32 G. Q. Tabios III DLSU Business & Economics Review 28(2018), pp. 32-39 RESEARCH ARTICLE Angat Multipurpose Reservoir with Increased Water Demand and Future Reservoir Sedimentation Guillermo Q. Tabios III Institute of Civil Engineering and National Hydraulic Research Center, University of the Philippines at Diliman, Quezon City and National Academy of Science and Technology, Philippines [email protected] Abstract: The Angat Reservoir is a multipurpose reservoir serving competing uses for Metro Manila’s domestic water supply and Bulacan’s Angat-Maasim River irrigation system with a privatized hydropower generation component. This study assesses the reliability of Angat Reservoir operations to deliver these demands with an increased domestic water supply requirement from 46 to 50 m3/sec and with future reservoir sedimentation in 2050. A reservoir optimization-simulation model is employed to simulate daily reservoir operations with long- term generated inflows from a watershed model. Then, the resulting daily reservoir releases for each scenario are subjected to flow duration analysis in the context of reliability analysis to assess the competing water uses of Angat Reservoir. Keywords: Competing water uses, reservoir optimization-simulation, reliability analysis, reservoir sedimentation JEL Classifications: C61, O21 and Q25 The Angat Reservoir is a multipurpose reservoir Competing Uses of Angat Reservoir by Its serving Metro Manila’s domestic water supply needs Physical Configuration and Water Rights under the Metropolitan Water Supply and Sewerage (MWSS) and Bulacan’s irrigation demand for about The Angat Reservoir system has essentially 24,000 hectares of riceland in the Angat-Maasim competing uses by virtue of its physical configuration River Irrigation System (AMRIS) under the National and water rights allocation for the different water uses. Irrigation Administration (NIA) including hydropower As shown in Figure 1, the main hydropower plant generation and flood control. (HP) generation with a capacity of 200 megawatts Copyright © 2018 by De La Salle University 3 original 22 CMS plus 15 CMS referred to as conditional water rights from NIA plus nine CMS Angat Multipurpose Reservoir with Increased Water Demand and Future Reservoir Sedimentation 33 from the Umiray River; (B) NIA-AMRIS water rights were reduced to 21 CMS, which is the (MW) is through releases to NIA for irrigation. The water from the Umiray River transbasin transfer, the originalauxiliary HP36 generationCMS minus with 15a capacity CMS ;of and 46 MW(C) isBulacan water b ulkrights water situation rights has evolvedare three into CMS the following:, which comes through releases to MWSS for domestic water supply. (a) MWSS water rights became 46 CMS, which is the fromHowever, the twice Umiray as much River power transbasin can be generated transfer at . Thesum aboveof the original water 22rights CMS plusallocation 15 CMS isreferred based to on the the main HP compared with the auxiliary HP for the as conditional water rights from NIA plus nine CMS from the Umiray River; (b) NIA-AMRIS water rights longsame -flowterm discharge, average since inflow at, say, of a thereservoir Angat elevation and Umiray watersheds of 70 CMS. of 205 meters (m), the head for power generation for were reduced to 21 CMS, which is the original 36 CMS the main HP is 187.5 m (that is, 205–17.5 m tailwater minus 15 CMS; and (c) Bulacan bulk water rights elevation),In while view that of forthe the above auxiliary chang HP ise 104in waterm areallocation three CMS, and which priority, comes from200 theMW Umiray capacity River of the (that is, 205–101 m tailwater elevation at Ipo Dam). transbasin transfer. The above water rights allocation mainHowever, HP NIA of Angatand MWSS is seldom water rights,reached, originally, since NIA’sis based allocation on the long-term has been average reduced inflow to of 21the CMSAngat from after Angat Dam was built in 1968, NIA had 36 m3/sec and Umiray watersheds of 70 CMS. its(CMS), original while MWSS36 CMS. had 22Al CMS,so, in a totalaccordance of 58 CMS, with theIn Philippine view of the Waterabove changeCode, induring water allocationwater shortage which is the amount equal to the water rights granted and priority, 200 MW capacity of the main HP of to the National Power Corporation to operate Angat Angat is seldom reached, since NIA’s allocation has conditions,Reservoir for theits hydropower order of priority generation of objectiveswater delivery been from reduced a multipurpose to 21 CMS from reservoir its original like 36 CMS.Angat is to provided that the MWSS (domestic water supply) and Also, in accordance with the Philippine Water Code, firstNIA (irrigation) satisfy domes water rightstic supply, or allocations followed are satisfied by ir rigationduring demandwater shortage and conditions,hydropower the order that ofis priority incidentally on a long-term, average daily basis. The long-term of water delivery from a multipurpose reservoir like generatedaverage is emphasized. here because NIA has seasonal Angat is to first satisfy domestic supply, followed by water requirements in contrast to MWSS, which is irrigation demand and hydropower that is incidentally fixed on a daily basis. Since 2000, with additional generated. Angat Dam HEPP Main HP @17.5m Bustos Dam NIA-AMRIS (irrig) 200 MW K-Water (Angat Afterbay 17,000-23,00 ha @ 17.5 m elev Regulation Dam) Monthly demand (ave 24.21 CMS) Aux HP @101 m 18 MW K-Water 28 MW MWSS Spillway Spillway Ipo Dam Angat River Low Level Outlet MWSS (domestic) 15 million people Constant demand of 46 CMS Figure 1. Physical components and water demands of the multipurpose Angat Multipurpose Reservoir System Figure 1. Physicalfor domestic cwateromponents supply, irrigation and wwaterater supply, demands hydropower of thegeneration multipurpose and flood control. Angat Multipurpose Reservoir System for domestic water supply, irrigation water supply, hydropower generation and 6 34 G. Q. Tabios III Angat-Ipo-Bustos-Umiray River Basin Delineation Figure 2. Watershed delineation and river network of the Angat-Ipo-Bustos-Umiray water resources system. Figure 2. Watershed delineation and river network of the Angat-Ipo-Bustos-Umiray water resources system. Angat Optimization-Simulation Model flow planes, the hourly evolution of soil-moisture with with Watershed Model or without rainfall results in overland and channel flows that are routed in each overland flow plane and into the Figure 2 shows the Angat Reservoir system with the channel or stream network of the basin from upstream Angat, Ipo, and Bustos watersheds and river network. to downstream. The daily rainfall record available in For purposes of watershed modeling, the watershed this study is from January 1974 to December 2012 for system is delineated into 153 sub-basins according a total of 38 years of daily flows that can be generated to basin topography, river network configuration, from the watershed model to conduct Angat Reservoir and slopes—considering as well the geology, soil operations and a reliability study. types, vegetation, and land use. The watershed model The WATPOW model originally developed by employed in this study is a continuous-time, distributed Tabios and Shen (1993) is used in the optimization- model consisting of various components: rainfall simulation study of the Angat-Ipo-Bustos-Umiray input, evaporation, surface detention, infiltration, water system. The model consists of a simulation overland flow, channel flow, and, most importantly, the module to represent the physical processes involved Sacramento soil-moisture accounting model. For each in reservoir operations and an optimization module sub-basin (see Figure 2) composed of two overland to specify the reservoir operating policies such as Angat Multipurpose Reservoir with Increased Water Demand and Future Reservoir Sedimentation 35 release schedules. The model geometry is defined by Optimization-Simulation Model Scenarios a node-link network representation of the Angat-Ipo- Bustos Reservoir system as shown in Figure 1 where The optimization-simulation studies were conducted the inflow points (as links) are the Angat River and for three (3) scenarios described in detail below: (a) Umiray River; local inflows between the reservoirs existing case or conditions with 46 CMS MWSS Angat and Ipo and, Angat and Bustos; the Angat Dam, demand; (b) with increased MWSS demand from 46 to Ipo Dam, and Bustos Dam as reservoir nodes); Main 50 CMS; and, (c) with future reservoir sedimentation and Auxiliary hydropower plants (as nodes); and the in 2050 with 46 CMS MWSS demand links to the demands nodes, NIA-AMRIS irrigation system of Bulacan and MWSS of Metro Manila. Scenario with Existing MWSS Demand at 46 CMS The objective function of the optimization- This optimization-simulation scenario is for the simulation model is a weighted multi-objective to existing or current water demands (on a daily basis) minimize water supply deficits (demand violations), from Angat Reservoir. In particular, the MWSS maximize hydropower generation, and minimize domestic water supply demand is 46 CMS, while the spills. The constraints include capacities of release NIA-AMRIS irrigation water demand varies from gates, storage volumes, operational constraints, and month-to-month. all other system constraints imposed implicitly in the simulation model. Specifically, the objective function Scenario with Increased MWSS Demand at 50 CMS to be maximized is given by This scenario is the same as the existing case but with increased MWSS domestic water supply demand F= pmin( QWS − QWD,0.0) + p min( QIR − QID ,0.0) − p QSP ++ b EM b EA WS t t IR t(1) t from 46SP to 50 t CMS. M t A t F= pWS min( QWSt − QWDt ,0.0) + pIR min( QIRt − QID t ,0.0) − pSP QSP t ++ b M EM t b A EA t Scenario with Reservoir Sedimentation in 2050 where QWSt and QWDt are MWSS water supply This scenario is to assess the reliability of the release (through the auxiliary HP) and MWSS water Angat Reservoir system to deliver water to MWSS demand, respectively, at time t with penalty coefficient and NIA-AMRIS with future sediment deposition in pWS; QIRt, and QIDt are the NIA-AMRIS irrigation Angat Reservoir around 2050.