Turning NEXRAD Rainfall Estimates Into Model Input for DuPage County, Illinois

Why radar rainfall

Next-Generation Radar (NEXRAD) has transformed precipitation estimates. Its fine detail in space and time is especially valuable where rain gages are sparse, and better rainfall estimates let hydrologic models produce reliable streamflow forecasts across the United States. The USGS uses National Weather Service (NWS) NEXRAD data in many studies, and has built similar processing tools, including an archive of NWS precipitation estimates.

Quantitative Precipitation Estimates (QPE). The NWS River Forecast Centers (RFCs) combine radar estimates with rain-gage measurements and satellite estimates, which usually improves on radar alone.

• QPE is archived in several formats, including GRIB (GRIdded Binary — the World Meteorological Organization's standard for gridded weather data), shapefiles and NetCDF.
• Each RFC distributes it hourly; the Advanced Hydrologic Prediction Service provides national coverage online.
• The grid is about 4 kilometers on a side, in the Hydrologic Rainfall Analysis Project (HRAP) projection (polar stereographic on a spherical datum).

The DuPage County model

The USGS and DuPage County's stormwater management division built a near-real-time streamflow-simulation modeling system for Salt Creek, a headwater basin, to test how flood-control structures should be operated. It links:

• HSPF (Hydrologic Simulation Program–FORTRAN), a continuous rainfall-runoff model;
• FEQ (Full Equations), which routes flow through channels and control structures.

In 2015 the system was being extended to a second basin, the West Branch DuPage River. It runs hourly, with rainfall in hundredths of an inch, and can draw on two rainfall datasets: a network of 27 tipping-bucket rain gages and NEXRAD-QPE from the North Central RFC (NCRFC).

The two basins and the NEXRAD grid cells used for the model's precipitation input. USGS.

From GRIB to model input

The NCRFC's raw hourly files cover much of the upper Midwest: 450 × 350 HRAP cells. The model needs 71: 36 for Salt Creek and 35 for the West Branch. The USGS Illinois Water Science Center converts them to HSPF's hourly observations format (HYDHR):

1. Download the hourly NEXRAD-QPE GRIB files from the NCRFC's secure file transfer site.
2. Convert them to text with the NWS WGRIB program.
3. Extract the 71 cells with a Perl script, converting millimeters to hundredths of an inch, and write one HYDHR text file per cell.
4. Publish: a second Perl script copies the updated files from an internal server to a public website every hour, for both basins.

Details

• Files keep the original HRAP projection and are named xx#####.hsp — wb (West Branch) or sc (Salt Creek) plus the five-digit NEXRAD cell number. Cell numbers are specific to the NCRFC regional grid.
• A shapefile of the NCRFC's HRAP grid, with cell numbers, was built by a published method.
• HSPF users load HYDHR files into Watershed Data Management (WDM) files — binary files holding many time series — using WDMUtil or ANNIE.

Sources

Based on Terry Ortel and Ryan Spies, "NEXRAD Quantitative Precipitation Estimates, Data Acquisition, and Processing for the DuPage County, Illinois, Streamflow-Simulation Modeling System," U.S. Geological Survey fact sheet (2015), prepared with the DuPage County Stormwater Management Department; a work of the United States government in the public domain. Its map is taken from the fact sheet's PDF. The download and website addresses it gives were current in August 2015.