| Summary: | A two-state deterministic DP (Dynamic Programming) model is developed to derive
the optimal reservoir operation policy for the Mangla and Tarbela reservoirs in Pakistan. The
analysis is carried out with two separate objective functions, (1) maximisation of energy
generation while treating the irrigation demands as constraints, and (2) maximisation of
combined benefits from energy production and irrigation water supply in monetary terms.
Historic data for a period of five years (1985-90) has been used in this research.
The Mangla and Tarbela reservoirs are built on the Jhelum and the Indus Rivers,
respectively, in northern Pakistan. Both the reservoirs are multipurpose reservoirs and are
operated on a ten-day time step. Water is released from these reservoirs to meet irrigation
demands of the agriculture sector as a first priority and generate electricity as a second
priority. The maximum live storage capacity of Tarbela reservoir (9.986 MAF) is almost twice
as that of Mangla reservoir (5.365 MAF). The current maximum plant output at Tarbela
(3500 MW) is more than three times of that at Mangla (1000 MW). Two of the four irrigation
regions served by these reservoirs are common to both Mangla and Tarbela, which emphasizes
the need to operate the two reservoirs in conjunction with each other in order to maximise
benefits from irrigation and energy production.
The optimisation results from the DP model are compared with the actual operation of
both the reservoirs during the period 1985-90. The model shows an increase in energy
production over actual energy production during the same period. However, the model shows
deficits in irrigation water supply in the months of May and June, which are critical from the point of view of irrigation, when 'maximisation of combined benefits in monetary terms' is
used as the objective function. This problem can be overcome by assigning a greater monetary
value to benefits from irrigation. The important characteristic of the model, when run using
this objective function, is that it maintains a more or less constant discharge through the
turbines during most of the one year period of operation which is quite important from the
point of view of energy generation. When run using the objective function of 'maximising
energy production with constraints on irrigation water supply', the model not only gives
higher energy production but also deals adequately with the irrigation demands. The results
also show that more water can be released for irrigation from the reservoirs during early
Kharif period (Apr-Jun) because both reservoir can fill to their respective maximum
conservation levels during the monsoon season (Jul-Sep). This would also help in flood
mitigation by providing more storage and reducing flood peaks.
The model can be used, with some modifications, for optimising the real-time
operation of Mangla and Tarbela reservoirs. These modifications would involve finer
quantisation of the state variables (reservoir levels), determining the limits on minimum
discharge outflows necessary to maintain suitable turbine efficiency, criteria to prevent salt
water intrusion and meet the requirements of hydro projects downstream of Mangla and
Tarbela, and consideration of the losses due to evaporation.
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