I have a site where I want to create a water harvesting/graywater irrigated urban food forest. Unfortunately, the site has shallow (26") heavy clay soil on top of bedrock (at least 6' of a calcium carbonate rock called caliche). I also want to concentrate 10s of thousands of gallons of roof runoff into about 10 basins. Three of those basins will receive graywater and so shouldn't have an overflow. (Though I may do that anyway for large rains.) How can I increase lateral percolation away from these basins? Since water can't go down, it has to go sideways at sufficient rate to prevent prolonged water logging. Because of cost, I don't want to just dig it up and replace or amend the soil. I would have to excavate and amend over 200 cubic meters. And in any case, the site is an occupied residential area and I don't want to dig up everything. I am considering a heavy crop of legumes and grasses (or other plants) with deep/large roots. Will this increase percolation out the sides of the basin, if I don't also add sand to physically break up the soil? What plants should I use that are adapted to alkaline soils and -8C to 42C temperatures? Since the problem is to get water out of the sides of the basins, Do I need to build some sort of French Drain around the edges of the basins? If so, can you suggest any designs or design issues?