Posted Sun, 07 Oct 2018 02:35:16 GMT by zebetheridge

I'm trying to create an expression which varies evaporation with moisture content in the elements at the surface of my model. In the first instance i'm just trying to create an expression which sets the rate to zero when moisture content is below a certain value and applies a constant rate at other times. I'm using the Source/Sink Material Property and creating an expression in the expression editor. But i'm not sure how to reference the top row of elements in the expression. I've tried it various ways - most recently by creating a user defined expression for the surface elements (which looks up the moisture content of those elements) and referencing that expression in my Source/Sink expression. But it doesn't switch off the flux when moisture content drops. I've also tried just assigning values to the top layer of elements as User data and referencing those elements in my expression using that value, but that didn't work either. I've attached a screenshot. Can anyone tell me what i'm doing wrong?
Posted Mon, 08 Oct 2018 06:45:19 GMT by Peter Schätzl
Hi Zeb,

Great to hear from you again :)

I suspect that ED.1 is a simple distribution rather than an expression (as expressions cannot be used in expressions). Thus its values are fixed and not depending on actual moisture content. I recommend to use a distribution for defining the top elements only (using a specific value, for example '0' for top elements). Then you can identify those elements by an 'if' statement in your expression and query current soil moisture by another if (or an AND statement).

Posted Mon, 08 Oct 2018 07:34:10 GMT by zebetheridge
Hi Peter,

Good to speak to you again, it's been years!

I figured after i sent the post that ED.1 is presumably like the reference distribution in Feflow 5, so i tried that but i still can't get it to work. I've attached a screen shot of the expression - is that the correct format for referencing the elements in ED.1?
Posted Mon, 08 Oct 2018 08:36:36 GMT by Peter Schätzl
You don't need the brackets for Saturation - just write 'Saturation >= 0.06. Brackets would be necessary only if you'd like to address a specific element (that is not the current one), but in your case you want to evaluate saturation of the current element wich is simply 'Saturation'.
Posted Mon, 08 Oct 2018 08:54:49 GMT by zebetheridge
Ahhhh, so that's what it was!  ;D ;D. Thanks heaps Peter.
Posted Mon, 08 Oct 2018 08:58:58 GMT by Peter Schätzl
You're welcome! Glad it works :)
Posted Wed, 19 Dec 2018 02:22:07 GMT by Barbara Tsuyuguchi Civil Engineer Student

For calculating evaporation, if I am using an expression that depends of water level, when simulation is running, does  the software recalculate the evaporation rate each time step using the current water level information?
I am implementing it using an expression on In/outflow on top/bottom. Is it right or should I do it another way?


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