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11.8 sqmi Browns Creek nr Nashville, TN

Nexrad 5-Minute Radar Urban Rainfall-Runoff Comparisons in Real-Time
With Radar Hourly Ratio Adjustments
(includes epic May 1-2, 2010 storm)

March 1, 2014 to ... Ended May 1, 2015.

Download this project (Zip 9 MB)



































Current NOAA N0R radar lite gif above with Browns Creek Basin overlay.  Below is zoomed N0Q radar, Wundergages, and adjusted N0Q at watersheds.  Note that the raw N0Q radar here almost always shows "ground clutter" because of being so close to the radar site. Ground clutter (0.05" to 0.1" a day) is just another simple matter that rain gage adjustment removes.  No rain at the rain gages for the hour then no radar rain.



Automated graphs and radar updates:     


Wundergages and radar:

Averages, ratio, adjusted, frequencies and runoff:



This is a quick study to show that with a day or 2 of your time that you can setup a real-time multi-subbasin radar rainfall compuatation using GetRealtime.  A  small 11.8 sqmi basin in subburbon residential Nashville, TN was selected with a USGS gage.  The initial lumped basin runoff coefficients were cookbooked and then evaluated with the USGS gage and revised as needed.  The revisions needed were for very large recession flows presumedly for deep soils with a lot of night crawler and gopher activity or an imperivious layer at the gage.  The CN loss was not adjusted but my normal large soil and groundwater losses had to be greatly reduced as well as speeding up soil drainage and weighting and extending my 3-triangular unit graphs recession and now all seems well.  Probably due to the large average residential lot size and not reworking the soils too much during construction and never dismiss them worm holes and varmits.




Next the basin was subdivided into 3 subbasins and the lag times and base flow were distributed with area size of the original lumped basin size.  I had thought one sub in paticular would need much higher % impervious but has not been adjusted yet (update, it was 4/3/2014).  (Note: If your lag times start to go below 0.5 hours you should switch your unit graph method to the 3 linear reservoir method which should give the same resultes around 0.5 hours but you'll be glad you did.  GetRealtime makes this an easy selection.)  Pretty simple, so let's see what I did and then how we do.


After selection of the basin, I fired up my free GetMapArea to create the lat/long boundary file needed for radar rainfall. I used 'Load Map File', 'EPA WATERS KMZ', which loads Google Earth and the catchments and streamlines and 2 diagonal Lat/Long points for scaling. I know for you GIS gurus this seems stone age but it still works and probably better than your methods at times and so what's your lag time smart ass?  Below is the 'Ctrl*PrtScr' captured image used in GetMapArea with diaganol scaling points:

My notes needed for creating the boundary and lag time:


P1,P2 Scalinng: -86.852670,36.061461,-86.676659,36.153219

11.8 sqmi so says the USGS, we shall see.... bingo on the dot!

Longest Stream Lenth= 5.13 (mi)

 Length Below Centroid= 2.56 (mi)

Upper Elev=800 ft

Lower Elev=443 ft


Lag=1.06 hours GetMapArea computation Lag Time=C*(L*Lca/S^.5)^.33


16 Houses in: 653 ft x 765=499545 sqft= 11.47 acres or 16/11.47=1.39 acre lots

(*oops*  I used 1.39 acre lots but should be 11.47/16=0.72 acre lots.)

Soil Groups: 72.7%= Silt loam= B

15.4%= Silt loam= C


Silt loam B, Carson's Recovery Factor=0.17

From SCS TR-55 table: 1 acre resedential on B soil, CN=68, % Impervious=20%, % Connected=40%

See The Cookbook: http://getmyrealtime.com/HarvardGultchExample.aspx


Now we need to convert our lat/long boundary to radar pixels for radar OHX Nashville. You use LatLongPixelsFromFile.exe to convert your GetMapArea boundary text file to pixles, copy to your GetRealtime radar image folder and create the point file for it using GetNexrad.exe. I also digitize the stream lines for radar display and do the same using DSID=1 but no point file is created.


Next we need at least 2 Wundergages to adjust the radar:

-10811 KTNNASHV31 Rand Lake Nashville  DOWN

Lat: N 36 ° 4 ' 7 '' ( 36.069 ° ) Lon: W 86 ° 47 ' 55 '' ( -86.799 ° ) Elevation (ft): 800


-10812 KTNNASHV42 Forest Hills Nashville  DOWN

Lat: N 36 ° 4 ' 56 '' ( 36.082 ° ) Lon: W 86 ° 49 ' 42 '' ( -86.829 ° ) Elevation (ft): 912


And here are 2 more in case wee need them (update and these are the only 2 operational now):

-10813 KTNANTIO2 Antioch, TN (-86.694, 36.066) UP

-10814 KTNNASHV1 Bellevue, TN (-86.938,36.077) UP

added 3/19/2014:

-10816 KTNNASHV24 Oak Hill, TN (-86.794,36.080) DOWN

added 5/19/2014:

-10817 KTNNASHV39 WCPike North Nashville, TN (-86.825,36.246 UP


(if needed:  KBNA airport (-86.6892, 36.1189 but I hate airports)


Use my free LatLongPixels.exe to create the Wundergage's boundary files and the point files.


And the subdivided subbasins:

SID 802 Browns Cr Sub 1 (Upper South) Area sqmi: 6.506471

SID 803 Browns Cr Sub 2 (Lower East) Area sqmi: 3.278148

SID 804 Browns Cr Sub 3 (Lower West) Area sqmi: 2.01575



Station_ID; Datatype_Site_ID; Datatype_Name; Site_Name; base1; shift1; formula1;....

***;***;****; *** USGS Gage ****


***;***;****; *** Radar ****

NEXRAD-OHX; -10801; Rainfall; Browns Creek Basin, TN; 0

NEXRAD-OHX; -10802; Rainfall; Browns Cr Sub1, TN; 0

NEXRAD-OHX; -10803; Rainfall; Browns Cr Sub2, TN; 0

NEXRAD-OHX; -10804; Rainfall; Browns Cr Sub3, TN; 0

NEXRAD-OHX; -10811; Rainfall; KTNNASHV31 Rand Lake Nashville; 0

NEXRAD-OHX; -10812; Rainfall; KTNNASHV42 Forest Hills Nashville; 0

NEXRAD-OHX; -10813; Rainfall; KTNANTIO2 Antioch Nashville; 0

NEXRAD-OHX; -10814; Rainfall; KTNNASHV1 Bellevue Nashville; 0

***;***;****; *** Forecasts ****

 FORECAST-NWS; 11801; Adjust Rain; Browns Cr Big Sub to Gage; -2; 36.0713,-86.777,2 FORECAST-NWS; 11802; Adjust Rain; Browns Cr Sub1, TN; -2; 36.0713,-86.777,2

FORECAST-NWS; 11803; Adjust Rain; Browns Cr Sub2, TN; -2; 36.0713,-86.777,2

FORECAST-NWS; 11804; Adjust Rain; Browns Cr Sub3, TN; -2; 36.0713,-86.777,2

***;***;****; *** Wundergages **** 

KTNNASHV31; 10811; Rainfall; Rand Lake Nashville

KTNNASHV42; 10812; Rainfall; Forest Hills Nashville

KTNANTIO2; 10813; Rainfall; Antioch Nashville

KTNNASHV1; 10814; Rainfall; Bellevue Nashville

***;***;****; *** Computations ****

COMPUTE-Hour; 10815; Rainfall; Nashville Avg of 2 Rain Gages; 0; 0; (P1+P2+P3+P4)/N

COMPUTE-Hour; -10815; Rainfall; Nashville Avg of 2 Radars; 0; 0; (P1+P2+P3+P4)/N

COMPUTE-Hour; 31815; Ratio; Nashville Gage/Radar; 0; 0.01, 1, 5; P1/P2

COMPUTE-unit; 11811; Adj Rainfall; Adjusted Rand Lake Nashville; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11812; Adj Rainfall; Adjusted Forest Hills Nashville; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11813; Adj Rainfall; Adjusted Antioch Nashville; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11814; Adj Rainfall; Adjusted Bellevue Nashville; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11801; Adj Rainfall; Nashville Basin Adjusted Radar; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11802; Adj Rainfall; Browns Cr Sub1, TN; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11803; Adj Rainfall; Browns Cr Sub2, TN; 0; -1, 1,0.75, 0.50; P1*P2

COMPUTE-unit; 11804; Adj Rainfall; Browns Cr Sub3, TN; 0; -1, 1,0.75, 0.50; P1*P2

***;***;****; *** Runoff ****

COMPUTE-unit; 30801; Runoff; Nashville Basin at Gage; 4; 1.06, 68, 0.1, 20, 11.80, 0.5, 0.8, 0.4, 4, 0.3, 28, 89, 58, 0.5, 0, 2, 0.1, 0.8, 0.03, 0.4, Lag0; P1; 0; Triangle Unit Graph


COMPUTE-unit; 30802; Runoff; Browns Cr Sub1, TN; 2.21; 1.06, 68, 0.1, 20, 6.506, 0.5, 0.8, 0.4, 4, 0.3, 28, 89, 58, 0.5, 0, 2, 0.1, 0.8, 0.03, 0.4, Lag0; P1; 0; Triangle Unit Graph


COMPUTE-unit; 30803; Runoff; Browns Cr Sub2, TN; 1.11; 0.5, 68, 0.1, 20, 3.278, 0.5, 0.8, 0.4, 4, 0.3, 28, 89, 58, 0.5, 0, 2, 0.1, 0.8, 0.03, 0.4, Lag0; P1; 0; Triangle Unit Graph


COMPUTE-unit; 30804; Runoff; Browns Cr Sub3, TN; 0.68; 0.5, 68, 0.1, 20, 2.016, 0.5, 0.8, 0.4, 4, 0.3, 28, 89, 58, 0.5, 0, 2, 0.1, 0.8, 0.03, 0.4, Lag0; P1; 0; Triangle Unit Graph


ROUTE-unit; 1805; Flow; NashVille Subs Combine 1,2,3, TN (note routing file was simple combines)

***;***;****; *** Post Current Radar and Map on Web **** 

SHELL-C:\GetRealtime\GetNexrad\GetNexrad.exe -2 myPassword; 0; Pic; Nashville Radar Pic; -1



GetAccess table Rsite:



5-minute radar N0Q cells are 0.57 x 0.71 miles (1km x 0.5 dBZ):


NOAA Atlas 14 frequency table used by GetGraphs above:


Interesting... did you note how the peaks of the 1960's-70's seem to be from a different population of mean and variance than the post 1990's.  Global Warming! I mean Global Cooling!  Or maybe they built more houses to keep warm?  Which caused the warming, I mean cooling. What ever pays the most I'm always for.  And seeing the 2010 outlier I decided to do this historical event as shown at the bottom.



The calibartion of March 2, 2014 Wundergages and OHX Radar using GetGraphs hourly display:

NOTE:  The violet colored series is always the 2nd series referred to.  As a quick example the graphs shown are the real-time graphs of my GetGraphs.exe and what a user sees in real-time.


The calibraion of the lumped and subdivided runoff, USGS is violet:

Two days to get things setup and  we are all set to see how things work out....




And we are off>>>Mar 16, 2014:

The radar at the gages displayed hourly... base flow raised from 4 to 8 cfs but reset to 2 after thinking that the soils just need some initialization and other factors will be changed soon enough.

The adjusted radar at the gages...

The adjusted radar runoff with 4-day forecast and USGS...

Adjutsted basin radar rainfall freqency and depth at x-scale minute durations....



Hourly runoff by month: 

(Month1:  Update 3/20/2014, Soils need initialization befor fine tuning so I have decided to just fit the longer recession for now and hope the peak recession fixes itself.  I will update the setups once this occurs.)


Month 2:  Northeast Sub 2 with industrial and commercial area percent impervious raised from 20% to 50% and directrly connected raised from 0.4 to 0.8.  This did not change the historic 5/2/2010 peak but doubled the small 4/3/2014 peak here.


Month 3:  From the Month 2 peaks it looks like I raised the % impervious a wee too much.  I will adjust when a new peak comes in for May.  Below is the current coefficients for the large Sub 1 which is almost the same as the the other two.  I increased the recover rate to steepen the recession and shifted the increased soil flow to the more rapid groundwater flows.  Base flow increased to 4 cfs and soil losses increased a bit.


Month 4:  No change.


Month 5:  Recovery factor lowered from 0.4 to 0.25 to lower recession, base triange lowered, dry CN raised, and groundwater recovery factor adjusted.  Hey, it's getting there.


Months 6 & 7:


Months 8 & 9:


Months 10 & 11:


Months 12 & 13:




SCS Curve Number method adapted for continuous simulation:


Current Sub1 runoff wizard coefficents:


More runoff to come....



***************** History in the Making *********************



I used the NWS Weather Toolkit to download the May 1-3, 2010 N0Q radar which is never fun but somebody has to do it (Iowa Mesonet missed these).  The Wundergages all crapped out so I used the nearby (2.5 miles east) KBNA airport Wundergage to adjust the radar as a last resort.  The USGS flow gage also crapped out so only their peak of 8,540 cfs on May 2, 2010 is available.  I had to develop a flow rating to fill in what is shown below Q=15.95*(GH-0.43)^2.4491 from USGS gage heights and should be very good.  The airport adjusted radar subdivided peak came in as 7,900 cfs so thats good.  The radar showed cells continously above hail condaminated dbz >55 with a continuous maxima about 58-60 dbz and up to 64 dbz on the scope but over all the gage corrected hail capped radar z-r seems adequate for this storm.  No reports of hail for this periord.  I deleted the N0H target type so forget about downloading it twice, dang!  I also tried using a daily Airport/Radar ratio instead of hourly but that really reduced both day's peak flows so no help there.  I searched and found 5 reporting widely scattered Wundergages and only one had a 5-minute rainfall above the hail cap and was just barely above at 6.12 in/hr so with storm cells moving at 63 mph you get one shot so no loss of sleep there.  Anyway what is eye-popping is the 2,250 year recurrance 2-day storm duration and this is for a 11.8 sqmi basin rainfall average (but lag time is 1 hour).  No wunder the gage washed out.  So when the next big one hits, you better have your radar trimmed and burning because the flow gages may not be there.


I wonder what the forecast was???  See below.


The first wave of the storm was a 169 year 12-hour event then followed the next day by a 148 year 6 hour event.  The odds of that occurring in Nashville apparently is 1 in 2,250 years from a linear rainfall vs log recurrance extrapolation beyond 1000 years in NOAA 14.  As for the 8540 cfs USGS peak flow, based on just the log Pearson stats ignoring regional skew, the mixed population, and this peak, the 8540 cfs has a recurrance somewhere out there past the age of the earth.


Here was Apr 30, 2010 at 0:00 Zulu (-5 hr dls so Apr 29 7 pm PM's shift, not grave yards) 3-day forecast of 3 inches which should have put up an alert to most in this business.  May 1, 2010 24 hour forecast at 0:00 Zulu at the start of the storm shows 1" for Nashville, nice try but 700% low.  Forcasting a 169 year event as 0.41 yr event is missing the bullseye a wee bit, but was headed in the right direction with a forecast 3" for next day May 2.  After the May 1 deluge, the May 2 0:00 Zulu forecast shows 6", which is dead on, and certain doom to follow. You really have to view the radar loop that shows a frontal prarrie fire of rain descend on Nashville out of nowhere and just continued to burn for 2 days.  What I find interesting on these extreme events in the Southeast is the lack of hail... or everyone has better things to do than report on it:


The official spin:  http://www.nws.noaa.gov/os/assessments/pdfs/Tenn_Flooding.pdf

"The primary recommendation identifies the need for improved communication and collaboration between WFO Nashville, the OHRFC, USACE Nashville (LRN) and the Tennessee USGS." -- Beggar thy neighbor. Sounds like they all should have been using GetRealtime on Windows with their GetAccess database in DropBox for better communication! 



Download the Nashville N0Q Radar images for May 1-2, 2010, 58 mb and view with GetNexrad and create return frequencies for any point you click.


If you would like to run the Nasville 3 day storm over your basin, download the OHX zip file, unzip, and rename or copy KOHX_*.* Kyyy_*.* where yyy is your radar id. In the now named HTX_N0Q_0.Kfw, change the lat/long values to somewhere to the northwest of your basin location. You can try different lat/longs to try and maximize rainfall over your basin.


6.27910352405173E-03 0 0





Now treat these files as if you downdloaded them as a KML download from the Weather and Climate Toolkit and convert or copy(?) your boundarys and point files to this folder and there you go. GetNexrad or even GetRealtime can read and compute the rainfalls for your basins


OR... put the Nasvhille hourlys in both runit and rhour with GetAccess and 'UPDATE database with Excel':


Pacific DST Adjusted Radar
5/1/2010 0 0.01159
5/1/2010 1:00 0 0.02813
5/1/2010 2:00 0 0.02263
5/1/2010 3:00 0 0.00961
5/1/2010 4:00 0 0.00704
5/1/2010 5:00 0.25407 0.39162
5/1/2010 6:00 0 0.10446
5/1/2010 7:00 0.54295 0.10388
5/1/2010 8:00 0.90796 0.97607
5/1/2010 9:00 0.3551 0.36927
5/1/2010 10:00 0.61261 0.35932
5/1/2010 11:00 0.5594 0.64481
5/1/2010 12:00 0.39387 0.39201
5/1/2010 13:00 1.7454 1.77084
5/1/2010 14:00 0.50911 0.50824
5/1/2010 15:00 0.28635 0.29699
5/1/2010 16:00 0.21741 0.21796
5/1/2010 17:00 0.05712 0.05522
5/1/2010 18:00 0.04482 0.04735
5/1/2010 19:00 0.00176 0.00186
5/1/2010 20:00 0.08825 0.12093
5/1/2010 21:00 0.21762 0.21808
5/1/2010 22:00 0.13344 0.18712
5/1/2010 23:00 0 0.03015
5/2/2010 0 0.03641
5/2/2010 1:00 0 0.03419
5/2/2010 2:00 0.01744 0.03003
5/2/2010 3:00 0 0.01608
5/2/2010 4:00 0 0.00248
5/2/2010 5:00 0 0.00216
5/2/2010 6:00 1.00344 0.98361
5/2/2010 7:00 1.4319 1.38859
5/2/2010 8:00 1.48191 1.49433
5/2/2010 9:00 0.45237 0.45875
5/2/2010 10:00 0.26444 0.29862
5/2/2010 11:00 0.56889 0.52879
5/2/2010 12:00 0.30404 0.3307
5/2/2010 13:00 0.7429 0.70951
5/2/2010 14:00 0 0.50561
5/2/2010 15:00 0 0.0812
5/2/2010 16:00 0 0.00626
5/2/2010 17:00 0 0.02783
5/2/2010 18:00 0 0.03433
5/2/2010 19:00 0 0.02432
Total 13.19 13.87



You could even use s Forecast Wizard to paste in just the values as a 44 hour forecast and save to all your subbasins.  Tip:  Paste the above in Excel and then copy the values column and paste in the Forecast Wizard.  After using the values and the forecast wizard for several of my projects I found something interesting.  Depending on lag times/basin size the runoff can be very unremarkable.  I don't know why the runoff frequency was so high on this small basin so give it a try on yours.  In New York it's a 124,000 year storm, in central Alabama just a 283 year storm, the 1-hour was 2.3 years, the 6-hour was 39 years, but for this 3.7hr lag 34 sqmi basin the runoff frequency was 1,360 years so dont let a low frequency short duration storm fool you if it happens 2 days in a row.  The same storm but transposed radar in northern Alabama had just a 20-year runoff frequency so who knows about yours.



Update Mar 27, 2014-- After being scolded by an expert for my hail cap at 55 dbz I raised the hail cap to 62 dbz.  This made a slight change to the runoff hydrograph but... Now the first wave of the storm was lowered from a 169 year to a 128  year 12-hour event then the following day was raised from 148 year to a 161 year 6 hour event.  The 2-day odds of that occurring in Nashville was raised from 1 in 2,250 years to 1 in 3530 years.  My first thought was that raising the hail cap would really increase the adjusted rainfall, but as you can see, rasing the R in the hourly G/R ratio will reduce the adjustment.  When applied to the basin increased radar rainfall it actually looks much better.  But like I say, minor changes to the ruonff with the minor changes for the positive as compared to my rated USGS flows.  (NOTE:  I would definately advise keeping a hail cap at 53 to 55 dbz WEST of the Rockies where tropical conditions are extremely rare (???)  I have had terrible results with this 62 dbz cap where my adjusting gage says 1.1 inches and the radar says 7.2 inches in an hour.)




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