Data: Long Period Average of South-West Monsoon Rainfall Has Reduced Over the Years
Sai Krishna Muthyanolla
July 13, 2023
Long Period Average of rainfall or LPA is the rainfall recorded over a particular region for a given interval (like month or season) averaged over a long period like 30 years or 50 years. Data indicates that India’s mean rainfall for the South-West monsoon season has dropped by 12.0 mm during 1971-2020 as compared to 1961-2010.
Every year, around the first week of June, the South-West Monsoon hits Kerala and gradually progresses towards the Himalayas and northwest region. By July, the Monsoon is expected to cover the entire county. The southwest monsoon is generally responsible for the major portion of annual rainfall over this sub-continent and contributes about 75-80% to the annual total rainfall in South Asia. The livelihoods of millions of people are dependent on these rains. Monsoon variability has a severe impact on agricultural production which in turn impacts the country’s economy and food security. Further, monsoon spells come as a relief after the hot summer and help in replenishing the groundwater levels and other surface water resources like dams, tanks, and reservoirs.
In the last two weeks, there have been reports of severe damage due to heavy rains in the northern parts of India. According to the Central Water Commission’s flood forecast portal, the water level in Yamuna River in Delhi has breached all-time record levels and even the danger mark. However, this year, according to Indian Meteorological Department (IMD), the rainfall was 10% lower than usual in the month of June 2023. But how is this measured?
Long Period Average reduced in the latest revision
All India’s monthly rainfall is the amount of accumulated rainfall received over India for a particular month. Likewise, the all-India seasonal rainfall is the amount of accumulated rainfall received over India for a particular season. For instance, the seasonal rainfall of the South-West Monsoon is for the four-month period from June to September. These values vary from year to year.
Long Period Average of rainfall or LPA is the rainfall recorded over a particular region for a given interval (like month or season) averaged over a long period like 30 years or 50 years. The value is used as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season. IMD which monitors the rainfall in the country has prepared this rainfall normal based on the data for the period of 50 years and is updated periodically once in every decade by incorporating the latest data from rain gauge stations.
At the national level, effective from South-West Monsoon 2022, the LPA for the South-West has been revised to 868.8 mm based on data from 1971-2020 instead of the erstwhile rainfall normal of 880.6 mm based on 1961- 2010. It was 89 cm or 890 mm for the period 1950-2000. In other words, India’s mean rainfall for the South-West monsoon season has dropped by 12.0 mm during 1971-2020 as compared to 1961-2010. Similarly, at the national level, the annual LPA has been reduced from 1176.9 mm to 1160.1 mm, by 16.8 mm.
It is based on this LPA that IMD categorises rainfall into large excess, excess, normal, deficient, and large deficient across spatial and temporal scales. For instance, if the realized rainfall is 60% or above of the LPA, the rainfall is large excess. If it is less than 20% higher or lower, the rainfall is considered normal. For the entire monsoon season at the national level, when the rainfall averaged over the country as a whole is within 90% to 110% of LPA, the rainfall is said to be ‘normal’ and when the rainfall is less than 90% or more than 110% of LPA, the rainfall is said to be ‘below or above normal’.
The trend in actual rainfall in South- West Monsoon during the monsoon season (June to September) since 1901 has been mapped for the national level as well as four different regions in this story. Data for this story has been taken from Factly’s Dataful.
Rainfall has reduced in some regions
Although there is variation every year, at the All-India level, it is seen that the actual rainfall has dropped below 800 mm and gone above 1000 mm more frequently since 1960. Prior to that, the majority of the years saw rainfall in the range of 800 to 1000 mm.
Across regions, while there is higher variability in the rainfall since the 1960s in the south peninsular region and central India, the actual rainfall in the monsoon season has dropped gradually in the northeast. In northwest India, since 2000, the rainfall has been below 600 mm in 17 out of 22 years. This is slightly higher compared to the earlier years.
Overall, the rainfall has declined in some regions as is evident in the case of the revised LPA.
Variation in patterns across months is evident
Data of the monthly actual rainfall during the South-West monsoon season indicates that in the month of June since 1901, the rainfall has dropped below 100 mm only 4 times- in 1905, 1926, 2009, and 2014. Since the 1950s, the rainfall has been below 150 mm in June more frequently. In the month of July, the year 2002 recorded the lowest rainfall since 1901. Until the 1970s, the rainfall was more than 300 mm more frequent than in the period since. In September, the rainfall crossed 250 mm only three times, first in 1917 and twice since the 1970s- in 1983 and 2019.
Monsoon forecast has improved
Forecasting monsoons is crucial as it helps the country to be better prepared to deal with the rains & plan the cropping season considering the significant number of people dependent on rainfalls in India. For instance, forecast helps IMD to issue early heat and rain warnings and alerts to states and districts. With the increase in extreme weather events in recent years due to climate change, the need for precise forecasts has only increased.
Factly compiled data of the actual date of onset of monsoons and the forecasted date from various sources like IMD’s journal and press releases.  Data is not available for 7 years- 1994, and the period from 1999 to 2004. IMD started publishing the said data periodically starting in 2005. Data clearly indicates that IMD’s forecast has improved in the last two decades. With the variation of 10 to 16 days until 1999 and up to 6 days in the last 15 years, the forecast precision has improved.
Featured Image: Monsoon Rainfall