6/3/2010 ER FFG Conference 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 ER FFG Conference 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

6/3/2010 ER FFG Conference 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

6/3/2010 ER FFG Conference Gridded Flash Flood Guidance

6/3/2010 ER FFG Conference 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

6/3/2010 ER FFG Conference Arkansas-Red Basin River Forecast Center Gridded FFG OHD RDHM Soil Moisture Dynamic NRCS Curve Number Variable Threshold Runoff Gridded Flash Flood Guidance Model Slope Length to divide Static CN Area Duration 2-yr, 3-hr DDF Gridded Runoff How much rain to produce runoff How much runoff to produce flash flooding

6/3/2010 ER FFG Conference Soil Moisture Accounting with a Distributed Hydrologic Model 4km gridded soil moisture products UZTWC UZFWC LZTWC LZFSC LZFPC Sacramento Model Storages Upper Zone Saturation Ratio National Weather Service Office of Hydrologic Development Research Distributed Hydrologic Model Arkansas-Red Basin River Forecast Center 4km gridded temperature 4km gridded precipitation

6/3/2010 ER FFG Conference Upper Zone Saturation Upper zone saturation, 5/1/2009 Upper zone saturation, 6/1/2009

6/3/2010 ER FFG Conference 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

6/3/2010 ER FFG Conference Arkansas-Red Basin River Forecast Center Varying Curve Numbers by Antecedent Soil Moisture Conditions Average Curve Number Soil Moisture Adjusted Curve Number * 75% Upper Zone Saturation

6/3/2010 ER FFG Conference NRCS Curve Number Graphic Arkansas-Red Basin River Forecast Center

6/3/2010 ER FFG Conference Threshold Runoff Computation  Development of Variable Threshold Runoff (Thresh-r)  Thresh-r is estimated at the HRAP scale  Ratio of flood flow, Q s to peak flow, Q p of the unit hydrograph.  Q s is approximated by the annual return period flow, Q2  Q p 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

6/3/2010 ER FFG Conference NRCS Triangular Unit-graph Variables CN = NRCS Curve Number y = average watershed slope Known Variables Calculated Variables Qp = peak discharge from unit hydrograph t p = lag time (time from center of mass of rainfall to Qp) T R = time to Qp from beginning of rainfall S = Abstraction = (1000/CN)-10 l = length to divide A = drainage area D = duration of rainfall ( 1 Hour for unit Hydrograph) Arkansas-Red Basin River Forecast Center

6/3/2010 ER FFG Conference NERFC Area Computed Thresh-r Q s = f (design storm, slope, curve number) =  Q p = f (slope, curve number) Thresh-r

6/3/2010 ER FFG Conference Arkansas-Red Basin River Forecast Center GFFG System FFG Calculation S = (1000/CN)-10Q = ThreshR

6/3/2010 ER FFG Conference 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 12ZMPE 12 hours ending 1/25 12Z SCHOHARIE COUNTY SWE about 3 inches Approaching rainfall

6/3/2010 ER FFG Conference 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/ Z GFFG system 1/25/ Z SCHOHARIE COUNTY 00Z 3-hr FFG = Z 3-hr FFG = 1.8

6/3/2010 ER FFG Conference 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/ ZLegacy system 1/25/ Z SCHOHARIE COUNTY 00Z 3-hr FFG = Z 3-hr FFG = 1.4

6/3/2010 ER FFG Conference 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

6/3/2010 ER FFG Conference Questions / Contacts John Schmidt (918) Tony Anderson (918) James Paul (918) Erick Boehmler (508)