Environmental flow assessment for the Ganges River

Abstract

This research work has been carried out to assess the e-flow of the Ganges River. First objective of this study is to identify the appropriate methodology among the established e-flow measurement techniques for the Ganges River and the second objective is to assess the e-flow for fisheries, maintenance of Sundarbans ecosystem and morphological equilibrium condition of the river. Based on detailed review on various methodologies, the Indicator of Hydraulic Alteration (IHA) method of Range of Variability Approach of Hydrologic method is used for assessing the impact on flow regime of Ganges River after the construction of Farakka Barrage. Building Block Method (BBM) is used to estimate the fisheries demand, ecological demand, Sundarbans‘ requirement, flushing flow and morphological equilibrium. Analysis shows that Golda fish requires 379 m3/s amount of flow during November to April, 2008 m3/s during May to September and 6023 m3/s during the month of October. Hilsha fish requires 9318 m3/s amount of water during July to October. Carp fish requires minimum of 853 m3/s during December to February and maximum of 12829 m3/s during July to August. The minimum requirement of Gangetic Dolphin is 2213 m3/s for November and 5177 m3/s in the months of May and October. Ghorial habitat requires a minimum flow of 664 m3/s in the month of March and maximum flow of 3775 m3/s during August and September. For sustainability of Sundarbans, a minimum of 542 m3/s flow is required during April and an amount of 22715 m3/s is required during August and September for natural river flushing. This study also identified that the observed minimum flow during the pre-Farakka period was within the range of e-flow requirements in the dry season. But in the post-Farakka period, the minimum flow falls below the dry season requirement. The dry season flow has been reduced drastically after 1975 but it was found that the average flow between the two time periods differs only by 706 m3/s. This study has shown a reasonable hydrograph (here it is called e-flow hydrograph) at Hardinge Bridge. The sustainability of Ganges River itself and its selected dependents may be secured if the required amount of water can be ensured at Hardinge Bridge.