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The quantity of water available is the most basic figure for any dam project. However, the amount of water actually available for the SSP continues to be debated hotly. Table 2 illustrates the existing controversy over this most basic figure : the amount of water actually available for use.

There is about 17 % less water in the river than planned for. The amount of water actually available for use at the dam site at 75 % dependability is only 22.69 MAF (Million Acre Feet) and not 27.22 MAF as assumed by the dam builders.

Table 2
75 % Dependable Yield of the Narmada
Years of Study Official Yield 1
Actual Flow Data 2
1966 27 22.28 (15 years of data)
1978 27.22 22.28 (22 years of data)
1983 27.79 23.16 (33 years of data)
1990 27.22 22.69 (42 years of data)

1 : From Patel 1991:60
2 : Calculated from CWC 1992

How did the project authorities fail to accurately measure this most basic quantity ? The Narmada Water Disputes Tribunal Award (NWDTA), on the basis of which the SSP was designed, used the amount of water available at the SSP dam site for 3 out of 4 years (called the 75 % dependabile yield) to apportion water between the states. When the award was made, actual flow data were available for only 22 years (1948-1969). The government estimated flow in the river using rainfall data from 1891-1947, a procedure known as hindcasting. The resulting combination of hindcasted and actual flow data led to an estimate of 27.22 MAF as the 75 % dependable yield (henceforth referred to as the yield).

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