AVERAGE MONSOON RAINFALL
1. Context
In its first-stage long-range forecast for the 2022 southwest monsoon, the India Meteorological Department (IMD) has forecast normal rainfall during the season. It has, however, downgraded the Long Period Average (LPA) for all-India monsoon rainfall from 88.06 cm to 87 cm, effective from June this year.
2. Trends of Rainfall in India
- Based on trends for 1961-2010, India’s normal annual rainfall is about 1176.9mm. Of this, nearly 74.8%, or 880.6 mm (88.06 cm), occurs during the Southwest monsoon from June to September. This is the LPA rainfall for the monsoon, the figure that has been revised.
- Before the revision, the distribution of the rest of the rainfall was 3.4% during winter (January-February); 11.2% in the pre-monsoon season (March-May), and 10.5% during the post-monsoon season (October-December).
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Image Source: The Indian Express3. What is LPA?
- It is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc.
- The amount of rain that falls every year varies from region to region and from month to month.
- The IMD also maintains LPAs for every meteorological region of the country.
- This number ranges from 61 cm for the drier Northwest India to more than 143 cm for the wetter East and Northeast India.
- The LPA of the season rainfall over the country as a whole for the period 1971-2020 is 87 cm.
4. When is the LPA revised?
- It is an international convention to verify the quantum of annual and seasonal monsoon rainfall once in a decade. The monsoon season’s LPA rainfall acts as a baseline figure calculated over 50 years.
- The LPA is revised if required, depending on any variations observed from the rainfall data obtained from the network of rain gauges.
- In 2002, the IMD operated 1,963 rain gauges located across 523 districts.
- As of 2020, rainfall data was being collected from 4,132 rain gauges spread uniformly across 703 districts.
- Between 2005 and 2010, India’s LPA was taken at 89.04 cm. Between 2011 and 2015, the IMD revised it to 88.75 cm.
- It was 88.06 cm between 2018 and 2021. From the upcoming monsoon, the revised LPA will be 87 cm.
5. How much rainfall does India receive in a year?
IMD maintains five rainfall distribution categories on an all-India scale. These are:
- Normal: When the percentage departure of actual rainfall is +/-10% of LPA.
- Below normal: when the departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA.
- Above normal: when actual rainfall is 104-110% of LPA.
- Deficient: when the departure of actual rainfall is less than 90% of LPA.
- Excess: when the departure of actual rainfall is more than 110% of LPA.
6. Why it has been Downgraded?
- The monsoon season rainfall shows an epochal behavior.
- The monsoon can shift between dry and wet epochs (30 to 50-year periods) in certain decades.
- The reduction in rainfall is thus due to the natural multi-decadal rainfall variability. Decadal variability between 1901 and 2020 shows the southwest monsoon rainfall underwent a dry epoch between 1901 and 1921. This was followed by a wet epoch that prevailed till 1970.
- Since 1971, the monsoon has been passing through a dry epoch that persists to date.
- The decadal mean value will reach near normal during 2021-2030. The decadal mean value for the ongoing decade is predicted to be around minus 1.4 to 1.5.
- Normally, the realized monsoon rainfall remains below normal for most years in a decade during a dry epoch.
- Rainfall is normal or above normal during most of the years in a decade when it is a wet epoch.
| For Prelims: India Meteorological Department (IMD), Long Period Average (LPA), Northeast India, Monsoon behavior, Climate Change, and Southwest monsoon rainfall. |
Previous year Questions
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1. Consider the following statements: (UPSC 2012)
1. The duration of the monsoon decreases from southern India to northern India.
2. The amount of annual rainfall in the northern plains of India decreases from east to west.
Which of the statements given above is/are correct?
A. 1 only
B. 2 only
C. Both 1 and 2
D. Neither 1 nor 2
Answer: C
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Source: The Indian Express
