1. INTRODUCTIONThe United States weather services program has been providing river and flood forecasts for the public since 1890 (Stallings and Wenzel, 1995). These forecasts have typically been provided for the protection of life and property in the vicinity of rivers. They have also been used for the assessment of extended drought scenarios. The public, private and other federal agency sectors are now insisting upon the expanded use of coupled National Weather Service (NWS) hydrologic/meteorologic analyses and products for the economic and environmental management of our Nation's water resources. To meet this need, the NWS intends to capitalize upon its modernization in remote sensing, data automation and advanced hydrologic/hydrometeorologic modeling. The Advanced Hydrologic Prediction System (AHPS) (Fread, 1995; Fread, et al., 1995) meets this objective and is to be operationally demonstrated beginning with the snowmelt/spring-runoff season of 1997 on the Des Moines River Basin, Iowa. AHPS will provide information regarding the relative uncertainty of hydrologic variables (i.e., river stage and discharge) with lead times out to several weeks and months. Currently, the NWS provides river stage forecasts out to 1, 2, or 3 days. Therefore, AHPS will greatly improve the capability of water facility and emergency managers to take timely and effective actions that will significantly mitigate the impact of major floods and droughts. The system will also provide products to water resource managers for the sustainable use of water. This discussion focuses on the modernized operations of the North Central River Forecast Center (NCRFC), Chanhassen, Minnesota and the Weather Forecast Office (WFO) in Des Moines, Iowa. In addition, the discussion elaborates on the need for new advances in informational displays and user requirements.
2. TRADITIONAL PRODUCTS AND SERVICESSince the first River Forecast Centers (RFC) were established, in the mid-1940s, hydrologic forecasts have been issued for a flood crest at a given point on a river or stream. Each flood forecast was based on a pre-determined flood stage where damage would occur in the reach surrounding the forecast point. These forecast products were issued as single point values for an expected time when the crest might occur. The watershed areas affecting these forecast points were generally large, on the order of thousands of square miles. In later years, the number of hydrologic forecast points increased and the hydrologic (rainfall-runoff) forecast areas have become increasingly smaller, 200 to 400 square miles as a rule in the upper Midwest.
Within a short period of time, after crest forecast products were introduced, most RFCs began issuing outlooks for spring snowmelt flooding and extended mainstem (navigable rivers) forecasts. These outlooks are prepared during February and March, accounting for winter snowfall accumulation and normal precipitation during the melt period. Using normal melt patterns, these data are used as input to produce one stage value that considers only the present water equivalent with no additional precipitation. A second stage value considers both the present water equivalent and normal precipitation for the melt period. Extended mainstem forecasts are prepared for the towing industry up to 28 days into the future. These are recession type forecasts that do not consider future rainfall beyond the Quantitative Precipitation Forecasts (QPF) incorporated in daily forecasts.
The RFCs also produce an "Advisory Product" for the WFOs that contains rainfall guidance values that the WFO meteorologist uses in heavy rain situations. The values are provided for counties and specific locations with known flood potential. The guidance value considers current hydrologic conditions and represents the amount of rainfall necessary where runoff response will cause river and streams to reach bank full stages. Ice advisories are issued periodically as conditions warrant on tributaries and weekly for mainstem rivers during cold seasons.
In order for the NWS to provide the best service, interagency coordination is on-going between the NWS WFOs and RFCs, with other "need-to-know" agencies (local, state and federal) requiring real-time knowledge and technical expertise in dealing with hydrologic conditions and trends. While the public release of forecasts and outlooks, via the news media, is the sole responsibility of the WFOs. These NWS offices are also the public contact point for information and specific explanations of hydrologic events.
3. EXPANDING REQUIREMENTS FOR WATER RESOURCE MANAGEMENTThe Great Flood of 1993 heightened the awareness of the devastation and strife that flooding will impose upon society and the environment. With this heightened awareness, our Nation's subsequent floods have reminded us that extreme events will continue to occur. Furthermore, structural modifications to the riverine environment and flood proofing of flood prone areas are not always viable solutions. Therefore, as society continues to experience population growth and people choose to live by the water, we have an ever increasing need to improve our predictions to support flood and drought mitigation activities.
As we proceed into the 21st century, individuals have matured in awareness to live in harmony with the environment; thus, we find the recently enhanced use of the term, "sustainable development" (President's Council on Sustainable Development, 1996). Within this context, the importance of water resources for all hydrologic regimes is increasing in importance. For example, in many regions of the world, including locations within the U.S., we find competing demands for the allocation of water among its users (i.e., fisheries, irrigation, hydropower and municipalities). This increase in demand for water looms as a National problem that requires improved water quantity forecasts for sustainable use.
AHPS products with extended forecast lead times (up to several months) will greatly improve the Nation's capability to take timely and effective actions that will significantly mitigate the impact of major floods and droughts. The system will also provide products to water resource managers for the evaluation of water availability and allocation for water supply, navigation, hydropower, ecosystems and agriculture. To meet these requirements, an integrated advanced hydrologic forecasting system will be developed which builds upon: partnerships with other water cooperators (federal, state, multi-state, quasi-governmental, and private sector organizations); the NWS infrastructure including the 13 RFCs and the NWS RiverForecast System (NWSRFS), a very large software system used by RFC hydrologists to produce forecasts of discharge or stage time series at selected locations (approximately 4,000 along the Nation's rivers); and the NWS Modernization which is providing NWS RFCs with Advanced Weather Interactive Processing System (AWIPS) hardware, a powerful suite of networked computer workstations with graphic capabilities. The modernization of the NWS is also providing national coverage with approximately 140 WSR-88D Doppler radars which provide the basis for multi-sensor, high resolution (space and time) precipitation estimates.
AHPS completes this integration for advanced hydrologic forecasting by providing the pathway to: (1) make critical software enhancements to the NWSRFS; (2) develop a National Oceanic and Atmospheric Administration (NOAA) Hydrologic Data System (NHDS); (3) increase the use of short-to-long-range weather and climate forecasts within the NWSRFS through appropriate hydrometeorological coupling algorithms; (4) effectively calibrate and field-implement the advanced hydrologic/hydraulic models within the NWSRFS; (5) implement a snow estimation and updating system (SEUS) which provides gridded estimates of snow water equivalent; and (6) provide more timely, accurate, and informative forecast products to government and quasi-government water and emergency managers and to private sector intermediaries who provide value-added services to specific industries (Figure 1).
4. NWS MODERNIZED OPERATIONS4.1 North Central RFC, Chanhassen, MN
The NCRFC is now staffing both a hydrometeorologic and a hydrologic function, covered from 600 AM to 1000 PM each day. These extended hours provide greater access for WFOs and cooperating agencies to talk with an operational forecaster. Within the context of these expanded services, benchmark forecast locations have been selected for the daily issuance of 3-day forecasts.
Additionally, the NCRFC has six Government Development Platforms (GDP) which provide workstation processing of hydrologic models and interactive graphic displays. This advanced technology provides the NCRFC with pre-AWIPS capabilities creating an environment for development of AHPS activities in addition to meeting day-to-day forecast requirements more timely and effectively.
4.1.1 Hydrometeorologic Function at NCRFC
The Hydrometeorologic Analysis and Support (HAS) function at the NCRFC has enhanced access to NWS modernization technologies. These technologies provide a suite of data quality control procedures, the ability to incorporate QPF from the WFOs, radar produced mean areal precipitation values taken from the WSR-88D radar network, and a wide selection of informational displays and products for users. Operational enhancement examples include: the use of displays to compare hydrologic/hydrometeorologic data from multiple sources, the use of 24 hour QPFs in hydrologic forecasts, taking advantage of NWS Stage 3 precipitation-processing of radar sensed data on a 4 x 4 km grid (Figure 2), and being able to create graphs and displays that can be viewed by other NWS offices and interagency interests over Internet. In addition, a discussion of current hydrometeorologic conditions is routinely available for WFOs and other users.
4.1.2 Hydrologic Function at NCRFC
The AHPS operational era is complemented with the implementation of several hydrologic software technologies of the NWSRFS. As a software system (over 400,000 lines of computer code), NWSRFS consists of many programs which are used to perform all steps necessary to generate streamflow forecasts. The system includes the Calibration System (CS), the Operational Forecast System (OFS), the Interactive Forecast Program (IFP), and the Ensemble Streamflow Prediction (ESP) System. The CS performs the tasks needed to process historical hydrometeorological data and to estimate model parameters for a specific basin. The OFS provides for the processing of input data and the performance of requested hydrologic and hydraulic simulations. IFP allows a forecaster to interactively combine their hydrologic expertise with computational tasks in producing a forecast hydrograph. ESP enables the hydrologist to make extended probabilistic forecasts of streamflow and other hydrologic variables. Specific NWSRFS modeling technologies being implemented by the NCRFC for the AHPS operational demonstration include: the Sacramento Soil Moisture Accounting Model (SAC-SMA) and the Dynamic Wave Operational (DWOPER) streamflow routing model.
The SAC-SMA is a physically based rainfall-runoff model that accounts for percolation characteristics in order to simulate and transpose runoff response into stream-flow. The distribution of available moisture through the soil mantle follows the approach where the mantle is divided into an upper and lower zone. Each of these two zones has a tension and a free-water component. Calibration of SAC-SMA model parameters, between the two zones, results in percolation characteristics which are used to determine the capacity of each zone. In addition, a frost index is contained as a function of the SAC-SMA model. The effect of the frost index is to account for any significant impacts frozen ground may have on the amount of runoff resulting from a snowmelt and/or rainfall scenario. The operational version of SAC-SMA used at the NCRFC uses approximately twenty parameters to control the direct runoff, surface runoff, interflow, and baseflow.
The DWOPER routing model, is being implemented at the NCRFC on selected reaches of the Mississippi and Illinois Rivers and on local reaches of the Des Moines and Raccoon Rivers in the city of Des Moines. DWOPER is a one-dimensional implicit, unsteady flow routing model that computes stage and flow at various locations along a specified reach. It lends itself to a single river or an entire river system. The accommodation of various boundary conditions make it flexible. Also accommodated are irregular cross sections located at unequal distances along a single multiple-reach river or several such rivers having a dendritic configuration. DWOPER allows for the variability of flow resistance, within river reaches, along with the change in stage or discharge. Model features include weir-flow channel bifurcations to simulate levee overtopping, varying lateral inflows, wind effects, bridge effects and off-channel storage. Time steps are chosen based on computational requirements. DWOPER iscomputationally efficient and its application for simulating slowly varying flood elevations of several days duration is effective.
The union of AHPS and AWIPS technologies has improved the tasks given to forecasters to assess watershed flow events with greater precision. This is occurring at the NCRFC, not only through the implementation of the advanced physically based models (i.e., SAC-SMA and DWOPER), but also with the implementation of analytical tools which provide interactive displays in a workstation environment. Two examples of these tools are the Interactive Calibration Program (ICP) and IFP.
ICP provides the NCRFC the capability to visualize the Manual Calibration Program (MCP3) output by means of a graphic interface. This provides hydrologists with various options, using control features, to invoke the MCP3 so that specific output is generated for the ICP. For example, an individual is able to view graphical displays illustrating the hydrograph and hydrologic model parameter relationships, check on the effect of parameter values used, edit the control deck, and to rerun MCP3 to test revised parameter values. This technique has provided significant improvement in the calibration process of the SAC-SMA at the NCRFC.
The NWSRFS IFP consists of two main applications. The first is the spatial display of the RFC area which allows subareas to be run for a chosen time window. This display and its associated menu system allow for the control of the NWSRFS's OFS run sequence. The second application automatically performs a series of hydrologic computations within NWSRFS. In this mode, model output is displayed allowing the hydrologist to interactively adjust the model's parameters. The NCRFC is evaluating IFP capabilities, its computational run-time on the GDPs and the interaction required for forecast preparations.
A further enhancement to the NCRFC hydrologic function includes implementation of the new National Weather Service Flash Flood Guidance System (NWSFFGS). This system can utilize WSR-88D gridded precipitation data and current model hydrologic states to derive guidance values of varying durations. Guidance is being produced for 1,3, and 6 hour durations; however, the capability to produce 12 and 24 hour values also exists. The NWSFFGS implicitly accounts for both liquid precipitation and snow effects by using current hydrologic information from the rainfall-runoff models in NWSRFS. In addition, NWSFFGS has the capability of providing real time guidance based on the latest hourly precipitation estimates.
4.2 Weather Forecast Office, Des Moines, IA
The Des Moines WFO's Hydrologic Service Area (HSA) has seen many advancements in recent years, particularly within the Des Moines River Basin which covers about one-third of the HSA. These advancements include enhancements in hydrological data collection and analyses. Furthermore, extensive data records are now available from many locations in the area. The record includes a worst-case scenario that manifested itself during the summer of 1993, when unprecedented flooding occurred over much of the basin.
Hydrometeorologic data collection within the Des Moines River Basin occurs through the cooperation of federal agencies (i.e., the U.S. Geological Survey (USGS), the U.S. Army Corps of Engineers (USACE), and the NWS). These agencies have developed an extensive array of stream and precipitation gaging sites. Within the area defined for AHPS implementation, there are 35 stream gage sites, 22 of which are NWS river forecast points. The majority of these sites are equipped with Data Collection Platforms (DCPs), Limited Automatic Remote Collectors (LARCs) and various other telemetry devices. Retrieval of hydrologic data from these devices is possible at time intervals of 1 hour or less. The Des Moines River Basin also features a dense network of some 100 cooperative weather observers. These observers provide key assistance in the supply of backup river observations and ground-truth rainfall reports. These reports supplement digital precipitation estimates generated locally by the WSR-88D. A high degree of involvement and coordination in the flood warning process is also provided by emergency management officials within the basin, particularly over northern headwater areas.
Another key input to the river forecast process includes the QPF provided, twice daily, to the NCRFC by Des Moines WFO forecasters. The QPF provided in the morning covers 24 hours divided into 6 hour time periods. The evening QPF is an update of the last two 6 hour periods of the morning forecast.
5. AHPS DEMONSTRATION5.1 Des Moines River Basin
The Des Moines River Basin was selected as the initial implementation site where the record flooding occurred during the summer of 1993. The basin totals 14,450 square miles of rolling agricultural terrain which originates in extreme southern Minnesota and flows southeasterly across central Iowa joining the Mississippi River a few miles below Keokuk, IA (Figure 3). A major tributary to the Des Moines River is the Raccoon River lying just west of the Des Moines River and joining the Des Moines River in the city of Des Moines, Iowa. The topography in the headwaters of the basin was formed mainly from outwash plains from the glaciers to the north.
A further description of the basin may be found in its hydrologic record. Good records of streamflow exist for the basin, for 30 years or more, with excellent opportunities for runoff, routing, and reservoir model evaluation. Notable years for instantaneous record peak flows on selected tributaries include June 1990, July 1982, April 1969, and April 1965.
5.2 User Response for Advanced Hydrologic Information
Customers/users of NWS hydrologic forecast products include: emergency managers, the news media, water supply managers, reservoir managers, other federal, state, and local water resource officials, as well as individuals in the private sector interested in water resource management. In order to bring these users into the AHPS era team, the Des Moines WFO provided them a questionnaire inquiring whether advanced hydrologic informational products being developed by the NWS will meet their future anticipated requirements.
A summary of the questionnaire responses indicates a great deal of interest in receiving river forecasts out to 10 days, during both flood and relatively benign river conditions. The responses also indicated an interest in obtaining forecast products, in addition to a crest date/time, e.g., graphical information, which provide further evaluation and understanding of existing text products. The majority of these users will have access to the Internet by the time of AHPS implementation.
5.3 Suite of Advanced Hydrologic Products
During fiscal year 1995, the NWS began AHPS implementation activities within the upper Mississippi River basin through a significant commitment by personnel of the NCRFC, the Regional Hydrologist and other staff of the NWS Central Region Headquarters, the Des Moines WFO, and the NWS Office of Hydrology. The AHPS short-term implementation goal is to demonstrate an operational long-term probabilistic forecast system for the Des Moines River Basin by the spring of 1997. AHPS functionality and associated implementation activities of the NWS include:
One enhanced product type planned to be available for the AHPS demonstration project on the Des Moines River Basin in the spring of 1997 will be probabilistic hydrologic products (Figure 1). These probabilistic forecasts will convey to the user the likelihood of a variety of flow scenarios. In addition, coupled with the ESP Analysis and Display Program (ESPADP) software will be utilities that permit the user to verify the effectiveness of the forecast over selected periods.
ESPADP will enhance forecast evaluation in several ways. First, the ease with which the analyses can be accomplished will lead to greater use of the ESP forecasting technique. Second, by providing a variety of interactive graphical displays the forecaster will be able to understand more easily and completely the probabilities generated by an ESP forecast. Finally, by providing more attractive and easily read graphical outputs, NWS cooperators will find it easier to utilize forecast products. ESPADP analyses will include forecast probability hydrographs, historical probability hydrographs, automatic forecast adjustment to account for model error, hydrometeorological analyses to link past and present years, and forecast verification. It is through these means of forecast evaluation and verification that the forecaster will be able to provide a measure of confidence to any specific forecast. This information is essential for water resource and emergency managers as they integrate a multitude of data into a single decision. In this manner, modernized hydrologic forecast products will not only provide the forecaster with a mechanism to impart critical hydrologic forecast information, but will also provide water resource managers risk analysis products for alternative hydrologic scenario decision making.
These new products are a huge step forward from the previous ESP output format. In the past, forecasters were forced to review tabular output for a limited period, they now will be able to easily review the expected flows over a range of future time periods. In addition, it will be possible to pass graphical displays on to decision makers, thus enhancing their understanding of the state of the hydrologic system. This is the type of easy to read detailed forecast products that emergency management officials and on-site disaster managers requested after the Great Flood of 1993.
Once AHPS has been implemented for the Des Moines River basin, its implementation in other NCRFC River Basins will occur. As an increase in resources become available, AHPS implementation can be expedited within the Mississippi Basin as well as early implementation in one or more additional basins in the United States.
6. FURTHER ADVANCEMENTS - LOOKING INTO THE FUTUREThe NWS issues forecast and warning products in near real-time to a wide variety of other federal, state, and local agencies. This information dissemination involves coordination issues, data exchange, product formatting and user services. Major cooperating agencies involved in these activities include the Federal Emergency Management Agency (FEMA), the USACE, and local and state emergency management agencies.
In most cases during the Great Flood of 1993, and our Nation's subsequent flood events, the coordination activities among the cooperating agencies were reported by users as exceptional. In the aftermath of many of these events, especially the 1993 flood, many meetings and conferences were held which provided recommendations involving hydrologic forecasts and information exchange. These recommendations for product and service enhancements include the dissemination of forecasts via interactive graphic displays and improved communication with cooperating agencies via teleconferencing.
Discussions continue on how the NWS offices can best serve other cooperating agencies in a flood scenario. This need is being addressed in the following manner: have an individual from the WFO on site in a Emergency Operations Center (EOC) at a USACE District office; field office staff dedicated to answering media inquiries; provide graphic product displays for local media's use; make use of NOAA Weather Radio (NWR), etc. These activities are leading to future needs and requirements as the Illinois Governor's Conference (Illinois, 1994) suggests they are "an integral ingredient to a more holistic view of floodplain management" and flood fighting capabilities.
Additional enhancements to NWS communication during a flood event are being made through implementation of the advanced technologies that AHPS will provide. The AHPS short-term design features include probabalistic long-range outlook hydrographs for stage, discharge and flow volume that have accompanying indicators of uncertainty. Long-term design features include gridded estimates of snow-water equivalent, soil moisture and flash flood guidance, and probabalistic flood inundation mapping capabilities. New product requirements are also being investigated in coordination with NWS WFOs and other federal agencies, such as USACE and FEMA.
With added flexibility and graphical displays available from ESPADP, a variety of envisioned forecast products and graphical displays, beyond those planned for the AHPS demonstration, can be generated by the RFCs. Investigations into possible products are underway to assess cooperator interest and system development including data input, data storage, software design and product formats. For example, several end users of NWS long range stage forecasts have requested that National Centers for Environmental Prediction (NCEP) long-lead meteorological outlooks be included in these long-range stage forecasts. Inclusion of such forecasts is happening through the development of new scientific algorithms, the definition of new input data streams, new data storage facilities and the development of appropriate displays of the forecast data.
Two long-term goals of the AHPS project are to develop the capability to generate inundation maps based on the probabalistic stage forecasts and to provide gridded estimates of a variety of state variables describing the hydrologic system. The first goal of inundation mapping will be to provide local emergency managers a clear definition of the areas that are likely to experience flooding and the depth of flooding likely to occur at a given location. Also, by coupling the mapped areas with probabalistic forecasts, emergency managers will be able to evaluate the risk of future inundation and recommend appropriate actions for the threatened areas. Therefore, the translation of the forecast river stage to actual locations on the ground will be more readily communicated with these types of inundation maps. The second goal, gridded estimates of hydrologic variables, will provide forecasters and users with an in depth view of the natural system. Gridded estimates of the snow cover and soil moisture will enable forecasters and managers to evaluate the probability of flooding in more localized areas.
7. SUMMARYNOAA has the national responsibility to provide river, flood and drought forecasts and outlooks to the public for the protection of life and property. These products provide a multitude of water resource managers with reliable hydrologic data and understandable displays which are utilized in the decision making process for the economic and environmental well-being of the Nation's water resources. AHPS will greatly improve and enhance NOAA's capability to provide more reliable, timely and accurate forecasts to meet these priorities. NOAA's customers, becoming better informed and equipped, will be in a stronger position to meet their objectives. Floodplain and water resource management decisions, as a result, will become more confidently determined than previously possible. Thereby, as these advanced products come on line, the implementation of AHPS will elevate NOAA's leadership role in fostering economic gains for environmentally sound decision making for all streamflow regimes and climatic scenarios.
8. ACKNOWLEDGMENTSThe authors wish to recognize those NWS employees dedicated to making the modernized NWS a reality which as an organization is technically equipped to meet the challenge of forecasting extreme and harsh natural disasters. Special appreciation is extended to Mike DeWeese and Steve Kruckenberg for their technical assistance and to Craig Edwards and Preston Leftwich for their review of the manuscript. Very special thanks go to Kathy Patterson for her patience, assistance and preparation of the final manuscript.
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