Standard names: Residuum of soil water content

[rpnaut]

Proposer's name Ronny Petrik and Klaus Keuler and Burkhardt Rockel Date 20.05.2021. In climatological applications the budget of the soil water is of big importance. As simulations are done over many decades, the conservation of soil water mass (and also other mass quantities and the energy) is required for a robust and physicall-consistent simulation. Up to now, only the budgets of ice sheets are considered with the CF standard names, e.g. _land_ice_surface_specific_mass_balanceflux or _land_ice_surface_specific_mass_balancerate

With these both terms only the right side of the budget equation seems to be considered and with the standard names starting with _change_overtime..._ only the left hand side of the budget equation is considered.

We propose here a quantity quantifying the residuum of a mass budget equation and with respect to the soil water content. Our suggestion up to now reads:

- Term residuum_of_soil_water_content - Description "Water" means water in all phases. "Content" indicates a quantity per unit area. The mass content of water in soil refers to the vertical integral from the surface down to those layer of the soil model which is the lowermost hydrological active layer. For the content between specified levels in the soil, standard names including "content_of_soil_layer" are used. "Residuum" is defined by the difference between the tendency of the soil moisture content over a specific time period (left hand side of soil water equation) and the sum of the sources and sinks of soil water over the same time period (right hand side of equation). A positive value means that the soil becomes falsely wetter in a model and a negative value means the soil becomes falsely dryer. It should be noted, that the residuum has to be calculated excluding climatological layers where the temperature or the soil water content is kept to a climatological value or a value given by another soil layer. - Units kg m-2.

Are there any comments or suggestions?

[GeyerB]

The new name is needed in the modelling community, thanks for the proposal!

It might be better to use residuum_of_soil_water_mass_balance instead of residuum_of_soil_water_content as the subject of study is the mass_balance.

My attempt to shorten the description was not successful, but perhaps the following is easier to understand: -Description "Water" means water in all phases. The "mass" of water in soil refers to the vertical integral from the surface down to the soil bottom. In models, "soil bottom" refers to the layer of the soil model which is the lowest hydrologically active layer (explicitly excluding climatological layers with prescribed temperature or water content). "Residuum" is defined as the difference between the tendency of the soil moisture content over a specific time period and the sum of the sources and sinks of soil water over the same time period. Non-zero values indicate a non-closed water balance. Positive values mean that the soil in a model is falsely becoming wetter and negative values mean that the soil is falsely becoming dryer.

[JonathanGregory]

Dear all

Thanks for the proposal. I support the idea. I have a few comments.

• Concerning the description, I would say that this might not only apply to models. You could have an observational budget, in principle.
• Although the description is clear about it, the proposed standard name doesn't indicate the sign convention.
• Without the description, it's possibly unclear what "residuum" means.

In view of the above, I would suggest something more explicit for the name, such as difference_between_change_in_soil_moisture_content_and_sum_of_estimated_contributions. In other standard names, difference_between_X_and_Y means X minus Y. By inserting "estimated" I mean to imply that there is room for error. There must be an error of some sort if there is a significant difference! If it's any clearer, we could say net_change, although that's what change alone means.

Best wishes

Jonathan

[GeyerB]

Thanks for you help, Jonathan!

I’m not a fan of using ‘difference’ in the standard_name as we lose the fact of unnatural description when we take it instead of residuum. We could add to my description: "‘balance’ means the sum of all contributions and sinks of soil moisture." as penultimate sentence for further clarification of the sign.

Have a nice day Beate

[JonathanGregory]

Dear Beate @GeyerB

Sorry, I'm not sure what you mean by "we lose the fact of unnatural description". Please could you explain? My concerns with "residuum" are

• It's not a familiar word. I don't think everyone would immediately know you mean the difference between the net value and the sum of the contributions.

• It doesn't indicate the sign convention. As you know, we try to make sign conventions explicit in standard names.

Best wishes

Jonathan

[taylor13]

I would agree that indication of the sign should be included in the name. I think some variant on "difference_between_change_in_soil_moisture_content_and_sum_of_estimated_contributions" would work (if we all agree that difference between A and B means A-B and not B-A). I might move "estimate" to rewrite it "error_difference_between_estimated_changes_in_soil_moisture_content_and_sum_of_contributing_fluxes_and_sources". Perhaps its too long, but is this what is meant?

[Somewhat tangentially, I'm surprised that in (well formulated) models the difference should be anything but zero. I can understand that observational estimates of soil water content and the fluxes of water into the soil might be independently obtained, and so they wouldn't necessarily be consistent. In a model don't the fluxes (i.e., the contributors to the tendency) determine the tendency (and the content) so conservation is assured?]

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[GeyerB]

Dear Jonathan and taylor13, we start now to downscale the CMIP6 GCM data to high resolution. Therefore it would be nice to complete our ncetCDF output with valid standard names. difference_between_change_in_soil_moisture_content_and_sum_of_estimated_contributions - is fine with me, after more than a year no solution. The long proposal of taylor13 with the moved 'estimated' does not improve the situation in case of simulated soil moisture, @JonathanGregory - what do I have to do, to get the proposal completed? Best, Beate

[taylor13]

Apologies for the long delay. I guess we were waiting for some clarification in response to my questions above. I didn't mean for them to hold up anything.

What I stumbled over is why in a model should the the sum of the contributions to a tendency not equal the full tendency. And what is meant by "estimated contributions". Do models calculate a total tendency through some bulk formula and then estimate how much different sources and sinks might have contributed to it if they had been explicitly included in the calculations? That seems quite odd to me.

[JonathanGregory]

Dear Beate @GeyerB

The procedure is that the standard name moderators (Ellie, Fran, Alison) will review the proposal, or notice when we've completed the discussion, and take action to add the new name to the table.

Karl's questions are good ones. Perhaps this quantity is the undiagnosed contribution to the change in soil moisture content?

Best wishes

Jonathan

[efisher008]

Dear Beate @GeyerB,

Apologies for the long wait in replying to this issue - I am now revisiting long-term open issues after the migration of standard name proposals from the cf-conventions/discuss repo to the newly created cf-conventions/vocabularies repo.

I have changed the entry in the CF editor from the original residuum_of_soil_water_content to your (@GeyerB) new proposed difference_between_change_in_soil_moisture_content_and_sum_of_estimated_contributions. I think Karl @taylor13 makes a valid point with his questions about the estimated_contributions term and why the sum of the contributions to the tendency does not equal the tendency itself (i.e. the change in soil moisture content). Is there an additional term which has not been accounted for in the model which describes the difference between the sum of soil moisture contributions and the change term?

Best wishes, Ellie

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[rpnaut]

Dear Allie, the naming change from residuum of soil water content to "difference between change in soil moisture content and sum of estimated contributions" is an acceptable alternative, although it is very long. The proposol takes into accout, that it is not one-hundred percently possible to summarize all contributions in an exact way as numerical discretizations lead to small inconsistencies to the analytical description of budgets. Thus, the contributions are "the best estimate of contributions". However, it is a very valuable quantity as it helps (from our experiences) to indicate regions and processes which do not conserve mass. Thus, I can live with the suggestions of @efisher008.

[efisher008]

Dear @rpnaut,

Thank you for your comments. I appreciate the proposed name has increased in length quite considerably, but I do believe that for the sake of disambiguation/making explicit the inconsistencies due to numerical discretizations as mentioned, this change will be valuable. Would you like me to add any of the above text to the description to aid the interpretation of this standard name?

Best regards, Ellie