Grass clippings are a bit green, but they certainly hold water in. If
you can find a tree company willing to dump some wood chips on your
driveway, you will be all set. The chips last much longer, and have
better P content, too. The only way to save more water is to limit sun
exposure. You can do it by having beds on the east side of the house,
so they are shaded in the afternoon. Or you can grow some veggies under
beans or tomatoes, whatever will give them dappled light. Even in
Michigan brassica like to be in partial shade in the summer.