bug in JULES for using the Van Genuchten params

[pmcguire]

Hi Imtiaz

I think I found a bug in JULES for using the Van Genuchten params.

in jules-vn5.2/src/science/soil/vmc_from_head_mod.F90
it has n_vg = ( 1.0 / b(i,iz) ) - 1.0
whereas in order to get the exponents to be the same at high Psi=head for the Brooks and Corey and for the Van Genuchten, this should be I think
n_vg = ( 1.0 / b(i,iz) ) + 1.0
Do you concur?
Patrick

[pmcguire]

Patrick:
I agree, that line is wrong, it would make n_vg negative since 1/b is always less than 1. The correct equation is n=1/b+1 which can be rearranged as b=1/(n-1). The Jules ancillaries store 1/(n-1).

Also in the same file there are these lines:
78

smvcst(land_pts,sm_levels)

79

! Volumetric soil moisture content at saturation (m3 water/m3 of soil).

214

! theta - theta_r = (theta_s - theta_r) / (1+(alpha*Psi)ⁿ⁾m

215

smvc(i,iz) = smvcst(i,iz) / &

216

( ( 1.0 + ( (alpha_vg * psi_metres) n_vg ) ) &

217

( 1.0 - 1.0 / n_vg ) )

Line 214 implies that smvcst=(theta_s-theta_r).

I will forward your bug find to Doug Clark.

Cheers

Imtiaz

[pmcguire]

Imtiaz Dharssi
Tue, Feb 12, 10:50 PM (2 days ago)

to Doug, Toby, Patrick

Hi Doug

Patrick McGuire? has found a possible bug in ​https://code.metoffice.gov.uk/trac/jules/browser/main/trunk/src/science/soil/vmc_from_head_mod.F90


213

n_vg = ( 1.0 / b(i,iz) ) - 1.0

Line 213 implies that n_vg is always negative since 1/b is always less than 1. We think that the correct code should be n_vg = ( 1.0 / b(i,iz) ) + 1.0

This corrected equation is consistent with line 72 ! For the van Genuchten model: b=1/(n-1) (m).

Also the following lines need amending to add smvcst=(theta_s-theta_r) for the van Genuchten model.
78

smvcst(land_pts,sm_levels)

79

! Volumetric soil moisture content at saturation (m3 water/m3 of soil).

The Jules documentation doesn't make it clear that Jules ancillaries store (theta_s-theta_r), (theta_w-theta_r) and (theta_c-theta_r). Would it be possible to explcitly add a line or two to say that.

Thanks

[pmcguire]

Hi Doug and Toby
It looks like that bit of Van Genuchten code is only being used when the ECOSSE model of soil biogeochemistry is chosen (soil_bgc_model = 3).
This new bit of code was added in JULES4.9, and is still under development.
Patrick