Cover Model becoming less efficient with increasing thickness

[cwainwright]

Hi there, I'm simulating the efficiency of different soil cover materials and thicknesses. When I keep all variables the same and only increase the thickness of the model, more water is present in my sum(vBot) column. I have thicknesses ranging from 12in to 36 in. Root depth is modeled to extend 12 inches in every model. Any guesses into why this is occurring?