- The country is likely to receive normal rainfallfor the fourth consecutive year, the India Meteorological Department (IMD) said in its first Long Range Forecast (LRF) for this year.
- Rainfall in the four-month June-September southwest monsoon season was in the normal range in 2019, 2020, and 2021 as well.
What is Long period average?
- The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
- According to the IMD, the “LPA of rainfall 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 IMD’s prediction of a normal monsoon was based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
- The IMD has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for the period 1951-2000.
- While this quantitative benchmark refers to the average rainfall recorded from June to September for the entire country, the amount of rain that falls every year varies from region to region and from month to month.
- Therefore, along with the countrywide figure, the IMD also maintains LPAs for every meteorological region of the country — this number ranges from around 61 cm for the drier Northwest India to more than 143 cm for the wetter East and Northeast India.
Why LPA is needed?
- The IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations. Because annual rainfall can vary greatly not just from region to region and from month to month, but also from year to year within a particular region or month, an LPA is needed to smooth out trends so that a reasonably accurate prediction can be made.
- A 50-year LPA covers for large variations in either direction caused by freak years of unusually high or low rainfall (as a result of events such as El Nino or La Nina), as well as for the periodic drought years and the increasingly common extreme weather events caused by climate change.
Why LPA has been downgraded?
- The decrease is part of the natural multidecadal epochal variability of dry and wet epochs of all India rainfall.
- Presently, the southwest monsoon is passing through a dry epoch that started in the decade of 1971-80. The decadal average of all India SW monsoon rainfall for the decade 2011-20 is -3.8% of the long-term mean. The next decade i.e. 2021-30 is expected to come closer to neutral and southwest monsoon would enter into the wet epoch from the decade 2031-40.
- Normally, the realized monsoon rainfall remains below normal for most years in a decade during a dry epoch. On the other hand, rainfall is normal or above normal during most of the years in a decade when it is a wet epoch.
Range of normal rainfall
In its forecast, the IMD said: “Southwest monsoon seasonal (June to September) rainfall over the country as a whole is most likely to be normal (96 to 104% of LPA. Quantitatively, the monsoon seasonal (June to September) rainfall is likely to be 99% of the LPA with a model error of ± 5%. The LPA of the season rainfall over the country as a whole for the period 1971-2020 is 87 cm.”
The IMD maintains five rainfall distribution categories on an all-India scale. These are:
* Normal or near normal, when the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA;
* Below normal, when 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 departure of actual rainfall is less than 90% of LPA; and
* Excess, when the departure of actual rainfall is more than 110% of LPA.
Factors Responsible for Inaccurate Monsoon Forecast:
- The lack of data due to insufficient monitoring stations.
- Automatic weather stations are of substandard quality. They need to be calibrated and cleaned regularly, which does not happen often. That affects data.
- Then, there are major data gaps, like those involving dust, aerosols, soil moisture and maritime conditions are not monitored.
- The models that we have brought from the west have been developed by western scientists to forecast in their region, little progress has been made is the fine-tuning of weather models to suit Indian conditions.
- Lack of competent software professionals and scientists working with the IMD.
- The population of India is increasing and to provide food security to the population, a large part of the monsoon water which is currently unutilized should be held at suitable locations for irrigation and power generation purposes.
- India needs to invest more resources in better prediction of Monsoon forecast in order to achieve reliability and sustainability.
- With a warming climate, more moisture will be held in the atmosphere, leading to heavier rainfall, consequently, inter-annual variability of the monsoon will increase in future. The country needs to prepare for this change.
- Thus, to secure and bring sustainability to the climate pattern of India we need to take effective and timely steps not just at the domestic front (National Action Plan on Climate Change) but also at international front (UN Framework Convention on Climate Change), as we live in a shared world with a shared future.