## Water uptake by vegetation

MOHID Land forum. Questions and discussion related to MOHID Land
epelde.ane@gmail.com
Posts: 172
Joined: Thu Jan 01, 1970 00:00

### Water uptake by vegetation

Hello!

I´ve seen that MOHID can calculate the potential water uptake by 2 methods: SWAT based and root profile based (which can be triangular or constant). I´ve read that Feddes et al (1978) assumed homogeneous distribution of Sp with depth, being this: Potential water uptake= Potential transpiration/root zone depth. .. the root zone depth would be the fraction between actual length/max length at maturity, no?
To be the Sp homogeneous means that no bother the layer, in the vertical Sp is going to be the same, no? and what happen in a layer where the roots extend just in the half of it? And which is the difference between triangular or constant root profiles in the formulas?

Thanks again! davidbrito
Posts: 187
Joined: Thu Jan 01, 1970 00:00

### Re: Water uptake by vegetation

Hello Ane,

SWAT based it means that it will distribute the potential transpiration in depth with exponential form with SWAT parameters to make the curve. And the actual transpiration will depend on water content.

Root Profile Based means that the potential transpiration will be distributed in depth according to root form (triangular or constant) form surface to root depth.
If triangular will be higher in surface and zero when it reaches the tip of root depth, if constant the potential is the same in all depths.
We only use Feddes for the stresses that are used to compute the actual transpiration. But you just need to know that the 4 "Feddes" suctions heads that you give the model mean that the first is the head that below the plant will not take water (too much water in soil), the second is the first optimum and the third is the end of the optium (no streess in water uptake) and the forth is too much dry soil and will not uptake.

If the root gets in between the layers the shape already takes it in account.
Just remember that what is being computed first is just what will be the potential transpiration in each layer. If you sum all the layers potential than you get the total potential, it was just a ditribution in depth.
Then the actual transpiration depends on the plant stresses (given by Feddes heads).

Hope it helped,
David

epelde.ane@gmail.com
Posts: 172
Joined: Thu Jan 01, 1970 00:00

### Re: Water uptake by vegetation

Hello david,

Thanks for the explanation. Ok, so whatever the roots form is, the potential transpiration will be the same in both methods obtained from root distribution option right? As it is calculated with (Tp=Etc*1-e^0.463Lai) formula.
What I was asking about feddes is that to calculate effective transpiration, you multiply a stress factor (obtained from the grapgh with feddes heads) for potential water uptake, and for the latter, would be the puntual root depth/root max depth fraction multiplied for potential transpiration, right?

Thanks!

davidbrito
Posts: 187
Joined: Thu Jan 01, 1970 00:00

### Re: Water uptake by vegetation

Yes Potential Transpiration comes before because it is the potential for that cell.
These processes first just distribute the Potential Transpiration in depth for each layer.

I am not understanding your point.
The Feddes stress is not related to root depth or root max depth. Is just related to the amount of water present in the soil. One root that finds water contents (in head) lower than H4 will not be able to uptake water (Stress factor is zero) because it is too dificult to create suctions like that; one root that finds water content higher than H1 will not be able to uptake water (Stress factor is zero) because it is too much water and there is no oxygen available; water contents in between H2 and H3 are optimal for the plant so it will be able to uptake the potential (Stress factor one).
And this is wherever the root we are looking is, it can be at surface, at middle or root end, or whatever is the plant development if it is just 10cm, 1m or 10m. It just depends on the water content surrounding the roots because that is the environment problem that may allow or not to uptake the maximum.

Best regards,
David

epelde.ane@gmail.com
Posts: 172
Joined: Thu Jan 01, 1970 00:00

### Re: Water uptake by vegetation

Ok, I think I wrote about feddes and you thought I was speaking about feddes stress, sorry for the misunderstanding.
So, just to let it clear, can you let me if I´m right?
--The potential transpiration (Tp) is calculated by
Tp=Etc*1-e^0.463Lai ,and this value is for the plant for a growing stage. Is a value that is distributed along the layers depending the root distribution (triangular, cte or exponential (the swar based)
--Then, Potential absortion (Sp) is calculated by
Sp=Tp*(root depth/root depth maximum) , and root depth would be central point of the layer with roots or down face of the cell with roots? (so, the root depth that is taken into account is the real one or as in swat the model calculates it for down face, top face, and then, calculates the mean value?
Last, the effective transpiration (Sh),
S(h)=alpha*Sp , being the alpha the feddes stress.

I hope now was clearer the doubt thanks!

davidbrito
Posts: 187
Joined: Thu Jan 01, 1970 00:00

### Re: Water uptake by vegetation

The confusion is because in MOHID ther is no Sp! Or we do not do it like that.

The Sp is the distribution of the total transpiration and we do it depending on root distribution and root depth of course (the actual root depth). So the process described previously is the Sp.

Best regards,
David

epelde.ane@gmail.com