SR SSD 2001-10

Technical Attachment

The National Basin Delineation Project and FFMP

Scientific Services Division
NWS Southern Region

(Ed. Note: Much of the following is transcribed and slightly edited from a Web site created and maintained by Ami Arthur, CIMMS/NSSL Research Associate, combined with information provided by Jeff Zimmerman, NWSH Office of Services/Hydrologic Services Division.)

Delineation Project Overview.

Last year NSSL entered into a cooperative project with the NWS to delineate river and stream basins nationwide from 1-arc-second (~30-meter) digital elevation data for use in the AWIPS Flash Flood Monitoring and Prediction Program (FFMP). Forecasters will use the FFMP, which will be included in AWIPS Build 5.1, to assist in their flash flood warning decisions. The FFMP is based on the Areal Mean Basin Estimated Rainfall (AMBER) Program developed by Bob Davis at WFO Pittsburgh and Paul Jendrowski at WFO Honolulu. An overview of the AMBER algorithm can be found at The algorithm accumulates areal average values of precipitation derived from the WSR-88D Digital Hybrid Scan Reflectivity product on a basin level, and updates these accumulations with each new volume scan of the radar. The accumulations are then compared to Flash Flood Guidance (FFG) values or to user-defined threshold values to assess the flash flooding potential.

Updated information about FFMP, including links to references and sample products, can be found on the FFMP Website at

The basin delineation is underway and is being performed using the ArcView and ARC/INFO Geographic Information Systems (GIS). It is based on the National Elevation Dataset (NED) produced at the EROS Data Center (EDC). The NED is a seamless mosaic of the highest-resolution, best-quality elevation data for the U.S. and it is available for use in this project through a Memorandum of Agreement between the NWS and the EDC.

In most areas, the NED will be adequate for delineating basins on a flash flood scale, roughly 10 square miles and smaller, but in particularly flat terrain or other areas where elevation data are not adequate, the NED will be modified based on streams in the National Hydrography Data Set. This data set was produced by the U.S. Geological Survey and the Environmental Protection Agency based on streams and water bodies in the USGS DigitalRaster Graphics (DRG) files and the EPA River Reach File Version 3-Alpha (RF3).

Basins are being defined and assigned unique IDs using the Pfafstetter system of basin delineation and codification (Verdin 1997). This system defines basins according to the natural topographic control of drainage and the topology of the river network. The Pfafstetter codification scheme makes efficient use of digits such that basins smaller than 4000 square kilometers can be uniquely identified for the entire globe using five-digit IDs. In addition, hydrologic ordering and relationships can be easily extracted from the Pfafstetter IDs, adding versatility to the basin data set. When the project is complete all offices will be provided with a number of results, all of which become integral to FFMP, including the following:

NED Base Data
Flow Direction Grids
Flow Accumulation Grids
Synthetic Stream Coverage
Basin Coverage
Associated Attribute Data

Status of Basin Delineations.

The table on the next page shows the status of NSSL's basin delineation work for Southern Region offices. Their efforts, and the order in which offices are listed in the table, largely reflect the impact flash floods have on WFO areas - and in turn the expected relative utility of the FFMP. In other words, we are most eager to being testing the utility of FFMP/AMBER at offices such as Austin/San Antonio (New Braunfels) and Morristown, and less so at Key West. Regardless, it is expected that over the coming months delineations will be complete for all Southern Region offices. The term "blind pass complete" in the table means the basic delineation work has been completed, up to the point of making any necessary customization changes (see below).

Training for Basin Customization.

As part of implementing the FFMP software the offices will likely need to work with the delineated basins to customize them for best use. This will require some training and familiarization, and plans at this time are to provide a short course during FY2002, most likely at COMET, to be attended by one individual from each office. The course will prepare the attendees to use ArcView in order to perform customization and localization at their offices. Those tasks are defined as follows:


- Manual definition/division of urban areas.
- Manual definition/division near lakes or reservoirs.
- Manual editing of basin boundaries to correct minor inaccuracies.
- Aggregation to form larger basins.
- Modification of attribute data (such as stream name).


- Adding or replacing shapefiles and attribute files as necessary.
- Remapping radar bins to basins and rerunning aggregation scripts
(to ensure each basin contains at least one bin).

WFO Basin Delineation Status

(as of March 2001)

New Braunfels, TX EWX In Progress
Morristown, TN MRX Blind Pass Complete
San Juan, PR SJU
Albuquerque, NM ABQ Blind Pass Complete
Atlanta, GA FFC Blind Pass Complete
Nashville, TN OHX Blind Pass Complete
Little Rock, AR LZK In Progress
San Angelo, TX SJT In Progress
Norman, OK OUN In Progress
Tulsa, OK TSA Blind Pass Complete
Jackson, MS JAN
Houston, TX HGX In Progress
Fort Worth, TX FWD In Progress
El Paso, TX EPZ In Progress
Slidell, LA LIX
Midland, TX MAF In Progress
Birmingham, AL BMX In Progress
Amarillo, TX AMA In Progress
Shreveport, LA SHV
Lubbock, TX LUB In Progress
Tallahassee, FL TAE In Progress
Memphis, TN MEG In Progress
Lake Charles, LA LCH
Corpus Christi, TX CRP In Progress
Mobile, AL MOB In Progress
Jacksonville, FL JAX In Progress
Brownsville, TX BRO
Tampa Bay, FL TBW
Melbourne, FL MLB
Miani, FL MFL
Key West, FL EYW

Localization refers to changes made to files in AWIPS as a result of customization, so the term should be reserved to mean performing AWIPS localization via mainScript.csh and other AWIPS specific tools to set up D2D, SCAN, etc. for a specific office. So for FFMP, anything that has to be run to set up FFMP and D2D with the office's basin data set.

Redelineation of basins is another issue. In some cases it may be desirable to redelineate some basins to handle specific flash flood problem areas that are not located at the branch of two streams. For example, bridges which cross streams tend to be dangerous flash points where cars may be swept into a creek. Low water crossings are similar trouble spots. In such cases only the rainfall (ABR) falling in the portion of the watershed upstream of the trouble spot contributes to flash flooding at that point. These examples would be the primary reason for any redelineation into smaller stream segment.

A problem is that redelineation requires ArcView with Spatial Analyst, or ARC/INFO, and aside from providing relatively expensive software to the offices, considerable additional training and expertise will be required for its use. Fortunately, redelineation should not be a major issue immediately. It will take some time for forecasters to become familiar with FFMP, and it will take even more time to determine if smaller basins or delineation to a specific point would improve the algorithm's utility for a WFO. The actual number and scope will not be known until the national data set is fielded and examined by local experts.

Following is a draft outline for the 3-day course follows.

FFMP Basin Customization Course Content (Draft)

Potential Audience: Service Hydrologist, Hydrology Focal Points or GIS Focal Points

Prerequisite: Completion of ArcView's "Quick start tutorial", or working knowledge/equivalent experience. The tutorial is found in chapter 2 of the "Using ArcView GIS" manual.

Day 1 Flash flood forecasting
- Basins vs grid points
- Operational benefits & scientific reasoning

FFMP Overview

Amber functionality

AWIPS - Display/Demonstration

GIS Basin Delineation Process

NSSL Basin Delineation Process (scripts/AML)
- Discuss different files on CD ROM

ArcView Software, scripts, and extensions

Day 2 Downloading Basins from NWSH
- NOAA2 Filer
- Dedicated Radar
- Uploading updated basins

FFMP Localization Issues

Introduction to WFO basins

Customization Part I:
- Working with ArcView editing techniques- attribute tables
- Working with local governments for stream naming, topo maps, etc...
- Using other high resolution GIS spatial data, i.e. schools, hospitals, street maps, etc...

Workshop: Editing attribute tables & adding other spatial data sets

Day 3 Customization Part II:
- Shapefile editing techniques
-EROS editing tools

Workshop: Urbanization and general editing of shapefiles

When to re-delineate, working with grids