Assessment of Environmental Flows for Various Sub-Watersheds of Damodar River Basin Using Different Hydrological Methods

Assessment of EFs is of paramount importance in the present era of modernization/development leading to increasing hydrological alternation through dams and diversions and, in turn, modification to the natural conditions of stream flow and ecosystem as well due to increasing water extraction for meeting human demands for uses such as industry, agriculture, recreation, hydropower generation, domestic water supply [1-3]. It is estimated that more than 60% of the world’s rivers are fragmented by hydrological alternation and modify the natural patterns of rivers or stream flow and this figure is projected to increase to 70% by 2025 [4]. The protection of aquatic resources against the impact of dam and water extraction in river is a challenging and elusive issue in sustainable river basin management. Now a day, EFs assessment has undergone a major paradigm shifts from a single hydrologic attribute (i.e. minimum flows) to a full range of flows (floods, average, and low flows) that account for seasonal and interand intra-annual variation in stream flow variability. In addition, its magnitude, timing, frequency, and rate of change also plays an important role in longterm sustainability of water resources and their proper utilization with river ecosystem in state of good health and other features (such as fish, wildlife habitat, environmental purposes, water quality) [5,6]. In last two decades, EFs is gaining more attention, because it is required for the longevity of a river, led to the development of more than 240 methods in published literature. Although [7] pointed out that none of developed methodology is considered as best and all methods have importance, depending on objectives for estimating EFs and hydro-geological condition of a watershed. Therefore, comparative approaches is ideal approach to provide an assessment of EFs in a river or stream because all categories of quantitative methods have importance and its use depends on the level of protection, environmental goals, and objectives of the study [8]. Further [8] pointed out that hydrological and hydraulic methods are useful in cases where there is a poor understanding of the ecosystem or where a high level of protection for an existing ecosystem is required. Based on this concept, a number of hydrological based comparative studies have been conducted throughout the world by researcher for different purposes. Table 1 shows only a few of them. In this view the objective of the study is assess EFs variability using three well known hydrological methods: i) Tennant, (ii) Tessman, and (iii) FDC in eight different sub-watersheds of the DRB, India.


Introduction
Assessment of EFs is of paramount importance in the present era of modernization/development leading to increasing hydrological alternation through dams and diversions and, in turn, modification to the natural conditions of stream flow and ecosystem as well due to increasing water extraction for meeting human demands for uses such as industry, agriculture, recreation, hydropower generation, domestic water supply [1][2][3]. It is estimated that more than 60% of the world's rivers are fragmented by hydrological alternation and modify the natural patterns of rivers or stream flow and this figure is projected to increase to 70% by 2025 [4]. The protection of aquatic resources against the impact of dam and water extraction in river is a challenging and elusive issue in sustainable river basin management. Now a day, EFs assessment has undergone a major paradigm shifts from a single hydrologic attribute (i.e. minimum flows) to a full range of flows (floods, average, and low flows) that account for seasonal and inter-and intra-annual variation in stream flow variability. In addition, its magnitude, timing, frequency, and rate of change also plays an important role in longterm sustainability of water resources and their proper utilization with river ecosystem in state of good health and other features (such as fish, wildlife habitat, environmental purposes, water quality) [5,6]. In last two decades, EFs is gaining more attention, because it is required for the longevity of a river, led to the development of more than 240 methods in published literature. Although [7] pointed out that none of developed methodology is considered as best and all methods have importance, depending on objectives for estimating EFs and hydro-geological condition of a watershed. Therefore, comparative approaches is ideal approach to provide an assessment of EFs in a river or stream because all categories of quantitative methods have importance and its use depends on the level of protection, environmental goals, and objectives of the study [8]. Further [8] pointed out that hydrological and hydraulic methods are useful in cases where there is a poor understanding of the ecosystem or where a high level of protection for an existing ecosystem is required. Based on this concept, a number of hydrological based comparative studies have been conducted throughout the world by researcher for different purposes. Table 1 shows only a few of them. In this view the objective of the study is assess EFs variability using three well known hydrological methods: i) Tennant, (ii) Tessman, and (iii) FDC in eight different sub-watersheds of the DRB, India.

Study Area
The DRB has area of about 23,170 sq. km. in the states of Jharkhand and West Bengal, India and lies between 22°15'N to 24°30' N latitude and 84°45' E to 88°30' E longitude Figure 1. The basin has two main rivers Damodar and Barakar, experienced exclusive anthropogenic activity and the riverbed is probably altered The water of the rivers is mainly used for agriculture, industry and domestic purposes and demand for water from these sectors would drastically increase in near future. Details of surface water availability, water use, and demand are summarised in Table 2. The basin hydrology is the product of its climate, geology, land use, topography and drainage systems. The flow response

Abstract
Environmental Flows (EFs) assessment is a global challenge involving a number of tangible and intangible segments of hydrology, hydraulics, biology, ecology, environment, socio-economics, and several other branches of engineering including the management of water resources. It has consequently led to the development of more than 240 methods available in literature. Required for the longevity of a river, EFs derived from a single method are usually not accepted. In the present study, the EFs variability was assessed using three hydrological methods: (i) Tennant, (ii) Tessmann, and (iii) Graphical and Stochastic Flow Duration Curve (FDC) for various sub-watersheds of Damodar River Basin (DRB), located in the states of Jharkhand and West Bengal. The estimated values from Tessmann and graphical FDC methods were not as reliable as those from Tennant and Stochastic FDC methods.
Comparative results indicate that minimum flow should be ranged from 1.04-16.08 cumec, was required to keep basic river ecosystem functions in different watersheds. Whereas, 7Q series were estimated using stochastic FDC recommended for design flows from low to high. The value of Q95 of 7Q10 and 7Q100 FDC were found appropriate as designed EFs during drought/low flows and normal precipitation years respectively. Similarly, the values recommended for 7Q50 and 7Q100 of probability exceedance of 90% and 95%, will help water resource planner and decision makers for development of new water resources projects such as the design of storage facilities, assessment of water available for municipal, agricultural or industrial purposes and operating rules that satisfy EFs in the DRB. Comparing results provide useful update and widest application [10] In-stream flow methods used in Australia Tennant, Flow duration (Q95& Q90), Constant yield.

Assessment of Environmental
Suggested different methods are suitable according to different conditions or objectives [11] Comparisons of hydrologically based in-stream flow methods in 70 rivers of Atlantic Canada Tennant, 25% of the MAF, Monthly (Q50, Q90) flow, Aquatic Base Flow (ABF), 7Q10 flow.
Q 90 and 7Q10 methods predict extremely low in-stream flows during winter and summer months; whereas, Q50 flows recommended for gauged and 25% of the MAF & ABF for ungagged basin. [12] Examine impact of temperature on low stream flow for 77 rivers in the Canadian Prairies and trends analysis Seasonal 7-day low flow, Seasonal 25% of mean flow, Seasonal Q80, Decrease in the magnitude and an increase in the frequency of low flow results in poor water quality and negative impact on aquatic life in river, while temperature has an increasing tendency.
Q50 method provide high level and whereas Q90, 7Q2 and 7Q10 methods low level of in-stream flow in small river.
[15] Assessment of EFRs in major Indian river basin. Default FDC Suggested 6 (From A to F) Environmental Management Classes (EMCs).

[16]
Link EFRs with EFs classes FDC Developed a software package named Global Environmental Flow Calculator (GEFC) for desktop assessment of EFs.
Assess optimal EFs in Tungabhadra river, India FDC Tennant 7Q10 FDC and 7Q100 FDC were appropriate methods for designed EFs during drought /low flow periods and normal precipitation years respectively.
Required more water improve water quality and livelihood support base of river ecosystem. [19] Computing the minimum water requirement to save biological activity in Safaood river, Iran. Tennant, Q95 from FDC, Hydraulic Q95 from FDC gave compatibility results with the rivers condition whereas; Tennant and Hydraulic methods gave overestimated results. [20] Comparisons of hydrological based methods for assess EFRs to maintain basic functions in Shahr chai river, Iran.
Tennant, FDC shifting, Low flow index, DRM, GEFC FDC shifting and DRM methods are more reliable methods in compare to Tennant, 7Q10 and Q90 of AFDC for maintains basic river functions.

Hydrological Methods Adopted to Assess EFs
Three hydrological methods: Tennant, Tessman, and FDC (traditional and stochastic) were used to assess EFs in eight sub-watersheds of the DRB. Details of Tennant, Tessman, and FDC methods can be found elsewhere, Whereas, described about stochastic FDC. Furthermore, the value of probability of exceedance equal to 95% (Q95) of FDC was chosen as "design EFs" in the DRB, because, the basin has extremely low flows during lean period and the ecosystem (flora and fauna) manages with the severity of flow from high to low flows very well.

Results and Discussion
Firstly, the Tennant method was applied to generate EFs requirements corresponding to different habitat condition in selected each watershed and recommended values are summarized in Tables 3 and 4. It can be noted that required values for optimum habitat status (60-100% of MAF) are maintaining in the DRB except in low flow season from April to July. Analysis shows that excellent habitat condition was maintain in downstream of Phusro, Maithion, and Panchet watersheds. On the other hand, reaches below Konar dam, Tilaya dam and, Barkisuriya did not satisfy required flow even 2.5% of the MAF throughout the year due to heavily flow regulation in Konar and Barakar watersheds ( Table 5). To make preliminary flow recommendations that take into consideration the needs of fish and other aquatic life, the Tennant method can be used in the DRB. However, it should be modified by adjusting the season of lowest flow to cover the period from April to July. Secondly, Tessmann method [22][23][24] was applied that mimic the natural flow on monthly basis and recommended values are shown in Figure 2. In the third method, FDCs of daily and 7-day were drawn for respective stations based on available period of record (POR). Figure  3 shows daily FDC at each monitored site. It has been observed that shape of computed daily FDCs at each sites were different, may be due to different in variability in precipitation, watershed characteristics, meteorological factors, urbanization and water abstraction or demand. The Damodar catchment is highly urbanized and having impervious surface which causes increase in storm water runoff and decrease in infiltration and ground water recharge.

Conclusions and Recommendation
The paper presents the preliminary EFs recommendations using three hydrological methods; (i) Tennant (ii) Tessman, and (iii) FDC in eight sub-watersheds in the DRB, which is the first attempt in this tropical river system. These methods are preliminary approaches, where insufficient ecological and hydraulic data are not available. Lack of eco-hydrological data makes it difficult to determine minimum flow thresholds and tipping points of different freshwater ecosystems across        the world. However, in some cases, hydrological methods gave results that were in agreement with each other, but in other cases different approaches yielded different results and threshold values.
The flow recommendations in this study are often used as firsthand information, because neither a flow as magnitude computed by Tennant and Tessman and duration computed by FDC nor frequency obtained by stochastic FDC analysis has a consistent relationship to habitat or production across a range of stream geomorphology. It is therefore, likely to generate results with low confidence and monotonous and there is no provision to integrate other associated aspects, for instancethe ecology, biodiversity, riverine communities etc. Thus, there is a need for a better methodology that could be implemented and describes relationships between discharge and width, discharge and depth, and discharge and velocity. Hence, simple historical based approaches used in assessing EFs requirements are not found suitable for all types of watershed. However, the results obtained provide detailed information, which can be used for estimating the water supply, the water demand for both anthropogenic and ecological, and the amount available for withdrawal in near future regarding the planned anthropogenic alteration and its consequences. The values recommended for 7Q50 and 7Q100, will help water resource planner and decision makers to develop new water resource projects such as the design of storage facilities, assessment of water available for municipal, agricultural or industrial purposes and operating rules that satisfy EFs in DRB.