Water harvesting/storage

We've got about 500L worth of storage and wish we had more, probably double would be OK. That's mostly for watering the greenhouse (10ft x 6ft) and outdoor containers. We ran out of water a few weeks ago and will likely run out again in summer if we have a prolonged dry, hot spell. I did water the raised beds this spring as the soil was getting very dry, most of the time I only water at planting. Part of the reason I prefer growing in the ground is the lower water requirement. In the greenhouse, the tomatoes and half of the peppers will be in the borders this year so hopefully they won't need watering so often.