You need to provide country, experience, skills, and so on. No one rate exists for PhD Scientists.

This is highly variable by technical profession, and there is many factors such as area of expertise, experience, marketability, etc.  Rareness of skills, funding and urgency of project may enter.  Team and leadership skills, ability, work ethics and ability to accomplish work may enter.  Writing skill important too.  An independent consultant without contacts may have difficulty finding work.  A well known expert may have more work than desired.  Google on the profession, such as engineer pay in USA, and pay will vary by locale also.  Working for a consulting firm is probably best if you are not well known or experienced, or do not have some spectacular skills that would set you apart.  Sometmes, you may have to take work to just live until you find the right position.  You may have to move to where demand is higher.  

Sometimes less pay can be better as cost of living varies by locale, company or supervisor quality, more enjoyable work duties or co-workers, community, housing, etc.

http://www.payscale.com/research/US/Job=Hydrologist/Salary

Try this site out.