Runoff and Evapotranspiration on SWAT - Part I

[Eduardo Jauch]

Hi,

I have some questions regarding Runoff and Evapotranspiration on SWAT.

By what I saw on the "Theoretical Documentation", when the metod for estimate runoff is the SCS, on the Ia (initial abstractions) term is included the precipitation intercepted by the plant canopy, and so it wasn´t calculated (I'm wrong?).

But if I use others methods than the Penman-Monteith for calculate the potencial evapotranspiration, I need to calculate the transpiration. And this is done with the equation:

Et = (E'o . LAI) / 3.0         (0 <= LAI <= 3.0)
Et = (E'o)                         (LAI > 3.0)

Where E'o is the remain evaporativa water demand, given by the equation:

E'o = Eo - Ecan

Where Eo is the potential evapotranspiration and Ecan is the evaporation of the free water on the canopy.
To calculate the sublimation and evaporation of the soil, the equations depend on E'o value too.

So, here is my question. If using the SCS method for calculationg Runoff, the amount of water intercepted by the canopy is alread present "implicity", what value will be used for the Ecan, so that the SWAT can get the value for the E'o?

Or the value for the Ecan will be calculated, regardless of the Runoff calculation method chosed?

Thanks for any help on this.

If is a "stupid" question, forgive me, I'm a "newbe" on this :)

[Eduardo Jauch]

Another question for this subject...

How will be calculated the Ea value (Actual evapotranspiration).
I mean, what termos will be "sumed" to achive the Ea value?

Ecam, Et, E's and Esub?

Thanks!

2009/2/18 Eduardo Jauch <eduard...@gmail.com>

[Fan]

Here's what I think: in the CN method, canopy interception is
implicitly included in the Ia term, but is not explicitly specified.
Regardless of the runoff calc method chosen, The Ecan is a function of
the parameter CANMX, daily precipitation, LAI and PET.

Ea is calculated at HRU level. The final watershed or subbasin Ea is
the weighted sum of HRU Ea, where the weight is fraction of watershed
or subbasin area in HRU.

On Feb 18, 11:00 am, Eduardo Jauch <eduardo.ja...@gmail.com> wrote:
> Another question for this subject...
>

> How will be calculated the *Ea* value (Actual evapotranspiration).
> I mean, what termos will be "sumed" to achive the *Ea* value?
>
> *Ecam*, *Et*, *E's* and *Esub*?
>
> Thanks!
>
> 2009/2/18 Eduardo Jauch <eduardo.ja...@gmail.com>

>
> > Hi,
>
> > I have some questions regarding Runoff and Evapotranspiration on SWAT.
>
> > By what I saw on the "Theoretical Documentation", when the metod for
> > estimate runoff is the SCS, on the Ia (initial abstractions) term is
> > included the precipitation intercepted by the plant canopy, and so it wasn´t
> > calculated (I'm wrong?).
>
> > But if I use others methods than the Penman-Monteith for calculate the
> > potencial evapotranspiration, I need to calculate the transpiration. And
> > this is done with the equation:
>

> > *Et = (E'o . LAI) / 3.0         (0 <= LAI <= 3.0)

> > Et = (E'o)                         (LAI > 3.0)

> > *
> > Where *E'o* is the remain evaporativa water demand, given by the equation:
>
> > *E'o = Eo - Ecan*
>
> > Where *Eo* is the potential evapotranspiration and *Ecan* is the

> > evaporation of the free water on the canopy.
> > To calculate the sublimation and evaporation of the soil, the equations

> > depend on *E'o* value too.

>
> > So, here is my question. If using the SCS method for calculationg Runoff,
> > the amount of water intercepted by the canopy is alread present "implicity",

> > what value will be used for the *Ecan*, so that the SWAT can get the value
> > for the *E'o*?
>
> > Or the value for the *Ecan* will be calculated, regardless of the Runoff