dr. kate baird, is measuring the amount of water used by vegetation near the san pedro river, a river that has been in decline over the past decade. for each tree, baird determines the volume of water traveling through that tree. to do this, she measures both the velocity of the water moving up the tree and the area of its xylem tissue. dr. baird: so, if i wanted to understand how much water is flowing through the tree or howuch water this tree reallyeeded in any day, we would simply peel off a small part of this bark, drill two very small holes into the tree, and insert this heat probe. narrator: a double probe is used to measure the velocity of water moving through the tree. the lower probe is heated to a known temperature. the other measures the temperature of both the tree and water in the tree. if water is not flowing, the upper probe records the maximum-temperature difference possible. the faster the flow of water, the more heat is dissipated, and the more the temperature is similar. in this way, researchers can determine the amount of water used by that tree. baird also takes small