Number theory: Past week’s rain covered deficit... but in areas that didn’t need it

A look at how heavy rains in the past week means India's rain deficit has dropped, but in regions that already had a surplus.

Monsoon rain has been intense in the past week, particularly, in the northern and north-western parts of the country, and along the west coast. This has led to widespread chaos, such as the floods seen in the national capital and the landslides seen in Himachal Pradesh and Uttarakhand. An HT analysis shows that the heavy downpour has helped fill the overall rainfall deficit and also reduced the area under deficit. However, this may not be the perfect conditions for farmers

The recent downpour has helped fill the overall rainfall deficit, data shows

The charts that matter

• 
  No more deficit for India overall, but only because of heavy rain

  India has received 250.9mm rain up to 8.30am on July 9, 4.8% more than the 1961-2010 average and the 46th highest since 1901, the earliest year for which India Meteorological Department (IMD) has gridded data for rainfall. This data makes it appear as if India is seeing a rather regular monsoon, but that is far from true. Higher intensity rain has been record-breaking this year. “Heavy” and “extreme rain” combined has a surplus of 14.6% and is ranked 28th highest since 1901. On the other hand, light and moderate intensities have a deficit of 6.1% and 1.9%, only 88th highest and 75th highest since 1901. Clearly, heavy rain has helped fill the deficit prevailing up to last week.
• 
  Rain in the past 5 days has helped in creating a surplus in a big way

  Intense rain falls in a shorter duration of time. Daily rain trends show this. There was at least a 20% surplus on each day except on the first two days of the week ending July 9. These five days were also ranked 33rd or higher for the day’s rain since 1901. The rain on July 9 (this is the 24-hour period ending at 8.30am on July 9 in IMD’s gridded dataset) had a surplus of 65.4%, the most for any day this season. Rain on both July 7 and July 9 rain is ranked the 5th highest for those days since 1901. This intense and continuous five days of surplus rains is what led to the condition for the disasters seen in the past week.
• 
  ‘Deficient’ rain area down, but not all of it increased area under ‘normal’ rain

  As is expected, past week’s rain has decreased the area under “deficient” rain (this is a deficit of 20% or more at the local level): from 46.4% of the total area up to July 2 to 38% by July 9. However, this eight percentage points decline did not all contribute to increasing the area under “normal” rain (rain with a deficit or surplus under 20%). The area under “normal” rain increased 5.1 percentage points and the area under “excess” (a surplus of 20% or more) rain increased 3.3 percentage points. This is expected because some of the surplus rain fell over regions that were not under deficit before. This is the case the most in Rajasthan, Haryana, Punjab, Himachal Pradesh, Jammu and Kashmir, Ladakh, Uttarakhand, and Uttar Pradesh. In the first five of them, the area under both “deficient” and “normal” category has decreased in the past week and only the area under “excess” category has increased. In the latter three, less than half of the decrease under “deficient” category has passed to the “normal” category, with the rest going to “excess” category. This created the conditions for the floods and landslides seen in the past week. To be sure, such an area-wise analysis is not possible for very small states that are covered by a single grid, such as Delhi. However, the rain for the Delhi grid in the past week was also more than required. The July 9 rain (it was 19 times the historical average and the highest rain for the day since 1901), for example, just added to Delhi’s already sufficient surplus, which increased from 32.6% on July 8 to 106.9% on July 9.
• 
  Crop sowing progress inching back to normal

  As is expected from monsoon’s performance since late June, sowing of crops is now closer to the levels seen at the same time last year. 21.9% of the season’s normal sowing of cereals and 23.3% sowing of pulses was complete as on July 7. This completion rate is 1.6 and eight percentage points behind the level seen on July 8 last year (the nearest date for which this data is available for 2022). To be sure, the lag in both cereals and pulses is because of the biggest individual crop by area of those categories: rice and arhar. Rice sowing is 4.4 percentage points behind last year and arhar sowing is 19 percentage points behind last year. This is to be expected. As the maps above show, more than half the area in Maharashtra, Jharkhand, Bihar, and West Bengal; and slightly less than a third of Uttar Pradesh (largely its eastern half) is “deficient” even after last week’s rain. These states are all important producers of at least one of rice and arhar if not both. While most of these states can recover from a deficit in July, IMD’s forecast up to July 20 suggests rain will continue to be below normal in eastern Uttar Pradesh and Bihar.