We are in a state of extreme drought here, on a 100% outdoor watering ban.
I've no idea how you can get more groundwater without drilling an actual well. But I know a little about rainwater and storing graywater.
Stored rainwater does work, it just depends on the size of your barrel and your roof size. Larger the roof, the more water you collect. An hour rain usually gets me about 50 gallons. From there I dole out a quart on some plants, 2 on some that really need it, every other day.
I've been keeping a 5 gallon bucket in my kitchen and shower to collect dishwater and any wasted gray water from the shower to put on my plants.
This is pretty labor intensive as I do have 1/2 acre to water, I just try to make sure everything gets some water. I'd love to haul a hose around, but for now it's buckets for me. Actually, I wish the skies would open up... for about three months. haha.