Toward a New Advanced Hydrologic Prediction Service (AHPS) (2006) / Chapter Skim
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3 Scientific and Technical Aspects of AHPS
3 Scientific and Technical Aspects of AHPS
Pages 31-50
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From page 31... ...
AHPS is predicated on these hydrologic models; therefore, hydrometeorological inputs, generally, and precipitation inputs, specifically, strongly influence AHPS hydrologic forecasts. AHPS hydrometeorological inputs consist of quantitative precipitation estimations (QPEs)
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From page 32... ...
QPFs are issued routinely by the NWS as a part of meteorological forecasts. Intuitively, QPFs would be useful in producing hydrologic forecasts, but there is no strong evidence that QPFs are being used that way to extend flood and streamflow predictions.
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From page 33... ...
. However, more updates are needed to: incorporate advanced hydrologic science into NWS models; provide forecasts for two weeks or further into the future; create information that is useful to assess risk to flooding; and provide other products and services promised by AHPS.
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From page 34... ...
Functions of NWSRFS Hydrologic Models NWSRFS allows hydrologists to combine the appropriate models in a manner that is descriptive of the basin, the available data, and the forecast products desired. NWSRFS hydrologic applications include conceptual rainfall-runoff models, snow, and the Antecedent Precipitation Index (API)
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From page 35... ...
The API empirical method estimates the amount of surface runoff that will occur in a basin from a given rainstorm based on an index of moisture stored within a drainage basin before a storm, physical characteristics of the basin, time of year, storm duration, rainfall amount, and rainfall intensity. Limitations of and Research Needs for NWSRFS Hydrologic Models The current lumped conceptual hydrologic models used by the NWS and in the NWSRFS are functional and relatively accurate (Reed et al., 2004)
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From page 36... ...
: overall, the lumped hydrologic models performed better than, or slightly inferior to, a well calibrated distributed model. The NWS initiated DMIP to assist with the distributed modeling choice for AHPS, but DMIP Phase I research results do not clearly delineate whether AHPS products should use lumped or distributed models.
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From page 37... ...
Another limitation for DMIP and similar exercises is the lag time between research and implementation into AHPS operations that may be too long to be effective. Protracted intervening time may inhibit AHPS developers from fully exploiting new modeling capabilities and achieve the AHPS goal of producing advanced hydrologic products.
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From page 38... ...
Although the lumped model outperformed distributed models in more cases than distributed models outperformed the lumped model, some calibrated distributed models can perform at a level comparable to or better than a calibrated lumped model (the current operational standard)
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From page 39... ...
The NWS should continue efforts to improve and expand AHPS calibration capabilities, accelerate the rate of transfer of the latest calibration techniques into its operational AHPS-NWSRFS version, and conduct adequate training of modeling personnel to ensure appropriate and consistent use of the new techniques. Like calibration advancements, model parameterization improvements in coupled climate/ hydrologic models need to be transferred into AHPS operation.
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From page 40... ...
Like their meteorological counterparts in the NWS, hydrologic forecasters should run hydrologic models primarily in a "model only" mode, make forecast adjustments with post-processing techniques, and document these adjustments for future verification purposes. Elements of current real-time hydrologic data assimilation are recognized as problematic.
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From page 41... ...
Knowledge of the current climatology is often used to weight the years of simulated streamflow based on the similarity between the climatological conditions of each historical year and the current year. More specifically, the ESP component blends together historical temperature and precipitation data sequences and deterministic meteorological forecasts to form ensemble inputs to hydrologic models that produce forecasts out to several months (Werner et al., 2005)
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From page 42... ...
Pre-processor Verification 1 (Med-term) Verification 2 Verification 3 CY 2004 CY 2005 CY 2006 CY 2007 Initial Condition Post-Processor 1 Hydraulic Model Model Parameters (Ensemble)
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From page 43... ...
Unlike meteorological forecasts, little is known about hydrologic forecasts and actual river forecast skill. The assumption that forecasts have been improving over time may not be true because it is not documented whether the forecasts have skill over simple persistence forecasts.
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From page 44... ...
Furthermore, site specific hydrologic conditions may require alternative or even multiple models and techniques to be applied at a particular location in order to optimize forecast skill. The NWSRFS is a barrier to the AHPS goal of producing more accurate products by incorporating advanced hydrologic science into the NWS model.
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From page 45... ...
Like with hydrologic models in NWSRFS, the step-by-step testing and evaluation plans are not defined for transitioning to distributed modeling for flash-flood forecasting. The NWS plans to implement a national verification program for its flash-flood monitoring and prediction (FFMP)
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From page 46... ...
Second, the modeling capability needs improvements for AHPS to produce more accurate products and incorporate advanced hydrologic science in the NWS hydrologic models. Also noted in the modeling evaluation was a gap between the state-of-the-art hydrologic modeling capabilities and those used in AHPS product development.
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From page 47... ...
· Like their meteorological counterparts in the NWS, hydrologic forecasters should run hydrologic models primarily in a "model only" mode, make forecast adjustments with post-processing techniques, and document these adjustments for future verification purposes. · AHPS developers should consider automating the OFS/IFP component of the AHPS-NWSRFS and develop a systematic mechanism to include new research results and error analysis techniques into the operational OFS/IFP component.
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From page 48... ...
1973. National Weather Service River Forecast System: Snow Accumulation and Ablation Model.
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From page 49... ...
2004. Draft: Advanced Hydrologic Prediction Service (AHPS)
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From page 50... ...
2005. Incorporating medium-range numerical weather model output into the Ensemble Streamflow Prediction system of the National Weather Service.
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