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Variational Assimilation (VAR) Presented by: Jerry Nunn Hydrologist In Charge West Gulf River Forecast Center October 28, 2003
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What is VAR? Software that produces initial estimates of soil moisture content for SAC-SMA. Based on variational assimilation popularized by widespread use in meteorology and oceanography. Solves large-scale inverse problems in dynamic systems via least squares minimization.
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Purpose for VAR Given the latest streamflow observation, MAPX and MAPE estimates, and the initial SAC-SMA state variables: – VAR adjusts the SAC-SMA state variables and the MAPX and MAPE estimates such that the resulting model simulation matches the observed. Allows the forecaster to start with more accurate SAC – SMA states for forecast runs. Possible replacement of the current OFS operation; SS-SAC.
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VAR VAR is currently a stand-alone program that runs independent of OFS, however, it uses data stored within OFS. – VAR ingests time series data from OFS in a text format. – VAR also utilizes special decks that include the SAC- SMA parameters and unit hydrograph definitions. – After VAR completes, it outputs time series data that can be ingested back into OFS.
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Schematic of VAR
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VAR The software was originally developed on the AWIPS – HP-UX system. July, 2003 - Software also made available in Linux format. WGRFC currently utilizing the Linux version to capitalize on the faster processing speed.
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WGRFC VAR Project Implemented in 2002 – Original project consisted of 6 headwater basins located in eastern Texas Expanded project in 2003 – Added 10 basins throughout Texas This was done to account for the varied hydrologic conditions throughout the WGRFC area.
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Project Locations
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Basin Criteria Basins selected for the VAR project had to meet the following criteria: – Headwater basin – Basin defined in operational model – Hourly stage data available Candidate basins also needed to vary in the following areas: – Basin Size (50 – 700 square miles) – Time to peak in the unit hydrograph (4 –80 hours) – Diverse basin shape and geomorphology
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WGRFC Project Requirements Hourly stage data Hourly MAPX Climatological MAPE Calibrated 1 hour SAC-SMA parameters 1 hour unit hydrograph OFS segment definition
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Project Steps Preparation of historical data sets for observed stage and mean areal precipitation. (WGRFC) Development of climatological MAPE using Calibration Assistance Program (CAP). (OHD) Development of 1 hour unit hydrograph. (OHD) Refinement of SAC-SMA parameters for use in a 1 hour model. (OHD) Creation of all required OFS components. (WGRFC)
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AB_OPT Software Created by OHD for assistance in the implementation of VAR. – Available in both a HP-UX and Linux version. Ingests historical stage and MAPX data. Software estimates long term biases in MAPX and MAPE climatology based upon the historical data. Uses these biases to develop a 1 hour empirical unit hydrograph and refined SAC-SMA parameters.
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Project Analysis Project Output – 1 hour VAR adjusted time-series (VAR) – 1 hour SAC-SMA time-series (OFS) Without modification With forecaster modification – 6 hour operational SAC-SMA time-series (OFS) All time-series mentioned above are produced and archived hourly for analysis purposes.
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VARPOST Forecast Group Output
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Future Enhancements Expansion to non-headwater locations – Routed flow locations – Reservoirs Inclusion of unit hydrograph theory in the VAR process – Currently VAR will only make modifications based on rainfall, evaporation and sac-sma parameters.
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Possible Applications for VAR Enhancement to RFC Models – Provides a way to maintain the SAC-SMA contents. – Better initialization of SAC-SMA contents will allow for improved forecasts. Distributed Modeling Development of new software for use by field offices – (i.e. AB_OPT)
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Possible Applications for VAR (cont.) Flash Flooding – Site Specific – Unattended Models
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Contacts Please contact the following people with any questions relating to VAR: – OHD Team : Dongjun SeoDongjun.Seo@noaa.gov Lee CajinaLee.Cajina@noaa.gov – WGRFC Team: Bob CorbyRobert.Corby@noaa.gov Tracy HowiesonTracy.Howieson@noaa.gov
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