Relationship between rainwater and solar radiation
Any relationship or correlation? That is quite a broad standard. Invoking Tobler's first law of geography, there is a relationship. I would question if there is such a thing universally, but there are some instances that we can see an overlap. For example, many deserts receive plenty of sunlight, but observe little rain.
One of the first things that comes to mind are instances where these overlap. A plant needs both water and radiation to grow.
Another thing that comes to mind is the investigation of land-atmosphere coupling metrics. There are plenty of metrics that try a variety of methods capturing a plethora of different dynamics. These are indirectly associated with sunshine, as the surface fluxes are driven by solar radiation.
As far as finding a specific spot for self-sufficient drinking water, I am afraid that any relationship you get will not be one that will yield such a desired result. If you want an environmentally friendly, generally reliable way of attaining water, try well water.