Water is the VERY first thing we should be designing around ALWAYS, without exception. Water is a broad topic within the PDC curriculum and for very good reason. Without water, life is simply not possible.
As with most Permaculture topics, by no means do I feel remotely confident yet in my abilities to harvest and manage water in all situations, but I certainly have had my eyes opened to a lot of different techniques. By far however, the most efficient, inexpensive way to harvest and store water for use by plants and trees is by way of the soil.
Feeding and nurturing your soil by adding lots of varied organic matter can improve the soil structure quickly, creating a spongey texture that will absorb and hold water. When combined with planting fast carbon pathways (plants to chop and drop or cover crops to work in), the soil will make dramatic improvements in its ability to store water (not to mention boosting fertility exponentially at the same time). Quite literally, THOUSANDS of litres of water can be stored in your soil without any collection or storage equipment, and if you have taken care to properly mulch your soil (to prevent evaporation), that captured water will be held in situ and released gradually to the plants as water is needed with absolutely NO involvement on your part. In some circumstances (and in some climates), soil building alone won't be enough to meet water requirements. However, improving your soil can go a surprisingly long way in dramatically reducing the amount of water required to collect, store and irrigate with.
I am slowly learning that by looking to the natural world, we can learn volumes about designing efficient systems. A truly brilliant permaculture design literally designs the labour (and the working parts) right out the system. Sustainable, intelligent design is patterned after nature and will function without your input (or at least with very little input on your behalf). I've got a LONG way to go in my learning, but this revelation is a certainly the firm foundation of good design...