Modeling Analysis Of Navajo Reservoir Operations Proposed .

16d ago
1.39 MB
49 Pages

January 2015Modeling Analysis of Navajo Reservoir OperationsProposed Modification of Available Water Calculation forDetermining Spring Peak Releases on the San Juan River1.0 IntroductionReclamation has proposed a modification of the calculation used to determine spring peak releaseoperations at Navajo Dam. To support this proposal, Reclamation has conducted extensive hydrologicmodeling with the purpose of: Understanding the recent changes in San Juan River Basin hydrology and its effect on NavajoOperations.Calculating the releases that would potentially have been made from Navajo since 1971under the 1999 Flow Recommendations as written.Calculating the releases that would potentially have been made from Navajo since 1971under the Proposed Method by SJRIP (San Juan River Basin Recovery ImplementationProgram) and Reclamation.Comparing the two methods and assessing changes in overall release volumes and timing,on an annual basis. Assessing the difference in resulting downstream river flows.Aid workshop participants and the SJRIP in making technical decisions for the Proposal’simplementation.2.0 TerminologyTerminology used in this report is shown below and was used for convenience in this report. Someterms used here may not apply to other outside purposes.Base Flow: Average flows through the critical habitat area along the San Juan River (from Farmington,New Mexico to Lake Powell in Utah).Daily Operations Model: Reclamation’s hydrologic model of the San Juan River Basin for dailymanagement and operation of the reservoirs within the basin.EOWYST: End of water year storage target. This is the reservoir storage that the reservoir will beoperated to begin and end each water year at, dependent on forecasting accuracy and hydrologicavailability. The equivalent reservoir elevation is used interchangeably with storage in this discussionand is also referred to as the EOWYST.Modified Daily Operations Model: The Daily Operations Model, modified to test the Proposed Method.NIIP: Navajo Indian Irrigation ProjectOperational Spill: A planned water released through the outlet works that is in excess of the sum of thespring peak release volume and the release required to maintain the minimum target baseflow.Original Method: The method of determining available water and the spring peak release using theoriginal 1999 Flow Recommendations.Reclamation WCAOPage 1

January 2015Proposed Method: The method of determining available water and the spring peak release using theproposed method by the SJRIP and Reclamation.SPR: Spring Peak ReleaseTBF: Target Base Flow. The SJRIP recommends a target base flow of between 500 cfs and 1,000 cfsthrough the critical habitat area along the San Juan River (from Farmington, New Mexico to Lake Powellin Utah). This is calculated as a weekly average of the minimum of the lower 3 of 4 USGS gages.Uncontrolled Spill: Use of the uncontrolled spillway. An undesirable condition.WY: Water Year. A 12 month period ending September 30 of the designated water year.3.0 Navajo Inflow HydrologyThe inflow into Navajo Reservoir has decreased significantly in recent years. While the hydrology used todevelop the Flow Recommendations and the Navajo Biologic Opinion include the wet decade of the1980’s and pre‐dam conditions, Navajo has often received below average inflow since 1999, when theFlow Recommendations were finalized. Forecasts in the basin and across the southwest indicate thelikelihood of continued below average hydrology in the future.Though the SJRIP Flow Recommendations have only been in place since 1999, the hydrology used in theanalyses consists of observed daily inflow data into Navajo Reservoir for water years (WY) 1971 through2013. An additional subset of the last 15 years (since 1999) was analyzed because of its significanceboth hydrologically and programmatically.Figure 1 is a plot of annual water year inflows into Navajo Reservoir. The median inflow is shown for theentire 42‐year record (1971‐2013), as well as for the more recent, and much drier, 15‐years of record(1999‐2013).The more recent range of hydrologic data, from 1999‐2013, had a median inflow into Navajo of 690,000af/year annually, compared with 850,000 af annually since 1971. The repeated years of lower hydrologyover the last decade has not allowed the reservoir to adequately recover from large spring releases, andthe risk of shortage to users has increased. It is important to consider the more recent subset ofhydrologic data because climatologic forecasts indicate that this dry pattern of hydrology is likely tocontinue. Though not represented in this study, WY2014 was yet another dry year for the San JuanRiver Basin, only receiving slightly more inflow into the reservoir than 2013.Reclamation WCAOPage 2

January 2015Figure 1. Annual inflow into Navajo Reservoir by water year (1971‐2013).4.0 Comparison of Original and Proposed Methodologies4.1 Original Method: 1999 SJRIP Flow Recommendations for determining Available Water and SPRsThe 1999 SJRIP Flow Recommendations Chapter 8 gives detailed guidance on recommended hydrographconditions in the San Juan River and recommended Reservoir Operating Rules that are designed toachieve these hydrograph conditions.The Recommended Reservoir Operating Rules use a Decision Tree to determine the appropriate SPR foreach year, based on forecast inflow and releases. The volume of water that is used to walk through thisdecision tree is called the Available Water. Under the Original Method, the Available Water iscalculated using the following equation:In this equation, t time of calculation, TBF target base flows, NIIP Navajo Indian Irrigation project. Carryover storage isbased on a Level of Development in the basin, as shown in Table 8.4 (page 8‐13) of the 1999 SJRIP Flow RecommendationsSimply stated, this equation states that the current storage in the reservoir, plus the forecast inflow,minus the forecast releases and evaporation, minus diversions to NIIP, minus a carryover storagevolume, is the volume of water available for a SPR this year.Reclamation WCAOPage 3

January 2015The Carryover Storage is based on the Level of Development in the basin, as described in Table 8.4 ofthe 1999 SJRIP Flow Recommendations. The Level of Development is the minimum carryover necessarythat the reservoir would need in storage in order to meet demands for one additional year.The Available Water is then walked through the Decision Tree (Figure 2). The Decision Tree flow chartuses a combination of Available Water, the occurrence of perturbations (based on the number of storm‐event days occurring over the previous year), the possibility of spill, and the size and frequency of SPRsin recent years. If appropriate, the Decision Tree may result in a recommendation that the SPR bereduced or skipped, even when there is sufficient Available Water, with the goal of increasing storage inthe reservoir for a potentially larger SPR the following year.The volumes in Figure 2 correspond to specific hydrographs that range from 1‐week at 5,000 cfs to a fullhydrograph that lasts for a month. (It will later be important to note that slight modifications have beenmade by the SJRIP to these hydrographs since the original report, including a reduced ramping rate andbase release volume, resulting in slightly different volumes than those represented in Figure 2.)Reclamation WCAOPage 4

January 2015Figure 2. Flow chart of Navajo Dam operating rules for a 5,000 cfs peak release. From 1999 SJRIP FlowRecommendations.Reclamation WCAOPage 5

January 20154.2 Proposed Method: Reclamation and SJRIP’s proposal for determining Available Water and SPRsReclamation’s proposal involves a slight modification to the Available Water calculation that is currentlyin the Flow Recommendations. This modification is a reflection of the impact recent hydrology has hadon the basin, reservoir supply, and shortage risk, and is an attempt to provide a level of insuranceagainst the threat of shortages to the water supply under expected future conditions. Having thisinsurance against shortage will increase the likelihood that the target base flows can be met, even in asustained and serious drought.Rather than using a carryover storage that represents a certain level of development in the basin, as laidout in Table 8.4 of the Flow Recommendations, Reclamation is suggesting using an annual End of WaterYear Storage Target (EOWYST).The EOWYST is the reservoir storage target on September 30th, which is the conclusion of each wateryear. If the reservoir is operated to begin and end each water year at the same elevation, thenoperations are based on the year‐to‐year hydrology, rather than speculating on the hydrology of futureyears. This method is also more consistent with the accuracy of long‐term water supply forecasts, whichtypically have a sharp decline in forecast skill beyond a few months.With this change, the Available Water Calculation becomes:In this equation, t time of calculation, TBF target base flows, NIIP Navajo Indian Irrigation project, EOWYST End of WaterYear Storage Target.The new Available Water calculation would be used in the Decision Tree to determine the appropriateSPR for the year. However, this new method negates the need to look back and reduce or skip SPRs incertain years, as suggested in the Decision Tree. These actions are designed to save up water for a largerrelease in the following year. By utilizing an EOWYST, and therefore operating the reservoir on anannual basis, the need for such predictive actions is unnecessary. This method mimics the actual annualhydrologic conditions in the basin.Questions posed for hydrologic modeling What is the difference between the Original and Proposed methods?How will Navajo’s releases change, both the SPR, and the base flow release?Will this increase or decrease the overall release from the reservoir? By how much?How would release timing change?Are the flow recommendations more likely or less likely to be met with the Proposed Method?What affect do different EOWYST’s have on the Available Water calculation?What affect do different EOWYST’s have on release type and timing?What would be the most conservative EOWYST with regard to the possibility of a multi‐yeardrought or shortage?Or conversely, with regard to spill?Reclamation WCAOPage 6

January 20155.0 Description of Model and RulesReclamation’s San Juan River Basin Daily Operations Model (Daily Operations Model) was used as thebasis for the hydrologic modeling effort. The Daily Operations Model includes the entire San Juan RiverBasin, including upper basin reservoirs and operations, and high elevation stream gages (Figure 3). It isused in forecasting operations of Navajo and resulting river flows on a day‐to‐day basis and is used inReclamation’s 24‐month study. The model was altered and updated with new inputs and rules for thepurposes of testing the Proposed Method and comparing it with the Original Method from anoperational standpoint.Figure 3. Daily Operations Model for the San Juan River Basin used for realtime daily forecasting and operations.For this modeling effort, the Daily Operations Model was modified to only include Navajo Reservoir, theAnimas River, and the San Juan River from the release point at Navajo to its confluence with the AnimasRiver, and downstream through the critical habitat reach (Figure 4). As such it is referred to as the“Modified Daily Operations Model”.Appendix B contains a full printout of the Riverware Model rulesets and a copy of the model will beprovided upon request.Reclamation WCAOPage 7

January 2015Figure 4. Modified Daily Operations Model used for comparisons in daily operation between Original Method andProposed Method.Inputs to the Modified Daily Operations Model include the followinga) Flows: Observed historic flow in the Animas River, historic observed inflow into Navajo.b) Losses: Observed diversions from NIIP for 2012 were used for each year under the Originaland Proposed Methods. The historic 75% exceedance statistical diversion in the river wasused, based on river flow and time of year. The 75% exceedance was chosen to beconservative, and to align more closely with higher water user take and drier conditions.c) EOWYST: Each model run used a single EOWYST for all years. This was the main parametervaried with each model run for comparison purposes. The EOWYST ranged from 6000 ft to6080 ft in the modeling runs. The reservoir was modeled to end each water year at thedesignated EOWYST.d) SPRs: SPR Tables are based on the hydrographs described in the 1999 FlowRecommendations. The prescribed SPRs include 3 day ramps up and down (as approved bythe SJRIP), except in the case of the Full Hydrograph, which has its original full ramps. Thehydrograph volumes are as follows (total volume including base):a. 1‐week at 5,000 cfs with 3‐day ramps up and down: 90,843 afb. 2‐weeks at 5,000 cfs with 3‐day ramps up and down: 150,347 afc. 3‐weeks at 5,000 cfs with 3‐day ramps up and down: 229,686 afd. Full Hydrograph, 3‐weeks at 5,000 cfs with full ramps: 418,512 afThe rules and priority order in the Modified Daily Operations Model follow the SJRIP RecommendedReservoir Operating Rules (1999 Flow Recommendations, Chapter 8) except where noted. Riverware willfire the rules at each time step in the order specified. An exhaustive list of the rules is provided inAppendix B. The rules pertinent to this discussion, listed in priority, are described below.1. Set release from Navajo to 350 cfs (2006 Reclamation’s ROD, goal minimum release).2. Increase the release, if necessary, to meet the minimum TBF (Minimum TBF is 500 cfs).3. Determine what volume of water over the EOWYST will be left at the end of the water year.This is the volume available for a SPR.4. Choose the maximum possible SPR from the Decision Tree and SPR tables based on thisAvailable Water volume.Reclamation WCAOPage 8

January 20155. After SPR inserted, remaini