Consider a Rain Barrel
A rain barrel is a relatively simple idea. Normally the water from your roof would run into a storm sewer and then out into a local river, leading to massive amounts of not-always-clean water hitting the river every time it rains. Then, when you want to water your lawn or garden, you turn on the faucet and use up a bunch of drinking water.
A rain barrel should mitigate both the water use and the runoff problem. It’s essentially a 60 gallon barrel that goes underneath your downspout. It has a spigot on it. When it rains, the barrel fills up. (And even a small roof should be have enough water to fill it up in a light rain.) Then, for the next few days, you can use the rain water to do the watering. There is a little spigot on the bottom of the barrel that a hose connects to.
As an added benefit, it should get the water that falls on the roof a long way from the foundation of the house, which is a great way to prevent your basement from flooding.