Dear All,
I would like to start a new thread to draw together discussions on 3 land water terms which are under discussion, and update the proposals to take into account some feedback from Hyungjun Kim below.
(1.1) mrtws: Terrestrial Water Storage [kg m-2]
(
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2018/060238.html)
(1.2) sw: Surface Water Storage (excluding snow) [kg m-2]
(
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2018/060300.html)
(1.3) qgwr: Groundwater recharge from soil layer [kg m-2 s-1]
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2018/060300.html
dgw [kg m-2], Change in Groundwater, requested by LS3MIP
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2018/060241.html
mrtws: Terrestrial Water Storage [kg m-2]
For Terrestrial Water Storage (which is a recognised hydrological phrase), Jonathan has suggested land_water_amount, following the pattern established in change_in_land_ice_amount. There is a distinction, however, that while the new term is intended to include snow and ice on land, it excludes the floating ice shelves which are included in change_in_land_ice_amount. This distinction is important, because Terrestrial Water Storage is used in budgets related to sea-level change, so needs to exclude any ice floating in sea water. We could use the name suggested by Jonathan and make the distinction in the description of the term, but it may be safer to modify the name slightly. It would also be possible to use an area type, e.g. "land_excluding_floating_ice_shelf", but this would not be strictly accurate: if we want a term which is relevant to the water budget for sea level rise calculations we should be consistent with the name land_ice_mass_not_displacing_sea_water introduced for ISMIP6, which includes a "port
ion of grounded ice-sheet mass exceeding the mass of any sea water displaced" (which is non-zero where the ice-sheet rests on ground below sea level). We could use: land_water_amount_not_displacing_sea_water. This would avoid the problems I've raised: can anyone see a better solution? As this is quite a specialised term, perhaps we should just use terrestrial_water_storage?
Adapting the wording provided by Hyungjun to take bring it into line with the ISMIP6 term:
'The quantity with standard name land_water_amount_not_displacing_sea_water, often known as "Terrestrial Water Storage", includes liquid phase surface water (water in rivers, wetlands, lakes, reservoirs, and vegetation including canopy interception), surface snow (including canopy interception) and ice (including ice on lakes etc, and the portion of grounded ice sheets not displacing sea water and but excluding ice shelves floating in sea water and a portion of grounded ice-sheet mass matching the mass of displaced sea water), liquid subsurface water and solid subsurface water.'
sw: Surface Water Storage (excluding snow) [kg m-2]
This term is intended to be the liquid, surface component of Terrestrial Water Storage. Alison has suggested:
land_surface_liquid_water_amount (kg m-2)
'The surface called "surface" means the lower boundary of the atmosphere. "Amount" means mass per unit area. The quantity with standard name land_surface_liquid_water_amount includes water in rivers, wetlands, lakes, snow, vegetation and reservoirs.' The inclusion of snow here is presumably a typo, and, following the discussion with Hyungjun I think it is worth adding a reference to canopy interception:
'... The quantity with standard name land_surface_liquid_water_amount includes all liquid phase surface water (water in rivers, wetlands, lakes, reservoirs, and vegetation including canopy interception)".
qgwr: Groundwater recharge from soil layer [kg m-2 s-1]
Alison has suggested liquid_water_mass_flux_from_soil_to_groundwater, but the groundwater includes a component of soil moisture: saturated soil if present. Hyungjun suggests expressing it as a mass flux through the water table, which divides saturated from unsaturated soil. Translated into CF format, this might be "water_table_downward_liquid_water_mass_flux". Alternatively, closer to Alison's suggestion, we could have "liquid_water_mass_flux_from_unsaturated_soil_to_groundwater". The latter term may be preferable, since the water table can move, and there is an ambiguity in "water_table_downward_liquid_water_mass_flux" as to whether it means the flux at the current location or the flux relative to the moving boundary (which is what is wanted here). Hyungjun also notes that contributions to CMIP6 should include the mass flux out of the bottom of the soil model if the model does not extend down to the water table. This implementation detail can be placed in the CMIP6 variable definitions, I don't think it nee
ds to be in the standard name description.
regards,
Martin
________________________________
From: Hyungjun Kim <hjkim at iis.u-tokyo.ac.jp>
Sent: 14 June 2018 09:26
To: Juckes, Martin (STFC,RAL,RALSP); Chris Jones; Jean-Yves Peterschmitt; Karl Taylor; Pamment, Alison (STFC,RAL,RALSP)
Subject: Re: Land and terrestrial water amounts in the CMIP6 data request
Dear Martin and all,
I am sorry about my delayed response.
Please find my opinion below.
1) This ambiguity might come from diverse model representation of land
water continuum.
If we preferentially focus on mass balance, it would be straightforward
to have:
+ soil moisture : total water in soil column
+ water table depth : depth of saturation
+ groundwater recharge : water flux at water table (which should be
base flow if water table depth is lower than model soil depth)
2) In terrestrial hydrology, cold processes and associated storage are
treated with liquid phase processes in general.
Therefore, it would be more useful to have them in separated variables.
3) I mostly agree with you suggestion, but because of reason 2), it
would be better to be defined as (although it is same to yours
physically) summation of:
+ liquid phase surface water (water in rivers, wetlands, lakes,
vegetation, reservoirs, canopy interception)
+ snow (&canopy interception), and ice, including grounded ice
sheets and ice on lakes etc, but excluding ice shelves floating in sea water
+ liquid subsurface water
+ solid subsurface water
Best regards,
Hyungjun
On 6/6/2018 12:17 AM, Martin Juckes - UKRI STFC wrote:
> Hello All,
>
>
> We have a few variables for water amounts, with some open questions. We really need to resolve these questions this week, as many modelling groups contributing to CMIP6 will not be able to deal with changes after that. Most of the questions are about LS3MIP, but some relate to PMIP and C4MIP requests as well, so please take a look and comment wherever you can.
>
>
> The main variables that these questions relate to are:
>
>
> mrtws [kg m-2], Terrestrial Water Storage, requested by LS3MIP, C4MIP and PMIP;
>
>
> sw [kg m-2], Surface Water Storage (excluding snow and ice), requested by LS3MIP;
>
>
> dgw [kg m-2], Change in Groundwater, requested by LS3MIP;
>
>
> mrso [kg m-2], Total Soil Moisture Content (CMIP5 variable, reused in CMIP6);
>
>
> qgwr [kg m-2 s-1], Groundwater recharge from soil layer (requested by LS3MIP);
>
>
> 1) We have opted for the following definition of groundwater: "Groundwater is subsurface water below the depth of the water table, including soil moisture and underground aquifers." It appears to be common usage to include soil moisture as part of groundwater, but, if that is the case, what is meant by the parameter qgwr, "Groundwater recharge from soil layer"? On the other hand, our proposed definition for Terrestrial Water Storage, ".... It includes surface water (water in rivers, wetlands, lakes, snow[, and ice], vegetation and reservoirs) and subsurface water (soil moisture, groundwater)", could be read as implying that soil moisture and groundwater are separate. Would it make sense to consider "surface water", "soil moisture" and "groundwater" as three independent water masses which combine to give terrestrial water storage? Or, when we talk of "groundwater" in CMIP models, do we mean the saturated portion of the soil moisture (i.e. there are no rock aquifers)?
>
>
> 2) The terrestrial water storage definition implies that "surface water" is "water in rivers, wetlands, lakes, snow, vegetation and reservoirs": does this definition make sense for the LS3MIP variable "sw"? (allowing for the fact that "sw" has the additional restriction that it only applies to liquid water and so would exclude snow and ice -- but the question is whether "sw" should include liquid water in all the other categories);
>
>
> 3) The majority of our land surface variables are defined over land including ice sheets and floating ice shelves. It appears that the parameter "Terrestrial Water Storage" (TWS) is most widely used for analysis of sea level rise, in which case it would make sense to exclude the floating ice shelves. It appears that many existing studies of TWS exclude Greenland and Antarctica, perhaps because analysis of the water budget in these regions is a different science. However, with the (possible) advent of global climate models that can represent the water budget consistently in these regions, we should try to define variables in such a way that they can be used globally. Hence, I propose clarifying the TWS definition to ".... It includes surface water (water in rivers, wetlands, lakes, vegetation, reservoirs, snow, and ice, including grounded ice sheets and ice on lakes etc, but excluding ice shelves floating in sea water) and subsurface water (soil moisture, groundwater)". Does this fit with expectations and r
equirements of the requesting MIPs?
>
>
> regards,
>
> Martin
>
Received on Thu Jun 14 2018 - 05:53:32 BST
