Background
The Threshold Runoff
Plan describes the motivation for this work by putting it in the
context of the Flash Flood Guidance program. An article accepted
for publication in the Journal of Hydrology, Carpenter et. al. (1999),
summarizes previous work on threshold runoff.
For flash flood guidance purposes, threshold runoff for a basin is the
stream flow at "flood stage" divided by the unit hydrograph peak flow for
that basin as shown in Equation 1.
Equation 1
R = Qf / qpR* A
In Equation 1, R is threshold runoff [inches], Qf is the flood discharge
in cubic feet per second [cfs], qp is the unit hydrograph peak flow in
cfs per unit area in square miles [cfs/sq mi], and A is the subbasin drainage
area [mi2].
The task at hand is to determine threshold runoff values for thousands
of small watersheds across the country. Nationally available GIS
databases can be used to define basins and compute basin characteristics
that will help in the estimation of Qf and qp.
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USGS digital elevation models can be used to define basin boundaries and
to derive topographic attributes such as drainage area, longest flow path
length, and longest flow path slope. 3 arc-second digital elevation
models have been available nationally for many years. The National
Operational Hydrologic Remote Sensing Center resampled a 3 arc-second DEM
to produce a 15 arc-second dataset of the United States that is more manageable
than the complete 3 arc-second data set on a national basis. For
the AV-ThreshR project, these 15 arc-second data were projected and resampled
to produce a 400 m DEM for each RFCs.
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The 2 year or 5 year return period peak flows (Q2 or Q5) can be used as
a surrogate for flooding flow. Regression equations to estimate these
quantities in all 50 U.S. states and Puerto Rico have been published by
the U.S. Geological Survey in Water Resources Investigations Report 94-4002.
To provide software that will estimate these flood flows for thousands
of basins in the United States, an extensive underlying GIS parameter database
is required. A contractor, Michigan Technological University, has
collected GIS layers that can be used to derive the required parameter
information and provided this data to HRL.
An ArcView application has been devloped to work with these datasets
and automatically calculate the required basin parameters. The major
improvements in this system over the initial GRASS-based threshold runoff
software are that a national terrain and regression parameter database
has been prepared and the software functionality has been increased to
estimate many more parameters. Using the popular and user friendly
ArcView interface will make it easy for RFCs to understand, update, and
correct derived parameter values as necessary. As with the earlier
GRASS-based system, there is no national database to support selection
of synthetic unit graph coefficients (e.g. Snyder's Cp and Ct) in the initial
release of AV-ThreshR, so estimation of these parameters still requires
local knowledge.
Documentation Overview
This is an overview of the online documentation for the ArcView-based Threshold
Runoff (AV-ThreshR) system developed at the National Weather Service Hydrologic
Research Lab. The purpose of the AV-ThreshR system is to derive gridded
threshold runoff values nationwide that can be used by the Flash-Flood
Guidance (FFG) software. The AV-ThreshR system is a package consisting
of both data and software. A unique GIS database is prepared for
each of the 13 River Forecast Centers. These databases are fairly
extensive. For example, the database for MBRFC includes 27 GIS data
layers and the database for ABRFC includes 22 data layers.
The philosophy in developing this system was to complete the one-time,
data preparation tasks that require extensive GIS experience before delivering
the system to the RFCs. With the database in place, the software
functionality can be thought of as consisting of three major components:
(1) defining subbasins and computing subbasin topographic and climatic
parameters (25 parameters are required for MBRFC), (2) for each subbasin,
calculating 2 year or 5 year return period flood flows (Q2 or Q5) (uses
as surrogates for bankfull flow) from regression equations, calculating
the unit hydrograph peak Qp, and computing threshold runoff, and (3) interpolating
results to the HRAP grid and exporting to FFG.
In addition to this overview, the AV-ThreshR documentation includes
"User Documentation," "System Documentation," and "Workshop Instructions."
The user documentation is the most extensive, describing the ThreshR database,
methods, and user options in enough detail so that the user can understand
the computational procedures without needing to understand the actual code.
The user documentation also provides information about the limitations
of the input data and software. With knowledge of how the programs
work from the user documentation, the system documentation provides
more detail about the codes and database so that subsequent developers
can add to or modify the codes and update the database.
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