ENVIRONMENTAL FLOW ASSESSMENT FOR VARIOUS STRETCHES IN THE CAU VERY BASIN

Abstract

The importance of environmental flow have been recognized worldwide and gaining prominence in India as well. Assessment of environment flow is a complex process and could be subjective as different available methods would result in widely differing estimates. In this study, a comparative analysis of some of the environment flow assessment methods have been undertaken utilizing data from 7 hydrological stations namely, Akkihebal, Biligundlu, Kanakpura, Kudigue, Kollegal, T.K.Halli, and T.Narasipura in Cauvery river basin. Data required for this study i.e. daily discharge data has been procured from central water commission (CWC) interface INDlA-WRiS version2 for period over 20 years for each station. Thereafter the flow duration curve (FDC) method, Tennant method, Tressman method, Modified Tennant Method for Multi-Habitat Condition-Ill (MTMHC-lII) have been used to assess the environment -. flow for Cauvery Basin. Results obtained indicate that the FDC method cannot provide reasonable rule curve for water release and at particular point because EF exceeds flow rate which is impossible to maintain. Similarly, the Tressman method, also cannot derive better rule curve at drought situation as the EF would exceed the flow rate. The Tennant method and MTMMHC-III are nearly similar, but on keen comparison with one another, it has been found that Tennant method seeks to release higher amount of EF in low flow season than that derived from MTMMI-IC-III and in high flow season, vice-versa. Therefore, the MTMMHC-lll has been adjudged better method in matter that it provides reasonable rule curve for EF release while getting other water demands form river ecosystem maintained. Limitation in previous tenant method i.e. region-specific nature, has been eliminated from this method. In addition to resolving previous dilemma of low flow years and high flow years in Tennant Method, it takes into account spatial and temporal variability. Further, it considers the intra-annual and inter-annual variability. Thus, the MTMMHC-lll which is recent version of modified tenant method and suits best among available method for (environmental flow assessment) EFA in Cauvery river basin; for both ephemeral and perennial streams. Additionally, the percentage value of EF varies from 10-60%, 20-90%, 0-60%, 0-50%, 0-50%, 0-60%, and 5-60% for Akkihebal, Biligundlu, Kanakpura, Kudigue, Kollegal, T.K.Halli, and T.Narasipura respectively. In general, EF range in MTMTMIlC-lll is 0-60% for poor habitat condition, which is higher than in other methods. Thus EF temporal variability is highest in MTMMTC-lll. Specifically, At Biligundlu; the most downstream station, the EF variability is 0-90% due to highest sediment deposits often settles at downstream point and widest range of EF is helpful to remove this sediment deposits.