dr. maddock: there are certain processes that can only be described mathematically. what we've been able to do is to introduce into ground-water-flow models transpiration processes which reflect the behavior of the plants if you raise and lower the water table. in the past, that hadn't been able to be done. narrator: transpiration is the process in which water vapor is lost from land plants to the atmosphere. this process causes movement of water through the plant from soil to air. water enters a tree through its root system, moves through xylem tissue -- the very thin straws lining the inside of the tree -- before evaporating into the atmosphere through its leaves. dr. maddock: some of the trees in the riparian system can use quite an astounding amount of water. it's been estimated that under say, perfect conditions, a healthy cottonwood can take up as much as 90 gallons per day of water which is about the same as half of a normal household here in the southwest. t wee tag aba irly substantial amount of water that's being transpired by the trees. one of the things t