Attached Link: http://www.evermotion.org/tutorials/particles/rain/

In this tutorial (which was made for 3dsmax) there are some excellent "wet" maps that can be used in Poser as well to test the results. Its an excellent example of a highly realistic "wet" ground material. He offers the 3ds scene files for download, and the maps themselves can be downloaded seperately for you to run tests with. You can't use them commercially of course, but you can use his maps as a reference to make your own reflection maps in photoshop that simulate the effect as seen in that tutorial, right in Poser. Creating the "rain" (if you wanted to) would be another issue, but Poser can probably simulate that too using the Metaparticles plugin. This example shows a "ripple and splash" effect from the rain hitting the ground, but you can use it's basic reflection map principles to produce any kind of wet surface effect. It's just a matter of creating a greyscale map of the ground surface that you plug into the reflection node of the ground material, telling it what parts of the ground are more "wet" than others. Using this map in the bump node as well should produce a great wet effect.

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Tools :  3dsmax 2015, Daz Studio 4.6, PoserPro 2012, Blender v2.74

System: Pentium QuadCore i7, under Win 8, GeForce GTX 780 / 2GB GPU.