Watermelon experiment

I’m trying to figure how to maintain a garden, primarily on harvested rain water. I dug these holes a couple inches below the ground and put some Watermelon seeds in them. I don’t get much summer rain but the relative humidity is a little high most days. My thinking is because it is a a lower level than the top soil, the soil will stay cooler and more moist. I did a similar thing with some corn. The only extra water I gave the corn was when I fed them. Has anyone else tried this?