Rockhopper,

I think what he is referring to is that water is depleted from the soil by tree roots/plants needs. Any tree in question needs some unspecified water to survive throught it's life cyle. You are absolutely correct in the assumption that trees and brush for that matter can act as an isulator/shield to the evaporation of ground water. Once water makes it's way to the ground the plants and trees act as a shield or buffer thereby retaining much of the ground water through shading and cooling -That includes both water in the stream, river, lake, pond & marsh as well as that moisture that fell and beaded or collected at the surface of the soil and plants. Still these plants all need H20 to survive. But the demands are set by the type of climate, geography, individual location, specie, their age and many other factors. And yes, some moisture is lost to the inevitable direct evaporation from the surface of the plants themselves. Snow is a great form of water collection and savings. Most snow falls to the ground and collects there. Only moderate amounts are typically transferred through solar radiation and increased temperatures/sublimation (often above 32 degrees F) relative to the overall water content in the snowpack. -And of course the result is often in the form of run-off besides the typical evaporation rates. Does that help explain things a little??? ;-)