Estimation of Environmental Flows Allocations for Tropical Reservoir System by Integration of Water Quantity and Quality Models

Author: P. Naga Chandi Priya 2019710004
Date: 2022-11-29
Report no: IIIT/TH/2022/142
Advisor:Shaik Rehana


Globally, due to the changes in climate, rapid urbanization, land use changes, and increased water demands, water quantity-quality management has become a significant sustainability problem. Maintaining a complete natural river flow is usually impossible due to the development of the water resources and variations in land and soil usage in the catchment. These developed resources can create differences in the balance of the ecosystem and socioeconomic activities. Stream flow is a prime factor affecting the river flow health and quality in such watersheds. Admitting many issues associated with the water quality of the rivers, stream flow plays a vital role as the changes in the stream flow can directly transmit the changes in water quality. The variations in the flow regimes over time relatively affect the riverine ecosystems of the watersheds. Environmental Flows (EFs) assessment, which depends on the flow regime, illustrates the quantity and quality of water flow required for the sustainability of freshwater, estuarine ecosystem, and human livelihood. In this context, Environmental Flow Allocations (EFA) of reservoir-river systems do not adequately address the connections between reservoir inflows, releases, and corresponding downstream river water quality, rendering water resources management difficult. Thus, this study aims to develop an integrated, holistic modeling approach for Eflow estimation by combining a hydrological model, reservoir release analysis, Global Environmental Flow Calculator (GEFC), and river water quality model. In this present study, the GEFC method is used to estimate the ecological flows by using the flow duration curves (FDC) generated from the given monthly discharge data of the river. The FDC in this system contains 17 fixed percentile points concerning the discharge. In the current study, we analyze the Tunga-Bhadra River basin’s Environmental flows (Eflows) by considering different discharge stations. The stations are Balehonuur, Haralahalli, Hosaritti, Shivamogga, Honalli, Rattihalli, and Tunga-Bhadra Dam, with a mean annual flow (MAF) of 36%, 24.8%, 27.2%, 16.2%, 23.3%, 21.1%, and 12.2% respectively to maintain the ecological conditions of the river. The monthly discharge data from 1995 to 2017 for those stations are obtained from the Advance Centre for Integrated Water Resource (ACIWR) in Bengaluru, India. River flow health is a study that helps understand the environmental variables that affect the river’s habitat structure, flow regime, water quality, and biological conditions. To estimate the flow health of the Tunga-Bhadra River, we used a tool called Flow Health which uses nine indicators to represent the Flow Health (FH) score for the stations Balehonnur, Haralahalli, Hosaritti, Shivamogga, Honalli, Rattihalli, and Tunga-Bhadra. This tool uses the gauge discharge data in the form of reference (1995-2005) and test periods (2006-2017), with Flow Health score of 0.72, 0.4, 0.72, 0.70, 0.58, 0.73, 0.71 and 0.72, 0.63, 0.63, 0.7, 0.66, 0.67, 0.66 for test and reference period with respect to stations. The study noted that most of the discharge stations along the Tunga-Bhadra River show moderate to low flow variations for the reference and test periods. Overall, Tunga-Bhadra river health, measured by the flow indices, declined from 1995-2005 to 2006-2017. A hydrological model, Soil & Water Assessment Tool (SWAT), is selected to simulate the inflows of the Bhadra reservoir. The calibration and validation of the SWAT estimated inflows are done using the SWAT-CUP tool, which shows satisfactory results. The SWAT findings were utilized in reservoir release analysis to determine the reservoir releases. Environmental Flow Management Classes (EMCs) for estimating and assessing the minimum flow to be maintained in a river were derived using the GEFC for the Bhadra watershed. The river water quality-modeling tool, QUAL2K, was chosen to simulate the Dissolved Oxygen (DO) corresponding to the EMCs developed from the GEFC for the Bhadra Reservoir. The study considered various plausible scenarios of inflows (10 to 20% reduction), pollution scenarios of Biological Oxygen Demand (BOD) (0 to 100 % treatment) for arriving drains by altering headwaters DO (4 to 8 mg/l). The study revealed that by maintaining the headwater DO (about 7 mg/l) in conjunction with BOD treatment of drains (25%) and by maintaining sufficient Eflow allocation (class D with 1269 Million Cubic meters (MCM)); downstream river water quality could be improved (6.5 mg/l of DO at Holehonnur monitoring point of Bhadra reservoir-river system, India). From all these performed analyses, we provided an insight into the reservoir supply management and maintaining the riverine ecosystem by combining the water quantity and quality models corresponding to the environmental aspects.

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