This year we actually got more than our usual rainfall, thanks to the storm of the week. In the past years, the Big Island had drought conditions for more than 3 years. As of November, 2018 we are ahead in total rainfall and are drought free.
This past summer was not as hot. Fewer days above 90 degrees, but with a lot of humidity. The summer storms meant that there was some rain every month. The heat does limit what will grow. That and disease has shortened my summer list of veggies. The tomatoes got TYLCV so now, I need to find resistant varieties. I can grow holy basil but not sweet basil and it actually tolerates the heat better. I am growing more tropical vegetables like yard long beans and chard since they produce better; have fewer disease problems, and tolerate heat better. Wild bitter melon is a weed in my yard. The citrus trees do need to be watered every 1-2 days because they are in pots, but otherwise the heat does not bother them. I usually plant throughout the year, but when it is a hot dry year, I harvest only June-August and start planting in September. It is actually still hot in September, but I need to plant broccoli in late summer so they will head up during the cooler weather.
I do have air conditioners but I have not used them in years. Even on hot nights, I only use the ceiling fans because it just costs to much to run them.
The gutter water does go into the rain barrels and I have connected rain barrels in tandem to increase storage. The thing with rain barrels is that you have excess water when you don't need it. One 55 gallon barrel will water my front yard once. Rain garden is an idea. Usually, if the rain comes down slowly, it gets absorbed well. Runoff is only a problem when the rain comes down hard and fast or has been continuous for weeks and has already saturated the soil.
I have had to turn off my sprinklers and it is survival of the fittest in the yard. Geraniums do well, in fact they do worse when there is too much rain since they get oedema and the stems can rot. The roses, most of the established trees and shrubs in the ground, and rosemary, are pretty drought tolerant. They can survive on very little extra watering. The worst thing of course has been the weeds, since they take over any bare ground. I have planted alyssum as a ground cover and that has helped since it does not need a lot of water. Although, I keep getting harassed by the HOA that does not like that the alyssum mounds onto the edge of the driveway. I don't feed the grass, since feeding it makes it grow and then I have to mow it more. It is a mix of dwarf St. Augustine grass and weeds and any water will set it off. When it does not rain for a while it gets quite yellow, but greens up fast with a good shower.
I am watering my garden less over all. I used to water it every day, but that can make the plants dependent on that next drink. So, I try to extend the time to a day and a half or two days when I can. Some plants can't but, sometimes it is because they need to be up potted. Up potting has helped some of the peppers in gallons that really needed to go up and I have a cutting celery that really needs to go in the ground already.
I tried soil polymers and while it worked fine in summer, I think. It was a mistake when the rains came and they just bubbled out of the soil and became a gelatinous smothering mess.
Plant spacing and deep but infrequent watering has helped the most. Mulching has also helped to keep a lot of the weeds down. I do have problems with mulch when I don't have the drip system on, since I seem to be spending a lot of time watering the mulch instead of the soil. Once the plants size up and shade the ground, they use proportionately less water than when the plants are small.
This is the drought monitor that indicates the regions of the US that are in drought or have varying degrees of dryness.
Happy gardening in Hawaii. Gardens are where people grow.