Harvesting the rain doesn’t stop at installing rain barrels, it involves the entire garden and its principal goal is to keep the water from running off the plot onto paved areas, only to eventually end up in the storm drains.
Careful planning can create places for the rain water to slow down enough to percolate into the soil, as well as ways to move it through the landscape and places for it to settle in. Depressed spots are not desirable features, since nobody wants to end up with a lawn full of puddles, so the water catchment area needs to blend naturally into the design and be populated with rain garden plants.
Where most of the property is covered by lawn water run off is not an issue, but for areas where lawn is not an option, use gravel and permeable pavers, or try stepable xeriscape plants.
If there is a place that can’t be left bare, it’s a slope. Slopes are always prone to water and soil run off, a problem easily fixed by stabilizing them with ground covers or in extreme cases, by creating terraces. Don’t underestimate the power tiny roots can hold, the plants will keep the dirt from eroding when all other methods have failed.
If you have clay soil in your flower beds, amend it, you’d be surprised how little water gets through it, especially if there is even the hint of an incline. If you need proof, remember how the mulch got washed away after the first rain. Don’t cover the clay with top soil, it too will get washed off, dig the flower beds to a full spade’s depth and carefully blend in the garden soil, both your plants and the city storm drains will thank you.