Both manual and automatic irrigation can be used with rainwater harvesting systems.
In basic systems such as rain barrels, manual irrigation is almost always the preferred method. Often a hose bib is incorporated into the bottom of the rain barrel, which makes it easy to distribute the collected rainwater across the landscape. Larger and more elaborate rainwater harvesting often utilize manual irrigation because it is much less expensive and easier to maintain.
Subsurface drip is often the preferred irrigation method to use with rainwater harvesting. The rainwater storage tank or cistern can be connected to a gravity fed drip system that provides water to a wide variety of plants and trees across the landscape. Such a system does require some filtration to ensure that the drip emitters don’t get clogged, but is generally easy to operate and maintain. This type of system typically does not operate on a timer and requires the user to operate a valve to start or stop irrigation.
Some complex rainwater harvesting systems do feature pumps capable of providing enough pressure to run spray irrigation emitters or even dirty water sprinkler heads. These systems are more expensive and more complex to install and require extensive filters to keep the pump from getting damaged and sprinkler heads from getting clogged.
Automatic irrigation tends to use a lot of water very quickly, hence it is only possible to use as part of very large rainwater harvesting systems that feature large storage tanks. These systems are typically designed and installed by professionals.