Monsoon normal despite heavy rainfall in September
Only about two weeks remain in the 122-day-long June-September monsoon season. Given this, cumulative monsoon rainfall this season is likely to remain in the normal zone. This is despite the fact that there were days in the last two weeks when the departure of daily rainfall from past averages was higher than any other day this monsoon. Such days, as has been the pattern this season, only made up the deficit which was created due to dry patches in previous weeks. However, rainfall in the last two weeks has ended the deficit in the 130 main reservoirs of the country.
High rainfall in the past two weeks has only reduced deficit
The India Meteorological Department (IMD) analyses trends in rainfall for any interval by comparing it with the average of rainfall for that interval in the 1961-2010 period (called the Long Period Average or LPA). As of 8.30am on September 18, cumulative rainfall’s departure from the LPA for this monsoon season was a deficit of 0.1% according to IMD’s gridded rainfall dataset, which is within the normal range. IMD considers a deviation of up to 20% on either side of LPA as normal, a positive deviation of 20% or more as excess, and a negative deviation of 20% or more as deficient rainfall.
IMD’s broad categories, however, do not detail how rainfall has varied this season. Cumulative rainfall remained in the normal range throughout since June 29, but has also varied within this range. On June 30, there was a 15.2% surplus in cumulative rainfall. This changed to a 2.21% surplus on July 31 and a 5.48% deficit on August 31, before arriving at the 0.1% deficit on September 16.
These changes in cumulative rainfall’s deviation from LPA have taken place on the back of wide variation in daily rainfall. In June, the daily rainfall was above the LPA on 18 of 30 days. Such days decreased to 14 in July and only 9 in August. The regular rainfall in September – 13 of 18 days have seen above LPA rainfall – has therefore only managed to make up for the deficit at the end of August.
One-fourth districts still rainfall deficient
The pattern in the average rainfall over India also glosses over the wide variation seen across states. While India has a 0.1% deficit, among states, cumulative rainfall’s departure from LPA varies from a 33.7% surplus in Delhi to a 60.6% deficit in Ladakh. A total of five states and UTs have received an excess, five have received deficient, and the rest have had normal rainfall. The excess regions are Delhi, Telangana, Tamil Nadu, Meghalaya, and Andhra Pradesh. The deficient ones are Arunachal Pradesh, Nagaland, Chandigarh, Manipur, and Ladakh.
To be sure, among smaller states or districts, the current distribution of excess, normal, and deficient units can possibly change in the next two weeks. Gujarat and Odisha were deficient on August 31, but have now moved to the normal range, having received 45% and 31% of their respective cumulative rainfall until September 18 after August 31. Delhi similarly was in the normal range on August 31, but is now in the excess range. It has received 27% of the cumulative rainfall until September 18 after August 31.
At the district level, the number of deficient districts has decreased this month. However, around 1/4th of districts remain deficient, data shows. Of the 614 of India’s districts (Delhi’s districts merged as one) from 2011 census for which this analysis is possible, 196 were found deficient, 278 were in the normal range, and 140 had received excess rainfall until August 31. On September 18, there were 140 deficient districts, 324 normal districts, and 150 excess districts.
Was this monsoon exceptionally heavy?
As discussed above, August and July saw long dry patches. The deficiency from these patches has been made up by heavy patches rainfall. Does this mean heavier intensity rainfall has had a very big share this year? This is not true at the country-level. The share of heavy and extreme rainfall until September 18 is 40.3%, which is less than the average of 42.9% for the June 1-September 18 interval during the 1961-2010 period.
However, since country-wide rainfall is the average of rainfall over different parts, it is theoretically possible that intense rainfall in parts of India does not add up to create a huge amount at the country-level. Although the share of heavy and extreme rainfall is less than the LPA at the country-level, it is more than the LPA in 12 of 33 states and UTs for which this analysis is possible. For example, the share of such rainfall is 56.7% in Delhi this year, compared to 35.7% in the LPA. In Nagaland, the share of such rainfall this year (10.6%) is half of the LPA of the share (21.3%).
The improvement in the last two weeks has filled up reservoirs
September rainfall has, however, managed to fill up reservoirs where there were deficits. At the country-level, the 130 main reservoirs of the country for which the Central Water Commission has data had a 6% deficit compared to the last 10 years’ average on September 2. By September 16, this was reduced to almost nil. The storage in these reservoirs was 127.05 billion cubic metres (BCM) on September 16. The last 10 years’ average for this time is 127.45 BCM.
The biggest deficit currently is in Punjab (44%), Himachal Pradesh (34%), and Madhya Pradesh (22%). To be sure, the current deficit in these states is still lesser than what it was on September 2. The biggest surplus regions are Andhra Pradesh (167%), Telangana (93%), and Tripura (23%).