You should feel truly lucky if there is an unlimited supply of water for your garden. You'll find parts of the world where water is rationed certain times of the year so it is not good for the plants. As soon as people discover how little water they have, many give up, but there are others who find ways to deal with the small amount of water they are allowed. You will notice it is possible to have a beautiful garden while conserving water.
Some of the solutions to keep your garden strong, include using mulch or compost to prevent drainage by insulating the water. An additional option would be to group plants based on the amount of water they need so that water can be focused on plants that need more. You might like to do something called drip irrigation which slowly and gradually drips water directly at the roots. These methods function very well especially when your area experiences droughts regularly. Since droughts are generally predicted, make sure you take steps when rain comes before the drought.
An individual who cares for their garden will likely have rain barrels to collect water from the rain. The foolish gardener may think it's a lot of effort and a waste of time, so won't do anything. After the rainwater barrels are set, you could potentially get several gallons of water. The most difficult part may be obtaining the rain barrels, but you can use garbage cans, or 55 gallon drums that are plastic from the local home improvement store. The primary expenses may be high and they'll get heavy if filled to the brim with water. You will also need a screen or cover to remove the debris and leaves. Positioning the barrels should be where the most water flows off of the house, so you should make sure you know where these places are in advance.
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An effective way to fill up your rainwater barrels is to make adjustments to your gutter so that water can go through several spots into a rain barrel. This is possible by slanting individual portions of the gutters, to divert the water to the corners. This will likely supply you with the most water from your roof structure. It does require you to do a little work at the front end but once you are done, there is not much maintenance involved.
This is probably the very best, and long term cost-effective, way to water your garden during dry times. The amount of work involved will be well worth it when you see how your garden will continue to thrive.