Report of Working Group to advise Water Quality Assessment Authority on the minimum flows in the rivers

This report of the Working Group to advise Water Quality Assessment Authority (WQAA) on the minimum flows in the rivers outlines the principles behind environmental flow assessments, provides a description of methods that have been used to assist with such assessments, and highlights the features that will increase the chance of successful implementation of environmental flows. 

The four components that constitute the minimum flows are low flows, flushing flows, special purpose flows and maintenance of impoundment levels. However, for the purpose of present study, only requirements for low flushing flows are specific to a particular reach and not a general requirement.

The main difficulties in quantifying the minimum flows are absence of an acceptable definition of ecological needs and, also, the absence of an agreement on the priority of Environmental Flow Releases (EFR). Moreover, while the domestic and agriculture requirements are easily quantified, it is difficult to decide as to what comprises the legitimate ecological needs.

A methodology similar to Tennant method needs to be adopted wherein certain percentages of the annual flows are prescribed as minimum flows as well as flushing flows during the monsoon. Certain studies are undertaken for Indian conditions and recommendations of the Working Group are based on the results thereof. The recommendations are as under –

• Naturally occurring minimum flows with 99 per cent exceedence can be taken as the minimum flows required for maintaining the in-stream environment. A range of minimum flows may be recommended as flushing flows during the flood period.
• It was also felt that since the Himalayan rivers carry large snow melt component during summer months the recommendations may be different for the rivers originating in the Himalayas and for others.
• From the studies, it is seen that in the case of Himalayan rivers the virgin flows are very high due to snow melt contributions. However, in Himalayan rivers where utilization is mainly through diversions and not through storages, it is felt that the minimum flow should be a function of lean period flow keeping in view the committed utilizations.
• For Himalayan rivers the minimum flow is to be not less than ten per cent of average virgin lean period flow (1^st Dec to 31^st May) expressed in cubic meters per second. One flushing flow is required during monsoon of not less than 250 per cent of 75 per cent dependable annual flow expressed in cubic meters per second.
• For other rivers, minimum flow in ten daily period to be not less than observed ten daily flow with 99 per cent exceedence. Where the daily flow data is not available this may be taken as 0.5 per cent of 75 per cent dependable annual flow expressed in cubic meters per second. One flushing flow is required during monsoon of not less than 600 per cent of 75 per cent dependable annual flow expressed in cubic meters per second.
• There is a need for extensive efforts on augmenting wastewater collection, enhancing treatment facilities and ensuring proper operation and maintenance of these facilities.
• Use of wastewater after treatment for irrigation, horticulture and other purposes is recommended.

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