One tends to think that plants are the essence of gardens. But no life can exist without water. So water is the essence of gardens.
Garden stores encourage the idea that water should be obtained from pipes and is distributed by pumps to garden fountains, cascades and waterfalls. This is a mistake.
Sustainable designers know that water comes from the sky and is distributed by gravity. This is how water was managed in pre-twentieth century gardens. It should also be the principle for water use in the gardens of the future. All the water which falls on a property should be stored and used. Roof water should go into water tanks, water barrels and ponds. If pumps are wanted, they can be solar powered.
Most roofs should be vegetated. Vegetation stores water and recycles it through the process of evapo-transpiration.
Most gardens should contain a boggy area, supplied by rainwater. This lets you grow water-loving plants, including aquatic and semi-aquatic plants. The water is also good for wildlife. Just do it.