Part of the reason for a lack of water in the underground table is that humans have built over so much land. Things like concrete and asphalt do not absorb any water, so none gets into the soil where it needs to go. Buildings and other huge structures also block the collection of water, and cause it to run off, causing dangerous erosion. Owning a collection tank can help with this issue.
These tanks are built specifically to collect some of this rain and hold it in storage for later use. This water is safe to use for dishes, laundry, toilets and landscaping, which means that drinking water is conserved for human consumption only.
These tanks can be installed in a variety of places, and pumps to use them can be purchased separately, often from the same dealer who sold the tank to you. The pumps are generally either manual or automatic, but with energy-saving features. This means less electricity used, which is yet another benefit of this type of system.
There are two main types of tanks, underground and above ground. An above ground tank is just what it sounds like. It does not need a hole dug to place it, so you can put it on top of concrete, wood, or just dirt. You can place it near gutters to maximize the amount of water collected.
Underground tanks work much the same way, but they are buried underneath the ground. This is great for areas where ground space is at a premium, and keeps them out of sight. People who want to store extra water for long term use often turn to an underground system for their storage needs.
There are various sizes and capacities available. If you wish only to use yours for gardening, a small 100 liter vessel might suffice. If you intend to use this is a source of water for household needs as well, you might look into a much larger 10,000 liter model.