The current study has not looked at the most crucial data — how much water, of the total, is utilisable
The Central Water Commission (CWC), in June 2019, released a report Reassessment of Water Availability in India Using Space Inputs. It re-assessed the average annual water resources of the country with technical support of National Remote Sensing Centre (NRSC).
“The average annual water resource of the basins for the study period of 30 years (1985-2015) has been assessed at 1,999.20 billion cubic metre (BCM). The mean annual rainfall of the basins for the same period is 3,880 BCM,” read the report.
The water availability at 75 per cent dependability is put at 1,705 BCM, added the report.
This report broadly confirmed figures of water availability in the country mentioned in earlier similar assessments, namely the CWC study of 1993 and the 1999 National Commission for Integrated Water Resources Development (NCIWRD) study.
“Since the present study is based on the most advanced methodology, it generates more confidence in the results achieved,” read the study.
The methodology used in the current report supports this assertion and its figures are more reliable, subject to the caveats and limitations outlined by the report itself.
The current study has not estimated the most crucial part — the “utilisable” part of this total water against what’s “available”, which is what the estimate of 1,999 BCM is. This is important because the CWC’s methodology of estimating the utilisable portion of surface flow is essentially based on how many dams, diversions and storage structures can be built in a river basin.
It would be important to see the use of a more modern and advanced methodology to assess the utilisable component, just as a more advanced method has been used to assess the total resource.
The current study essentially uses a hydrological model and water balance, with as disaggregated data as possible (spatial and temporal, meteorological and other). It uses daily rainfall data of 0.25o X 0.25ogrids, daily temperature data of 1o X 1o grids, and Land Use Land Cover (LULC) map between 2004-05 and 2014-15 prepared under Natural Resources Census (NRC), a project of NRSC.
This methodology is certainly advanced than the methodologies used in the earlier assessments. Yet, it needs to be emphasised that the method is only as good as the data used – whether it is of the rainfall, soil characteristics or abstractions, diversions and uses. And this probably remains its biggest limitation.
The limitations and assumptions of the study could significantly influence the accuracy of the estimates. Some of the limitations include estimating the water utilisation for irrigation rather than actual withdrawal data, absence of land use maps for some of the years of the study period, approximating groundwater flux to basin boundaries when measurement is with respect to district boundaries etc.
Probably, the biggest source of error could be the fact that the model used for this report is calibrated with respect to the observed (and measured) flows at the Gauge Discharge sites of the CWC. The errors of these measurements that are well accepted will, therefore, affect the accuracy of the model estimations too.
Moreover, the calibration of the model has been done on an annual basis. This is important because the calibration is supposed to take care of the limitations and errors of the model to a good extent.
The Confidence Interval reported in the study at 90 per cent indicates that the model may not be calibrating well with the observed parameters, possibly due to the many limitations and assumptions. Ideally, a confidence interval of at least 95 per cent would have been expected.
Some other figures also raise doubts about the CWC estimates. For example, in the basin-wise estimations, the water resources of the Narmada river at 75 per cent dependability are given as 45.24 BCM, significantly higher than the 34.5 BCM, which the Narmada Water Disputes Tribunal determined as the total water in the river.
Last but not the least, the CWC report does give the water resource estimates basin-wise, but it would have been very important if the estimates were also given season-wise (monsoon, lean season etc). Making available the detailed outputs from the model would also help others to better understand and analyse it.
These issues must be addressed in the subsequent reports.
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