Ground Slab/basement Modelling-water reservoirs and wells

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Sorry for replying so late.

Basically I have a rainwater cistern made up of waterproofed boundary walls and slab, below the conditioned zone. The water level varies over the year so if we look as a section we will have (top to bottom): slab, air layer, water, slab, ground.

do you think that considering half air layer and half water (in term of thickness) is a correct assumption, don't you?

That's a starting point, but the cistern has side walls which add another dimension to the heat transfer, and possibly more important is the effect of rain water entering the cistern.

So if I am not mistaken I need to assess in such a way the 3d through the entire assembly and then add it into eplus as an equivalent construction? Do you think I would need a finite element analysis in order to model the ground slab?

Finally What about the thermal mass of the entire equivalent assembly? in this case is not appropriate to use a R-value material, to my knowledge. What do you think?

thanks in advance

Given that the thermal mass varies as water enters and leave the cistern, and these flows move energy in and out of the cistern, it's not clear that an equivalent assembly will come anywhere close to representing the dynamics of the cistern. Even a simple spreadsheet model of the cistern would be better than trying to mash it into an equivalent 1-d surface. With a model of the cistern outside EnergyPlus, you could produce a schedule of surface temperatures to apply using OtherSideCoefficients.