Where to enter LAI for Interception when E and T are seperate BCs

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mfd
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Joined: Wed Oct 21, 2020 8:32 am
Location: Germany

Where to enter LAI for Interception when E and T are seperate BCs

Post by mfd » Wed Mar 17, 2021 4:21 pm

Dear Prof. Simunek, dear all,

1. I selected to input Ep and Tp seperately in HYDRUS 1D (because having Hydrus split ETp was not giving me the values I wanted), by not entering met. data and by not clicking "Input PET and LAI".
However, when I do this, there is no longer a possibility to input LAI and thus no interception is calculated. I would be grateful to learn the reason the code no longer asks for an LAI to calculate interception when one chooses to input E and T seperately. I would think Interception is still relevant and needs to be deducted from P and added to T? Or do I need to do so myself outside of Hydrus in this case?

2 A second question relates to Interception: the coefficient a is recommended to be 0.25mm/d and when it can be specified in "Meteorological parameters" there is a unit given [mm]. However when interception is specified in "Water flow boundary conditions" in Atmospheric BC, no unit is given, and for example the preset value in the file TEST2 (when activated) is 0,025. So is the unit for the coefficient in "Water flow boundary conditions also mm, or cm?

Thank you and kind regards
Mariel

Jirka
Posts: 5216
Joined: Sat Mar 16, 2002 3:47 pm
Location: USA
Location: Riverside, CA

Re: Where to enter LAI for Interception when E and T are seperate BCs

Post by Jirka » Thu Mar 18, 2021 4:35 am

1. That’s correct. If you do not use meteorological data and/or do not ask the code to divide potential ET into potential E and T using LAI, the code does not allow you to consider interception, since in that case it does not know LAI. The Interception model needs LAI, and that is an input only if you use the meteo data or if you ask the code to divide potential ET into potential E and T using LAI.
2. When the Interception is entered with the meteo data, then it has units of mm. Variables in the Meteo module have fixed units (as indicated in the GUI). On the other hand, in the rest of the HYDRUS code, the user can select the time [T] and space ([L] e.g., cm) units and then these units need to be used for all variables. Thus, if the interception value is entered in the Water Flow BC window, then it should have units of [L/T], and the recommended value indeed is a=0.025 cm/d for Equation 2.78 (e.g., van Dam et al., 1997).
J.

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