Let's face it, everyone would love it if their home could be powered by moving water.  It looks cool.  It sounds cool.  It's free and essentially endless (forget about droughts for a moment). But the truth is only a teeny tiny percent of the population have optimal conditions for installing a residential micro hydro generating system.  Just like a residential wind turbine is a miniature version of a utility wind turbine, a residential micro hydro system is a miniature version of a utility hydro system.

HomePower Magazine has a great feature on Microhydro Electricity Basics.  They say that the power output of a micro hydro system "is the combination of head and flow. Both must be present to produce electricity."  In layman's terms:

• Head (or Pressure Head) is how much "oomph" the water has.  Think of a ball falling.  As we all know, as you raise the book higher and higher, the force that it hits your head with is going to be harder the higher the ball goes.  The higher something is, the more potential energy (static head) it has because the accelerating force of gravity will have a longer time to work on that object.  The higher a lake or stream is above the turbine, the more force it will have it will hit the turbine
• Flow is how much water is flowing through your turbine.  You can have a water source a mile higher than your turbine, but if you only let a few ounces of water fall, you obviously aren't going to get that much power output.  You could also have as much water that flows in the Mississippi River flowing through your turbine, but if there isn't a large elevation drop in your backyard, you won't get much power.

But I don't even know why I'm trying to simplify this for you, because the HomePower article does a great job of explaining both:

Head is water pressure, which is created by the difference in elevation between the water intake and the turbine. Head can be expressed as vertical distance (feet or meters), or as pressure, such as pounds per square inch (psi). Net head is the pressure available at the turbine when water is flowing, which will always be less than the pressure when the water is turned off (static head), due to the friction between the water and the pipe. Pipeline diameter has an effect on net head.

Flow is water quantity, and is expressed as "volume per time," such as gallons per minute (gpm), cubic feet per second (cfs), or liters per minute (lpm). Design flow is the maximum flow for which your hydro system is designed. It will likely be less than the maximum flow of your stream (especially during the rainy season), more than your minimum flow, and a compromise between potential electrical output and system cost.

Just because you have a creek in your yard, doesn't mean that you have optimum conditions for micro hydro.  You need a creek/river/"lake with a pipe to your turbine" with a large volume of water flowing and at a considerable force.

Regarding the combination of "head" and "flow" in micro hydro systems, the Renewable Energy Handbook (REH) says:

Most sites are located in an either/or situation.  A high head site is generally located in a very hilly or mountainous region, where the water might flow through several hundred to several thousand feet (100 m to 1000 m ) of plumbing line.  Fortunately a high head site requires little flow, so the pipe and turbine in this instance are fairly small.

The opposite is true of low head sites.  Commonly known as "run of river" designs, low head sites can be sized to operate from 2 to 10 feet (0.6 m to 3 m ) of head.  What we gain in lower head we pay for in more water flow and a larger turbine; nobody rides for free.

The Renewable Energy Handbook also mentions Energy Systems and Design, which makes micro hydro turbines and their website has some nice information.

Finally, the REH mentions a Zero Head turbine, which basically operate in streams/river with minimal head.  I like how they suggest predicting power output of these locations:

Throw a ping-pong ball into the stream.  If you can keep pace with the bal at a walk the site is too slow.  A brisk walk to partial jog indicates a power output of 6 amps at 12 volts potential (1,700 watt-hours per day).  If you need to increase your speed to a jog, the power output will be 9 amps at 12 volts (2,500 watt-hours per day).

If you have water flowing on your property then consider micro hydro!  There is obviously way more to discuss on this topic and we are just scratching the surface.

I'd like to hear about anyone's experience with micro hydro in the comments below!