of 38 districts of the state, 1 received deficient rainfall (-20% to -59% of its 1961-2010 period LPA), 16 districts received normal rainfall (-19% to +19% of its LPA), 18 received excess rainfall (20% to 59% of its LPA), 3 received large excess (60% or more of its LPA) rainfall. Figure 2(a) and 2(b) show the time series of variation of % departure of seasonal and annual rainfall for the state for the period 1901-2021 respectively. The departures are calculated with respect to the LPA base period of 1961-2010. For the monsoon season & the year 2021, the state received 103% & 127% of its LPA rainfall respectively. Figure 2(c) depicts the district-wise rainfall trends for annual rainfall. It is seen that among all the districts, 6 districts show significantly decreasing trend while rest of the districts show non-significant trend.

Standardized Precipitation Index (SPI)

The SPI is based on precipitation and used for measuring drought. This index is negative for drought and positive for wet conditions. As the wet and dry conditions become more severe, the index becomes more positive or negative. Extremely wet/severely wet conditions were observed over parts of West Champaran, Darbhanga, Gaya, Gopalganj, Madhubani, Muzaffarpur, Samastipur, Siwan, Vaishali districts while mildly /moderately wet conditions were observed over other districts of the state except Purnea district which observed mildly dry condition (Fig. 3).

Extreme Weather Events

Heavy rainfall events were recorded over some stations of Bihar during the period January to December 2021 (Figure 4). Extremely heavy rainfall (> 204.4 mm) events were recorded over some stations of Bihar during the period January to December 2021. Table 1 shows the extremely heavy rainfall values with the date of its occurrence and the location. Locations of impact occurred associated with Major Extreme Weather Events occurred during 2021 (Figure 5). Table 2 shows the extreme weather events and loss of Human Lives occurred during 2021 in Bihar.

Figure 1: District Annual rainfall departure Map for Bihar (Source: India Meteorological Department).
Figure 2(a): Percentage departure of southwest monsoon rainfall averaged over Bihar state (1901-2021) (Source: India Meteorological Department).
Figure 2(b): Percentage departure of annual rainfall averaged over Bihar state (1901-2021) (Source: India Meteorological Department).
Figure 2(c): District-wise annual rainfall Trend (Source: India Meteorological Department).
Figure 3: District wise Annual SPI Map for Bihar (Source: India Meteorological Department).
Figure 4: Location of Heavy Rainfall (64.5-115.5mm), Very Heavy Rainfall (115.6-204.4 mm) and Extremely Heavy Rainfall (more than 204.5 mm) reported stations over Bihar during the period January to December 2021 (Source: India Meteorological Department).
Figure 5: Locations of impact occurred associated with Major Extreme Weather Events occurred during 2021 causing loss of life (Source: India Meteorological Department).
Table 1: Extremely heavy rainfall recorded over some stations of Bihar during January – December 2021.

Date

Station Name

Rainfall (in mm)
28 MAY 2021 Manihari 251.6
28 MAY 2021 Purnea 212.6
29 MAY 2021 Tribeni Balmiki 215
15 JUN 2021 Chanpatia 275.8
15 JUN 2021 Ramnagar 277.6
30 JUN 2021 Gaunaha 214.6
3 OCT 2021 Thakurganj 210
20 OCT 2021 Forbesganj 257.4
20 OCT 2021 Hasanpur 235.2
20 OCT 2021 Nirmali 248.6
Table 2: Extremely heavy rainfall recorded over some stations of Bihar during January – December 2021.

Event

Date

Season

Affected Districts
Flood and Heavy Rain 7 to 17 August 2021 Monsoon (June to September) Begusarai, Bhagalpur, Bhojpur, Buxar, Darbhanga, Katihar, Khagaria, Lakhisarai, Munger, Muzaffarpur, Patna, Saharsa, Samastipur, Saran, Vaishali
Lightning & Thunderstorm 12 May; 23 to 28 June; 30 July; 7 August; 2, 27 September; 1, 2, 17, 19 October Pre-Monsoon (March to May), Monsoon (June to September) & Post-Monsoon (October to December) Araria, Aurangabad, Banka, Begusarai, Bhagalpur, Bhojpur, Darbhanga, East Champaran, Gaya, Gopalganj, Jamui, Katihar, Khagaria, Lakhisarai, Madhepura, Madhubani, Munger, Muzaffarpur, Nalanda, Nawada, Patna, Purnia, Rohtas, Samastipur, Saharsa, Saran, Sheikhpura, Sitamarhi, Siwan, Supaul, West Champaran, Vaishali
Heavy rainfall and strong winds associated with the Very Severe Cyclonic Storm YAAS 23 to 28 May Pre-Monsoon (March to May) Begusarai
Historical Rainfall in Bihar (1961-2019)

Several extreme rainfall events were recorded in the past (1960-2019). Maximum rainfall (about 1000 mm) takes place during the Monsoon season (June, July, August, and September) followed by summer (March, April, and May), Post Monsoon (October-November), and Winter (December-February).

Figure 6: Seasonal variability of rainfall (1961-2020) over Bihar

Strong interannual variability of rainfall was observed in the period 1961-2019. The highest range has been observed in 1971, an extreme flood event, and 1966 and 1992, the severe drought years.

Figure 7: Annual variability of rainfall (1961-2020) marked by significant drought and flood events over Bihar.

As per the definition of IMD, any day having rainfall of 2.5 mm or more is considered a Rainy Day. Bihar observed an average of 80 rainy days per year in last sixty years (1961-2019). However, the districts in the Northeastern part of the state, particularly Kishanganj and Purnia have recorded more than 100 rainy days per year.

Figure 8: Number of Rainy days (>2.5 mm /day) in Bihar over the last 60 years.

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