Hi Elad,
there's not one 'best way', but there are several constraints to consider when defining a concentration for recharge:
:( You can use a first kind condition only in the case that at the top nodes recharge is the only flow. If you have additional lateral flow, and recharge only contributes a little bit to the total amount of water in the top slice, you'd get too much mass into your model by putting a first kind bc.
:( Source/sink is not applicable as it defines a source/sink per volume of the element - so your known values of recharge concentration won't fit.
:) So the only way is to use a 2nd kind boundary condition (unit is something like g/(m²d) all over the top. You can calculate the value you need by recharge multiplied with recharge concentration.
[size=14pt][color=red]![/color][/size] You need to use 'divergence form' of the transport equation when using 2nd kind mass bcs. Using 'divergence form' might lead to difficulties at outflowing boundaries like pumping wells.
Best regards,
Peter
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Dipl.-Ing. Peter Schätzl,FEFLOW Support Services
WASY Gesellschaft für wasserwirtschaftliche Planung und
