An Overview of Gridded Flash Flood Guidance A

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 Ø Produce a flash flood guidance product at a resolution closer in scale to basins in FFMP that: Ø Reflects the spatial variability of the physical characteristics that impact the susceptibility of a location to flash flooding Ø Uses freely available (or easily derived) data with nationwide coverage Ø Requires minimal calibration effort Ø Fits seamlessly into the existing RFC and WFO operational flash flood warning infrastructure 6/3/2010 ER FFG Conference

Gridded Flash Flood Guidance Ø Advantages of GFFG approach are: Ø Improves the spatial resolution of soil-moisture conditions and flash flood thresholds Ø 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 Ø Gridded FFG has a significant precedence Ø 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

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 6/3/2010 Variable Threshold Runoff ER FFG Conference Arkansas-Red Basin River Forecast Center How much runoff to produce flash flooding

Soil Moisture Accounting with a Distributed Hydrologic Model 4 km gridded temperature 4 km gridded precipitation National Weather Service Office of Hydrologic Development Research Distributed Hydrologic Model Sacramento Model Storages 4 km gridded soil moisture products 6/3/2010 ER FFG Conference 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 ER FFG Conference

Runoff Computation Ø Development of Dynamic Curve Number ØCombined to create an average (ARCII) Curve Number grid at HRAP scale Ø 30 m National Land Cover Dataset Ø 1000 m NRCS Hydrologic Soil Groups ØUtilized upper zone saturation ratio from RDHM output with NRCS curve number model (NEH Part 630, chap. 10) to formulate an automated curve number adjustment ØConvex of average for wet conditions ØConcave of average for dry conditions 6/3/2010 ER FFG Conference

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

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

Threshold Runoff Computation Ø Development of Variable Threshold Runoff (Thresh-r) ØThresh-r is estimated at the HRAP scale ØRatio of flood flow, Qs to peak flow, Qp of the unit hydrograph. ØQs is approximated by the annual return period flow, Q 2 ØQp is derived through the use of NRCS’ triangular unit hydrograph method ØAdjustment 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 Known Variables CN = NRCS Curve Number S = Abstraction = (1000/CN)-10 l = length to divide y = average watershed slope 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 6/3/2010 ER FFG Conference Arkansas-Red Basin River Forecast Center

NERFC Area Computed Thresh-r Qs = f (design storm, slope, curve number) 6/3/2010 = Qp = f (slope, curve number) ER FFG Conference Thresh-r

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

Limitations of the GFFG system Ø GFFG system limitations include: Ø No projection of snow-melt runoff within applicable duration of FFG value RDHM modeled SWE 1/25 12 Z MPE 12 hours ending 1/25 12 Z SCHOHARIE COUNTY SWE about 3 inches Approaching rainfall 6/3/2010 ER FFG Conference

Limitations of the GFFG system Ø GFFG system limitations include: Ø No projection of snow-melt runoff within applicable duration of FFG value GFFG system 1/25/2010 00 Z GFFG system 1/25/2010 12 Z SCHOHARIE COUNTY 00 Z 3 -hr FFG = 1. 9 12 Z 3 -hr FFG = 1. 8 6/3/2010 ER FFG Conference

Limitations of the GFFG system Ø GFFG system limitations include: Ø No projection of snow-melt runoff within applicable duration of FFG value Legacy system 1/25/2010 00 Z Legacy system 1/25/2010 12 Z SCHOHARIE COUNTY 00 Z 3 -hr FFG = 2. 0 12 Z 3 -hr FFG = 1. 4 6/3/2010 ER FFG Conference

Limitations of the GFFG system Ø GFFG system limitations include: ØCN adjustment for runoff response is consistently overestimated during cool season months ØRDHM models upper zone tension water at capacity ØUpper soil zone contents to capacity ratio remains high 6/3/2010 ER FFG Conference

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