Do you even bother to research a little to see if the question you asked can already be satisfactorily answered? I did explain where the energy comes from, but you missed it somehow. So.....
"The cohesive forces among the liquid molecules are responsible for this phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in every direction by neighboring liquid molecules, resulting in a net force of zero.The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This attraction between molecules forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submerged."
"Surface tension pulls the liquid column up until there is a sufficient mass of liquid for gravitational forces to overcome the intermolecular forces."
It is surface tension that causes the liquid to draw up the tube. There may be a way to harness this energy in some way, but it is a very tiny force which is only cabable of moving a few milliliters of water a few centimeters. IT does not seem likely that this will ever comprise any substantial power source.
As for your reservoir idea of energy....it doesnt really work that way with gravitation. Imagine I throw a baseball at a target. Chemical energy in my muscles is converted into kinetic energy to move my arm, and some of this energy is transferred to the ball. As the ball travels through the air, some of the energy is also transfered in the form of friction. Ultimately the ball strikes the target, and the original chemical energy has been converted to kinetic. This is where you are getting your "reservoir" idea, and it is perfectly accurate in this arena.
Gravity works a little differently. All matter distorts the fabric of spacetime and attracts all other matter. What determines the strength of the attraction is the mass of the objects and the distance between them. There is no conversion of energy really, so there is no "reservoir" to be depleted. Every atom of matter in the universe is constantly attracting every other bit of matter in the universe, simultaneously. This attraction causes an acceleration as two objects get closer and the force of gravity increases. This causes an apparent increase in the kinetic energy of the objects, but this is coming from the attraction itself and not the conversion of energy from one type to another. Since gravitational attraction is an essential property of all matter, there is no reservoir to drain or account for.