[Biogenics] Improve the Soil Moisture/Drought Gamma

[drnimbusrain]

It has been shown that the current MEGANv2 or v3 soil moisture/drought gamma is does not capture the 2011 or 2012 drought effect on isoprene emissions (Potosnak et al., 2014; Sindelarova et al., 2014; Seco et al., 2015; Huang et al., 2015) when using the default soil data (i.e. soil types and wilting points from Chen and Dudhia, 2001) due to the low wilting point values recommended by Chen and Dudhia (2001). This would be the case in the current canopy-app implementation of gamma_sm, and using the GFSv16/Noah LSM soil moisture and wilting point parameters for the correction.

At some point, recommend updating to include another option using Jiang et al. (2018) approach, based on physiological effects of drought stress on plant photosynthesis and isoprene emissions for use in the MEGAN3 biogenic emission model.

[drnimbusrain]

@quaz115 Thank you for the reference on improving drought response to biogenic emissions paper: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022MS003174

I will read through it, and lets connect soon on what you may want to do to improve current implementation (based on Guenther et al. 2012)