A DAY, as we know, is approximately 24 hours long. I say "approximately", because, the time taken for the Earth to complete a revolution on its axis varies slightly, and an occasional "leap second" has to be inserted or deleted now and then to keep our clocks in phase with the rotating planet. Some of this variation, apparently, arises form the fact that our increasing appetite for water has shortened all our days, a consequence of a principle known as the "Conservation of Angular Momentum".
Momentum - roughly, "weight multiplied by speed" - represents something like a "quantity of motion". Angular, momentum, by extension, represents a "quantity of rotation" it depends on the rate of spin of an object, and on the distance of the various parts of the object from its axis of rotation. And it is a cardinal principle of physics, from the atom to the cosmos, that in the absence of any external influences, angular momentum is "conserved": it remains constant. If, in the case of a rotating system, there is a shift of weight away from the axis of rotation - thereby increasing the "distance" part of the equation - there will be a compensating reduction in the rate of spin, and vice versa, so that the product of the two quantities always stays unchanged.
The principle can be seen at work, for example, in the case of a swimmer somersaulting from a high diving board; she can increase her rate of spin by doubling up her body. Likewise, a ballet dancer doing a pirouette with arms outstretched can spin more rapidly by drawing in his limbs.
Now during the past 100 years or so, the march of civilisation has resulted in thousands of reservoirs being built" around the world to supply water to the growing urban population. On average, it seems, some 10 thousands billion tonnes of water are stored in this way at any given time. Moreover, this water is not distributed evenly around the globe, but tends to be concentrated in the middle latitudes of the northern hemisphere.
This artificial interruption of the normal hydrological cycle, whereby rainwater drains away quickly to the sea, has in effect taken a massive quantity of water previously distributed in the equatorial oceans, and moved it closer to the poles, nearer to the axis of rotation of the Earth. As in the case of the ballet dancer, since the "distance" part of the equation has diminished, the rate of spin must increase - and so it has, to the extent that it has been calculated that a day, for this reason and excluding other influences, is now some eight millionths of a second shorter than it was about 40 years ago.