Dig a hole in your garden area about the size and depth of a large coffee can (8 in. x 8 in. x 8 in.) and fill it with water. Time how long it takes for the water level to drop. If in one hour the water level has dropped by 1/2 in., you can figure the soil drains an inch in two hours. At this rate, the garden soil will handle 12 in. of water in a 24-hour period, making the ideal depth of this garden 12 in.