A Comparative Study on Environmental Flows Assessment Methods in Lower Reach of Mahanadi River

Environmental flows assessment is a process which determines the allocated water for maintaining aquatic habitats and ecological processes in a environment. The river system attains zero flow 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 for releasing the adequate water in the river throughout the year as well as a flushing flow once in a year. Thus environmental flows assessment is done in Lower Mahanadi sub-basin and recommended to provide the EFRs on average 26 % of MAF with a range of Low flow 56% of mean low flow & High flow 21% of mean high flow to be ensured at any circumstances to avoid any degradation of river ecosystem. In the present study the assessment of the environmental flows on the basis of Tennant method, Tessman method, VMF method, Q90_Q50 method, Smakhtin method, low flow index (7Q10) method and FDC (EMC shifting technique) method using Global Environmental Flow Calculator (GEFC) software.


Introduction
The environmental flows assessment (EFA) methodologies began in USA in the late 1940s and picked up during the 1970s.Outside the USA, the development of EFA methodologies gained in significant ground in 1980s or later.In India the issue of minimum flows was first highlighted in a judgement of Supreme Court of India in (I) Healthy aquatic ecosystems provide human several important silent services such as retention, storage & consequent supply of water, the dilution, removal and purification of wastes.(II)Aquatic ecosystem also supply commercial and subsistence products i.e. fish & plants.

Review of Environmental flows assessment methods:
In a comprehensive study of Environmental flow methodologies Tharme (2003) documented the existence of more than 207 Environmental Flow Methods (EFMs) in 44 countries & it classified these into four general categories: hydrological methods; hydraulic rating methods; habitat simulation methodologies; holistic methodologies.These methods were mainly developed after studies have been conducted for rivers, wetlands, estuaries, forest and grassland ecosystem.

Hydrological methods
Hydrological approaches have been developed for broad scale planning & which are fully dependent on the historical flow records.Among all these methods the best known method is Tennant method (1976), which defines seven classes ranging from severe degradation to outstanding ecological condition.According to Tennant classification a different percentage of the annual flow is allocated during the high flow periods and low flow periods.The Tessman method (1980) another EF method which depending on the different flow season (high, intermediate, low flow months).The low flow index method usually based on annual minimum flow thresholds i.e. 7Q10 (the lowest flow that occurs for seven consecutive days once in ten years) .The EFR can also be calculated using Range of Variability Approach (RVA), which evaluates flow regime based on a comparison of 33 flow statistics for the regulated as well as natural flow regimes.The other hydrological methods are based on FDC (EMC shifting technique) method through Environmental Global Flow Calculator software.The EFR is calculated by using 17 fixed probabilities and aim to maintain an ecosystem for default EMC classes varied from natural condition to critical modified condition of river.
Hydrological methods are typically inexpensive, rapid & desktop approaches.They are highly appropriate at the reconnaissance level of water resources development and for planning purposes, providing routine and low resolution estimates of quantities to be set aside for environmental maintenance.

Hydraulic rating methods
Hydraulic rating methods ( termed by Loar et.al 1986) are used at a local scale, when river cross section measurements are available at each river section.In this method consider site specific information on hydraulic parameters, such as wetted perimeter or other limiting river cross section.The recommended flows are commonly set at a break point in the hydraulic parameter-discharge curve, interpreted as the flow below which habitat decreases rapidly with a decrease in flow & above which habitat increases slowly with an increase in flow.

Habitat simulation methodologies
These are based on the simulation of physical habitats, such as the instream Flow Incremental Methodology (Bovee,1982).These methods are an extension of the hydrological methods.It uses hydraulic condition, which meet specific requirement of biota for determining flow requirement.These methods use to find a relation between different flow regimes and the habitat quality requirements of fish species.

Holistic methodologies
The fourth methodologies labelled "holistic" have appeared more recently as a response to a changing conception of water resources management.This method considers the whole riverine ecosystem.In this method the Environmental flow requirement is maximum depth across riffle or other limiting river cross section.The recommended flows are commonly assessed for all biotic and abiotic components of river ecosystem.It is not only focuses on the rivers physical features but also the wetland, estuaries and ground water which are associated to the rivers ecosystem.The Building Block Method is a well documented method in Holistic methodologies for estimating EFRs at either the local or basin scale (King & Louw, 1998;King & Brown, 2003;Tharme,2003;) .The flow blocks encompass low flow & high flow and both of are defined for normal & dry years .

Material & method Study area
The Mahanadi is one of the major east flowing river of the country & it is originating from Dhamtari district.The basin is broadly divided into three sub basin i.The FDC shift by one step means, flow which was exceeded 99.99 percent of the time in the original FDC will be exceeded 99.9 percent of the time.The flow at 99.9 percent becomes the flow at 99 percent, the flow at 99 percent becomes the flow at 95 percent etc.The entire shifting procedure can be accomplished in a spreadsheet.For Smakhtin method, IfQ90>30%MAF,HFR=0, If20%MAF<Q90<30%MAF,HFR=7%MAF.If10%MAF<Q90<20%MAF,HFR=15%MAF, IfQ90<10%MAF,HFR=20%MAF. Once such curve is determined, then converted to actual environmental monthly flow time series in terms of percentage of mean annual run off (MAR).Global Environmental Flow Calculator (GEFC) is a software package which is developed by International Water Management Institute (IWMI,2006).The GEFC uses the shifting technique to estimate EF.The higher the EMC, the more water is to be need for ecosystem maintenance.The six EMCs are used in the calculators ranging from "unmodified" to "critically modified", i.e.EMC A,B,C,D,E&F.Water resources currently target class of D or above.Each EMC is represented by unique FDC.The best suited EMC for the river to be selected based on expert Judgement.

Results & discussion
The average daily flow data series of 24 years (1991-92 to2014-15) at Jobra gauging staion, Mahanadi river was used to develop FDC and to generate flow requirements corresponding to different levels of ecosystem values, i.e. from unmodified natural condition to critically modified condition.3) and on average low-flow requirements is 79 m3/sec equivalent to 56% of mean low flow, while high-flow requirements is 412 m3/sec equivalent to 21% of mean high flow.Percentage of Low flow requirements are normally higher than high flow requirements, as the low flow season is longer than four months.The correlation between the calculated EFRs with the six selected methods are shown in Figure -4.From the figure all the simulated EFRs were highly correlated with calculated EFRs.The FDC method, VMF method and Tessman method recorded the highest correlation co-efficient (R 2 =0.92 above), while the Smakhtin, Q90_Q50 & Tennant methods showed a correlation (R 2 =0.75).The low flow index 7Q10 (7 day mean flow obtained in 10 year return period) flow was calculated using 7 daily mean flow data of 24 hydrological years (1991-92 to 2014-15).In 7Q10 flow duration curve the EFRs value is 5.7m3/sec , which is equivalent to 0.77% of MAF.This flow rate showing clearly is far less than other above mentioned methods & this method can be applied for determination of flow rate for habitat protection during drought condition.Lastly in WQAA method (CWC, 2007) the minimum flow calculated is 11m3/sec, which is equivalent to 1.5% of MAF & one flushing flow is 2496 m3/sec, to be released in high flow season.

Conclusion
In the present study six different hydrological environmental flow methods have been applied & recommended the Environmental flows requirements is 190 m3/sec, which is equivalent to 26% of MAF and the range of EFRs varies from 79 m3/sec to 412 m3/sec, which is equivalent to 21% to 56% of Low Flows & High Flows respectively, with one flushing flow 2496 m3/sec to be released in October month for maintenance of river ecosystem.The range of flow is necessary as High flows important for channel maintenance, bird breeding, algae control, wetland flooding etc; Moderate flows important for fish migration, cycling of organic matter from river banks and Low flows for fish spawning, water quality maintenance & the use by local people for maintaining fair ecological condition.On average Smakhtin method and Q90_Q50 method resulted the lowest EFRs i.e.10% of MAF and 12% of MAF respectively.The VMF method resulted EFRs on average 34% of MAF, which is higher than the Tennant method (27% of MAF) & lower than Tessman method (43% of MAF).The FDC method resulted 16.6% of MAF for EMC C (moderately modified) condition of river,30.8% of MAF for EMC B (slightly modified) condition of river & 57.9% of MAF for EMC A (natural) condition of river for maintaining basic function of river ecosystem at each level of river towards the Bay of Bengal.From the analysis the VMF method, Tessman method and FDC method showed good correlation with calculated EFRs from case study and these three methods also recommended especially in case of variable flow regimes.The main aim of this study is to ensure the minimum value of flow at any circumstances to avoid any degradation of river ecosystem and if the excess water is stored, then that can be used further for the expansion of irrigation land, in industries, hydropower sector, recreation and many more.
e. Mahanadi upper sub-basin, Mahanadi middle sub-basin, Mahanadi lower sub-basin.The Mahanadi river basin lies between 80° 28´ to 86° 43´ East longitude & 19° 8´ to 23°32´ North latitude.The total length of the river is 851 km from origin to Bay of Bengal.The Mahanadi basin extend over five states i.e.Chhattisgarh (52.42%),Odisha (47.14% which is approximately catchment area 65847 Sq Km & length 494 Km ) & smaller portion of Jharkhand, Maharashtra & M.P. with total catchment area of 139681.51Sq Km.The basin have maximum length 587 km & width have 400 km respectively.The Mahanadi Delta extends about 101 km lies in the Mahanadi basin.The annual rainfall of the Mahanadi catchment is about 1417 mm.The three major tributaries are the Seonath, IB , Tel & others are Jonk, Hansdeo, Mand, Ong.The main distributaries are Birupa, Kathajodi and others are Paika, Luna, Chitrotpala, Kuakhai, Devi, Bhargavi, Daya, Biluakhai, Kandal, Kushabhadra.The lower Mahanadi basin compromises an area 57958 Sq Km.The longest Dam is named as Hirakud Dam (21°32´N,83°52´30´´E) of 4800 m length, which was constructed in 1957 across Mahanadi river near Sambalpur and the longest barrage is named as Mahanadi barrage of 1928 m length, which was constructed in 1991 across Mahanadi river at Jobra .In the present study the gauge station at Jobra of Mahanadi river is taken for the analysis of Environmental flow assessment.The thematic map of lower Mahanadi sub-basin & its sub watersheds (Figure-1) are generated using Arc GIS 10.2.2 software.Description of the Hydraulic features of study area For the assessment of Environmental flows, the daily discharge data of Mahanadi river has used in this study.The flow regimes of the study area were analysed using several hydrological indicators (Table-1) i.e.Base Flow Index (BFI) = ((Q90)/ MAF) --(Eq-1) & Hydrological Variability Index (HVI) = ((Q25-Q75) / Q50) ---(Eq-2).Where Q90,Q75,Q50,Q25 are the annual flows equalled or exceeded for 90%,75%,50%,25% of the time respectively, MAF = Mean annual flow and MMF= Mean monthly flow.The higher the variability index, the more variable in river flow regimes.
Environmental flow Researchers have stated the hydrological methods based on morphology, ecology & location of rivers are very successful in application.Hence the commonly methods are used in this study for estimation of Environmental flow.The Tennant method assumes, some proportion of the average / mean Annual flow (MAF) is required to sustain the biological integrity of a river ecosystem.Tennant (1976) recommended percentage values of MAF for 'low'&' high' flow season and a periodic flushing flow of 200% of the MAF.Looking into the importance of the flow variability in the river system, the constant allowance for environmental flow based on the mean annual flow (MAF) will be not adequate for the Indian River system which has a large variation in the flow during the monsoon and non-monsoon periods.According to Tessman in 1980 modified the Tennant's Method to include a more detailed monthly variation of environmental flow.

Figure 1 :
Figure 1:Location map of study areaThe low flow index( 7Q10) method (Jha, R. 2008) is interpreted as the 7day low flow with a 10 year return period using daily discharge data.This method has been applied in various countries as the 7Q10 use for flow regulation purposes, ranging from protection or regulation of water quality from waste water discharges or

Figure 2 :
Figure 2 : Flow Duration Curves of Environmental flows of default EMC A,B,C,D,E,F.The calculator estimated in Figure-2,an average annual EF allocation of 123 m3/sec (equivalent to 16.6% of natural MAR), for EMC C river i.e. water released from the Jobra barrage to maintain the D/S stretch of the Mahanadi river, in moderate condition and to keep basic ecosystem function intact.Table 3. Computation of annual average environmental flow requirements (EFRs) by various methods corresponding to study area.The EFR, LFR, & HFR are expressed as a percentage of mean annual discharge of river in natural condition, mean annual low-flow & mean annual high-flow respectively.Hydrologic-al season SEP OCT NOV DEC JAN FEB MAR APR MAY Monthly Flow (m3/s)

Figure 4 :
Figure 4: Relation between the monthly calculated EFRs & the locally calculated EFRs.(a)Tennant method (b) Tessman method (c) Smakhtin method (d) Q90_Q50 method (e) Variable monthly flow (VMF) method (f) FDC(shifting)method for EMC C. In each sub figure, each dot represents EFRs for one month for the case study.

Table - 1
: Hydraulic features of flow of the study area.
Comparison of EF methods with natural flow.The results indicate the overall annual average Environmental flow requirements (EFRs) is 190 m3/sec equivalent to 26% of MAF (Table-3,Fig-