Many sites do not recommend putting a barrier under the ground due to the lack of drainage but I have found none mentioning the advantages of reducing water loss.
My garden/property is situated on 200ft of glacial till (gravel and boulders) topped with 1-3 ft of well-draining loam in most places and I am irrigating with drip from a well producing only 1gal/min of hard water. Our area is is northern Washington (cold desert with sage scrub) 1300' zone 5 a little below the pine elevation. A couple years ago I dug out either 50 gal holes or 32" wide x 24" deep x 40' rows filled with peat and this helps but still the beds still require alot of water during the 117F hot dry days of summer.
This last year I was planting a row of honeyberries and like my blueberries dug out 3ft wide x 2ft deep of what was mostly rocks but this time I lined the holes with leftover black plastic up to 6-12 inches below the soil line. This effectively creates an underground water collection/pond. I watered with 2 x 0.5 gal/h drippers 90 minutes or 1.5 gal/day.
The area around the honeyberries stayed continuously wet. I co-planted with eggplant and tomatoes around the small berry plants. The berries and vegetables produced and some moss grew under the plants. Water loss seemed predominately at the surface and some hard water deposits accumulated on the soil surface. In late summer I thought there might be water logging and salt build up from the water and fertilizer so I pushed a steel digging rod through the peat to puncture the plastic in a few places at about 2.5 ft depth around the sides. This provided some drainage and still, the soil stayed wet.
Over winter we had a couple of feet of snow which hopefully helped flush out some salts. I doubled the number of holes with some in the middle at the bottom of the plastic and reduced it to 1 dripper per plant and still, the soil still stays moist around the plants. After fine-tuning the drainage the system seems to work as planned. The berry plants are growing well this spring with less hard water deposit accumucation on the surface.
I haven't seen much of anyone trying this sort of technique to reduce water loss and so far it doesn't seem like a total failure. I'm thinking of re-doing my raspberry/blackberry beds by making a 40ft long underground plastic-lined trough filled with a peat loam mixture.
I guess this simpler/cheaper variation of an in-ground wicking bed.
Any ideas?