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Environmental Flow Characteristics of the Gorai River, Bangladesh Article · June 2015 DOI: 10.12983/ijsres-2015-p0208-0218
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International Journal of Scientific Research in Environmental Sciences, 3(6), pp. 0208-0218, 2015 Available online at http://www.ijsrpub.com/ijsres ISSN: 2322-4983; ©2015; Author(s) retain the copyright of this article http://dx.doi.org/10.12983/ijsres-2015-p0208-0218
Full Length Research Paper Environmental Flow Characteristics of the Gorai River, Bangladesh Sanjida Hoque Moly, Mirza A.T.M. Tanvir Rahman*, A H M Saadat Department of Environmental Sciences, Jahangirnagar University, Savar, Dhaka 1342, Bangladesh *Corresponding Author:
[email protected] Received 10 April 2015; Accepted 12 June 2015
Abstract. Environmental flow ensures ecological health status of a river. The Gorai River is the major distributary of the Ganges River in the right bank and the main source of upland freshwater inflows to the southwestern region of Bangladesh. The aim of the research is to estimate environmental flow of Gorai River and to assess flow characteristics of the river through the comparison between past and recent time. Mean daily discharge data and mean daily water level data recorded by Bangladesh Water Development Board has been collected and analyzed for two periods i.e. G1 (1976-1990) and G2 (19912006). Mean Annual Flow (MAF), Flow Duration Curve (FDC) and Constant Yield (CY) methods have been used and IHA software (version 7.1) has been exercised. According to MAF, FDC and CY methods, estimated environmental flows are 229.6 cms, 230.5 cms and 241.2 cms respectively and by averaging these values; environmental flow is estimated as 233.8 cms which reveals insufficient flow condition remains from December to May. Mean annual flow decreases about 21.8% from G1 to G2 periods where August and April are the highest and lowest flowing months respectively for the both periods. In Range of Variability Analysis it has been found that very high reduction in flow occurs in July whereas high reduction occurs in January, October and December comparing the flows between G1 to G2 periods. Key words: EFCs, Flow Duration Curve, IHA, Mean Annual Flow, Range of Variability Approach
are needed to evaluate the overall ecological health of the river and the degree of hydrologic alteration caused by a particular operating policy (Gao et al., 2009). Environmental flow in a river or stream describes the quantity, timing, and quality of water flows required to sustain fresh water and estuarine ecosystems and the human livelihoods and well being that depend on these ecosystems (Brisbane Declaration, 2007). The health and integrity of river systems ultimately depend on environmental flow components, which may vary seasonally (Mathews and Richter, 2007). Even though different natural factors such as origin, gradient and flow influence water distribution (Farooque, 2004), this situation is even more critical when a river is trans-boundary, where ‘natural geography’ overwhelms the hydrological needs (Wolf, 2001). Owing to geographic location, rivers in Bangladesh have very high flow in monsoon and low flow in dry season. Considering the current state of river water management, analyses of the original flow characteristics based on historic flow data and estimating environmental water requirements (EWR) for the rivers are critically important for Bangladesh (Mullick et al., 2010). But the concept of
1. INTRODUCTION Most rivers around the world are highly variable. Variability of river flow influences water quality, energy cycles, biotic interactions and habitat of rivers (Naiman et al., 2002). More recent time the river scientists refer to the flow regime in freshwater systems as a master variable due to its strong influence on the environmental factors e.g. water chemistry, physical habitat, biological composition and interaction (WB, 2008). Although human manipulation of river flow provides many societal benefits, it also degrades and eliminates valuable ecosystem services and threatens freshwater biodiversity (Bunn and Arthington, 2002; Magilligan and Nislow, 2005). The environmental flow emphasizes the need to maintain or restore the range of natural intra and inter-annual variation of hydrologic regimes in order to protect native biodiversity and the evolutionary potential of aquatic, riparian and wetland ecosystems (Arthington et al., 1991; Sparks, 1992; Richter et al., 1996, 1997; Stanford et al., 1996; Poff et al., 1997). Assessment of river flow characteristics is essential for understanding and predicting the biological impact of both natural and altered flow regimes on riverine biota. Indicators 208
Moly et al. Environmental Flow Characteristics of the Gorai River, Bangladesh
environmental flow is relatively new in Bangladesh. By imposing importance on the environmental flow, the Government of Bangladesh (GoB) makes one of the seven strategies to assess environmental flow requirement after prioritizing twelve major issues/problems as the main sources of coastal and marine pollution (DoE, 2007). Most rivers in southwestern region (SWR) of Bangladesh depend on water flow from Ganges River. Gorai river is the right bank distributor of the Ganges river which provides interface between freshwater and brackish water in the estuary connecting the Bay of Bengal and major source of upland freshwater supply in the SW region of Bangladesh. After construction of Farakka Barrage on the Ganges River in 1975 in India which is 17 km far from the Bangladesh border, Ganges water flow has reduced significantly in the downstream (Islam and Gnauck, 2011). The water shortage and sedimentation process in the Gorai basin has been ridged and the width has been shrink and spacious and depth has deduced in different section of the river (Islam and Gnauck, 2011). For the shortage of Gorai basin, sea saline water are penetrating in the upstream and increasing in the upstream direction through capillary upward movement. As a result the river system of the Southwest region of Bangladesh starts to be affected by coastal saline water inflow in November, and reaches to a maximum in April and May (Mirza and Sarker, 2004). This shortage of fresh water in the Gorai basin is also the root cause of salinity intrusion and damage of mangrove ecosystems and its services of the Sundarbans region.
Highest salinity ever recorded was in 1992 for February, March and April when the flow in the Gorai River was close to zero. The April salinity rose to 29,500 micro-mhos/cm (Mirza and Sarker, 2004). But salinity threshold is recommended by FAO (2003) is 750 micro-mhos/cm for irrigation. Present study aims to estimate environmental flow of the Gorai River which can be used for future reference in management purposes and to assess changes in flow characteristics of the river through the comparison between past and recent time. 2. MATERIALS AND METHODS 2.1. Study area and data collection The Gorai River catchment area is 15160 km² and is located between 21° 30’ N to 24° 0’ N latitude and 89° 0’ E to 90° 0’ E longitude (Islam and Gnauck, 2011). It flows through Kushtia, Jessore, Faridpur, Khulna, Pirogpur, Borguna districts of south western region of Bangladesh. Gorai river is very old river and is formed of three offshoots of the Padma. The course of Gorai River is wide, and meandering (Bari et al., 2012). Gorai River used to discharge into the Bay of Bengal through the Mathumati and Baleswar Rivers and thus serves as a crucial instrument for maintaining both the environment and economy of the region (Islam and Gnauck, 2011). Mean daily discharge (cms) and mean daily water level (m) data had been collected from Bangladesh Water Development Board (BWDB) for the years 1976 to 2006.
Table 1: Percentage of MAF for various Habitat Qualities (Tennant, 1976) Habitat Quality Percent of Mean Annual Flow (MAF) Low Flow Season High Flow Season Flushing or maximum 200 200 Optimum 60-100 60-100 Outstanding 40 60 Excellent 30 50 Good 20 40 Fair 10 30 Poor 10 10 Severe degradation
