Permaculture: Yeoman's keyline system for storage of water by opening up the land.
http://www.youtube.com/watch?v=OpAPRlg2Zgs
http://www.youtube.com/watch?v=YTaNCpmp7Yk
The cheapest form of storage is the actual soil. Modern agricultural practices involve heavy machinery and plows which compact soil and subsoil creating a plow pan. The work of Peter Andrews on the Australian story demonstrates a better method of using the natural contours and diverting streams to flood what were once flood plains.
http://www.youtube.com/watch?v=a7pMDHJ1sbY
Permaculture - Bill Mollison is all about capturing energies passing through the eco system.
http://www.youtube.com/watch?v=vUr4uPe9WBk
Reclaiming the desert by desalting and re-greening the desert. Desertification is caused by the removal of vegetation and subsequent over irrigation of soil. See
http://www.youtube.com/watch?v=sohI6vnWZmk
All water is diverted away from property and roads due to planning and building regulations. This legislation prevents water from being put to its full potential. It would naturally be stored in the soil, trees, and in other plants.
What about small scale rainwater harvesting? Water use including agricultural, manufacturing, building and residential use of potable water when grey water could be used. If we did not rely on chemical cleaners, additives, processing methods and products.
Obvious answer to deforestation is to replant deforested areas with trees. Erosion - coastal erosion is caused by altering the natural flow of water by erecting inappropriate sea defenses. Soil erosion would be cured by attention to all of the above.
As far as returning the wilderness to nature and supporting ourselves there is still a lot of work to be done on this subject. However, good Permaculture Design is a start.