Skewed July rain explains floods in some states, dry reservoirs in others

July's normal monsoon rain narrowed rainfall deficit, but analysis shows showers were uneven, triggering floods in some regions and dry reservoirs in others

New Delhi: With half of the June-September monsoon season over, country-level deficit in rainfall has decreased substantially compared to earlier in the season, although it remains significant. This is thanks to July rain, which has been usual. However, an HT analysis of the gridded rainfall data of the India Meteorological Department (IMD) shows that the cumulative country-level trend hides the concentration of July rain in a few places and in about half the days of the month. This is why

Parts of the country are reporting devastating floods (AP Photo)

• A small surplus in July has decreased monsoon deficit

  Country-level rain in July this year is 281.9 mm, according to the gridded rain data of the India Meteorological Department (IMD). This is around 2.2% more than the 1971-2020 average, which IMD currently considers as the Long Period Average (LPA) for tracking rain’s performance; and ranked 68th highest (or 59th lowest) since 1901, the first year for which IMD has published gridded data. This modest surplus has helped decrease the cumulative monsoon deficit from 31% in June to just 10.5% now. To be sure, the rank and departure might be somewhat different in the summary statistics IMD publishes because of differences in weather stations used and the process of calculation.
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  But this surplus was made possible by just 18 days of rain

  To be sure, the small surplus in July rain was accumulated in just 18 days that recorded a surplus. In total, these 18 days received 209.3 mm rain compared to the LPA of 156.3 mm for those days, a 34% surplus. The other 13 days, all of which recorded a deficit and have a cumulative LPA of 119.6 mm, received just 72.7 mm rain, a 39% deficit. This implies that the rainfall was intense. This can be seen in the departure in different intensities of rain. Light (rain up to 7.5 mm at a place in 24 hours in a day) and moderate (7.6-35.5 mm rain at a place in 24 hours in a day) rain have deficits of 4.9% and 6.2% while all categories of heavy intensity rain (at least 35.6 mm rain at a place in 24 hours in a day) taken together have a surplus of 12%. In other words, July owes its surplus entirely to rain of heavy intensity concentrated in a few days.
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  Area under large surplus and large deficit was almost equal in July

  IMD classifies a local deficit of 60% or more as “large deficient” and that of 20%-60% as “deficient”. Similarly, a surplus of 60% or more is classified as “large excess” and that of 20%-60% as “excess”. Rain within 20% of the LPA is classified as “normal”. The area under large excess and large deficient categories was almost equal in July: 12.2% and 12.6% respectively. This further shows that July was a month of extremes, with parts of India extremely dry and other parts extremely wet. To be sure, despite the extreme contrast, the month was wet in more places than it was dry, with less than a majority of the country (44% of India’s area) in the deficient or large deficient category in the month. This decreased the area under these two categories for cumulative monsoon rain from 67% on June 30 to 46% on July 31.
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  This is reflected in the geographical skew in reservoir levels

  The regions of large excess in rainfall in July – such as south Gujarat, northwestern parts of Maharashtra, Uttarakhand, Nagaland, and some parts of Assam – are correlated with the places that have reported floods in the month. It also explains the geographical trends in the storage of 166 reservoirs monitored by the Central Water Commission (CWC). For example, resevoirs monitored in Odisha, Chhattisgarh, Gujarat, and Maharashtra have a surplus. On the other hand, reservoirs monitored in all peninsular states and those in the Gangetic plains are running a deficit. And this also explains the latest round of charges and counter charges in the water wars between Tamil Nadu and Karnataka, both states with deficient rainfall as of July 31.