the national basin delineation project
TRANSCRIPT
E VOLUTION OF THE NEED FOR A FLASH-FLOOD-SCALE BASIN DATA- SET. Floods and f lash f loods, on average,
cause more fatalities and property damage annually
than any other weather-related hazard. It is possible,
though, to reduce these fatalities and property dam-
age through accurate and timely warnings. On the
flooding scale, the National Weather Service (NWS)
River Forecast System and the Advanced Hydrologic
Prediction Service, which is currently being imple-
mented at the River Forecast Centers, provide a suite
of hydrologic products that are used to determine
flooding potential and assist in flood-warning deci-
sions. Tools to assist NWS forecasters in warning
decisions for shorter temporal scale f lash f loods,
however, have historically been lacking. A f lash-
flood guidance product (Sweeney 1992) is issued by
the River Forecast Centers on a county-by-county
basis, or, in some areas, on a gridded basis. Based
on current soil moisture conditions, these guidance
values are estimates of the average amount of rain-
fall for given durations required to produce f lash
flooding in a particular county. Until recent times,
flash-flood guidance, radar precipitation estimates,
and rain gauge amounts were the primary sources
of information upon which forecasters at the NWS
Weather Forecast Offices (WFOs) could base flash-
flood-warning decisions.
To address the need for a more hydrologically
based flash-flood-monitoring tool, the Flash Flood
Monitoring and Prediction System (FFMP) has been
developed by the NWS. The first step to determin-
ing whether or not a rainfall event will cause flash
flooding is to pinpoint where the rain is falling and
how it will travel over the terrain. Based on radar pre-
cipitation estimates, the FFMP system monitors the
amount of precipitation that has fallen in a particular
basin, bridging a significant gap by enabling forecast-
ers to interpret precipitation estimates hydrologi-
cally. This system is the offspring of the Areal Mean
Basin Estimated Rainfall (AMBER) program (Davis
and Jendrowski 1996), which was developed at the
Pittsburgh, Pennsylvania, WFO during the past two
decades. A brief history of AMBER follows.
THE NATIONAL BASINDELINEATION PROJECT
BY AMI T. ARTHUR, GINA M. COX, NATHAN R. KUHNERT, DAVID L. SLAYTER, AND KENNETH W. HOWARD
A fruitful collaborative effort has produced a seamless digital dataset of flash-flood-scale
basin boundaries that is an important component of flash-flood-warning decision tools
in forecast offices throughout the nation.
AFFILIATIONS: ARTHUR, COX, KUHNERT, AND SLAYTER—Coopera-tive Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, Oklahoma; HOWARD—NOAA/National Severe Storms Laboratory, Norman, OklahomaCORRESPONDING AUTHOR: Ami T. Arthur, National Severe Storms Laboratory, 1313 Halley Circle, Norman, OK 73069E-mail: [email protected]:10.1175/BAMS-86-10-1443
In final form 28 May 2005©2005 American Meteorological Society
1443OCTOBER 2005AMERICAN METEOROLOGICAL SOCIETY |
From 1983 to 1992, the first version of the AMBER
program computed average basin rainfall rates and
accumulations from Radar Data Processor II pre-
cipitation estimates. These calculations were made
for relatively small basins with areas on the order
of 10–50 km2, which had been manually delineated
from hundreds of U.S. Geological Survey (USGS)
7.5-min topographic quadrangle maps. The Radar
Data Processor II polar grid was drawn on these
topographic maps to determine which radar bins
corresponded to each basin. The basins and their cor-
responding radar bins were stored in input files and
formed the basis of the AMBER calculations.
Over time, AMBER was upgraded to take advan-
tage of the higher spatial and temporal resolution
of the Weather Surveillance Radar-1988 Doppler
(WSR-88D) precipitation estimates in the Digital
Hybrid-Scan Ref lectivity (DHR) product (Fulton
et al. 1998). Years of operational use at the Pittsburgh
WFO, and many case studies, indicate that among the
most significant factors in flash-flood initiation are
rainfall intensity and duration (Davis 1998). Flash
floods generally occur over small areas during short
time periods. The ability to monitor smaller areas in
AMBER over short intervals of time would potentially
improve the accuracy and timeliness of flash-flood
warnings. For these reasons, and because the higher
spatial resolution of the WSR-88D made monitoring
smaller basins feasible, further subdivisions of the
existing basins at the Pittsburgh WFO were manually
delineated from the USGS topographic maps.
As the news of AMBER spread throughout the
NWS community, forecasters at many WFOs were
anxious to implement the system in their own opera-
tions. In response, the NWS decided to include an
AMBER-like functionality in its FFMP system (Smith
et al. 2000). Implemented in the NWS Advanced
Weather Interactive Processing System, FFMP is a
set of tools that forecasters at the WFOs can use to
monitor precipitation in individual basins prior to a
flash flood. To make this happen, however, a data-
set of flash-flood-scale basins was needed for every
WSR-88D radar coverage area in the country. In the
interest of consistency, and considering the time
constraints faced by WFO staff, it was agreed that
the delineation of small basins should be done at a
central location rather than by each WFO.
The National Severe Storms Laboratory (NSSL)
became involved with the AMBER program in the
late 1990s when it developed and tested a proto-
type AMBER display. As part of this work, NSSL
delineated basins for two WSR-88D coverage areas
associated with the Sterling, Virginia, and Tulsa,
Oklahoma, WFOs. This was accomplished using the
ESRI ArcView (version 3.1) geographic information
system (GIS) software, and was based on USGS three-
arc-second (approximately 90 m)-resolution digital
elevation models (DEMs). Shortly thereafter, NSSL’s
help was enlisted by the NWS to delineate f lash-
flood-scale basins for the United States, including
Puerto Rico and Guam, in support of FFMP. FFMP
would require a separate GIS basin dataset for each
radar coverage area in the country. In late 1999, NSSL
embarked on the National Basin Delineation Project
(NBDP) to fill this need.
The NBDP was shaped by specific requirements
within FFMP, as well as lessons learned from years of
operational use of FFMP’s predecessor, the AMBER
program, at the Pittsburgh WFO. The requirement that
most significantly shaped the NBDP was that, in most
regions of the country, AMBER and FFMP calcula-
tions are considered to be most meaningful for basins
that are a few square kilometers in area. To accurately
delineate such small basins using GIS, it is necessary to
have a high-quality, high-resolution DEM, such as the
USGS one-arc-second (approximately 30 m)-resolution
National Elevation Dataset (NED).
To obtain access to the NED, NSSL formed a part-
nership with the USGS Earth Resources Observation
Systems (EROS) Data Center (EDC). As part of the
agreement, NSSL assisted the EDC in the initial stage
of its Elevation Derivatives for National Applications
(EDNA) dataset (Kost and Kelly 2001). Derived from
the NED, EDNA is a multilayered raster and vector
dataset, which includes f low direction, f low accu-
mulation, shaded relief, slope, catchments, synthetic
streamlines, and other layers. Because several of these
layers were also necessary for the NBDP, the partner-
ship was developed to eliminate duplication, while
maintaining consistency in elevation derivatives
used among federal and state agencies. The result
was a seamless dataset of elevation derivatives that
has fulfilled the requirements of both EDNA and
the NBDP.
The purpose of this paper is to explain how the
NBDP was carried out and describe the flash-flood-
scale basin dataset that was produced. Details of
the general basin delineation procedure, techniques
applied in regions of the country requiring special
attention, and assembly of the final FFMP datasets are
provided. Potential errors in the dataset and regions
that may require additional attention are described,
and examples are provided on how individual us-
ers can edit or customize the dataset to maximize
its utility in flash-flood-warning decisions or other
applications.
1444 OCTOBER 2005|
NBDP PROCEDURE. The procedure for delin-
eating basins in a GIS begins with a digital elevation
dataset. For the NBDP, basin and stream datasets
were derived from the one-arc-second (approximately
30 m)-resolution raster elevation data in the NED using
the ESRI ArcInfo (Version 7.2.1) and ArcView (Version
3.2) GIS software. Produced at the USGS EROS Data
Center, the NED is a seamless mosaic of the highest-
resolution, best-quality elevation data for the United
States. It is based primarily on USGS 1:24,000-scale
DEMs and is assembled using data-filtering tech-
niques to ensure edge matching, fill sliver areas of
missing data, and minimize artifacts from the source
DEM production methods. In addition, the NED is a
dynamic dataset that is updated periodically by the
EDC as new and improved source
DEMs become available.
Scripts were specially tailored by
EDC scientists to fulfill the require-
ments of both the EDC’s EDNA
project and NSSL’s NBDP. These
scripts automated the delineation
process using the drainage analysis
tools in the ESRI ArcInfo GIS soft-
ware. ESRI’s drainage analysis tools
include the algorithms described
by Jenson and Domingue (1988),
which have been widely adopted as
a standardized method for extract-
ing topographic structure from
DEMs. The drainage analysis tools,
in conjunction with the specially
tailored scripts, automated the basic
delineation procedure to the greatest
degree possible. Certain customized
tasks and the compilation of the final
FFMP datasets, however, required
significant manual intervention.
Basic delineation procedure. The first
step in any basin delineation project
is to define the area of interest and
a “processing unit” or manageable
area for processing. For this project,
the simplest approach would have
been to designate each radar cover-
age area as a processing unit. The
large file size of the NED tiles, cou-
pled with the computing resources
available at the time, prohibited this,
however, and it was necessary to
define smaller processing units that
could be mosaicked for each radar
coverage area. The processing units selected were the
USGS eight-digit hydrologic unit basins (Seaber et al.
1987), also known as cataloging units. The catalog-
ing units for all of the United States were derived
from the 1:500,000-scale USGS state base maps and
were published in a series beginning in 1974. They
are widely accepted as a standardized base for use
by water-resources organizations in planning and
describing water-use activities and in geographically
organizing hydrologic data. The average cataloging
unit area is approximately 4200 km2, which is a rea-
sonable area to process using the NED. In addition,
a 25-km buffer was applied to each cataloging unit
to ensure adequate terrain data for correctly defining
basins along the boundaries (Fig. 1, step 1a). Thus, the
FIG. 1. Schematic diagram of the National Basin Delineation Project delineation process.
1445OCTOBER 2005AMERICAN METEOROLOGICAL SOCIETY |
buffered cataloging unit became the processing unit
for the NBDP (Fig. 2).
For each buffered cataloging unit, the necessary
1° latitude × 1° longitude NED tiles were mosaicked
to create a single NED grid (Fig. 1, step 1b), and
were clipped to eliminate data values outside the
processing unit (Fig. 1, step 1c). The resulting NED
grid was projected (Fig. 1, step 1d) from geographic
coordinates to an Albers Equal Area Conic Projection
based on the North American Datum 1983 (NAD83),
with units in meters (Alaska NED tiles were based
on the North American Datum 1927). Because the
algorithms that were used work in Cartesian space,
projecting the elevation grid before delineating basins
ensures the accurate measurement of lengths and
areas and the correct derivation of flow direction.
After projecting to the Albers Equal Area Conic
Projection, it was necessary to “hydrologically con-
dition” the NED grid (Fig. 1, step 2). Unconditioned
elevation grids often contain sinks, which are one or
more grid cells with an undefined drainage direction
(all neighboring cells are a higher elevation). Most
automated basin-delineation algorithms function
with the assumption that every grid cell must be part
of the drainage network. Grid cells with undefined
drainage direction are not part of the network and,
therefore, must be modified so that all grid cells are
connected, and ultimately flow to one or more outlet
points. The elevations of the sink grid cells are in-
creased in an iterative process until all grid cells have
a defined drainage direction. In reality, these sinks
may exist either as small-to-large “natural sinks” or
as “closed basins,” such as those in the Great Basin
region. A closed basin is defined as a basin with no
outlet point. The Great Salt Lake and Sevier Lake in
Utah are well-known examples of closed basins. To
maintain their hydrologic integrity, closed basins
and natural sinks were processed separately in a
customized step.
From the hydrologically conditioned DEM, a grid
of flow direction values was derived (Fig. 1, step 3).
Each grid cell in the conditioned DEM was assigned a
value, representing one of the eight principal flow di-
rections, based on the greatest slope to its neighboring
cells. The logic described by Jenson and Domingue
(1988) is used to assign f low direction when the
“greatest slope to” a neighbor is shared by multiple
cells. The resulting values define the flow direction or
drainage network grid from which synthetic stream-
lines and basin boundaries are extracted.
Before the synthetic streamlines and basin bound-
aries can be defined, it is necessary to know the up-
stream area contributing drainage to specific points.
The upstream area, known as flow accumulation, can
be calculated by totaling the number of upstream grid
cells f lowing into a specific grid cell. Based on the
drainage network represented in the flow direction
grid, the flow accumulation value for each grid cell
is calculated and stored in a flow accumulation grid
(Fig. 1, step 4). In this grid, cells with values of zero
represent topographic ridges, and cells with high
values are areas of concentrated flow where stream
channels likely exist in the real world. Thus, the flow
accumulation grid is integral to defining the synthetic
stream network.
The synthetic stream network is defined by ap-
plying a minimum flow accumulation threshold to
the flow accumulation grid (Fig. 1, step 5). Grid cells
with a flow accumulation equal to or greater than the
threshold define the stream network. Selection of an
appropriate minimum flow accumulation threshold
is critical to any basin delineation project and is
dependent on many factors, such as topography and
rainfall regime. For projects with a large scope, such
as the NBDP, it is difficult to determine a single flow
accumulation threshold that is appropriate for all ar-
eas. For the sake of simplicity, it was decided that the
FIG. 2. USGS cataloging units for the conterminous United States and the NED grid of elevation data for buffered cataloging unit 15060202 (upper Verde River basin in Arizona). Buffered cataloging units were the processing units for the NBDP.
1446 OCTOBER 2005|
FIG. 3. Synthetic stream network for cataloging unit 15060202, defined from a minimum flow accumulation threshold of 5000 grid cells (4.5 km2).
best approach for the NBDP would be to select a single
threshold for the nation that was small enough to
fulfill the needs of those areas that would benefit from
monitoring very small basins on the urban scale. This
threshold would be applied nationwide to develop the
base set of delineated basins and streams. For those
areas that would benefit from a higher threshold,
the base set of delineated basins and streams could
be filtered based on a more appropriate threshold,
resulting in larger and fewer basins to monitor. This
postprocessing of the base set of delineated basins and
streams could be accomplished relatively easily by
staff at the local forecast offices using GIS tools. Using
this approach, and based on various case studies and
forecasters’ experiences using AMBER (Davis 1998), a
minimum flow accumulation threshold of 5000 grid
cells (4.5 km2) was selected for the NBDP. Thus, all
points along the NBDP synthetic stream network are
guaranteed to have a flow accumulation value of at
least 5000 grid cells (a minimum upstream drainage
area of 4.5 km2). This small threshold produced a
dense synthetic stream network (Fig. 3).
Basin outlet points were derived from the f low
accumulation and synthetic stream network grids
(Fig. 1, step 6). Located just before the junction of
two or more stream segments or “reaches,” an outlet
point is defined as the grid cell with the maximum
flow accumulation along each stream reach.
Finally, based on the outlet points and flow direc-
tion grid, boundaries for the small basins correspond-
The synthetic stream network den-sity is an important consideration in any basin-delineation project. Because it directly affects the general size and number of delineated basins, the appropriate density will vary depending on the scope and purpose of the project. The density of the synthetic stream network can be varied by adjusting the minimum-flow-accumulation threshold. The minimum-flow-accumulation thresh-old is a filter for the flow accumula-tion grid that identifies grid cells as either being “stream” or “non-stream.” All grid cells with flow accu-mulation values greater than or equal to the threshold become part of the synthetic stream network. Thus, a high-flow-accumulation threshold produces a sparse synthetic stream network, resulting in basins that are larger in area and fewer in number. A low-flow-accumulation threshold produces a dense synthetic stream
MINIMUM FLOW ACCUMULATION THRESHOLDnetwork, resulting in basins that are smaller in area and greater in number.
A common misconception is that the minimum-flow-accumulation threshold determines the minimum basin area. In actuality, the minimum-flow-accumulation threshold deter-mines the minimum upstream drainage area (not individual basin area) of the delineated basins. In headwater basins, the upstream drainage area is the same as the basin area. Thus, the NBDP headwater basins are guaranteed to have both an upstream drainage area and an individual basin area of at least 5000 grid cells (4.5 km2). Headwater basins are the only basins for which the minimum-flow-accumulation threshold determines the minimum basin area. Moving downstream, each subsequent basin will have a greater upstream drainage area than the previ-ous basin. The individual basin area of each subsequent basin will vary widely though. An NBDP interbasin (a basin
defined between the inflow points for two tributaries along a main stem) may have an area as small as one grid cell (0.0009 km2).
It is important to note that the syn-thetic stream network does not neces-sarily represent the existing stream channels. A synthetic stream reach, or segment of a stream correspond-ing to a specific defined basin, may not have a corresponding stream channel in the real world, and vice versa. The synthetic stream network is simply the result of applying the minimum-flow-accumulation threshold as a filter to the flow accumulation grid, which itself was derived from a hydrologi-cally conditioned digital representation of terrain. Regardless of whether or not there is a corresponding stream channel in the real world, the synthetic stream network generally represents the direction that water will travel over the terrain.
1447OCTOBER 2005AMERICAN METEOROLOGICAL SOCIETY |
ing to every reach in the synthetic stream network
were defined (Fig. 1, step 7). These basins are different
from the large drainage areas that typically come to
mind when the term “basin” is used. The definition
of a basin in hydrology often refers to an area that
drains many stream reaches or a network of streams.
The small basins defined by the NBDP, however,
represent either headwater or local flow into a single
stream reach, depending on whether the reach is a
first- or higher-order reach. Because the synthetic
stream network for the NBDP was relatively dense,
the defined basins were relatively small (Fig. 4). Most
were on the order of several square kilometers, which
is essential for meaningful FFMP-averaged basin
rainfall calculations in many areas of the country.
Coastal areas. Because the NED includes elevation
values over portions of the ocean, automatically de-
lineated basins and synthetic streams that are derived
from the NED extend into the ocean in coastal areas
(Fig. 5a). Using these basins in FFMP would result
in erroneous average basin rainfall values, because
precipitation estimates in those portions of the basins
that are over the ocean would be included in the cal-
culations. To ensure accurate FFMP-averaged basin
rainfall calculations in coastal areas, basin and stream
definitions must end at the coastline. For the NBDP,
ocean polygons from the National Hydrography Da-
taset (NHD) were used to distinguish between ocean
elevations and nonocean elevations in the NED. The
1:100,000-scale NHD is the result of a cooperative
effort between the USGS and the U.S. Environmen-
tal Protection Agency (EPA), combining elements
of the USGS digital line graph hydrography files for
spatial accuracy and the EPA Reach File version 3.0
for attribute information. Along the coasts, NED
grid cells corresponding to the NHD ocean polygons
were eliminated, and basins and streamlines were
defined strictly from the nonocean NED elevations.
This produced basins and streamlines that end at the
NHD-defined coastline (Fig. 5b), which will result in
more accurate FFMP calculations.
Closed basins and natural sinks. As previously dis-
cussed, all sinks in the NED grid were filled to ensure
FIG. 4. Basin boundaries defined for the 5000-grid-cell-threshold synthetic stream network (cataloging unit 15060202).
FIG. 5. (a) NED-derived basins and streams for Cape Cod (cataloging unit 01090002). Without coastline data, irregularly shaped basins and streams extend into the ocean. These irregularly shaped basins and streams also appear in areas adjacent to and coinciding with lakes, reservoirs, and swamps. (b) Cape Cod basins and streams derived from NED data clipped with NHD coastline data. After clipping, the basins and streams do not extend further than the coast.
1448 OCTOBER 2005|
of assistance in projects from local to national scales.
For more information about the NBDP dataset and
its accessibility, please contact the corresponding
author.
ACKNOWLEDGMENTS. The Nat iona l Basin
Delineation Project was funded by NWS Eastern Region
Headquarters, NWS Office of Climate, Weather, and
Water Services, NWS Office of Hydrologic Develop-
ment, and NOAA–University of Oklahoma Cooperative
Agreement NA17RJ1227. The authors extend special
thanks to Thomas Donaldson, Peter Gabrielsen, Thomas
Graziano, Michael Mercer, and Mary Mullusky for their
support of this national effort. We are also grateful to
the reviewers, Dr. J. J. Gourley, Dr. David Schultz, and
Greg Stumpf, for their constructive comments and help-
ful suggestions.
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