The fallacy of long-term average reservoir levels

As of 15 September, the water storage at 91 major reservoirs stood at 107.7 billion cubic metres (bcm), 16% higher than the year-ago reading


But the falling trend in the long-term average—down from 97% on 8 September to 92% last week—means 10-year average figures are as unreliable as the monsoon rains
But the falling trend in the long-term average—down from 97% on 8 September to 92% last week—means 10-year average figures are as unreliable as the monsoon rains

Despite the monsoon rains being lower than earlier anticipated, the improvement in reservoir levels is seen as a bright spot. As of 15 September, the water storage at 91 major reservoirs stood at 107.7 billion cubic metres (bcm), 16% higher than the year-ago reading.

Crucially, the 10-year average storage reading rose to 92% this month compared to 77% a year ago. The improvement in reservoir levels and the long-term average reading are seen to augur well for winter crops (rabi), which are mostly river- fed.

But much is left unsaid. The storage level is only 68% of total storage capacity of the 91 reservoirs covered, which means almost one-third of the reservoir capacities are unfilled. Second, consecutive years of low reservoir level readings mean the 10-year average, based on which the long-term average is calculated, has moved far away from the actual storage capacity levels.

Data from the ministry of agriculture says the 10-year average reading as of 8 September stood at 111.8 bcm. It is only 71% of actual reservoir capacity level or the long-term averages are calculated on less than three-fourths of the reservoir capacities. The 10-year average reservoir reading till 15 September is not readily available. But the falling trend in the long-term average—down from 97% on 8 September to 92% last week—means 10-year average figures are as unreliable as the monsoon rains.

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