I'm trying to simulate the influence of solar energy on groundwater temperature in an annual trend. Therefore I have set up a model of 3 different units (different cp and thermal conductivity) with a thickness of 5 m for each. As input for solar energy I used temperature BC as well as heat transfer BC combined with a time series of temperatures. Both simulation showed changes in temperature of +/- 0.3 K. According to mesurement values I expect changes of at least +/- 3 .

Does any one has some ideas to solve the problem, or should I rethink my model concept?
There may be multiple influences on groundwater temperature over the year. With your BCs you simulate the dependency on air temperature, either assuming that air temperature is equal to the temperature of the soil top, or by applying an additional resistance (heat-transfer BC). Depending on local conditions, you may have influences of solar radiation (on blacktop, for example), infiltration of rainwater, evaporation, ... Typically, however, I'd assume that by using a temperature BC you'd rather overestimate than underestimate the influence of air temperature. So hard to tell why in your model you observe such a small impact. I guess you've checked cp and thermal conductivity already. If there's an unsat zone, it could have an influence, but with typical porosity values, this would be small, too (FEFLOW assumes air to be non-conductive for heat).
Yeah, the overestimation by application of T-BC was in my mind as well. That's why I've tried to use transfer BC. Do you have any recommendations regarding this values. Let's say I know thermal conducitivty, what should I use for thickness? Both, model layer thickness as well as lithological layer thickness seem not optimal...

I think I'll try a high discretization to see changes at lower depths.

Anyway, as you've already mentinoned  there are many processes that influence groundwater temperature. So it might be a possibility to adjust my input temperature as kind of calibration of my observation. So all these processes are involved by some degree?

With some internet research, you might be able to find some ballpark numbers for thermal resistance between air and soil. These values depend on vegetation, wind speed, etc. - but at least you'll get an idea (I remember having found some values about two years ago - but I don't have access to them anymore). Indeed all processes are involved by some degree, but typically air temperature will dominate.

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