GLDAS surface water component derivation

[gauravdumoga]

I have gone through many papers where it has been told that they have used surface water from GLDAS's NOAH, VIC or some other LSM, but I am not able to find the surface water component. It has surface runoff, but I don't know how they are doing it. Any suggestions

[andrew.m.fisher]

Can you provide examples of paper's citing "GLDAS surface water"? There may be more of an explanation somewhere in the paper. Surface water could have different meanings. It could be surface runoff, it could be runoff plus baseflow, it could be water-covered area... etc.

[gauravdumoga]

Thank you, Andrew, for your suggestion, I also thought same as you have suggested but when I went skimmed through the paper I found nothing. This is one of the paper you can see section 2.3- https://doi.org/10.1080/02626667.2020.1716238 .

[GES DISC - jschaperow]

Hi Gaurav,

Like Andrew said, surface water might mean different things in different contexts. In the context of the paper you linked, surface water (storage) is defined as the residual in the water balance equation (Equation 2 in the paper) when you subtract groundwater storage, soil moisture, and snowpack from "total water storage" (e.g. as observed by GRACE).

In the readme file (linked below) for the GLDAS data product, there are descriptions of each variable. For example, in the CLSM GLDAS outputs, there are variables for storm surface runoff, baseflow-groundwater runoff, snow water equivalent, and groundwater storage.

Toward what end are you looking to find GLDAS surface water data? (i.e. what are you hoping to do with this data?) This will help us better assist you.

Jacob

GLDAS readme file: https://hydro1.gesdisc.eosdis.nasa.gov/data/GLDAS/GLDAS_NOAH025_3H.2.1/doc/README_GLDAS2.pdf

[gauravdumoga]

Thanks jschaperow, README file helped. I am just trying to get groundwater storage anomaly data: simply by GRACE (TWSA) - GLDAS derived (Canopy water storage + Soil moisture + Snow equivalent + surface water storage). Just one component, surface water storage, was missing, but I think maybe they have used surface + subsurface runoff as its proxy.

[GES DISC - jschaperow]

Hi Gaurav,

I am not sure that using runoff as a proxy for river and lake storage is a good strategy. In the GLDAS models, runoff, along with precipitation and evapotranspiration, is a flux out of (or into) a grid cell. To calculate groundwater storage change (dGW), you are using an equation like this:

dTWS = d(SM + SWE + CAN + GW + Rivers + Lakes)

Let’s say we ignore SWE and CAN (canopy storage) for now and combine rivers and lakes into one variable called (SURF).

dTWS = d(SM + GW + SURF)

GRACE measures dTWS and GLDAS gives you dSM. But there are still two unknowns (dGW and dSURF).

If you want to bring in runoff, that is a flux, not a component of water storage. It relates to total water storage as follows:

dTWS = P – ET – R

But this does not tell us any new information. We already know all four elements of this equation (dTWS from GRACE and P, ET, and R from GLDAS). In other words, runoff tells us about TWS change as a whole, but it does not tell us about the components of TWS change (GW vs. SWE vs. rivers and lakes, …).

Ideally, you’d have a land surface model that explicitly represents rivers and lakes with a runoff routing model, but none of the GLDAS models do this. That said, in some areas of the world, you may be able to assume surface water storage is small compared to TWS (see Getirana et al. 2017, GRL) https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017GL074684 .

Also, keep in mind that soil moisture variations are sometimes overestimated by land surface models, so that when you subtract modeled soil moisture from GRACE TWS, the remaining (groundwater plus surface water) variations are underestimated.

You may be interested in the following:
• Monthly TWS anomalies from GLDAS published at PODAAC: https://podaac.jpl.nasa.gov/dataset/TELLUS_GLDAS-NOAH-3.3_TWS-ANOMALY_MONTHLY# (also lists some papers citing the data, such as Lv et al. 2020, J. of Hydromet.).
• TWS calculation example from GLDAS Noah outputs:https://grace.jpl.nasa.gov/data/get-data/land-water-content/
• Panda and Wahr, 2016, WRR, where they estimate groundwater storage by subtracting GLDAS NOAH SM from GRACE TWS and their justification citing Tewari et al. 2009 in GRL. https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015WR017797
• The GLDAS Readme file, see (4) in Section 3.5 Data Interpretation. It sheds some light on how TWS is calculated in the CLSM and Noah models.

Hope this helps!

Jacob

[ayep]

Hi,
Your explanation was very helpful, Jacob, and it helped me to clarify some concepts.
I am also trying to estimate groundwater storage anomalies, and I was wondering whether I should include the GLDAS runoff component...

What if I am working in basins like the Amazon and Paraná in South America, where surface water storage is a key component of TWS variability and the contribution of rivers to water storage cannot be neglected? (Getirana et. al, 2017, doi.org/10.1002/2017GL074684).
In that case, should I also consider the GLDAS canopy water storage component?

Thanks!

[GES DISC - jschaperow]

The GLDAS data do not quantify the amount of river, lake, and reservoir storage. You will likely want to consider another data source. If you wanted to take a model-based approach, you could use a model that explicitly represents river, lake, and reservoir storage (perhaps you could force the model with GLDAS runoff), or you could try to find observational data on surface water storage, such as lake stage/lake storage records.