On large dry-farms, water storing fallows are used. Tillage or herbicides are used to keep all vegetation down for a whole growing season, thus allowing the soil to store water for the following year.
What do you suppose the minimum size of plot would be to get an advantage from this effect? On a really tiny patch, I'm guessing nearby vegetation would suck it all up. Would a quarter acre (100 by 100 feet) be sufficient? Would using a chisel subsoiler around the edges keep tree roots out? What about a trencher?
Of course, clear tillage for a whole summer or spraying herbicides is not sustainable. I'm thinking that a tall, dense, drought tolerant and frost sensitive cover crop such as sorghum could be grown one year to suppress weeds, and left standing as residue, which according to research would actually trap more water than bare ground. The cover crop would have to be timed right so that seed didn't mature, and the ground would have to be fairly weed free for this to work.
In short, could this result in extra water storage beneficial for a dryland subsistence garden?
Our fenced garden is 100 x 100 10,000 square feet. I totally think that is large enough. On my grandpa's old dryland wheat farm the wheat is planted right up to the edge of perennial grass. I am sure there is an edge affect but it isnt super obvious. Grandpa tilled, his successors use glyphosate. Edge effect would depend on the lateral spread of grass roots and it would be less the further from the edge you were. Grandpa cropped religiously every other year. His successors use a soil moisture meter.
From what I understand(and please forgive/educate me if I misunderstand), they rotate from crop to fallow every other year, and they leave crop stubble and chaff on the field after harvest for the fallow year - which in terms of conventional commodity farming, is a very novel approach; in terms of permaculture this is a very simple principle - they are using mulch to retain moisture! Granted, it is a VERY thinlayer, using the chaff on hand, and likewise, only retains a LITTLE extra moisture, comparatively, but in terms of hundreds or thousands of acres, the retention is huge, and fully mulching all that land would be cost prohibitive.
So, in terms of a 1000 sq ft garden you're going to have VASTLY better water retention with full mulch than using this fallow system. but, even this fallow system is better than bare ground.