Physics for Geologists, Second edition

Gravity 39
Figure 3.7 The 1970 potentiometric map of Australia's Great Artesian Basin.
The contours are lines of equal energy of the artesian reservoir,
and the surface maps its energy - the higher the surface, the higher the
energy. (Habermehl 1980, fig. 15, reproduced courtesy Geoscience
Australia.)
aquifer with salt water (at best!). This was predicted over a hundred years
ago, and acted upon successfully (see Chapman 1981: 97).
Artesian basins are yet another example of water flowing from low pres-
sure (atmospheric in the recharge area) to higher - if there is leakage in the
basin.
The siphon
More than two thousand years ago at Pergamon in Greece, water was piped
across two valleys in what they called a siphon, and we still call a siphon.
The energy of the water at the higher side of the valley is greater than that
at the lower side, and the water flows from the higher side to the lower.
The pressure in the water increases as it flows down into the valley until it
reaches a maximum at the bottom of the valley and it flows from smaller
pressure to larger pressure. On the other side of the valley, it flows from
larger pressure to smaller. Clearly pressure is not the significant aspect of
the flow. It can flow from lower pressure to higher provided the energy at
the higher pressure in a continuous body of liquid is less than that at the
lower pressure, that is, the elevation at the higher pressure is less than that
at the lower pressure. It is essentially a gravity-driven process.
Look at it now the other way (Figure 3.8), with the connecting tube ris-
ing above the liquid containers - also called a siphon, as when you siphon
Copyright 2002 by Richard E. Chapman