Wilting points in MEGANv3.2

MEGAN
1

Hi,

The wilting points for different soil types are shown in the Figure (from MEGSEA.EXT). I want to ask how these wilting points were determined.

wilting point
wilting point808×292 42.6 KB

The soil moisture in California in July 2021 (drought period) and July 2019 (non-drought period) are shown in the following Figure.
soil moisture
soil moisture1198×678 116 KB

It can be seen that soil moisture is lower than the wilting points in central and southern California in both drought and non-drought periods.
When turning on soil moisture stress (GAMSM_YN=Y), MEGAN generated isoprene emissions are shown in the following Figure.
isoprene emissions
isoprene emissions1352×1008 182 KB

It can be seen that soil moisture stress decreased isoprene emissions a lot in both drought and non-drought periods. There is no isoprene emission in central and southern California, which is not realistic.
Therefore, I’m thinking if the wilting points in MEGANv3.2 may be not applicable to California.
The weather conditions I used as input to MEGAN are from WRF simulations, which were driven by ERA5 (ECMWF Reanalysis v5). According to Muller et al (2008), when using the ECMWF global weather model, it was necessary to use the ECMWF wilting point dataset. So I’m wondering if I should use ECMWF wilting points. But I don’t know where to get them. Or I can use Chen and Dudhia (2000) wilting points, as shown in the Figure.
Chen and Dudhia wilting point
Chen and Dudhia wilting point1494×618 191 KB

Thanks,
Huazhen

2

What land surface model are you using?

3

I used the Noah land surface model when conducting WRF simulations.

4

The top layer of Noah is 10 cm, which I think is the correct depth for the GAMSM estimate. So the soil model probably isn’t the issue. You could try changing SOIM1 to SOIM2 in megsea.f:

    IF ( .NOT. READ3(MGNMET,'SOIM1',ALLAYS3,IDATE,ITIME,SOILM)) THEN

That will pull the 25 cm depth moisture from the Noah model, which is probably less vulnerable to drought. Other than that you will probably need to verify soil moisture is accurate and modify the wilting points as you suggested. ECMWF doesn’t use Noah for their LSM so using their wilting points makes as much sense as calculating your own.

5

Thanks for your suggestions. I’ll try changing SOIM1 to SOIM2 first.

Huazhen