Every week, the Central Water Commission releases to the public and to government departments the numbers that describe how much water is stored in 85 reservoirs in India. These are the reservoirs designated as nationally important, because of their roles in providing water for large irrigated command areas and for generating hydro-electric power (37 of these dams).

These readings are taken as the authoritative compilation of the current weekly state of water storage, and are widely used in government. Amongst the uses is to recommend the rationing of water consumption in the states and urban settlements. Another use is to help determine what advice to broadcast to our farmers about when they can sow seeds (if their fields are irrigated by the canals that radiate outwards from these dams).

But of course these are only 85, even if they are the biggest. Whether Bargi in Madhya Pradesh is half full does not interest in the least a farmer in Purnea, Bihar. Whether Tungabhadra in Karnataka is filling up well in the last two weeks is of no consequence to the residents of Rohtak in Haryana. That Yeldari in Maharashtra has water at a level very much lower it should be at this time of the year does not affect the cultivators of Virudhanagar, Tamil Nadu.

Bihar has 24 large dams (and groundwater) and Tamil Nadu has 116 while Madhya Pradesh has 898 large dams other than Bargi, but we have no weekly or monthly information about how much water these hold, in the first week of June or at end July. And this is what we don’t know for all the 4,839 dams – in the national register of large dams – that are not amongst the list of 85. The Central Water Commission tells us that it uses the combined readings for the 85 large dams because, with their total of about 155 billion cubic metres (bcm) of water storage (if they are all full), they form a large portion of the approximately 254 (bcm) of total reservoir storage available in India (excluding tanks, ponds and traditional water storage structures). But, for the smallholder cultivator and the town council of a Class 2 urban settlement, it is the 50 million cubic metres reservoir in the next taluka that they depend on for water, not on any of the big 85.

What can we then do with the weekly reservoir storage bulletin from the CWC? My approach is to treat it as an indicator of the collection of rainfall by reservoirs in the same meteorological region and agro-ecological zone. When the list is divided into ten groups, by reservoir size, we see far more clearly the effect of the last two weeks of rainfall on the storage levels. Until we can persuade state and central governments to invest in widespread and cheap monitoring of as many of our water storage receptacles as possible, we may use the CWC bulletin as an indicator.

By Rahul Goswami

The panel of charts shows water storage ranges (at full capacity of the dams) for the groups of reservoirs. Starting with the smallest first: 56 million cubic metres (mcm) to 176 mcm (tenth group); 184 mcm to 380 mcm (ninth group); 399 mcm to 523 mcm (eighth group); 524 mcm to 735 mcm (seventh group); 767 mcm to 927 mcm (sixth group); 935 mcm to 1.436 billion cubic metres (bcm) (fifth group); 1.456 bcm to 1.994 bcm (fourth group); 2.171 bcm to 2.676 bcm (third group); 3.046 bcm to 5.378 bcm (second group); 5.649 bcm to 9.745 bcm (first group).