This 100-gallon Rain Barrel has a mosquito-proof lid
and a self-siphoning faucet to fill your sprinkling can.

It is filled from the white downspout of a nearby roof.
An inexpensive 55-gallon Barrel will do just as well.
Several will do even better.

Unless you get your water from a spring, there is much to be said for Rain Water. And if your spigot spurts that recycled chlorinated stuff, there's even more to be said. And if you have a Water Meter. Enough said !

Rain is absent of many minerals which clog greenhouse misters, and may eliminate the need for costly filtering. A minimal pump can pressurize your system. An inexpensive Fountain Pump may be adequate for Drip-irrigation.

If your Greenhouse or Garden are close to a roof with a downspout, all you need is a barrel to store the rain water. A window screen over the barrel hole will admit rain but not debris and mosquitoes. Why stop at one barrel?

Inverted "U"-shaped Siphons will automatically fill neighboring barrels to the same level. Removing water from any barrel will refill it from neighboring barrels. They can be filled or drained in any order. Overflow can be diverted to your bird bath or fountain. 

Siphons can be made from plastic pipe with elbows secured to prevent air penetration which would render them ineffective. The pipe ends are cut off at an angle to elevate the opening above sediment. The Tap should be high enough to get a container under it, but the higher it is, the less water will be available to siphon out.

Siphons are filled with water and very quickly inverted into the two Barrels. This works easiest if one Barrel is full and the other empty. A piece of paper towel loosely plugging the ends may also be helpful. Hold them with your fingers, as you quickly invert the Siphon. They will be washed out by the moving water.

Barrels are much easier to monitor if they are transparent or opaque. Gallons markings are a plus. If you don't have see-through Barrels, a Floating Gauge(plastic Water Bottle + 1/4" wood Dowel) can be helpful. Siphons can also be made from 8-foot lengths of 5/8" Garden Hose. It is cheaper, quicker, and MUCH easier to prime because it's flexible. 

With a piece of Garden Hose, 
1 - Hold both ends in one hand. 
2 - Fill them with water.
3 - Plug one end with your finger.

4 - Quickly plunge the other end into the full barrel.
5 - Quickly lower the plugged end to the ground.
6 - Allow a quart of water to drain out(with air).

7 - Stop the draining water with your finger.
8 - Quickly insert the end into the bottom of the empty Barrel.

9 - If the Barrel does not start to fill, go back to step 1.
Speed is your friend. Air is your enemy.

Note that Barrels should all have their tops even, so they all fill and overflow at the same time. Unless you want overflow to irrigate a certain area, then that Barrel top should be one inch lower than the others. 

Barrels can be far apart. A Garden Hose can be used for a Siphon, being sure that all air is removed from it. If air gets into your Siphons, they will quit working. Also make sure it is not pinched shut where it hangs over hard objects, like the edges of Barrels. We sell an automatic Siphon Valve HERE.

Your Barrel should be raised as high as possible to increase water pressure when you use a Garden Hose to siphon water from the Barrel. A stiff wire inserted inside the Hose will allow you to create a gentle curvature out of the Barrel, without kinking the Hose which would minimize water flow.

The larger diameter a Siphon is, the more water it will move, but the harder it is to get initial flow started. The longer a Garden Hose is, the more resistance to water flow is created by resistance and turbulence, and the less flow is realized.

Of course, the simplest method is dipping the water out with a Bucket. In Winter you may have to add some material that compresses when the water freezes, to prevent damage to the barrels. Or you may prefer to empty them, depending on the severity of your climate.

To fill a Sprinkling Can, submerge it with the Spout facing up.
This allows air to escape without gurgling and splashing.
It also skims floating debris from the surface.