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WHFS SITE-SPECIFIC MODEL
STATUS REPORT

Project Background
At the HIC/DOH conference in August 2000, a concept for a rudimentary WHFS site-specific river forecast model was presented, based on a suggestion from Lee Larson. MBRFC offered to assist with the development of this concept and is working with Russ Erb to create the application.

Lee's primary objective was to create a simple model which could generate forecasts for significant short-fused rainfall events. It would replace headwater tables and possibly other local site-specific tools now used by many WFOs. He also wanted to develop an algorithm which would generate a complete forecast hydrograph, not just a crest stage forecast . It also needed to use data, parametric information, and functions which were either already available within the current scope of AWIPS (particularly WHFS and the IHFS_DB arenas) or could be easily created.

When Lee studied the parameters and data required to create and use a headwater table, he produced a simple relationship that could be used to calculate storm total hydrograph as a function of flood discharge (from flood stage and rating curve), peak unit-hydrograph discharge, the mean areal precipitation (MAP) over the headwater area, and the current headwater guidance value (FFH). All of these parameters and data are already in the WFO AWIPS hydro system or can easily be installed.

For each site, a "flood" hydrograph is determined by multiplying the ordinates of the unit-hydrograph by a factor of flood discharge divided by the peak unit-hydrograph discharge. For an event, the ordinates of this hydrograph are multiplied by a factor of the total MAP divided by the appropriate FFH value. The resultant discharge time-series can then be processed with existing AWIPS applications to generate a forecast stage hydrograph and generate a product if required.

This concept has a few limitations and restrictions, most notably the use of a total event MAP value. This approach assumes that there is a linear relationship between MAP and runoff for a given FFH value, which is not the case. The degree of non-linearity is affected by the initial soil moisture conditions and how easily the site is flooded, as reflected in the magnitude of threshold runoff. Although these variables might be estimated for each site, it is difficult to determine the degree of non-linearity for various conditions.

It also diminishes the flexibility of the model to account for significant temporal distribution effects. As the duration increases, the potential for distribution errors increases significantly. It does not account for "multiple" events, cases where several hours of significant rainfall occurs, followed by a short break and another round of significant rainfall, all within the same FFH valid period. This is a problem often encountered using headwater tables.

Additionally, the duration of the MAP and FFH values must always be consistent. Since the MAP value used in Lee's equation is an event "storm total", and FFH guidance is normally issued as 1, 3, and 6 hour values, the MAP value used must likewise be "fixed" at 1, 3, or 6 hours . Events with durations other than these will not be processed as well as we would like.

A further issue is that the duration of the unit-hydrograph must also be consistent with that of the FFH and MAP values used. This means that multiple unit-hydrographs would have to be defined for each site, increasing the development, installation, and support effort.

Proposed Model Concept
In an attempt to offset the above-mentioned limitations and philosophical considerations, the approach we are taking bases all calculations on an hourly time-step. This requires only a single 1-hour unit-hydrograph per site. This significantly reduces development time and minimizes installation and support requirements. More importantly, it can be applied to events with a variety of durations and precipitation patterns without requiring any special decisions by the user.

The parametric information required for each site is...
* Basin boundary definition for the headwater site
* Flood stage
* Rating curve
* 1-hour unit-hydrograph

The data required for each event are...
* Current headwater guidance value
* Mean areal precipitation time-series for the headwater area

The model will consist of 3 modules or functions. The first will retrieve parametric information and data for selected site and determine the MAP time-series to be applied. The second will calculate the discharge time-series for the event. The third will display the stage hydrograph derived from the discharge time-series and provide the user with tools to adjust the hydrograph and generate desired products.

Current constraints with WHFS and IHFS_DB limit this initial version of the model to a single basin boundary per headwater site. For now, locations which may actually have multiple zones contributing to the site will have to use a composite total basin boundary definition. It is recognized that this limitation adversely affects spatial and temporal distribution of the runoff which degrades the performance of the model to a degree. Later versions should take advantage of more detailed basin boundary definitions.

Precipitation Processing Function
1. The user will select a headwater location from an option menu of valid location Ids or by clicking on the site in a WHFS display.

2. The flood stage, rating curve, 1-hour unit-hydrograph and most recent 1-hour headwater guidance (FFH) value will be retrieved from the IHFS_DB for that location. If any of these data are missing it will not let the user continue with this location.

3. The flood discharge will be calculated from the rating curve using the flood stage.

4. The 1-hour threshold runoff will be calculated by dividing the flood discharge by the peak unit-hydrograph discharge.

5. The user will press a "Calculate MAPs" pushbutton which will then calculate a 1-hour MAP time-series on the fly (there is no MAP preprocessor like NWSRFS) from the gridded Stage II data in the IHFS_DB. The default duration of the time-series will begin at the starting hour of the valid period of the FFH being applied and end at the top of the current hour. The time-series will be displayed for the user to view and/or edit. Individual hourly MAPs may be adjusted, a multiplier may be applied to the entire time-series, or user-specified hourly values may be added to extend the time-series (such as adding a few periods of QPF).

6. The user will then be able to press a pushbutton to run the "Discharge Time-series Generation Function". The threshold runoff value, 1-hour unit-hydrograph, 1-hour FFH value, and 1-hour MAP time-series will be passed to the discharge time-series generation function

Discharge Time-series Generation Function
1. Based on the FFH and threshold runoff values, a "soil moisture indicator" (SMI) will be calculated using rainfall-runoff equations from the MBRFC API model.

2. Beginning with the first hour of the MAP data, hourly accumulated MAP will be computed. Applying the SMI value using the equations from step 1, hourly accumulated runoff will be determined, and incremental hourly runoff values can be calculated.

3. The discharge time-series will be calculated using incremental runoff and the 1-hour unit-hydrograph ordinates and returned to the main program.

Hydrograph Analysis Function
1. The 1-hour discharge time-series will be displayed as a 1-hour forecast stage hydrograph

2. The user will be able to adjust the forecast hydrograph. See discussion later in this report about potential features in this step.

3. The forecast stage hydrograph will be saved in the IHFS_DB as a forecast time-series along with the input parameters which produced this output. Existing WHFS tools will then be used to create and transmit any desired product.


Obviously there will be requirements for user options to adjust or modify the discharge time-series computed by the model. These adjustments may be in the form of direct changes to the time-series, input data modifications, or returning to the precipitation processing function for re-analysis of the MAP data.

Since this is an "event-oriented" model, there are certainly issues surrounding the ability to account for initial flow conditions at the start of the discharge time-series computation. How to manage "baseflow" and hydrograph "continuity" from one FFH valid time period to the next are foremost questions.

Potential adjustments are a user-defined baseflow definition and/or point-and-click time-series changes similar to IFP hydrograph adjustment. Options to specify the date/time of the FFH value to be used or modifying the FFH value itself may be useful. Beta testing will confirm a few initial ideas, but continued monitoring for enhanced requirements after operational implementation will most certainly be required. Current Project Status
Russ is creating the first and third modules and we are formulating the middle section. Russ has made great progress with his 2 components and we have completed the testing of the hydrograph generation equations and validated that they yield very good results. Not only does this approach provide a better "crest" forecast than headwater table use, but the entire hydrograph can be used to determine other attributes of the event that may not be available from current methods. Information such as times to reach or fall below flood stage and duration of flood, can be easily determined.

The intent is to complete the coupling of all 3 modules within the next few weeks and install a test version on the WFO system here in Pleasant Hill. We can then test both the physical functionality as well as the hydrograph generation accuracy for future events in their HSA. We can also run tests for any case studies in other RFC's if we are provided the appropriate site parameters and event data. More about that later.

Keep in mind that this model is not intended to produce forecast hydrographs with the accuracy or completeness of a full-scale model like NWSRFS. It is intended as a local PRELIMINARY, assessment for significant events while keeping installation and support requirements at an acceptable level. It definitely performs better than the headwater table approach, and if it will provide at least similar results as other site-specific models used around the country, it is a step in the right direction. We realize that not all RFC's provide site-specific guidance to their WFO's and that not all WFO's have a site-specific model need. However, if you are interested in helping shape this initial site-specific approach, please feel free to comment.


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