1. Introduction
Reservoir development helps the entire world satisfy its food and fibre demands by providing water for agriculture and hydropower, generating rapid monetary growth, and enabling flood management. Simultaneously, current reservoir storage capacities are dwindling worldwide, posing a hazard to agricultural and hydropower generation. With mean yearly reservoir storage capacity reductions ranging from 0.1% to 2.3% owing to sedimentation, there is a scarcity of accurate data on reservoir sedimentation rates worldwide. Despite this, the global annual storage loss is approximately 1.0% [
1].
Storage loss in China’s largest reservoir is 2.3%, while the lowest storage loss in the UK is 0.1% [
1]. According to the International Commission on Large Dams, there are over 42,000 big dams in the world [
2], and the global rate of storage loss and storage capacity owing to sedimentation concerns is around 0.5% to 1% and 7000 km
3 annually, respectively. In 2015, 50 km
3 of storage capacity was added worldwide annually to compensate for this loss rate, costing over 18 billion dollars [
3]. A global crisis of water availability may occur, if communities continue to allow reservoir storage capacity to reduce [
4]. The world’s ever-increasing population exacerbates the problem. Although, worldwide, storage volume diminishes, water storage requirements increase as the population expands [
5]. Due to a decline in dam construction rate and sedimentation of reservoirs, there is a net reservoir storage capacity deficit [
6].
In Venezuela, an extreme event occurred within 15 years of the reservoir operation starting, when the available storage capacity of the Camaré reservoir was lost due to sedimentation [
7]. The storage capacity of Zuni Dam in New Mexico declined by 20% of the designed capacity within 25 years of operation commencement. Most of the dams in the United States west of the Mississippi River have yearly sedimentation rates of more than 1.2 percent, with storage loss rates of more than 2 percent in most states [
8]. As gross storage decreases, net irrigation releases and production from the reservoir decrease.
Tarbela is an important part of Pakistan’s Indus Basin plan, which intends to provide better and more consistent water to millions of acres of fertile land, hydroelectricity generation, and flood management. It is one of the world’s most important water resource development projects. The dam is located on the Indus River supplementing 50% of the country’s total food basket and making a 30% contribution to the overall energy benefit. The releases, power production, and durations will decrease with time due to the formation of a massive silt delta in the reservoir. The passage of irrigation water via the tunnels with turbines generates electricity from the Tarbela reservoir, which is Tarbela’s secondary priority. Since construction, however, the energy gains have outweighed the irrigation benefits. The plant is used to provide a base load from early June to October (when considerable, regular discharges occur) and as a peaking supply for the rest of the year. The monthly irrigation and electrical consumption curves from the Tarbela Reservoir are depicted in
Figure 1 and
Table 1 [
9].
The capacity of the Tarbela Reservoir in Pakistan was reduced by 20% during the first two decades of its operation because the dam traps a huge amount of sediment from the Indus River [
3]. It has now declined by one-third in 40 years [
10]. Modelling of the associated sediment [
11] was used to investigate the amount of sediment deposited in the reservoir and the dynamics of the sediment delta as it changes shape and position with incoming flows (in the past as well as in the future). According to these studies, unless remedial action (such as the current study) is taken, the reservoir’s dead and live storage will be largely filled by 2030, with a gradual reduction in the reservoir’s regulated production up to that date, and the intakes will be blocked well before [
12].
The Tarbela Reservoir serves as a vital water source for irrigation, flood control, and power generation. The reservoir’s initial capacity has been reduced by 39.04 percent due to sediments [
13]. When the Tarbela Reservoir was constructed in 1974, it had a gross storage capacity of 14.34 BCM and a live storage capacity of 11.94 BCM. subsequent reductions have seen the gross and live storage capacities drop to 8.74 BCM and 7.61 BCM, respectively, in 2017 [
14].
Sedimentation is diminishing reservoir capacity globally. Annual global reservoir storage capacity losses range from 0.1 to 2.3 percent, with an average of around 1.0 percent. With a mean annual sediment deposition rate of 0.134 BCM/annum, the Tarbela reservoir’s gross storage capacity diminishes by the day [
14].
The Tarbela Reservoir annually accumulates around 200 million tonnes of sediments (550,000 tonnes per day on average). The sediment delta pivot point is advancing towards the dam at a rate of roughly 0.386 km per year, according to current reservoir operations. The Tarbela Dam is a key component of the “Indus Basin Water Treaty” alternative works program, which began in 1960. The Tarbela Reservoir stores water from a 169,650-square-kilometre upper catchment area [
15]. The Tarbela Dam, with four units generating 700 MW of energy, was completed in 1974. In 1982, four units were added to the power plant, increasing its capacity from 4 to 8 units. Two more 350 MW units, 9 and 10, were constructed in 1985, followed by four 1728 MW units 11–14 in 1992–1993, giving the Tarbela Reservoir a total generation capacity of 3478 MW [
16,
17,
18,
19]. For agricultural reasons, the Tarbela Reservoir releases 75.10 BCM of water. The Tarbela Reservoir is the main component of IBIS. During low-flow seasons, it is crucial to meet irrigation demand [
20]. The reservoir will be full of sediment by the year 2030 if the current sedimentation trend continues, and the sediment delta will be close to the dam face. The expected sediment inflow rate in the reservoir was 0.294 BCM per year [
20] meaning that sediment will fill the Tarbela Reservoir storage volume to 90% in 50 years. Subsequently, the reservoir will only provide 1.2 BCM of live storage.
By following WAPDA’s current operating procedures, the delta is progressing rapidly towards the dam body. The sediment delta pivot point is moving at a rate of 0.386 km/year and is now at a distance of 8.77 km and a height of 422.026 m. Tarbela’s minimum pool level was 396.3 metres at completion, and it is now maintained at 422.8 metres in 2017 [
14]. According to the Pakistan WAPDA, spills (uncontrolled discharges) account for 70% of water discharge and do not generate energy [
21]. This is concerning because Pakistan’s energy demands are increasing at a 7.9% annual rate (Tarbela now produces 30% of the country’s hydropower). Uncontrolled releases (when the Indus River’s flow surpasses 11,326 m
3/s) have also resulted in 13 floods, causing significant damage to the agriculture industry.
The operation of the Tarbela Reservoir is based on a criterion of minimization of the irrigation release deficit. This use of a single criterion is a common approach employed globally but has limitations regarding the strengths and weaknesses of reservoir operations [
22,
23]. The dam is situated in a region vulnerable to the impact of climate change due to precipitation and snowmelt variability [
24,
25,
26]. Previous research has indicated that climate change impact may shift the of peak inflow event to an earlier time [
27]. If such conditions prevailed, the reservoir filling would have to be initiated earlier to attenuate flood peaks. Other research has indicated that climate change may change the crop sowing pattern to an earlier month to achieve maximum yield [
28]. Early reservoir irrigation releases and a similarly limited storage volume for the falling limb would be required for this shift in sowing patterns. Due to a low storage coefficient of 0.11 (ratio of live storage to mean annual inflows) and a high draft ratio of 0.97 (ratio of annual demands to mean annual inflows), climate change, in addition to the operational stressors mentioned above, places additional strain on Tarbela Reservoir’s physical aspects [
21]. Because of its low storage coefficient, the reservoir’s ability to control excessive hydrological flow fluctuation is limited.
In the 1980s, WAPDA tried to develop a decision support tool, the Indus Basin Model (IBM), for the assessment of the system’s performance aiming for maximization of net economic benefit from agriculture. However, it did not take into account the impact of standard operating procedure on long term releases and hydropower benefits [
29]. In addition, previous Tarbela research has mostly concentrated on rule curve optimization approaches for improving reservoir performance [
30]. For example, using a genetic algorithm for multi-objective optimization to minimize the irrigation shortfall in Tarbela. This research anticipated a low reservoir level for all months except August and September (as opposed to the present rule curve). Although this strategy reduced the irrigation deficit by a little amount (1.23 km
3), there was no improvement in hydropower generation or flood damage mitigation [
31]. Taking a similar irrigation-centric rule curve optimization technique recommended lowering the minimum reservoir level from 417 to 396 m. Because Tarbela was built to optimize irrigation deficits, the primary justification for this technique was that rule curves resulting from irrigation deficit reduction should be preferred. To date, no research has been done to inspect the potential effects of modifying the physical reservoir features and evaluating the existing WAPDA Standard Operating Procedures (SOPs) on the future irrigation release patterns and their multiple benefits. The present study primarily emphasizes different operational strategies for a reservoir to manage sediments for the determination of future release hydrographs along with power and energy benefits through optimal utilization coupled with the impact of existing static SOPs on the long-term operation of the reservoir. As a case study, we selected the Tarbela Reservoir in Pakistan. The generation of future optimized release patterns and power benefits were tried. Further, the impact of existing the Standard Operating Procedures (SOPs) on future reservoir operation was assessed, which was not done in earlier studies [
30,
31].
Simulation of reservoir operation can be efficiently carried out using HEC-ResSim computer model [
32]. Assessment of the Jiroft storage dam, Iran, used the HEC-ResSim model. It was concluded that the present system of operation led to serious deficiency and failures in 25% of the operation periods, which could be mitigated by enhancement of irrigation efficiency from 30% to 50% increasing the system resilience by 17% [
33]. To optimize hydropower generation and flood management, HEC-Res-Sim modelling was employed to simulate cascade dams across the Omo Gibe River basin. New operation rules were selected which increased the energy generation by 28–45% [
34]. The HEC-ResSim model performed hydrologic routing and determined multiple benefits for the Kajakai reservoir in Afghanistan [
35]. The HEC-ResSim model was developed for the Lancang Cascade to determine the influence of the cascade on the Lower Mekong River [
36]. The HEC-ResSim model was developed for the Mosul dam to simulate the operational behaviour of the reservoir. It was found that the real operation using the current rule curve did not match the original design operating curve [
37]. The HEC-ResSim model was used for the Tucuruí Dam located in Brazil. The rules for different zones of the reservoir were assessed based on the capacity of the spillways, the reservoir storage and the flow capacity of the river drainage system downstream [
38].
In the Pakistani context, HEC-ResSim was employed for reservoir simulation modelling to study the impact of climate change on the hydropower generation for the Mangla reservoir. As the Mangla reservoir was raised by 12 m in the year 2009, the model was set up for two scenarios, i.e., pre-raising and post raising. Based on the simulations , it was concluded that in future the increase in annual hydropower generation was expected to be 16.6 to 20.4 percent for the pre-raising scenario and 13 to 15.3 percent increase for post-raising scenario. The developed model lacked the capability for updating the elevation capacity relationship caused by long term sedimentation in the reservoir [
39].
The numerical model HEC-RAS was employed for the Tarbela Reservoir, Pakistan for the assessment of sedimentation for future operations. It was concluded that the life of reservoir may be extended by 27 years by decreasing the minimum operating level from 457 m to 401 m. However, the study did not estimate the impact on hydropower generation for future scenarios [
40]. A GA-based optimization model was developed for maximization of hydropower generation for cascade reservoirs and applied to the Tarbela and Bahsa dams in Pakistan. It was concluded that average annual benefits from hydropower generation can be maximized up to 2724.7 Million US
$ by the joint operation of the Tarbela and Bahsa dams [
41]. The impact on hydropower generation by varying the operation of the Tarbela Reservoir was carried out using the Evaluation and Planning (WEAP) model. It was concluded that a 13% increase in hydropower generation would be expected. However, the study did not cover the impact of sedimentation for future operational strategies [
42].
To achieve plan 2030, global goals or the Sustainable Development Goals (SDGs) have been developed by UNDP in 2015 to mitigate poverty and protection of the planet. Out of 17 SDGs, some of them are covered by efficient, innovative use of the reservoir operation, which include poverty and hunger alleviation, clean and affordable energy, protecting ecosystem from degradation [
43]. Use of an efficient operation regarding sedimentation will alleviate food and energy issues along with other socio-economic benefits.
Owing to variability in sediment deposition in the reservoir, the physical condition in the reservoir will fluctuate, which will affect the reservoir operation, power production in peak hours, and hydropower generation. Rapid variation in seasonal high flow and reservoir levels will require new restrictions in operation of the reservoir system. Otherwise, they may have major impacts on power plant machinery and physical components of the dam, like the spillways, power, and irrigation tunnels, and the downstream river conditions and reservoirs. The latter is a new aspect that has been considered in this study. Therefore, determining the most suitable operational strategy based on the reservoir’s physical condition, considering the sediment deposition rate, is a worthwhile aim from the dam manager’s viewpoint. Hence, for the efficient operation of the reservoir, there is a need to explore future optimized reservoir’s operation to gauge future storage, irrigation releases, and power and energy production to achieve sustainable development goals. These include poverty alleviation, clean and economic energy, efficient irrigation, the use of innovative techniques, and preservation of the ecosystem.
The paper aims to review and appraise the existing operation of the Tarbela Reservoir; to carry out an assessment of optimal future irrigation outflows; to optimize the likely future power production and energy benefits and to inspect the impact of the WAPDA static SOPs on the future optimized operation, bearing in mind the time-based reservoir sedimentation and other physical constraints. To accomplish this task, the HEC-RAS 1-D sediment transport model was set up along with the HEC-ResSim reservoir operation model. The prediction of the optimized future operation was assessed on a 5-year basis from 2025 to 2035 for determining storage capacity, irrigation releases, power production and energy generation. Further, the impacts of the operational strategy during Kharif (April to September) and Rabi (October-March) irrigation releases were predicted for future application years along with the impact on reservoir capacity upon sedimentation. The outcome may assist water managers to improve the Tarbela Reservoir operation for its sustainable use to attain future societal benefits. Further, the optimum operation of the reservoir is necessary to achieve sustainable development goals, i.e., poverty and hunger alleviation, clean and affordable energy, protecting ecosystem from degradation.
6. Simulation Procedure and Results
The simulation module was used to separate the output results from the development of the model. After the definition of the alternatives and completion of model, the module was used to run the simulation. It was required to define the simulation time window, alternatives to be assessed and simulation run interval. The DSS file consisted of all the DSS records that shows the input and output time series data for the considered alternatives. Besides this, elements may be saved and edited for further simulations.
The main concepts of the theoretical development of the reservoir network consists of two parts: A physical part that includes the Tarbela dam and reservoir, spillways, dam structure, tail waters information and correct information of all these is very important for an adequate result. The second part of the reservoir network is the provision of operation rules for the Tarbela Reservoir—that is a basic step in the model development. The description of this portion is really complicated and require a lot of information and hydrological and hydraulics equations are used for calculation. Conditional portrays like if, or, then, and rules are used for the better development of the Tarbela Reservoir simulation model.
The physical part is a very important part in the HEC-ResSim model as even minor changes affect the behaviour of the reservoir system. The physical part input consists of dam parts and reservoir details that consist of the Tarbela power plant detail, spillways, reservoirs storage and pool surface area, dam crest, length of crest of dam was included. The mathematical water flow models for large areas include water inflow in reservoir, flow in open streams, and release from the outlet tunnels (spillways, power, and irrigation tunnels) of dam and power production in hydropower plants. Various types of flows that are considered in this study are:
Flow in the Indus River reaches that are interlinked with the Tarbela reservoir.
Changes in Tarbela pool storage.
Releases from both spillways of Tarbela dam.
Releases from irrigation tunnel no. 4 and 5.
Releases and power generation from power tunnel no. 1, 2 and 3.
Spillways are the controlled/uncontrolled outlets of a dam to pass the flood safely to d/s stretches of river safely. During the high flow season, the reservoir level increases and results in overflowing of the dam crest. In such condition, the turbine or spillway will cater to pass this surplus discharge to restrain towards the guide curve. In the case of the Tarbela dam, Auxiliary and Service spillways are used to spill the extra water from the reservoir in the flood season, and both spillways are put into operation when the reservoir level reaches 458.72 m. It is safe for the dam structure to operate both spillways up to a level of 472.44 m.
The releases from spillways were estimated by Equation (2). These calculations were crucial in determination of the operational rules and physical considerations. The maximum increase in reservoir level depends on the discharge capacity of both spillways at different reservoir elevations. The operation rules in operation sets for the discharge capacity of both spillways were used to spill excess water at different elevations. Spill from the Auxiliary and Service spillways were calculated with Equation (2):
where ‘
Q’ is discharge (m
3/s), ‘
C’ is discharge coefficient (0.75 for streamlined and 0.6 at the sharp edge), ‘
B’ is spillway width, ‘
a’ is spillway gate above the threshold, ‘
g’ is acceleration and ‘
Do’ is water depth over spillway. Equation (3) was used to determine the releases from power tunnels no. 1, 2 and 3 for power generation:
where ‘
En’ is the energy generation (MWh), ‘η’ is the efficiency, ‘
h’ is Gross head (m), ‘
t’ is time (h) and ‘
Q’ is reservoir release (m
3/s).
Pan evaporation of Tarbela lake was used to determine the change in pool storage due to evaporation. End pool storage after seepage, evapotranspiration and releases from spillways, power and irrigation tunnels was determined by the HEC-ResSim model from continuity Equation (4):
where ‘
S1′ is end of period storage, ‘
Evap’ is evaporation during time interval, ‘
Outflow’ is power release and leakage, ‘
CQS’ is discharge to storage conversation.
Tarbela Reservoir standard operation procedures were used in this study defined by WAPDA rule curves and IRSA according to crop water requirement and power generation d/s of dam that specify the Tarbela pool storage or required releases based on the time of year and the current reservoir storage in the reservoir. Tarbela Reservoir has upper and lower rule curves either in dry or wet season throughout the year.
11. Results and Discussion
To investigate the operation patterns of the Tarbela Reservoir, data for irrigation releases, power generation and energy generation from the power plant were collected. The results of the model were compared with these data. These results are presented in the form of a comparison between the observed and predicted dam releases, power, and energy for the existing WAPDA operating scenario for the future years.
According to 6th Periodic Inspection Report of Tarbela Reservoir [
14], the minimum operating level of the Tarbela Reservoir is needed to keep the minimum operating level of the reservoir 2.1 ft. (0.640 m) higher than the pivot point elevation of Tarbela sediment delta. The current sediment delta pivot point elevation is 422.6 m, and the minimum operating level is 423.31 m.
The HEC-ResSim Model was calibrated for the irrigation releases and power generation for the year 2012 and 2013. Results of the Model calibration are indicated in
Figure 21 and
Figure 22 for irrigation releases and power generation, respectively.
The coefficient of efficiency, index of agreement and run-off ratio indicate the satisfactory values for accuracy of the model as shown in
Table 6.
After calibration of the model for the years 2012 and 2013, its validation was carried out using the years 2008 and 2009, see
Figure 23 and
Figure 24 for irrigation releases and power generation, respectively. The results of validation coincide with the observed value within the satisfactory limits. The resemblance of the simulated values of the model were very closer to the observed values for irrigation releases and power generation, as shown by the IOA, RR and COE in
Table 7.
Table 7 shows the quantitative estimation of the deviation between the simulated and observed data. It also includes numerical values of the coefficient of efficiency, index of agreement and run-off ratio also the comparison between the observed and the simulated model for validation of 2008 and 2009.
After the calibration and validation of the model to a satisfactory level, it was employed to predict the possible future irrigation releases and power generation according to WAPDA standard operations procedure. The simulations for predicting future conditions of Tarbela Reservoir were carried out on a 5-year basis for the years 2025, 2030 and 2035 to investigate future irrigation releases, power production and energy generation, as shown in
Figure 25,
Figure 26 and
Figure 27.
With the reduction in storage capacity, the releases during the high inflow period will be more, which will enhance the power production and energy generation as releases are preferred to be passed through the power tunnels.
Application of the model was done on a 5-year basis for 2025, 2030 and 2035. The Tarbela Reservoir was operated according to the existing WAPDA operations for the future years at different minimum operating levels (432.2, 438.2, 444.3 and 450.49 m) and maximum operation level of 472.44 for the year 2025, 2030 and 2035, respectively.
The model was optimized for maximum releases, power, and energy generation according to the modified upper and lower rule curves. When the reservoir capacity is reduced to 20%, then the storage reservoir will be transformed to a run-off-the-river project. The predicted irrigation releases, power, and energy generation of the application model for the year will be reduced, but the power generation and releases from the dam would be maximum (in surplus) during the high flow season.
Based on the comparison of inflow and outflow hydrographs of past years (2008, 2009, 2012, 2013) and future years (2025, 2030, 2035), it was observed that there was an increase and decrease in the release patterns in the Kharif and Rabi periods, respectively, by the reduction in storage capacity of the reservoir. As the capacity was reduced due to deposition of high volume of sediments, the releases from the reservoir would increase and up to 2035, it is expected that the inflows would be equal to the outflows from Tarbela (including releases from spillways, irrigation tunnels and power tunnels) due to the reduced storage capacity of 2.87 BCM.
In the past years 2008, 2009, 2012 and 2013, when the storage capacities were 9.932, 9.783, 9.335 and 9.186 BCM, respectively, the storage capacities of Tarbela were enough to operate the reservoir according to the WAPDA standard operation procedures and enough to fulfil the irrigation and power demands d/s of the reservoir throughout the year, as shown in
Figure 28,
Figure 29,
Figure 30 and
Figure 31.
In the future years 2025, 2030 and 2035, when the storage capacities will be 5.855, 5.337 and 2.87 BCM, respectively, the storage capacities of Tarbela would be exhausted such that it would be difficult to operate the reservoir according to the WAPDA standard operation procedures and not enough to fulfil the irrigation and power demands d/s of the reservoir throughout the year, as shown in
Figure 32,
Figure 33,
Figure 34 and
Figure 35.
Based on the comparison of flow and power duration curves, as shown in
Figure 36,
Figure 37 and
Figure 38, releases were regulated according to the demand during high, medium, and low flows. However, based on
Figure 39,
Figure 40 and
Figure 41, it can be observed that the flat portion of the flow duration curve predominates, which means that with the passage of time due to the reduction in storage capacity, natural releases would be sustained through the year for river flow regulation, instead of outflows being regulated as per demand as it was in the past years of operation.
The flow and power duration curves of the future application years indicate that the pattern of upper region of both curves in
Figure 36,
Figure 37 and
Figure 38 shows the reservoir would follow the pattern of WAPDA standard operation procedures (when capacity of the reservoir would be enough) and releases and power generation would be according to demand. The medium and lower regions of the flow and power duration curves in
Figure 39,
Figure 40 and
Figure 41 show that, during the low flow season, the inflows in the reservoir would be difficult to fulfil the irrigation and power demand d/s of Tarbela. While the flat pattern of the upper, medium, and lower regions of the flow and power duration curves for year 2035 (when the storage capacity of Tarbela would be expected to fully exhausted) shows in
Figure 41 that the inflow would be equal to outflow, and the gross storage would approach 2.87 BCM approximately as shown in
Figure 42.
It is observed from
Table 8 that the gross storage capacity in year 2035 would be 28% of the original gross storage capacity. This would lead to surplus releases during the kharif season and a deficit in the Rabi season, as demand would be more. The predicted long-lasting irrigation releases in the future year 2035 are shown in
Figure 43.
After 2035, when the gross storage capacity of the reservoir would be expected to be fully depleted, the reservoir levels will be higher and most of releases will be preferred to be released through the power tunnels, auxiliary and service spillway. Gross power and energy generation would increase during the high inflow season and will decrease in the low flow season and will not meet the power and energy demand of the country. Power and energy generation would be high in summer season and low during the winter season. The pattern of predicted power and energy generation in 2035 is shown in
Figure 44 and
Figure 45, respectively.
The simulated results from the study were compared with those of various researchers in the Pakistan region for Tarbela Reservoir [
30,
31,
40,
42,
45]. The impact on hydropower generation by variation in the operation of Tarbela Reservoir was carried out using the Evaluation and Planning (WEAP) model [
42]. It was concluded in that a 13% increase in hydropower generation would be expected by altering the operation and it was recommended that the reliability could be increased or enhanced by restricting releases to through the power units instead of the spillways. Further, it was concluded that shifting of the reservoir level to April-May will raise hydropower generation. However, the study did not compute releases and power generation for future operations. Moreover, the study was devoid of considering impact of sedimentation while altering operational strategies.
The performance assessment of the rule curves for the Tarbela Reservoir was carried out by [
45]. Three vital indicators, reliability, resiliency and vulnerability [
61], delineating several facets of operational performance, were computed using 44 years of daily time-step dataset from 1976 to 2019. Tradeoff criteria were assessed by earlier researchers [
22,
62,
63] who concluded that the reliability of Tarbela in meeting the irrigation demands could be increased by up to 70% based on present SOPs of WAPDA. The potential impact due to a low performance of Tarbela was evaluated, which entailed that an average annual shortage of 85 MCM was expected to be encountered and this would lead to a reduction in hydropower generation as well. Further, it was demonstrated through intra-annual analysis of maximum and minimum rule curves of Tarbela that maximum variation occurred in the month of April with CV ranges from 0.017 to 0.024 while minimum variation was encountered in August and September with an average CV of 0.003. The greater CV in April showed reduction in performance in fulfilling the downstream demand with a decreasing trend in CV at 0.1% annually [
45].
Recently, the RESOOSE model [
41] was developed and implemented for maximization of hydropower for cascade reservoirs, i.e., Tarbela and Basha reservoirs. It was concluded that an average annual benefit of 2724 Million US
$ by joint operation. Khan et al. developed the ROSSE model [
31] for optimization of reservoir operation for Tarbela. It was concluded that while giving priority to hydropower generation, an 8% increase in the net economic benefit may be achieved. In was recommended in the study to establish the model sing HEC-ResSim to validate the results.
Therefore, the present study considered the reservoir operation model coupled with a sediment model. It was found, based on simulation results, that with the predicted reduction in storage capacity, the releases during the high inflow period will be more, which will enhance power production and energy generation as releases would be preferred to be passed through the power tunnels. Application of the model was done on a 5-year basis for the future years 2025, 2030 and 2035. The Tarbela Reservoir was operated according to the existing WAPDA operation procedures for the future years at different minimum operating levels (432.2, 438.2, 444.3 and 450.49 m) and maximum operation level of 472.44 for the year 2025, 2030 and 2035, respectively. The model was optimized for maximum releases, power, and energy generation, according to the modified upper and lower rule curves. The simulated irrigation releases, power, and energy generation of the application model for the year will be reduced but the power generation and releases from the dam would be maximum (in surplus) during the high flow season.
The results of flow and power duration curves for the future application years, see
Figure 36,
Figure 37 and
Figure 38, shows the reservoir would follow the pattern of WAPDA standard operation procedures (when capacity of reservoir would be enough) and releases and power generation would be according to demand. The medium and lower regions of the flow and power duration curves in
Figure 39,
Figure 40 and
Figure 41 shows that during the low-flow season the inflows in the reservoir would be difficult to fulfil the irrigation and power demands d/s of Tarbela. While the flat pattern of the upper, medium, and lower regions of the flow and power duration curves for year 2035 (when the storage capacity of Tarbela would be expected to fully exhausted) shows in
Figure 41 that the inflow would be equal to the outflow and the gross storage would approach to 2.87 BCM. The gross power and energy generation would increase during the high inflow season and will decrease in the low flow season and will not meet the power and energy demand of the country. Power and energy generation would be high in the summer season and low during the winter season.
The present simulation study has revealed that while the reservoir consumed 80% of its live storage, power production will be expected to be enhanced during the summer season by 4% and reduced by 37% in the low flow season. However, the impact of gross storage capacity in future will cause a reduction of 6.5% in average annual energy generation because of the limitations imposed by the SOPs of WAPDA being implemented for safely operating the Tarbela dam. The results suggest that with the passage of time, even using optimized rule curves, the reservoir storage capacity will be depleted due to sedimentation.
The flat pattern of the upper, medium, and lower regions of the flow and power duration curves for year 2035 indicate that, by then, the storage capacity of Tarbela would be expected to fully exhausted and inflow would be equal to outflow. The flatness of the flow and power duration curves reinforces the comment that in the month having high flows, inflow becomes equal to outflows due to insufficient storage, while in the low flow season, due to demands higher than inflows, water could not be stored in the reservoir, as irrigation was the primary purpose for construction of the Tarbela reservoir. In 2035, the storage capacity would be exhausted such that it would be difficult to operate the reservoir according to WAPDA standard operation procedures and releases will not be sufficient to meet the irrigation demands throughout the year.