U.S. Department of the InteriorBureau of Reclamation

Workshop on the San Juan Basin Hydrology Model

February 17, 2005

Purpose of Workshop

• To provide an opportunity for interested parties to gain a better understanding of how the SJBHM works.

• Discuss Model Capabilities and Limitations

• Discuss Interpretation of Model Results

AGENDA

I. Basis of Hydrology Modeling

II. Overview of the Generation II Model.

III. Rationale for the Transition from Gen II to Gen III - Including capabilities and limitations of Gen II.

IV. Overview of the Gen III Model - Including capabilities and limitations of Gen III.

V. Interpreting the Results - Discussion on the Precision and Accuracy of the Model(s).

VI. Open Discussion, Q&A, etc.

What Is A Hydrology Model?

A computer representation of the physical processes associated with consuming and moving water moving through a river basin.

• Simulation model

• Natural Flow Based– Natural flow estimation – Period of Record (1929-1993 for the present SJBHM).

• Calibration Process– Apply historical demands to natural flows– Calibrate to replicate gage flows– Calibrate on one period of record– Validate on another period of record

What Is A Hydrologic Model?(continued)

• Assumptions in Simulation– Use historical hydrology to represent future conditions– Substitute future expected demands for historic demands– Historical and future statistics similar

• Baseline configuration– Basin conditions expected to occur in the future. – Foundation to which conditions of a future project are added to determine

incremental impact.

• SJRIP Baseline – Initially established with first ALP consultation 1991 – Updated with new input for flow recommendations – Updated by adding new projects as approved or new information received.

Purpose of SJBHM

Provide a tool to analyze ways to manage flows in the

San Juan River Basin for the benefit of Endangered

Fish while allowing water development to proceed.

• Provide technical input to the development and periodic evaluation and analysis of flow recommendations

• Assess the impact of project development on basin depletions and recommended flows for endangered fish

SJBHM Overview

• First generation model– Monthly timestep (1929-1993)– Implicit ALP– Reclamation natural flows– Post processing required to assess impacts

• Second generation model (May 2000)– Operating ALP with interaction with Navajo Reservoir– Other conditions same as first generation

• Third generation model– Daily timestep in decision model– StateMod natural flows

Overview of 2nd Generation Model

• Computer Modeling Tools• Basic Model Data Input• Model Adjustments and Operating Rules• Model Verification and Calibration• Simulation Scenarios and Model Runs

Computer Modeling Tools

• RiverWare – Generic hydrologic modeling tool that allows the user to build basin specific models.

• MS Excel and Basic Programs are used to post-process monthly model output to find daily flows necessary to compute flow statistics

Riverware Workspace

Overview of 2nd Generation Model

• Computer Modeling Tools• Basic Model Data Input• Model Adjustments and Operating Rules• Model Verification and Calibration• Simulation Scenarios and Model Runs

Basic Model Data Input• Reclamation estimated Natural flows (1970 -1993)

• Extended Natural Flows 1929 to 1969 using a combination of spatial and temporal disaggregation methods.

• Historical or Baseline Non-Agricultural Depletions.

• Agricultural demands were calculated from acreage, crop water demand and efficiencies

• Reservoir Evaporation is calculated using Reclamation’s Upper Colorado Region methods.

• Reach gains and losses are computed as water balance closure terms

• Data came from Colorado, New Mexico, BIA, USGS, Reclamation & other published sources

Overview of 2nd Generation Model

• Computer Modeling Tools• Basic Model Data Input• Model Adjustments and Operating Rules• Model Verification and Calibration• Simulation Scenarios and Model Runs

Model Adjustments

• Off-stream depletion adjustment to natural gains and losses.

• Return Flow Lag Adjustment

• Corrections Due to Missed Colorado Depletions in Natural Flow Calculations

• CRSS Data correction in the Natural Flow extension

Operating Rules

• San Juan - Chama Project• Pine River Project (Vallecito Reservoir)• Navajo Reservoir (historical and future rules)• Florida Project (Lemon Reservoir)• La Plata River Compact• Mancos Project (Jackson Gulch Reservoir)• Animas-La Plata Project (Lake Nighthorse)• Simplified Criteria

Overview of 2nd Generation Model

• Computer Modeling Tools• Basic Model Data Input• Model Adjustments and Operating Rules• Model Verification and Calibration• Simulation Scenarios and Model Runs

Model Verification and Calibration

• Verification Model (1970-93)– Actual depletions and reservoir releases

– Check for correct model configuration

– No rules are involved

• Calibration Model (1970-93)– Non-shorted depletions and computed diversions

– Adds rules that operate reservoirs, and compute depletions and shortages.

Navajo Reservoir Inflow

Overview of 2nd Generation Model

• Computer Modeling Tools• Basic Model Data Input• Model Adjustments and Operating Rules• Model Verification and Calibration• Simulation Scenarios and Model Runs

Depletion Base

• Irrigated acres and depletions• Ground water storage and return flow lagging• Non Irrigation depletions• Transmountain diversions• Animas-La Plata Project

RIP Flow Recommendation Simulation

• Navajo release decision tree– Navajo peak release hydrographs– Conditional selection of release hydrograph– Average base release implied in hydrograph volumes– Navajo release center date

• Minimum allowable release• Maintain target base flow • Avoid Reservoir Spills• Avoid Induced Shortage

Simulation Scenario for Project Analysis

1. Request is made for a model run with a new or changed project depletion which is provided by the project proponent through the approved SJRIP process.

2. Modeler examines the project data to see if it is compatible with the model and there is sufficient provided data.

3. If it is compatible then the SJBHM is configured.

4. Model is Run.

Simulation Scenario (cont.)

5. Test against minimum reservoir level, minimum release, minimum base flow.

6. Post-process data to generate flow statistics if Step 5 passes

7. Daily flows at Four Corners are used to evaluate the flow statistics.

8. If the flow criteria are met then the process ends and the results are reported.

If criteria are not met -In consultation with project proponent:

• Iterate by adjusting release parameters until statistics are met.– Minimum available storage, target space, weighted outflow etc.– Or manipulate project operation.

• Adjustments to the project demand may be necessary.

• If flow criteria are still not met, the results are provided to the project proponent for consultation with FWS.

Qualifications and Objections

• State of New Mexico– Use of the Modified Blaney-Criddle Methodology– Use of the Northwest Unit Assessment Study– Not to be used for anything but flow recommendation analysis

• Other interested Parties– Documentation of the CRSS Data

Rationale for the Transition from Gen II to Gen III

• The Gen II model cannot make a release decision based on past conditions (limitation of post-processing)

• Data set not easily extended beyond 1993• Concerns with natural flows• No return flow lagging

• Operates on a daily timestep• Allows inclusion of past statistics in release decisions• Can make release adjustments on the fly• Hydrologic period more easily extendable• More closely simulates actual operation• Compatible with and linked to Colorado flow model• Based on improved natural flows• Capable of lagging return flows• Better simulation of base flows

• Mix of RiverWare and StateMod Hydrologic Models

SJBHM – 3rd Generation Overview

SJBHM Gen III Modeling Steps

Compute Naturalized Flows Using StateMod

Compute Colorado Baseline Water Supply Using StateMod

Operate SJC By All Operating Criteria and Compute Forecasting Data Using RiverWare Migration Model

Disaggregate Monthly Data to Daily Data Using RiverWare Models

Operate RiverWare Daily Decision Model

Gen III Natural Flow Computations

• StateMod Advantages– Automatic management of missing data

– Use of Mixed Stations Model to extend natural flows

– Ability to spatially distribute flows to ungaged areas

Migration Model (full Basin)

Daily Decision Model

Gen III – General Operations

• StateMod Baseline Operation– Water rights solver– Modified Direct Solution Algorithm– Standard operations– Special operations – La Plata Compact– Establishes water supply for most Colorado water users

• San Juan Chama• Animas La Plata Project

– Normal operations– Mitigation Operations

• Navajo Normal Operations– NIIP– Downstream Demands– Fill and Not Spill– Flood Control

SJBHM – RIP Operations

• Baseflows – 7 Day running average of 500 cfs at combination of gages

– Three Gage

– Two Gage Maximum

– Two Gage Minimum

• Peak Releases– Event types targeted toward primary criteria

– Decision tree used to make and track decisions

– Can use previous statistics to make release decision

– Can adjust release based on in-year condition

SJBHM RIP Operations

• Third Generation Enhancements– Ability to look back at actual events rather than releases

– Prioritization of target events

– More frequent decision dates

– Inclusion of Animas forecast in decision process

– Alternative adjustments (in lieu of nose adjustments)

– Release curtailments or extensions based upon realized conditions

Interpreting the Results

Discussion on the Precision and Accuracy of the

Model(s)

Model Precision:The ability of a measurement to be consistently

reproduced

• Simulation models are typically very precise

• Given a set of inputs, the results are predictable and every drop of water is accounted for.

• Output precision is to the nearest acre-foot

• If a criteria is missed by 1 acre foot once in 65 years, it is deemed missed.

• The precision sometimes leads to a false sense of the model accuracy.

Model Accuracy: The ability of a measurement to match the actual value of the quantity being measured .

• Model accuracy limited by the accuracy of:– Input data– Model assumptions– Ability to simulate complex interactions

• For Example:– Gage accuracy for the best gages is 95% ( ±5%)– Historical gage record sometimes show impossible water

balance– Assumptions such as return flow lag are approximations– Future hydrology may not maintain statistics of historical

hydrology

Implications to Model implementation

• Precision Issues– All water projects should be accounted for as precisely as

possible in the model, regardless of size• Small Projects should be included in minor depletions accounting,

particularly when depletion patterns are not well defined.

• Projects with adequate definition and sufficient magnitude should be explicitly modeled.

– It may be advisable to output rounded values in the model to avoid the implication of unrealistic accuracy. (e.g. 500 cfs or 510 cfs rather than 504 cfs or 507 cfs)

Implications to Model ImplementationAccuracy Issues:

• Critical to develop criteria and evaluate projects with the same model, configuration and time series

• Important to understand the accuracy of the basis for the criteria in interpretation of the assessment of meeting the criteria.

• Most error in the model is random, with equal probability of over and under prediction.

• Computation of an error band around the model output is problematic, but cannot be better than about ± 5%.