I would think that conducting separate experiment tests simulating the real situation you are dealing with will help you to find the interception % component (evaporation from the vegetation and the infiltration part). This is the only way to have precise answers related to your own case.

Dear Dr. Samani

Thank you so much for your answer. In my study, I have calculated the evapotranspiration from satellite imagery using SEBAL method which considers the energy balance so we have the entire evapotranspiration of the area i.e. evaporation from soil and transpiration from plants.  I think that SEBAL can even consider the evaporation of water intercepted on plants. If so, is it necessary to calculate the interception at all?

 great answer Jamal M. V. Samani

I suggest you look at your situation from the physical basis of the processes occurring. Water is either evaporated, infiltrated into the soil and used by the plant, goes down below the root zone of the plant (deep percolation or also called leached water) or runs off the site. If it is intercepted, it still must take one of the pathways just mentioned, right?  Therefore, why is it even a concern?

In Quantifying the Interception component in the water budget, you should take into consideration the watershed land cover. In areas of shorter vegetation interception storage is likely to be small, and the rate of loss may not exceed the potential evaporation rate. Thus, interception storage is unlikely to be a measurable quantity in the water balance. So as other researchers suggested, it all depends on your own judgment and understanding to the contribution of this source to the water balance of the watershed under study. Please refer to the following references for more details.  

That's an interesting question. The answer depends on the time scale. Intercepted precipitation can:

- evaporate (some would call this interception loss and would be accounted for in your ET term)

- drip from the canopy (indirect throughfall)

- run down the stem/trunk of the vegetation (stemflow).

Indirect throughfall and stemflow will eventually reach the soil, from which a fraction will evaporate (accounted for in ET), some will infiltrate (accounted for in infiltration), and what remains may end up as surface runoff (accounted for in Runoff). 

Rainfall-Vegetation interaction  is likely to be finished by hours after the precipitation event and might not be all that important for a water balance developed in time scales of weeks or months. Since you don't have storage changes, I assume your time scales are longer than days, therefore interception might have been already accounted for in ET, infiltration and runoff. 

SEBAL gives you the total ETa, it does not separate them into components. You might be needing experiments for this.

Before making any decision, it worth checking very carefully if the estimated actual evapotranspiration using the SEBAL method contains interception or not. For example, the actual evapotranspiration estimated in the GLEAM data has the interception component, and there is no need to re-calculate the interception when using the GLEAM  evapotranspiration for hydrological simulation.

Thanks,