You will have better results if you use a tree with broad non-waxy leaves, like a maple or popular. *nerd alert* It happens because of a process called transpiration, plants have special openings called stomata which open and close letting water out and CO2 in. On a hot day a large tree can loose several gallons of water through this process, although to get all that you would have to have the whole tree in a bag. Several factors will effect the amount of water you get this way, like temperature, how much water the plant has avalible to it, and the amount of sun the plant is getting. I would guess that on the second and third day you may get more water due to the plant opening the stomata wider to try and get more CO2 as the level of CO2 dropped in the bag. I'm sure there would have to be a break over point where water production would drop off. It would be wise to try several differant plants in your area so you would know which worked best if the need ever arose.