It's a great argument except for one, little thing.The water is coming out of the hose at a steady rate as our model assumed!That is, we can use carbon-14 dating on a given tree-ring (the 8000-year sequence having been assembled from the overlapping tree-ring patterns of living and dead trees) and compare the resulting age with the tree-ring date.
When the dipole moment is strong, carbon-14 production is suppressed below normal; when it is weak, carbon-14 production is boosted above normal.
What the magnetic field does is to partially shield the earth from cosmic rays which produce carbon-14 high in the atmosphere.
That is, the equilibrium point should have long since been reached given the present rate of carbon-14 production and the old age of the earth.
The next step in Henry Morris' argument was to show that the water level in our barrel analogy was not in equilibrium, that considerably more water was coming in than leaking out.
Now, the fuller that barrel gets the more water is going to leak out the thoroughly perforated sides, just as more carbon-14 will decay if you have more of it around.
Finally, when the water reaches a certain level in the barrel, the amount of water going into the barrel is equal to the amount leaking out the perforated sides.
(The barrel is made deep enough so that we don't have to worry about water overflowing the rim.) Henry Morris argued that if we started filling up our empty barrel it would take 30,000 years to reach the equilibrium point.
Thus, he concluded, if our Earth were older than 30,000 years the incoming water should just equal the water leaking out.
Figure 19.5, curve C, shows the dipole field strength calculated from measurements of magnetism of lava flows and of artifacts such as pottery and bricks, whose age can be determined.