Multi-Reservoir Optimization for Maximization of Releases for Hydropower Generation Considering Environmental Flow

Water is the soul of the world. It is the most important element for the sur-vival of humans, animals, birds, plants and all other living things on earth. Water is essential for the beginning of life as well as regular availability of water ensuring the survival, growth and overall nourishment. Thus, proper planning and use of reservoir water are essential for all. To tackle this issue different optimization techniques underline their need and importance in the reservoir operations. In the present study, multi-reservoir optimization model is developed using Python programing language considering the objective of maximization of total annual release for hydropower generation. Model is applied to 3 reservoirs from Godavari River basin from Maharashtra state India. Water essential for conservation of environment has also been made available in river as environmental flow as per the recommendations of Central Water Commission (CWC) India. Developed optimization model provides optimal monthly operation policies.

non-monsoon season in India. Most of the rivers in Maharashtra have to face this crisis every year. In case of Godavari River basin which is situated at Maharashtra State of India, similar situation is experienced in Marathwada region many days of the year. On the other hand, demand for electricity is also high due to increasing industrialization and development works. While electricity is being generated from conventional power plants to meet the demand, a huge number of natural resources are being used and pollution is also increasing. Due to this increased pollution, global temperature is constantly rising. Also, global warming further becomes cause of changes in nature, causing problems of erratic rainfall. It is necessary to take timely steps to get out of all this vicious cycle. For this, it is necessary to adopt a balanced policy of sustainable development.
Electricity generation from non-conventional sources also promotes sustainable development as they do not cause environmental pollution. The hydropower plant is a good example of this. Power generation by hydropower plants does not deplete any natural resources and also curbs pollution. In this paper, an attempt has been made to present an optimal operation policy for multi-reservoirs considering the objective of maximization of the hydropower generation from the Jayakwadi Stage-I dam constructed at Paithan, Jayakwadi Stage-II situated at Majalgaon and Yeldari reservoir constructed at Yeldari from Godavari River basin. Additionally study also aims to maintain the minimum essential environmental flow conditions relative to the Central Water Commission (CWC) India recommendations to preserve the river's ecosystem.
Labadie [1] has presented a review on optimal operation model for multi-reservoir systems by all different methods of optimization for water resources operations and management. Wurbs [2] has discussed different reservoir-system simulation and optimization models to give guideline about different methods and its usefulness in various types of decision-support situations. Kim et al. [3] have developed a monthly operating rule for single-reservoir operation for Soyanggang dam basin in Korea Peninsula, with objectives of minimization of shortage and maximization of sum of Hydropower production. The simulation results are obtained by using the developed piecewise-linear operating rule.
Chen et al. [4] have suggested an interactive dynamic programming model for optimizing reservoir operation to support the policy makers for balancing humans and environmental water requirement. Mousavi et al. [5] have presented reservoir operation model using a dynamic programming fuzzy rule-based approach. This model was applied to the reservoirs system in Iran. Developed model gives better values of the simulated objective function with the higher reliability of meeting the demand. Nagesh Kumar et al. [6] have developed a reservoir operation model for flood control purpose with the help of folded dynamic programming. Regulwar and Anand Raj [7] have proposed a multi-reservoir operation model using genetic algorithm under fuzzy environment considering multi-objectives. Regulwar and Kamodkar [8] derived a fuzzy constrained reservoir operation model for multipurpose reservoir. Model is useful in dealing with imprecise constraints. Jager and Smith [9] presented a review of the hydropower generation study along with ecosystem conservation. The study suggests three measures required for environmental sustainability while planning for reservoir operations. It advised to carry out research to determine the characteristics of flow difference necessary for the health of the river, develop an assessment method to assess river health and develop a software that predicts water balance and its effects on ecosystem. Ju-Hwan [10] has suggested an optimization model by linear programming for maximization of hydropower generation as an alternative to nonlinear model. Arunkumar and Jothiprakash [11] have developed a non-linear programming model for optimal reservoir operation for hydropower generation. In the study this hydropower generation in three different seasons was analyzed for different dependable flow conditions.
Morankar et al. [12] presented integrated approach for multi-objective fuzzy optimization for planning of sustainable irrigation by fuzzification of objectives of maximization of net benefits, crop production and labor employment. Jothiprakash and Arunkumar [13] have presented optimization model developed for maximization of the hydropower production in multiple hydropower plants using a non-linear programming for multi-reservoir system. Aboutalebi et al. [14] have derived monthly operating optimal reservoir operation rules for hydropower generation with support vector regressions and nondominated sorting genetic algorithm (SVR-NSGA). The main advantage of SVR-NSGAII is its ability to determine the various combinations of parameters and variables that are useful for reservoir operators to optimize reservoir releases. Ghimire & Reddy [15] have presented Particle Swarm Optimization (PSO) technic based optimal reservoir operation policy for hydropower generation using and sustainability analysis. In this study weekly operated model was developed for Upper Seti hydropower reservoir system in Nepal for maximization of annual hydropower production. Rahimi et al. [16] have elaborated monthly operating model developed for simultaneous optimization of hydropower energy and flood control for multi-reservoir system using imperialist competitive algorithm. Objective considered for study is to increase revenue generation from hydropower and also control the flood situations for the Ostour and Pirtaghi reservoirs of Iran.
Environmental Flow: Brisbane declaration [17] provided a clear idea of Environmental Flow (E-Flow) and it can be defined as "The quantity, timing, and quality of water flows required to sustain freshwater and estuarine ecosystems and the human livelihoods and wellbeing that depend on these ecosystems." This declaration was formulated in 2007 during the 10th International River Symposium and International Environmental Flows Conference held in Brisbane, Australia, with participation of delegates from more than 50 countries. Jain and Kumar [18] have highlighted Indian scenario of environmental flow assessment with different Indian River case studies. Lokgariwar et al. [19] have discussed a case study of Upper Ganga River cultural water requirements for environmental flow assessment. S.K. Jain [20] has suggested adaptive management of environmental flow, which involves estimation of environmental flow requirements, the consideration of those requirements in reservoir operation poli-cies and monitoring the results after implementation to revise the estimation.
Indian scenario: The Indian summer monsoon typically lasts from June -September with large areas of western and central India receiving more than 90% of their total annual precipitation during the period, and southern and northwestern India receiving 50% to 75% of their total annual rainfall. These trends increase the tendency to store more water in reservoirs during this season to make it available for remaining period of the year. It also affects the availability of base flow in river throughout the year. In most of the rivers no flow condition is observed during many days in non-monsoon season because of shortage of rainfall and the storage made by upstream reservoirs. Such situation degraded the river ecosystem badly and need a sustainable approach to make minimum flow available for all seasons. Government of Maharashtra Water Resources Department published a report on Integrated State Water Plan for Maharashtra [23] about importance of water for eco-system it describes river, water sources, aquifers and wetlands which will be regarded as natural systems and must be protected from overuse, extinction, pollution. In consultation with experts, stake holders and NGO essential steps shall be planned like preparation of annual action plan by incorporating necessary environment flow for restoration and maintenance of river health.
After reading all the above studies, it was found that required quantity of environmental flow releases should be estimated and incorporated in reservoir operation model to achieve a sustainable development policy in reservoir planning. In present study from the past inflow data, it has been observed that availability of inflow in Godavari basin is insufficient to provide flushing flow from reservoirs during the monsoon, also no flow condition has been observed in river during many days of year. It shows inability to satisfy both minimum required environmental flow release conditions recommended by CWC, so the first condition of a flushing flow of not less than 600% of the 75% dependable annual flow is not feasible to adopt. In present model, condition of 0.5% of the 75% dependable annual flow is considered as release for environmental flow and provided constant for all months.

System Description
Godavari is an important river in India. Godavari originates at Tryambakeshwar in Nashik district of Maharashtra state. It flows through Marathwada region Godavari passes Aurangabad, Beed, Parbhani and Nanded districts. Godavari is the lifeline of this region with its great religious significance. For this study Jayakwadi Stage-I (P1), Jayakwadi Stage-I (P2) and Yeldari (P3) projects were considered as shown in Table 1 Jayakwadi Stage-I (P1) and Jayakwadi Stage-II (P2) projects are multipurpose reservoirs which releases water for different purposes like Irrigation supply, supply for hydropower generation, drinking water supply etc. Similarly, Yeldari (P3) project releases water mainly for the purpose of hydropower generation through installed hydropower plant of capacity 15 MW. Figure 1 shows location sketch of all hydropower plants.

Model Development
Study aims to develop optimization model to maximize the total annual releases where, i denotes the number of reservoirs and t represents the number of time step. Optimization model involves total of three reservoirs. Objective function is subject to some constraints as follows:

Irrigation Release
Irrigation supply is provided from Jayakwadi Stage I & II reservoirs for the irrigable command area of around 2355.25 square kilometer combined. Accordingly, for all reservoir irrigation releases (IRR) must be less than or equal to maximum irrigation demand (Max_IRD) for all time period. Similarly, irrigation water supply must be greater than or equal to minimum irrigation demand (Min_IRD). This irrigation release constraint is given by:

Turbine Capacity Constraints
Constraints for turbine release are considered according to capacity of turbine installed in power house for hydro-power production. The water release for power production (HPR) should be less than or equal to maximum capacity of turbine (TC) for all month. Also, the water releases for hydro-power production for all month should be greater than or equal to the firm power (FP) requirement. Mathematically these constraints are given by:

Storage Capacity of Reservoir
Reservoir storage (RS) for all months should be less than or equal to the maximum reservoir storage capacity (Max_RSC) of the reservoir and should be greater than or equal to Minimum Storage Capacity (Min_RSC) of the reservoir.
It can be given as:

Environmental Flow Release
Environmental flow is essential to preserve the river health and river ecosystem and must be considered in reservoir planning. Environmental flow (EFR) required for Godavari sub basin is also considered. Relative to the CWC study report guidelines it is suggested that the quantity of minimum environmental flow should be taken as 0.5% of 75% dependable annual flow. The quantity of environmental flow is estimated in reference to the recommendations of CWC and written as.
Jayakwadi Stage-I (P1) is a multipurpose reservoir, constructed for fulfil water availability for Marathwada region. Reservoir supplies water for irrigation also it supplies water for drinking and industrial use to nearby towns and villages and to municipalities and industrial areas in Aurangabad and Jalna districts. Another feature of this project is the bird sanctuary. Many species of birds from all over the country visit the Nathsagar Reservoir created by the dam. The abundance of waterfowl-loving swamps, algae, aquatic plants, small fish, insects and a wide variety of aquatic habitats attract domestic and exotic birds to reservoir. About 200 endemic and about 70 migratory birds of different species are found in the vicinity of this reservoir. Also, more than 50 species of fish are found in Jayakwadi reservoir. Total 183,322 Ha, of command area receives irrigation supply from Jayakwadi Stage-I project through left and right bank canals. Pump storage type hydropower plant of 12 MW capacity is installed at Jayakwadi Stage-I project. Water released through power plant is stored in a pool downstream and pumped back to reservoir. This volume of water is also considered in model with 10% of transition loss.
Godavari is also an important river in Maharashtra from a religious point of view. Kumbh Mela in Nashik is one of the largest religious events in the world. Millions of devotees from all over the world take holy dip in Godavari water at this moment. Paithan, where the Jayakwadi reservoir was constructed, also hosts a number of Hindu ceremonies throughout the year. Nanded is a very important religious place for Sikhs. All these holy places are situated on the banks of Godavari. Therefore, having a definite uninterrupted environmental flow in the Godavari throughout the year is essential for the devotees in terms of religion as well as health.
Jayakwadi Stage-II (P2) is constructed on Sindaphana river which also receives water from Jayakwadi Stage-I reservoir through feeder canal. The feeder canal release (FCR) from Jayakwadi stage-I is added in model with 10% transition loss. A canal hydropower plant is also installed at Jayakwadi Stage-II project with capacity of 2.25 MW. Jayakwadi stage-II project irrigates around 93,800 Ha. of area through right bank canal. Yeldari Reservoir (R3) is constructed on Purna river which is a tributary of Godavari. Yeldari project generates hydropower through installed hydropower plant of capacity 15 MW which is purely a hydropower project. Table 2 shows the monthly inflow value and demand of water for hydropower plant for all reservoirs. Journal of Water Resource and Protection

Result and Discussion
Optimal reservoir operation model was developed for maximizing the total annual releases for hydropower generation for multi-reservoir system on monthly operating basis. Developed model provides optimized sustainable multi-reservoir operation policies, which increases the hydropower production and also protects the river ecosystem by providing minimum essential environmental flow throughout the year. Python programming language was used for optimization model development. Latest version of Python 3 was used with an open-source linear programming package PuLP library. Developed optimization model provides maximum total annual supply for multi-reservoir hydropower generation which is 1503.7362 million cubic meters (MCM). Results from Table 3 show, in case of Jayakwadi Stage-I reservoir release are made available as 100% of maximum demand for all months. From table 4 it is seen that, at Jayakwadi stage-II releases for hydropower will be 32.50% and 95.84% of maximum demand for the month of June and July respectively. From August to April developed model provides hydropower releases to run power plant with full capacity and for May releases available as 97.85% of maximum demand. Results in Table 5 show, for Yeldari project hydropower release of 79.28% of maximum demand is available in June. From July to February 100% demand of power generation was fulfilled by developed model. And for remaining months' supply is available as 42.75%, 20%, 20% of maximum demand for March, April and May respectively, shown in Figure 2.
Similarly total annual irrigation releases in Jayakwadi stage I & II are 622.31 MCM shown in Figure 3. Annual water supply releases are 446.16 MCM made Journal of Water Resource and Protection    available from all reservoirs. As a part of sustainable reservoir operation approach annually total 19.075 MCM of environmental flow is also released to sustain the river ecology.

Conclusion
Maximizing power generation from environmentally friendly power generation projects to meet the rising demand for electricity in the future can be an important option to prevent pollution and global warming. Hydropower projects which generate electricity at low cost and without harming the environment in any way can play an important role in this. An attempt has been made in this study to provide a sustainable solution to meet different water demands of humans with the consideration of environmental water requirement by reservoir optimization. Optimization model provides total annual releases of 1503.7362 million cubic meters for hydropower generation and 622.31 million cubic meters for irrigation. Similarly, annually a total of 19.075 million cubic meters of flow is made available as environmental flow to sustain the river ecology. Water management of the reservoir in this way will definitely help in conserving the river environment and at the same time sustainable development will be achieved.