Add new irrigation dataset to GEOGRID.TBL

[weiwangncar]

This PR adds a new dataset of irrigation to the table as an optional entry. This dataset is
0.0833 degree (1/12 degree) resolution, about 9 km. The generated field, named irrigation, is a 2D field of actual percentages (0 - 100).

In anticipation of other irrigation data sets becoming available, this data is located under
the generic directory "irrigation" and then in the data-specific "fao" subdirectory.

The data is sourced from
http://www.fao.org/aquastat/en/geospatial-information/global-maps-irrigated-areas.

The dataset supports the new WRF model option sf_surf_irr_scheme =1, 2 and 3.

The following image is such a field over CONUS.

[davegill]

@mgduda @barlage @weiwangncar
Wei,
Can you plot this irrigation field from the geogrid output for any convenient (non ocean) domain that you have. The WRF registry says that this field is an "irrigated land percentage". We should add a simple description of this field to the PR commit message. Also, is the field really 0-100 (an actual percent) or is it 0-1 (a fraction), for example.

The WRF registry also has this irrigation information as a 2d array, with no month index. That restriction should also be part of WPS commit message.

[weiwangncar]

@barlage Mike, since you have the data under irrigation/fao/, do you expect other sources of data becoming available at some time?

[barlage]

Yes, locally we use a much higher resolution 500m dataset over the US, but we are not ready to publish this yet. I feel having an "irrigation/" directory will keep the geog directory more organized.

[weiwangncar]

@davegill I've made a plot and updated PR message.

[weiwangncar]

@barlage Thanks.

[davegill]

@mgduda @jimbresch @weiwangncar
Michael and Jim,
I am good with this PR.

[mgduda]

Do we need to add similar entries to other GEOGRID.TBL files, or is GEOGRID.TBL.ARW sufficient?

[weiwangncar]

@mgduda I know we don't need to add it for NMM, FIRE, noahmp, but iffy for chem.

[davegill]

@mgduda
Michael,
I would just make the minimum change, and only do the single file GEOGRID.TBL.ARW. The chemistry group is unlikely to be interested in this capability.

[weiwangncar]

@mgduda @davegill Thanks. Shall I merge or would one of you do so?

[mgduda]

I'll merge this soon. I'm running a couple of final tests.

[weiwangncar]

@mgduda Thanks.

[davegill]

@mgduda
Michael,
Wei manufactured some metgrid files for me which included the irrigation variables. I used those metgrid files in a few regression tests in the WRF model to test the irrigation PRs wrf-model/WRF#980 and wrf-model/WRF#1149.