New calculation for accounting of root distribution on the transpiration

[ccarouge]

The calculation of ssnow%evapfbl is changed significantly by the ground water work for the non-Haverd2013 case. From what I understand, it is trying to spread the demand for water through the soil layers due to the root distribution.

The current calculation is:

CABLE/src/science/canopy/cbl_dryLeaf.F90

Lines 518 to 521 in 630e2e1

	ssnow%evapfbl(i,kk) = MIN( evapfb(i) * veg%froot(i,kk), &
	MAX( 0.0, REAL( ssnow%wb(i,kk) ) - &
	1.1 * soil%swilt(i) ) * &
	soil%zse(kk) * density_liq )

The new calculation is:

CABLE/src/science/canopy/cbl_dryLeaf.F90

Lines 530 to 547 in 42bf0c5

	! MMY to fix the inconsistence in root water extraction between ! FEEDBACK(MMY?)
	! cable_gw_hydro and above
	xx = 0._r_2 ! demand : transpiration
	xxd = 0._r_2 ! difference : demand - supply
	diff(:) = 0._r_2 ! supply : water available
	DO kk = 1,ms
	xx = evapfb(i) * veg%froot(i,kk) + diff(kk-1) ! demand for layer kk
	diff(kk) = max(0._r_2,ssnow%wbliq(i,kk)-soil%swilt_vec(i,kk)) * soil%zse(kk) * 1000.0
	! supply for layer kk
	xxd = xx - diff(kk) ! deficit in layer kk
	if (xxd .gt. 0._r_2) then ! demand > supply
	ssnow%evapfbl(i,kk) = diff(kk) ! transpiration in layer kk is supply
	diff(kk) = xxd ! deficit in layer kk
	else ! demand < supply
	ssnow%evapfbl(i,kk) = xx ! transpiration in layer kk is demand
	diff(kk) = 0._r_2 ! no deficit in layer kk
	end if
	END DO !ms

I have absolutely no idea if it's a good calculation to introduce. If it's a good idea, I don't know if we want to introduce it now or park it away for later.

@har917 I think that's the last one from the ground water work where I need some input.

[har917]

On this one - yes this is probably a good idea to pull across - it avoids over extraction of liquid water from a layer with lots of ice in it.

But I'm a bit confused as to how this intersects with the code in cbl_remove_trans and remove_transGW (for the non-Haverd2013 case) - this seems only to be setting %evapfbl which isn't then used.

A couple of comments:

• the 1000. at line 537 needs to be a density_liq
• I'd be happier if the factor 1.1 was included - comments from Matthias indicates that this safety net is needed.
• this method does conceptually allow for extraction of water from soil layers where there are no roots.
• this method doesn't actually ensure that evapfb can actually be satisified - or that wbice/wb < frozen_limit