New Estimations of India’s Water Resources by CWC – Better Methodology But Questions Remain

The Central Water Commission (CWC)  has recently released (in June 2019) a report “Reassessment of Water Availability in India Using Space Inputs” that re-assesses the average annual water resources of the country with technical support of NRSC on similar lines to that of the pilot studies completed in June 2013 by CWC and NRSC on Godavari and Brahmani-Baitarani river basins.

As per the report, “The average annual water resource of the basins for the study period of 30 years (1985-2015) has been assessed as 1999.20 BCM. The mean annual rainfall of the basins for the study period of 30 years is 3880 BCM. However, utilisable water resources estimation was not in the scope of current study.” (Page xii)

 This report broadly confirms the figures of earlier similar assessments of the total water availability in the country, namely, the CWC study of 1993 and the study by National Commission for Integrated Water Resources Development (NCIWRD) of 1999. The current report asserts that “…. since the present study is based on the most advanced methodology, it generates more confidence in the results achieved.” (Page 77)

A perusal of the current report and the methodology used supports this assertion, and we can take these figures with a higher level of reliability and confidence (subject to the caveats and limitations outlined by the report itself at page 96 and some questions raised by us later on in this note.)

Comparing with Earlier Studies

The comparative figures for available water resources of India for this and earlier studies are given below, compiled from the Report itself (Table 2 at Page 20).

Sr. No.	Year	Agency	Water Resources Assessed at (BCM – Billion Cubic Meters)	Remarks
1	1901-03	First Irrigation Commission/using coefficients of runoff	1443	For pre-independence India
2	1949	Khosla's empirical formula	1673
3	1960	Central Water and Power Commission	1881	Statistical analysis of flow data wherever available and rainfall-runoff relationships wherever data were meagre
4	1988	Central Water Commission	1880	General water balance approach
5	1993	Central Water Commission	1869
6	1999	National Commission for Integrated Water Resources Development (NCIWRD)	1953
7	2019	CWC + NRSC	1999

The differences in the last three or four studies are not much and can be attributed to some lack of data, some assumptions and other factors.

However the current study has not estimated the most crucial part – that is, the “utilisable” part of this total water, as against “available”, which is what all the above estimates are. This is important because the methodology of estimating the “utilisable” portion of surface flows by CWC is essentially based on how many dams, diversions and storage structures can be built in a river basin. The details of the methodology and the way it has been applied to each basin are not available. It would be important to see the limitations and assumptions of this methodology as well as the use of a more modern and advanced methodology for assessing the utilisable component, just as a more advanced method has been used for assessing the total resource. 

Superior Method

So what is the method used by the current study? It essentially uses a hydrologically model and water balance, with as disaggregated data (spatial and temporal) as possible. As the study says, it “emphasises on quantifying basin scale water wealth by transformation [of the methodology] from presently adapted basin terminal gauge site discharge aggregation [method] to meteorological data based water budgeting exercise through hydrological modelling approach.” (Page xi)

It uses daily rainfall data of 0.25^o X 0.25^o grids, daily temperature data of 1^o X1 ^o grids, and Land Use Land Cover (LULC) map for the period from 2004-05 to 2014-15 prepared under Natural Resources Census (NRC) project of NRSC using IRS AWiFS satellite data (56 m resolution). Soil textural map, LULC map, daily rainfall map, daily temperature map, water body map and command area maps were integrated in modified Thornthwaite-Mather modelling framework to compute the monthly soil moisture, evapotranspiration, surface runoff. A software tool namely Water Resources Assessment Tool (WRAT) was developed by NRSC for computation of water balance components in modified Thornthwaite-Mather modelling framework using geo-spatial datasets. Abstractions for all uses (irrigation, domestic, industrial, others) were estimated. (Page xii). More details of the methodology can be seen in the report.

This methodology is certainly an advance on 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.

Addressing Concern Raised in Mihir Shah Committee Report

The Committee set up by the Ministry of Water Resources for “Restructuring the CWC and CGWB”, headed by Dr. Mihir Shah, gave its report in July 2016. It flagged an important issue related to the estimation of India’s water resources. It raised the issue that (Page 23 of Mihir Shah Committee Report)

“…recent calculations based on higher estimates of the amount of water lost to the atmosphere by evapo-transpiration are less comforting. Narasimhan (2008) has recalculated India’s water budget, using an evapotranspiration rate of 65 per cent, which compares with worldwide figures ranging from 60 per cent to 90 per cent instead of the 40 per cent rate assumed in the official estimates. The result also summarised in Figure 1.1 is sobering. After allowing the same 48.8 per cent for ecological flows, his estimate of water utilizable for human use comes to only 654 BCM, which is very close to the current actual water use estimate of 634 BCM.”

In effect, what is suggested by the results of Narasimhan (as outlined in the Mihir Shah report) is that India’s official water resource estimation assumes an effective total evapotranspiration of 1539 BCM leaving 2301 BCM as available water; whereas if evapotranspiration figures of Narasimhan are taken, then total evapotranspiration is really 2500 BCM, leaving only 1340 BCM as available.

First of all, there seems to some error in this due to double counting of groundwater. Groundwater resources are included in the total water resources. If this error is corrected, evapotranspiration figures of official estimates come to be 1971 BCM. Though this narrows the gap, the issue raised by Narasimhan remains.

In this context, though the current CWC report does not mention it, the methodology, by including specifically the evapotranspiration in terms of actual data (or better estimates using meteorological and other data) and using water balance studies, would address the concerns raised by Narasimhan. This is because the central concern raised has been the underestimation of evapotranspiration by the (earlier) official studies. If details of the figures from the modelling exercise done by CWC for this study are available, this could be further cross-checked.

Some Other Issues of Concern

There are some other important issues of concern with the CWC assessment and report.

First of all, the Report does give the limitations and assumptions of the study, and looking at these, it is clear that there these could significantly influence the accuracy of the estimates. The Confidence Interval reported in  the study at 90% 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% 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% dependability are given as 45.24 BCM (Page 71). This is 36.66 Million Acre Feet (MAF), significantly higher than the 28 MAF which the Narmada Water Disputes Tribunal determined as the total water in the river. This raises some questions on the dependability of the estimates. (While the Narmada Tribunal calls its estimates at the “utilisable” water at 75% dependability, from the way it has been  estimated, we can see that it what the CWC report calls “available” water.)

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).

We hope that some of these issues can be addressed in the subsequent reports as this should clearly be an ongoing work.

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Note: This post has been slightly modified mainly to include a comment on the  Confidence Interval. Further, this note may be treated as a Preliminary Comment, as the author is working on some further aspects based on several comments received. Comments of Manoj Misra, KJ Joy and Jeevananda Reddy are acknowledged, as is a tutorial on statistics and modelling methods by Ann Josey and Mokshda Kaul.