Environmental flows allocation in two main distributaries of Mahanadi river

Abstract

The environmental flows concept mainly recognizes, needs of fresh water system to maintain the ecological integrity and provide goods and services to society &dependent communities. In the Mahanadi river basin the environmental flow method was first introduced in Chilika Lagoon, downstream of Naraj Barrage, Odisha by World Bank Environment Department. In 2002 the EFA project was successed to integrate key water quality concern, particularly salinity within the lagoon, for functioning the lagoon ecosystem while it was not successed to influence in operation of the Barrage at Naraj. The river system attains zero and very low flows in low flow period due to construction of hydropower generating structures, water retaining structure and withdrawal of water by water users, which possesses a tremendous threat to the environment, ecology & aquatic life. Therefore, a need arises to regulate the reservoirs and barrages for releasing the adequate water in the river throughout the year. Thus, environmental flows assessment is done in Lower Mahanadi sub-basin and its two main distributaries for providing the Environmental Flow Requirements (EFRs), with a range of Low Flow Requirements (LFRs) and High Flow Requirements (HFRs) to be ensured at any circumstances to avoid any degradation of river ecosystem. The paper analyse the applicability of several desktop hydrology-based environmental flow assessment methods such as Tennant, Tessman, Variable monthly flow (VMF), Range of variability Approach(RVA), FDC shifting technique using GEFC software. In this approach daily discharge data of Mahanadi River are used to assess Environmental flows in percentage of Mean Annual Flow (MAF) and range of flow in percentage of mean Low flow and mean High flow respectively. The comparative results indicate that a minimum flow i.e. 624m3/sec (equivalent to 40%of MAF) for main stream of Mahanadi river, 203m3/sec (equivalent to 26% of MAF) for Kathajodi river and 23m3/sec (equivalent to 33.5% of MAF) for Birupa river respectively. This paper also promote the requirement of environmental water allocation in planning of water resources development project in Mahanadi river system.