Standard names: Additional evapotranspiration names

[GeyerB]

Proposer's name Beate Geyer Date 2021/06/18

On the way to a new model version, we found two additional variables where we need a new standard_name: water_potential_evapotranspiration_amount and water_potential_evapotranspiration_amount. We adjusted the definitions from water_evapotranspiration_flux and water_potential_evaporation_amount.

-Term water_evapotranspiration_amount -Definition Water means water in all phases. "Evapotranspiration" means all water vapor fluxes into the atmosphere from the surface: liquid evaporation, sublimation, and transpiration. Evaporation is the conversion of liquid or solid into vapor. Transpiration is the process by which water is carried from the roots of plants and evaporates from the stomata. (The conversion of solid alone into vapor is called "sublimation".) "Amount" means mass per unit area. Unless indicated in the cell_methods attribute, a quantity is assumed to apply to the whole area of each horizontal grid box. -Units kg m-2

-Term water_potential_evapotranspiration_amount

-Definition "Water" means water in all phases. "Evapotranspiration" means all water vapor fluxes into the atmosphere from the surface: liquid evaporation, sublimation, and transpiration. Evaporation is the conversion of liquid or solid into vapor. Transpiration is the process by which water is carried from the roots of plants and evaporates from the stomata. (The conversion of solid alone into vapor is called "sublimation".) Potential evapotranspiration is the rate at which evaporation would take place under unaltered ambient conditions (temperature, relative humidity, wind, etc.) if the supply of water were unlimited, on the surface as if from an open water surface and unlimited in the soil. "Amount" means mass per unit area. -Units kg m-2

Thanks for the work to the team!! Beate

[ngalbraith]

These look fine at a glance, but I have one suggestion. OK, maybe 2. I'm only addressing the second term, but these comments would apply to both.

On content, 'Potential evapotranspiration is the rate at which evaporation would take place' - shouldn't that be 'the rate at which evapotranspiration would take place'? Otherwise, why isn't the term 'potential evaporation'?

Second, although these definitions follow the standard pattern, they're perpetuating some problems with our definitions. I'd really like to see the 'meat' of the definition moved to the beginning as we add and/or modify terms. Specific terms can be defined within the definition in order of their potential to be misunderstood and/or in order of importance. This makes it MUCH easier to figure out what a standard name means, without needing to read an essay.

So, with these changes, the definition of water_potential_evapotranspiration_amount would become

Potential evapotranspiration is the rate at which evapotranspiration would take place under unaltered ambient conditions (temperature, relative humidity, wind, etc.) if the supply of water were unlimited, on the surface as if from an open water surface and unlimited in the soil. Evapotranspiration means all water vapor fluxes into the atmosphere from the surface: liquid evaporation, sublimation, and transpiration. Evaporation is the conversion of liquid or solid into vapor. Sublimation is the conversion of solid alone into vapor. Transpiration is the process by which water is carried from the roots of plants and evaporates from the stomata. Water means water in all phases. Amount means mass per unit area.

Note I removed the quotation marks from the definitions of sub-terms, because I have no idea why they are used sometimes used but not always, and I find that they distract from the definitions. If there's a rule for when they should be used, I'd like to know what it is.

[GeyerB]

Thanks for the help!! - The changes are fine with me.

[JonathanGregory]

I am concerned that the existing terms containing water_evaporation may actually be understood to mean evapotranspiration. It looks like this must have been discussed before, because water_evaporation_flux is already defined as an alias for water_evapotranspiration_flux. I wonder whether this means the same should be done with the other water_evaporation names. If they are needed, new standard names could be defined for evaporation explicitly excluding transpiration. Jonathan

[GeyerB]

Do we need another CF-history expert to solve this? Who? @japamment? Several changes would have to be made, which would not be easy in terms of backwards compatibility...

[JonathanGregory]

We should avoid backward incompatibility if we possibly can. Do you agree that people may understand "evaporation" to mean "evapotranspiration" in existing names? What do others thing of this? Thanks. Jonathan

[GeyerB]

In communication with my colleagues, we came to the result, that the alias ‘water_evaporation_flux’ is definitely misleading. We understand that the construct is a mirror of the development of both: the complexity of models and the CF-conventions themselves. BUT: without that background information, it is strange that physically different variables can have the same standard_name. A solution would be to introduce ‘water_evaporation_flux’ with proper description (similar to the description until version 56) and a standardized sentence hinting that former versions are incomplete.

Definition Water means water in all phases. Evaporation is the conversion of liquid or solid into vapor. (The conversion of solid alone into vapor is called "sublimation".) The process of transpiration is not included. In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called "flux density" in physics. Unless indicated in the cell_methods attribute, a quantity is assumed to apply to the whole area of each horizontal grid box. Previously, the qualifier where_type was used to specify that the quantity applies only to the part of the grid box of the named type. Names containing the where_type qualifier are deprecated and newly created data should use the cell_methods attribute to indicate the horizontal area to which the quantity applies. In the versions before 56 transpiration was not mentioned and for versions 56 to 71 it was allowed to use water_evaporation_flux as an alias. Beate

[JonathanGregory]

Dear Beate

Since it is quite a long time since we defined water_evaporation_flux as an alias for water_evapotranspiration_flux I think we ought to leave that unchanged. It was ambiguous before then, and we made a particular choice, probably the best guess of what people had meant by it. For consistency all the existing water_evaporation names ought to be made into aliases of corresponding new water_evapotranspiration names, and we also need new names for the corresponding quantities excluding evapotranspiration. Would this be acceptable as a way to tidy up a mess?

Best wishes

Jonathan

[taylor13]

there is relevant discussion at https://github.com/cf-convention/vocabularies/issues/34 and https://github.com/cf-convention/vocabularies/issues/79

[larsbarring]

Another aspect is that the distinction between actual and potential evapo[-transpi-]ration should be made for both suggested standard names. I.e. to change the first one to water_actual_evapotranspiration_amount, and adjust the description.

Moreover, I think that it is problematic to prolong and expand the indistinction between evaporation and evapotranspiration as suggested by @JonathanGregory

For consistency all the existing water_evaporation names ought to be made into aliases of corresponding new water_evapotranspiration names