Curve Number raster creation: handle null values in either landcover or soils datasets

[gassc]

For the tool that supports the creation of a curve number, have a way to handle null values in soils hydro classes and landcover classes. It's most likely we'll encounter soil layers without hydrologic classifications. Possible solutions:

• determine and build in the logic for an appropriate fallback
• expose a user-provided parameter in the toolbox for providing a fallback
• ensure that the lookup table itself can be used to handle nulls, e.g., with a blank soil hydro value in the lookup row, the tool will fall back to the provided curve number for the landcover class when the soil is null:

land cover class	soil hydro	curve number
2		45

This could also work to handle conditions where landcover is null but soil is known, or both are null.

That last approach is probably the easiest to implement. The change would happen around this line:

peak-flow-calculator/core/logic/gp.py

Line 215 in ba57c73

cn_raster_component = Con((landcover_raster_obj == rec.utc) & (soils_raster == rec.soil), rec.cn, 0)

Using the ArcPy Spatial Analyst / map algebra toolbox, an IsNull function following or proceeding that line could do the work.

Also: create docs around the Curve Number raster creation process, describing how that lookup table is made and adapted for other landcover datasets.