An Overview of Gridded Flash Flood
Guidance; A Spatially Distributed Runoff and
Threshold-Runoff Based Approach
Erick Boehmler
NERFC
6/3/2010
ER FFG Conference
Eastern Region Flash Flood Guidance
6/3/2010
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Goals of Gridded FFG Development
 Produce a flash flood guidance product at a
resolution closer in scale to basins in FFMP
that:
 Reflects the spatial variability of the physical
characteristics that impact the susceptibility of a
location to flash flooding
 Uses freely available (or easily derived) data with
nationwide coverage
 Requires minimal calibration effort
 Fits seamlessly into the existing RFC and WFO
operational flash flood warning infrastructure
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Gridded Flash Flood Guidance
 Advantages of GFFG approach are:
 Improves the spatial resolution of soil-moisture
conditions and flash flood thresholds
 Applies a spatially consistent soil-moisture
accounting model across RFC areas and
independent of RFC forecast basins
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Gridded Flash Flood Guidance
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Gridded Flash Flood Guidance
 Gridded FFG has a significant precedence
 It is an accepted and operationally utilized method for FFG grid
displays from Southern Region RFCs beginning in 2007
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Gridded Flash Flood Guidance Model
Length to
divide
Slope
OHD RDHM
Soil Moisture
Area
Static CN
Gridded Runoff
Duration
2-yr, 3-hr DDF
Dynamic NRCS
Curve Number
Gridded
FFG
How much rain to
produce runoff
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Variable
Threshold
Runoff
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Arkansas-Red Basin River Forecast Center
How much runoff
to produce flash
flooding
Soil Moisture Accounting with a Distributed
Hydrologic Model
4km gridded temperature
4km gridded precipitation
National Weather Service
Office of Hydrologic Development
Research Distributed Hydrologic Model
Sacramento
Model
Storages
4km gridded soil moisture products
6/3/2010
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Arkansas-Red Basin River Forecast Center
UZFWC
LZTWC
LZFSC
LZFPC
UZTWC
Upper Zone
Saturation Ratio
Upper Zone Saturation
Upper zone saturation, 5/1/2009
6/3/2010
Upper zone saturation, 6/1/2009
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Runoff Computation
Development of Dynamic Curve
Number
Combined to create an average (ARCII)
Curve Number grid at HRAP scale
 30m National Land Cover Dataset
1000m NRCS Hydrologic Soil Groups
Utilized upper zone saturation ratio from
RDHM output with NRCS curve number
model (NEH Part 630, chap. 10) to formulate
an automated curve number adjustment
Convex of average for wet conditions
Concave of average for dry conditions
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Soil Moisture Adjusted Curve Number
Varying Curve Numbers by
Antecedent Soil Moisture Conditions
100
90
75% Upper Zone Saturation
80
70
*
60
WET
NORMAL
50
DRY
40
30
20
10
0
0
10
20
30
40
50
60
Average Curve Number
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ER FFG Conference
Arkansas-Red Basin River Forecast Center
70
80
90
100
NRCS Curve Number Graphic
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Arkansas-Red Basin River Forecast Center
Threshold Runoff Computation
Development of Variable Threshold
Runoff (Thresh-r)
Thresh-r is estimated at the HRAP scale
Ratio of flood flow, Qs to peak flow, Qp of the unit
hydrograph.
Qs is approximated by the annual return
period flow, Q2
Qp is derived through the use of NRCS’
triangular unit hydrograph method
Adjustment for runoff anticipated within the
next 6-hours from rainfall in progress through
latest available MPE grid
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NRCS Triangular Unit-graph Variables
Known Variables
CN = NRCS Curve Number
S = Abstraction =
(1000/CN)-10
l = length to divide
tp 
l 0 .8 ( S 1)0.7
1900 y 0.5
y = average watershed slope
TR  D / 2  t p
A = drainage area
D = duration of rainfall ( 1 Hour for unit
Hydrograph)
Calculated Variables
tp= lag time (time from center of
mass of rainfall to Qp)
TR = time to Qp from beginning of rainfall
Qp = peak discharge from unit hydrograph
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Arkansas-Red Basin River Forecast Center
484* A
Qp 
TR
NERFC Area Computed Thresh-r

Qs = f (design storm,
slope, curve number)
6/3/2010
=
Qp =
f
(slope, curve number)
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Thresh-r
GFFG System FFG Calculation
0.4S  Q  Q 2  4QS
FFG 
2
S = (1000/CN)-10
6/3/2010
Q = ThreshR
ER FFG Conference
Arkansas-Red Basin River Forecast Center
Limitations of the GFFG system
 GFFG system limitations include:
 No projection of snow-melt runoff within applicable
duration of FFG value
RDHM modeled SWE 1/25 12Z
MPE 12 hours ending 1/25 12Z
SCHOHARIE COUNTY
SWE about 3 inches
Approaching rainfall
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Limitations of the GFFG system
 GFFG system limitations include:
 No projection of snow-melt runoff within applicable duration of
FFG value
GFFG system 1/25/2010 00Z
GFFG system 1/25/2010 12Z
SCHOHARIE COUNTY
00Z 3-hr FFG = 1.9
12Z 3-hr FFG = 1.8
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Limitations of the GFFG system
 GFFG system limitations include:
 No projection of snow-melt runoff within applicable duration of
FFG value
Legacy system 1/25/2010 00Z
Legacy system 1/25/2010 12Z
SCHOHARIE COUNTY
00Z 3-hr FFG = 2.0
12Z 3-hr FFG = 1.4
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Limitations of the GFFG system
GFFG system limitations include:
CN adjustment for runoff response is
consistently overestimated during cool season
months
RDHM models upper zone tension water at
capacity
Upper soil zone contents to capacity ratio remains
high
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Questions / Contacts
John Schmidt
(918) 832-4109
john.schmidt@noaa.gov
Tony Anderson
(918) 832-4109
tony.anderson@noaa.gov
James Paul
(918) 832-4109
james.paul@noaa.gov
Erick Boehmler
(508) 824-5116
erick.boehmler@noaa.gov
6/3/2010
ER FFG Conference