Very little. (Full disclosure. I’m in Houston right now, and have lived here half my life.)
Houston’s region has a recorded history of flooding all the way back to the Civil War. Although there’s no question that over-urbanization is part of the problem, most of Houston sits on a very very dense clay soil that is not very absorbent. I can tell you anecdotally that any decent rain that lasts an hour and drops perhaps an inch leads to pooling and runoff in backyards.