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Catchment Area

Summary: SSP's catchment is under heavy pressure, and there is a distinct possibility of premature siltation of the reservoir, as has happened in many other Indian projects. Catchment Area Treatment has been initiated, but appears to be far behind schedule in Maharashtra and Madhya Pradesh. The problem of lands identified for treatment is as yet unresolved. Non-completion of upstream projects like Narmada Sagar, Omkareshwar and Maheshwar, would increase the catchment area of SSP, with a concomitantly higher silt inflow.

All rivers carry silt in varying amounts and all reservoirs get silted up over time. The rate of siltation generally determines the lifetime of a reservoir. Data from several large reservoirs in India show that the actual rate of siltation is on the average 200-400% higher than the siltation rate assumed while planning the project (ICR 1972; PAC 1983). Siltation occurs in the area below the irrigation canal take-off level (dead storage) as well as above that (live storage). Siltation in the live storage reduces the amount of usable water in the reservoir, and this is frequently very high. One of the main causes for increased siltation rates is deforestation of catchment areas, and poor soil conservation practices. There is no reason why the SSP case should be any different: Satellite imagery of the last two decades shows significant loss of vegetative cover all over the catchment, a trend which is only likely to continue in the near future. There is a possibility of siltation rates in the SSP reservoir being higher than currently estimated, in which case the lifespan of the dam would be affected. Here too, further comment is not possible in the absence of comprehensive documentation on this aspect.

To combat siltation, the SSP plan envisages catchment area treatment (CAT), to be carried out simultaneously with dam construction. Consensus on the area to be treated was achieved only after much wrangling and argument between states and various ministries. Finally it was decided that only the critically erodable land directly draining into the SSP reservoir will be treated at project cost (NCA 1993). This is about 185,000 ha and represents only about 7.5 % of the total catchment area below the Narmada Sagar Dam, and about 27 % of the critically erodable land in the catchment. Since the "high and very high" erosion categories form a relatively small part of the catchment, the absolute magnitude of the silt contributed by them could be substantially less than that contributed by the other eroding areas. The project authorities and indeed the World Bank have expressed the "hope" that the rest of the area will be treated by the relevant state governments (mainly Madhya Pradesh), but so far, as far as we are aware, there is no plan, and no allocation of funds.

A related point is that CAT of the area upstream of the SSP reservoir catchment is to be done under other proposed projects such as Omkareshwar, Maheshwar, and Narmada Sagar. However, these are all substantially delayed; Omkareshwar has received conditional environmental clearance only in 1994. This virtually ensures that treatment of the catchment upstream of the SSP reservoir will not done in time, and a substantial part of the silt load

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