Square Foot Gardening in Extremely Dry Soil
I am doing my first round of square foot gardening in my yard. The soil in our region is known for being very dry due to lack of rain for the past three years. As such, local gardeners have warned that the water will be and is soaked up by the dry earth, even when using garden beds. I've been told by a farmer that the only way to prevent precious water from being soaked up (and keep our water costs down) is to use some sort of barrier in my square foot garden that will separate my storebought soil in my raised bed from the dry earth in the ground.
I'm already using some sort of weed mat to ensure that weeds aren't able to penetrate the soil in my raised beds. What can I use to keep the dry earth below from 1) absorbing all of the water, 2) letting weeds grow into my growbed, and 3) still keep the weed mat in my growbed?