Comparing NEXRAD and Gauge Rainfall Data Nate Johnson CE 394K.2 Final Project April 26, 2005

Motivation USGS performs hydrologic and water quality modeling using rainfall data from gauges. 9 km 6 km 10 km >20 km Sometimes there is greater than 20 km between gauges NEXRAD provides rainfall estimates on 4 km 2 grid Could more acccurate hydrologic simulations could be performed with “accurate” distributed rainfall information? How does NEXRAD precipitation data compare to gauges used for hydrologic modeling?

Outline Project involved 2 major tasks: 1.Transferring data from NEXRAD files to something more accessible 2.Comparing NEXRAD rainfall estimates to rain gauge measurements a)Cumulative Difference b)Storm Bias

Background – File Structure Space – Time – Variable dimensions Gridded File – One Time, all space TimeSeries File – One Location, all time To create a TimeSeries file (for a single location) from a collection of Gridded files (for a single time) must access thousands of gridded files! 24 hours * 365 days = 8,760 files/year

Background – Rainfall Estimates Compare NEXRAD to gauge based on… Point Comparisons (Grid Cell immediately above) Within cell spatial variability? Spatial Averages Interpolate between gauges? This project only considers point comparisons… Stay tuned for Jessica Watt’s presentation containing spatial comparisons

Methods – Extract NEXRAD Data Extracted 2 months of hourly NEXRAD data for about 1,800 cells near San Antonio. August and September of 2001 were very rainy ~100,000 non-zero timeseries records

Methods – Compare NEXRAD/Gauge First method – Cumulative difference over two months For each hour, calculate difference between Radar and Gauge and add to pervious total Removes biases from timing discrepancies

Results – Cumulative Differences August – September 2001 > 11 inches of rain!

Results – Cumulative Differences S.A. Airport Gauge – 11.9 inches total NEXRAD – Rain Gauge

Results – Cumulative Differences Government Canyon Gauge – 10.3 inches total NEXRAD – Rain Gauge

Results – Cumulative Differences Bulverde Rain Gauge – 10.8 inches Total NEXRAD – Rain Gauge

Methods – Compare NEXRAD/Gauge Second method – Storm-to-Storm Comparison Based on an analysis by NCAR and NOAA in 1979 Considers each event independently and looks for systematic biases Uses Ratio ‘G/R’ of Gauge to Radar for total storm- event precipitation

Methods – Compare NEXRAD/Gauge Average G/R Coefficient of Variation Average Difference Average Difference (Storm Bias Removed) G/R < 1 G/R > 1 G/R < 1

Results – Storm Bias Comparison Original Difference Improvement? Not enough gauges? Too much scatter?

Conclusions / Future Work Some significant differences between NEXRAD and Gauge measurements May be too much scatter in G/R to remove “Storm Bias” from NEXRAD Still need to consider effects of spatial averaging Investigate the effects on hydrologic modeling results

Background – Rainfall Estimates Hydrologic modeling traditionally relies upon estimates for “Average Drainage – Area precipitation Some question yet as to whether or not it is appropriate to use NEXRAD for hydrologic modeling Studies have shown some disturbing differences between NEXRAD and gauge measurements

Methods – Extract NEXRAD Data First – Get NEXRAD data into Arc Hydro Geodatabase Second – Get Arc Hydro into Timeseries format Arc Hydro GDB

Background – File Structure NEXRAD data are stored in a gridded file structure Hydrologic models require data in a TimeSeries Structure Gridded File – One TimeTimeSeries File – One Location