The operation of the reservoir follows a certain procedure so that the competing water users would be able to get their equitable share based on the agreed policy such as the amount allocated to each user. Water allocation is usually dictated by rule curve that is derived from historical data of river flows and water demands. A rule curve shows the minimum water level requirement in the reservoir at a specific time to meet the particular needs for which the reservoir is designed. It is important to note that rule curve shall be followed except during periods of extreme drought and when public interest so requires.
When the reservoir serves two or more purposes, water allocation becomes more complex. During normal flows, the reservoir will be maintained and kept at the rule curve level. During heavy flows, the water level may be allowed to rise above the rule curve. If exceptionally high inflows are expected, the reservoir is drawn down below the rule curve before the water arrives. If low flow conditions prevail, the reservoir may be drawn down below the rule curve to release the design dependable flow to satisfy the downstream needs. During drought periods the reservoir may be completely emptied.
DOWNSTREAM FLOW REQUIREMENTS
Irrigation - Water requirements for irrigation depend on the type of irrigation system, the kind of crop, and irrigation area. They usually vary seasonally (but remain fairly constant on an annual basis), i.e. during the dry cropping season (Nov – Mar) and wet cropping season (Jun – Oct). The total water requirement consists of water needed by the crop and the losses associated with the delivery and application of the water. Upon the advice of the field offices of the National Irrigation Administration (NIA), Angat, Magat and Pantabangan Reservoirs release water for irrigation purposes. Although Binga Reservoir is solely for power generation, it is often requested by NIA to release water to satisfy the irrigation needs of farmers downstream.
Municipal and Industrial Uses of Water - The demand for municipal and industrial water is influenced largely by present population and estimated future growth and by present and anticipated uses by industries. Requirements for municipal and industrial water tend to be more constant throughout the year except during summer months. It is the Metropolitan Waterworks and Sewerage System (MWSS) that determines the municipal and industrial water requirements of Metro Manila.
Hydropower - From the standpoint of power generation, it is desirable to use the water according to the electric power demand. This is particularly appropriate to single-purpose reservoirs (Ambuklao and Binga Reservoirs) where the principal output is hydroelectric energy. However, in multi-purpose reservoirs (Angat, Magat and Pantabangan Reservoirs) the irrigation and/or water supply requirements dictate the amount of electric power and energy that can be generated by the power station.
ANGAT RESERVOIR
In the case of the Angat Reservoir, there are two rule curves used, the upper rule curve which control the surcharge storage available and the lower rule curve which determine the minimum level at which the requirement of all users are satisfied. The upper rule curve or the flood operation rule curve is subdivided into two periods: the Normal High Water Level (NHWL) of 212.0 m (from Dec – April) and the Flood Season High Water Level (FSHWL) of 210.0 m (from May – Nov). The lower rule curve or the normal rule curve is a guideline for the operation of the reservoir for day to day water releases. NPC, NIA, MWSS and NWRB jointly establish the lower rule curve.
Water Level Above The Upper Rule Curve- All demands for water supply, irrigation are met and electricity can be generated at the full capacity of the turbine units. Excess inflow is discharge through the spillway. Water releases through the spillway is controlled and regulated by the NPC Dam Office personnel.
Between Upper And Lower Rule Curves - All demands for water supply and irrigation are satisfied. Generation of electricity is limited to the released water for water supply and irrigation. Further water releases for power generation is allowed provided that the auxiliary units are utilize first before the main units.
Water Level Below Lower Rule Curve - The remaining water in the reservoir is reserved for water supply and irrigation. Generation of electricity is limited to these water releases. No further water releases for power generation is allowed.
Off-Irrigation Period - A minimum release for power generation of 5 MW is allowed.
MAGAT RESERVOIR
As for the upper rule curve of Magat Reservoir, the NHWL is 193.0 m (from Oct – May) and the FSHWL is 190.0 m (from Jun – Sep). Again, it is NPC, NIA and NWRB that established the normal curve for the day to day water releases.
Water Level Above The Upper Rule Curve -At this level, operation of the turbine units is maximized. Excess inflow is discharge through the spillway. Water releases through the spillway is controlled and regulated by the NIA Dam Office personnel.
Between Upper And Lower Rule Curves -Demands for irrigation are satisfied. Hydropower production is confined to the released water for irrigation. However, water stored between the curves may be used for power generation in excess of the irrigation requirements provided that such operation is coordinated with NIA and the water level at Maris Diversion Dam will not reached 104.40 m.
Water Level Below Lower Rule Curve - The remaining water in the reservoir is reserved for irrigation only.
Off-Irrigation Period - A minimum release for power generation of 600 MWH is allowed.
PANTABANGAN RESERVOIR
As for the upper rule curve of Pantabangan Reservoir, the NHWL is 193.0 m (from Oct – May) and the FSHWL is 190.0 m (from Jun – Sep). Similarly, NPC, NIA and NWRB jointly established the normal curve for the day to day water releases.
Water Level Above The Upper Rule Curve -At this level, operation of the turbine units is maximized. Excess inflow is discharge through the spillway. Water releases through the spillway is controlled and regulated by the NPC Dam Office personnel.
Between Upper And Lower Rule Curves -Supplementary water releases for power generation in addition to irrigation requirements is allowed provided that the resulting reservoir elevation will not fall below the lower rule curve.
Water Level Below Lower Rule Curve -The remaining water in the reservoir is reserved for irrigation only.
Off-Irrigation Period -A minimum release for power generation of 5 MW is allowed.
AMBUKLAO AND BINGA RESERVOIR
A simple rule curve exists for Ambuklao and Binga Reservoirs since they operate solely for power generation. Water level may deviate from the curve since the reservoirs are particularly well suited for peaking operations. However, energy production of the reservoirs should strictly adhere to the seasonal demands of electricity. For example, energy demands are greatest in summer when the inflow may be lowest. This means that high inflow during the wet season should be stored to augment the low inflow of the dry season. Meanwhile, excess inflow is discharge through the spillway. Water releases through the spillway is controlled and regulated by the NPC Dam personnel.
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