This is a very pretty picture! You have composed it nicely. I think the only thing I would have done differently is lower the exposure a bit to avoid having a washed out sky and then use a strobe for fill light. I love the way the sun is illuminating her hair though. Good job!
Yes, strict use of natural light has its up's and down's but you seem to utilize it very well. So instead of using a strobe for fill.. think about using a large reflective surface to bounce the light back up on to her face, if you do not already use that technique. I have recently started using a large piece of cardboard wrapped in tin foil for my outdoor portraits and the results are great!