Value of Climate Forecasts for the Management of Lake Lanier

Transcription

1 Value f Climate Frecasts fr the Management f Lake Lanier Aris Gergakaks and Huaming Ya Bufrd Chattahchee West Pint Walter F. Gerge W Apalachicla Gergia Water Resurces Institute March 2001

2 Value f Climate Frecasts fr the Management f Lake Lanier 1. Intrductin and Overview Are climate mdel predictins ptentially useful fr reservir management? Can adaptive frecast-decisin systems imprve reservir perfrmance? These questins addressed in this reprt using Lake Lanier (Bufrd Dam) n the Chattahchee River (Gergia) as a case study. Figure 1 depicts the mdeling framewrk used in this assessment. The principal mdules pertain t (a) inflw frecasting, (b) reservir management, and (c) scenari assessment. The inflw frecasting mdel t be used is an adaptatin f the Sacrament mdel that has been develped by the Hydrlgic Research Center (HRC). The mdel can generate 16-week inflw frecast ensembles (baseline frecasts) that can be cnditined n frecast infrmatin prvided by the General Circulatin Mdel f the Canadian Climate Analysis Center (CGCM). The hydrlgic mdel and the cnditining prcess are described in the first part f this reprt. Reservir management is based n a decisin system that includes three cupled mdels pertinent t turbine lad dispatching, shrt-range energy generatin scheduling (hurly time steps), and lng/mid-range reservir management (weekly time steps). The decisin mdel has been develped fr the Apalachicla-Chattahchee-Flint (ACF) river basin (f which Lake Lanier is a part) and is briefly described in the fllwing sectin. The assessment prcess quantifies the respnse f the system ver a lng time hrizn, assuming that reservir releases are made based n the use f the frecast-cntrl scheme. The assessment is perfrmed fr a histrical inflw realizatin frm 1950 t In the fllwing sectins, we first describe the features f the ACF-DSS and then discuss the results f the assessment. 1

3 2. General Descriptin f ACF-DSS Cmpnents i Water resurces planning and management fr the ACF River basin is a cmplex undertaking invlving inter-dependent physical prcesses and water users. Thus, a cmprehensive decisin supprt system wuld have t represent the interactin f hydrlgic (surface and grundwater) and land use prcesses, the demands f the varius water users (water supply fr municipal, industrial, and agricultural purpses; navigatin; hydrpwer; water quality; river eclgy; and recreatin), and the integrative respnse f the river system. It shuld als have the ability t assess the benefits and cnsequences f different planning and peratinal plicies and present this infrmatin t basin planners and decisin makers in meaningful ways. This sectin discusses the cmpnents f a decisin supprt system that the Gergia Water Resurces Institute (GWRI) has been develping fr the ACF River basin. The system already includes hydrlgic, river simulatin, reservir cntrl, uncertainty analysis, and plicy assessment mdules, and is being used t supprt the tri-state (Gergia-Alabama-Flrida) negtiatin prcess fr the develpment f a water allcatin cmpact. Current wrk aims t expand the scpe f the DSS by including remte sensing, grundwater simulatin, water quality, and agricultural planning mdules. A shrt descriptin f the existing cmpnents fllws. Figure 2 prvides a schematic f the mdeled ACF elements and water uses. i I Streamflw Frecasting The purpse f the streamflw frecasting cmpnent is t frecast the upcming reservir inflws and prvide an appreciatin f the frecast uncertainty thrugh multiple frecast traces. These traces represent equally likely inflw realizatins reflecting characteristics f the histrical inflw sequences. Their tendency is t spread ut as the frecast lead time increases and the hydrlgy f the basin mves away frm its present cnditin. In this particular applicatin

4 Lake Lanier inflws are frecasted by the HRC mdeling system; frecasts fr the ther 22 inflw ndes are generated based n histrical analg methds. Reservir Cntrl Due t the several times scales ver which water uses are relevant, the reservir cntrl cmpnent includes three mdules. The purpse f the turbine cmmitment and lad dispatching mdule is t ptimize hydr plant efficiency by determining the pwer lad f each turbine such that the ttal plant utflw is equal t a given discharge level and ttal pwer is maximized. The inputs t this mdule include beginning-f-the-perid reservir elevatins, varius turbine and reservir characteristics (e.g., elevatin vs. strage and tailwater vs. discharge relatinships, pwer vs. net hydraulic head vs. discharge curves, and peratinal turbine ranges, amng thers), and minimum and maximum discharge requirements. This mdule utilizes Dynamic Prgramming as the ptimizatin prcedure and has a dual rle: First t prvide the feedback infrmatin needed by the ther reservir cntrl (dynamic) mdules in the frm f an ptimal relatinship amng ttal system utflw, ttal plant lad, and reservir elevatin, and, secnd, in an peratinal mde, t determine the actual turbine lads which realize the discharge assigned t a particular plant while maximizing pwer generatin. The first utput is the cnnecting link between this and the ther tw decisin mdules. The secnd reservir cntrl mdule is cncerned with determining the best hurly pwer sequences fr each hydrpwer statin in the system ver a perid f ne week. The bjective it 3

5 I t maximize energy generatin (given that all ther stated bjectives are met) with r withut dependable capacity cnstraints. The inputs t this mdule include physical reservir layut and characteristics (e.g., strage vs. elevatin curves, strage and release cnstraints, etc.), the relatinships between minimum discharge, reservir elevatin, and plant lad, determined by the turbine lad allcatin mdule, ther peratinal requirements (such as thse f the service units) and dependable capacity cmmitments. - The ptimizatin methdlgy f this mdule is als based n Dynamic Prgramming. In additin t the best hurly generatin schedules fr the system reservirs, the mid/shrt range cntrl mdule als derives the ptimal weekly energy generatin functins. These functins are passed n t the third cntrl mdule. - Lastly, the purpse f the lng range cntrl mdule is (1) t quantify lng term system perfrmance and tradeffs and (2) develp ptimal, system-wide release schedules fr each system reservir ver a perid f several weeks. Such tradeffs are usual between water uses (e.g., between water withdrawals and instream flws, upstream and dwnstream water uses, energy generatin and drught management, and navigatin and lake fluctuatin, t mentin but a few). The tradeff curves are imprtant infrmatin that decisin makers and basin planners must review befre selecting a water allcatin and peratinal plicy. Fr these determinatins, this mdule requires input n weekly inflw frecasts, urban and agricultural water demands, reservir characteristics, instream flw and navigatin requirements, and energy generatin targets. The ptimizatin peratins are carried ut by the Extended Linear Quadratic Gaussian (ELQG) cntrl methd (develped by A.P. Gergakaks and assciates), a trajectry iteratin cntrl algrithm suitable fr multidimensinal uncertain systems.

6 The three mdules cnstitute a multilevel cntrl structure with an peratinal flw that fllws tw directins: The lwer level mdules are activated first and generate infrmatin that is used by the upper levels regarding perfrmance functins and bunds. In the curse f this upward flw, the decisin system generates apprpriate peratinal tradeffs evaluating the cnsequences f varius lng- and mid/shrt-term plicies. At these key pints, the mdel halts and requests the input f the management authrities regarding their mst preferred plicy selectins. Once these decisins are taken, the cntrl levels are activated in the reverse rder t generate the best turbine hurly sequences and lads implementing these decisins cnsistently acrss all relevant time scales. Amng, the unique features f this decisin system are the explicit treatment f uncertainty at all peratinal levels, the detailed mdeling f all water uses, the high cmputatinal efficiency and the design philsphy which incrprates the preferences f the decisin authrity thrugh the evaluatin f tradeffs. Plicy Assessment The purpse f the plicy assessment cmpnent is t replicate the actual weekly peratin f the ACF system under varius water allcatin plicies, management strategies, and climate scenaris. Namely, at the beginning f each week f the simulatin hrizn, this cmpnent invkes the inflw frecasting and reservir cntrl cmpnents, determines the mst apprpriate reservir releases, simulates the respnse f the system fr the upcming week, and repeats this prcess at the beginning f the fllwing decisin time. At the cmpletin f the frecast-cntrl-simulatin prcess, the prgram generates sequences f all system perfrmance measures. These sequences can be used t cmpare the benefits and cnsequences f alternative water allcatin and peratin plicies. 5

7 I i ACF-DSS Advantages ver Traditinal Simulatin Mdels The decisin t develp the ACF decisin supprt system was prmpted by the limitatins exhibited by traditinal simulatin mdels (i.e., HEC-5 and STELLA) that were initially develped fr the tri-state water negtiatin prcess. The advantages include: better representatin f imprtant water uses, such as hydrpwer and eclgical flw requirements; ability t frecast, assess the severity f, and manage drughts; ability t assess the benefits f basin-wide reservir crdinatin strategies and the tradeffs amng the water uses; ability t mre exhaustively identify the water allcatin scenaris that culd be f interest t the tri-state negtiatin prcess, namely, scenaris that culd ptentially cmprmise better amng the demands and lead t the negtiatin f shared-visin water use agreements. The ACF decisin supprt system represents a mdern river basin planning and management tl that effectively addresses the limitatins f traditinal river simulatin mdels. It is develped t establish a unifrm basis fr evaluating varius water allcatin and river management scenaris and a cmmn cmmunicatin language amng the technical staff f the negtiatin teams. ACF-DSS Sftware Package. i I The ACF-DSS cmputer sftware is a graphical user-friendly package, and runs n persnal cmputers under the Windws perating systems (95, 98, 00, and NT). ACF-DSS can assess the ACF system respnse t water withdrawals, minimum flw targets, lake fluctuatin ranges,

8 - navigatin requirements, flw regimes sensitive t river eclgy, hydrpwer bjectives, and drught management plans, amng thers. The user can access and mdify all input parameters thrugh a graphical user interface. Once a particular simulatin run finishes, ACF-DSS displays all imprtant sequences and statistics pertaining t system cnditins and uses. Such infrmatin is essential fr understanding the capacity f the basin t meet the varius water uses and fr develping the infrmatin base fr technically feasible and equitable water use agreements. ACF-DSS has been prvided t the tri-state technical teams cmplete with technical reprts, user manuals, and hands-n training. 3. Assessments i The cupled frecast-cntrl scheme was used t assess the perfrmance f Lake Lanier ver a 3-year histrical hrizn frm 1950 t The assessment was cnducted fr baseline frecasts as well as fr GCM-cnditined frecasts (CGCM). The ACF-DSS management bjectives are as fllws: Meet municipal, industrial, and agricultural water supply withdrawals (prjected fr 2030); Meet instream flw requirements (pllutin abatement); Meet 2-hrs peak pwer generatin per week day; Maintain as high lake levels as pssible (fr hydrpwer ptimizatin and drught management); Avid spillage at least 95% f the time. Figures 3 and shw the Lake Lanier levels and releases fr the tw assessment runs (baseline and CGCM frecasts). The figures shw that the sequences d nt exhibit marked differences. Hwever, die lake levels fr the GCM-cnditined frecasts are smewhat lwer than fr baseline frecasts. This happens because the ensemble bands f the GCM-cnditined frecasts are smewhat wider than thse f the baseline, causing the decisin system t make slightly higher releases t maintain the same spillage reliability. This effect is als seen n Figure 5, depicting the frequency curve frm f the time sequences shwn n Figures 3 and. The tp graph f Figure 5 clearly shws that the CGCM lake

9 1 i levels are slightly, yet cnsistently, lwer than the baseline levels. The explanatin fr this is prvided by the bttm graph f Figure 5, shwing that GCM releases are higher than baseline releases fr high releases (high lake levels), and lwer fr lw releases (lw lake levels). Table 1 summarizes the results f the tw runs with respect t energy generatin and minimum flw vilatins. The results indicate that the differences are nt appreciable, but they are cnsistent with the previus cmments. Namely, the baseline run prduces slightly mre energy generatin (primary as well as secndary) and causes fewer vilatins f the instream flw targets.. Cnclusins This assessment shws that the climate frecasts prvided by the Canadian General Circulatin Mdel d nt imprve the management f Lake Lanier- Namely, the use f CGCM infrmatin des nt imprve frecast skill cmpared t the skill f hydrlgy-based frecasts. Additinal reasns cntributing t this result are that (1) lake strage and turbine utflw capacity are large in relatin t inflws, (2) management decisins are primarily driven by demands and reservir levels, and (3) frecast skill differences are mitigated by the adaptive features f the ACF-DSS. Hwever, the previus cnclusin is strictly assciated with the Canadian GCM. Further wrk shuld be undertaken t assess the frecast skill f ther GCMs fr the Sutheastern US. Acknwledgments The wrk reprted herein was partially supprted thrugh NOAA grant NA76GP0352 as part f the GCIP Prgram. We wish t thank Dr. Rick Lawfrd fr supprting this effrt. We are als grateful t Dr. Knstantine Gergakaks and his team at the Hydrlgic Research Center fr prviding us with the results f their inflw frecasting mdels and wrking with us n this prject.

10 References - Gergakaks, A.P., "Operatinal Tradeffs in Reservir Cntrl", Water Resurces Research, 29(11), , Gergakaks, A.P., Ya, H., and Y. Yu, "A Cntrl Mdel fr Dependable Hydrpwer Capacity Optimizatin," Water Resurces Research, 33(10), , Gergakaks, A.P., H. Ya, and Y. Yu, "Cntrl Mdels fr Hydrelectric Energy Optimizatin," Water Resurces Research, 33(10), , Gergakaks, A.P., H. Ya, and Y. Yu, "A Cntrl Mdel fr Hydrelectric Energy Value Optimizatin," ASCEJ. fr Wat. Res. Plan, andmgt, 123(1), 30-38, Gergakaks, A. P., H. Ya, M.G. Mullusky, and K. P. Gergakaks, "Impacts f Climate Variability n the Operatinal Frecast and Management f the Des Mines River Basin", Water Resurces Research, 3(), , I - * 9

11 Inflw Mdel Baseline CGCM S ill II ill ill Lng/Mid Range Cntr Mdule (Weekly! Reiervir & River Simulatin I Mdels I I ELQG IT Lang Range Tradeffs Seasnal Water Management Decisin Mdels i si I J. Eltivtun* DMint 111 ill II SliQrt Hangs Cntrl M Lie [Hurly) Reservir* River! Simulatin Mdels TT Hurly Release and Pwer Sequence* I l\ il II Turbine Lad Ci spate rung Wdule Dynamit Prgramming IT Turbine Lads Spillway Outflws Frecast-Cntrl Hrizn (16 weeks) 1/01/1950 thrugh 12/31/1993 Figure I: Elements f the ACF Decisin Supprt System (ACF-DSS)

12 Lake Lanier 105 MW Nrcrss Range: (ft) Vl# 7. (bcf) Mrgan Falls 16.8 MW Atlanta Whitesburg West Pint Lake 82 MW West Pint Bartletts Ferry Gat Rck Oliver N. Highlands Clumbus W.F. Gerge 168 MW Albany Newtn Mntezuma Majr Water Use Water Supply (M/l/A) I lydrpwer Recre; Navigatin Envirnmental/Ecsystem M Andrews Bainbridge J. Wdruff 36 MW Chattahchee Bluntstwn Sumatra Range: (ft) 6. (bcf) Legend Reg. Reservir Runff Reservir 0 Cntrl Pint 1^ Lcal Inflw fl Withdrawal Figure 2: ACF System Schematic

13 Level (ft) CO CO en -p» en en I \ J J J \ \ A ] J \ ] ] ] OOH n») gi- S 3' CO II?? T! CQ -> "D O O 0) r <. 0)

14 Release (cfs) Q) CO c ^ 03 7C DG cp_ 03 C/3 O 3 TJ 03 U>' =j I" ' I] 2. ft) 0) 0)

15 Lake Lanier Level Frequency Frequency f Exceedance (%) 100 Lake Lanier Release Frequency Baseline CGCM Frequency f Exceedance (%) 100 Figure 5: Lake Level and Release Frequency Cmparisn

16 Table 1: Energy Cmparisn Energy Generatin Cmparisn J Frecast Energy (GWH) Lake Lanier West Pint Gerge Wdruff System Primary ^81 1 Baseline Secndary I Sum Reliability (%) Primary I CGCM Secndary Sum Reliability (%) J Energy Generatin frm Private Reservirs I Frecast M. Falls B. Ferry G. Rck Oliver N. Hlands System I I Baseline [ CGCM J31(M I Minimum Flw Vilatin Time Frequency (%) I Frecast Atlanta Clumbus Chattahchee I I Baseline [ CGCM