I am looking for some hard data on the ability of Western red cedar to intercept rain water and the timing of release. Got numbers?
If I am understanding your question correctly, you are asking about the extent to which Thuja plicata (Western red cedar) allows rainwater throughfall. I discovered that there was some research of possible relevance to your topic being done at
Evergreen State University as well as at
University of Waterloo.
The Miller Library has a book entitled Water in Environmental Planning by Luna Bergere Leopold (1978), which might also be relevant to your search.