What is the relationship between drainage density and
a) Rainfall
b) Vegetation
c) Soil Erodibility
Since this is not my area of expertise, I searched Google under the key words "factors affecting drainage density" to get these possible sources:
https://www.google.com/search?client=safari&rls=en&q=factors+affecting+drainage+density&oq=drainage+density+is+determined+by&gs_l=psy-ab.1.0.0i71k1l4.0.0.0.121567.0.0.0.0.0.0.0.0..0.0....0...1..64.psy-ab..0.0.0....0.z1OttTXgcys
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
http://www.hackcollege.com/blog/2011/11/23/infographic-get-more-out-of-google.html