Basin Aggregation

• What is it?

- Merging two or more small basins to produce one larger basin.

• Why aggregate?

- Two reasons…..

Basin Aggregation

1) FFMP requires every basin to be associated with at least

one radar bin.

No radar bins associated with these basins

Basin Aggregation

2) Basins that are too small may be lost in the resolution of

the display. – Basins <2 mi2 in area were considered "too small".

Basin Aggregation

General aggregation rules followed for the FFMP basins (****_aggr_basins):

- An interbasin <2 mi2 or an interbasin without an associated radar bin is merged with the next upstream interbasin.

- A tributary basin that does not have an upstream basin is merged with the interbasin into which it flows.

- If the resulting merged basin is <2 mi2 or does not contain a radar bin, it is merged with the next upstream basin.

Aggregation Rules

An interbasin <2 mi2 or an interbasin without an associated radar bin is merged with the next upstream interbasin.

Interbasin <2 mi2 Next upstream interbasin

Final merged basin (>2 mi2)

Aggregation Rules

A tributary basin that does not have an upstream basin is merged with the interbasin into which it flows.

Tributarywithoutradar bin

Interbasininto whichthe tributaryflows

Final merged basin

Basin Aggregation

Aggregation can result in basins that are confusing from a hydrologic standpoint.

Ideally, the tributary shown in the merged basin to the left should have its own averagerainfall calculation. However, because it was not associated with a radar bin, it has been lumped with two interbasins. In a hydrologic sense, what does the average rainfall value for this basin tell us?

Basin Aggregation

Aggregation can also exacerbate the problem of poorly represented rainfall amounts.

The rainfall estimates contributing to the basin calculation above do not represent any precipitation thatmight be falling over the tributary portion of the merged basin.

The average rainfall value for the merged basin below is represented by a single radar bin at the most upstream reach.

FFMP Shapefile Assembly

1) Edgematch HUC basins

2) Aggregate basins (merge basins <2 mi2 and basins without radar bins)

3) Merge original basins, aggregated basins, streams, and NHD reach and region shapefiles

4) Flag reservoirs

5) Spatially join counties/CWAs and RFCs to basin outlets

6) Extract stream names from NHD (as available)

7) Spatially join aggregated basins to radar bin center points

8) Convert final shapefiles to Geographic coordinates and calculate centroids

FFMP Shapefile Assembly

Edgematch HUC basins

FFMP Shapefile Assembly

Edgematch HUC basins

Final FFMP Shapefile Assembly

Merge individual HUC shapefiles to produce one basin shapefile (original and aggregated basins).

Final FFMP Shapefile Assembly

Select aggregated basins within the radar coverage area and convert to a new shapefile.

Final FFMP Shapefile Assembly

Select and flag basins intersecting lakes and reservoirs that are >0.25km2.

Final FFMP Shapefile Assembly

Spatially join counties/CWAs and RFCs to basin outlet points.

Final FFMP Shapefile Assembly

Extract stream names from the NHD reach shapefile.

• Names are assigned based on the most prevalent name within a basin (based on stream length).

• Thus, the name assigned may not be the name of the main stem.

***Important: These names are a

starting point--they MUST be checked

for accuracy and changes/additions

should be made as necessary.

There may be more than one NHD reach corresponding to each DEM-derived stream reach.

Not all NHD reaches are named (red reaches show those with names).

Final FFMP Shapefile Assembly

Spatially join aggregated basins to radar bin center points.