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Groundwater recharge CLM5

cskinner

New Member
Hi,

I am trying to determine a groundwater recharge value in CLM5. In CLM5, there is a zero-flux boundary condition at the base of the soil column. However, in the CLM5 history fields, I see a QCHARGE (aquifer recharge rate) variable. How is this calculated?

The CESM2 data from CMIP6 archived on Cheyenne's /glade/ system does not include the QCHARGE variable. This archived data does include output variables, "mrros - the total surface runoff leaving the land portion of the grid cell" and "mrro - the total runoff (including drainage through the base of the soil model) leaving the land portion of the grid cell". Am I correct in assuming that mrro - mrros would give the total sub-surface runoff in each grid cell? If yes, is this effectively the groundwater recharge value?

Thank you for any clarification.

Best,
Chris
 

swensosc

New Member
In CLM5, the separate aquifer layer was disabled. The QCHARGE variable still exists, but it is not calculated and should just be zero. Similarly, the WA variable still exists, but it should be a constant as it no longer interacts with the rest of the system. In CLM5, there is only a soil column, so the concept of recharge to a deeper aquifer isn't really appropriate or meaningful. The soil column was made deeper (up to 8.5 m) and this seems to be adequate to capture most of the natural variability in the total water storage. Total runoff minus surface runoff does give subsurface runoff, but this is still runoff that is passed to the river routing model; it is not recharge. I don't know that one can calculate a recharge rate in CLM5 that is consistent with the way the term was used in CLM4.5.
 

cskinner

New Member
Thank you! This was very helpful.

Just to confirm, all subsurface runoff is directly placed into the river routing model (and subsequently discharged somewhere)? And this subsurface runoff only happens when the lowest soil layer and one or more layers above it in the soil column become saturated.

Thank you,
Chris
 

cskinner

New Member
*correcting what I wrote above, the subsurface runoff could happen even if the lowest soil layer was the only layer saturated.
 

swensosc

New Member
That is correct. Subsurface runoff is a function of water table depth, which is diagnosed from soil moisture. If the lowest layer is not saturated, there is no subsurface runoff. Subsurface runoff is passed to the routing model, which models runoff routing to the ocean.
 

cskinner

New Member
Hi,

Following up with a related question: When subtracting mrros from mrro to get subsurface runoff, I see that some grid cells have negative values. In the context of CLM5, what does a negative subsurface runoff value mean? I don't see how this is possible given that there is no lateral runoff between grid cells.

Thank you,
Chris
 

oleson

Keith Oleson
CSEG and Liaisons
Staff member
In CLM5, there are other terms that come into play here. The mrro (QRUNOFF in CLM5 history file) is equal to:

drainage (QDRAI) + sfc runoff (QOVER) + runoff from glaciers, wetlands, & lakes (QRGWL) + surface water runoff (QH2OSFC) + drainage from perched water table (QDRAI_PERCH) - irrigation (QIRRIG).
 

cskinner

New Member
Are the individual runoff terms from the CMIP6 simulations (historical and the scenariomips), in particular QDRAI, archived by NCAR?
 

oleson

Keith Oleson
CSEG and Liaisons
Staff member
This was resolved offline, but yes, this data is on the Earth System Grid, and on campaign store on glade, e.g., for the first historical ensemble member:

/glade/campaign/collections/cmip/CMIP6/timeseries-cmip6/b.e21.BHIST.f09_g17.CMIP6-historical.001
 

MrIgnition

Member
In CLM5, there are other terms that come into play here. The mrro (QRUNOFF in CLM5 history file) is equal to:

drainage (QDRAI) + sfc runoff (QOVER) + runoff from glaciers, wetlands, & lakes (QRGWL) + surface water runoff (QH2OSFC) + drainage from perched water table (QDRAI_PERCH) - irrigation (QIRRIG).
Dear Oleson,
Based on this, how would you advise me to calculate groundwater recharge? I have been doing GWR = RAIN-(QFLX_EVAP_TOT + QOVER). Please I need your advice on this. The reason is because I also get negative values for some grid cells.
Thanks.
 

oleson

Keith Oleson
CSEG and Liaisons
Staff member
Please see the earlier posts on this thread, e.g., "In CLM5, there is only a soil column, so the concept of recharge to a deeper aquifer isn't really appropriate or meaningful."
 
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