[thedoc (OP)]

Jenny Young asked the Naked Scientists:
   In a drought/water shortage situation does watering a garden borehole affect  the general reservoirs or water storage systems?
What do you think?

[evan_au]

Drawing water out of a backyard borehole to water your lawn draws water out of the community's water table.

Depending on how porous the bedrock is, some of the water sprayed on your garden may percolate back down into the water table. But it you water at midday, much of it will evaporate - either directly from the soil, or by plants sucking it up via the roots and releasing water vapor through their leaves.

If water is drawn out faster than it is refilled from natural sources, the water table will fall.
- This is a nuisance if you have to deepen your bore occasionally
- This is more of a problem if a commercial operation steals all the water from traditional users (as has been alleged for some "mineral water" bottling operations)
- In severe cases, withdrawal of water can result in ground shrinkage and cracking of buildings
- This is a severe health problem where the falling groundwater levels allows excess levels of toxic minerals in the ground (eg arsenic and fluoride compounds) to dissolve in drinking water

Hydrologists can sample radioactive isotopes to estimate how "old" the water is. If the water is thousands of years old (or estimated at millions, in parts of Australia's great artesian basin), then it is likely that we are drawing out the water much faster than it can be replenished; we are "borrowing from the future", and our children will have to pay back the loan.

See: http://water.usgs.gov/edu/gwdepletion.html

[evan_au]

PS: See The Tragedy of the Commons.

It is a limitation of our economic system that we do not account for "externalities": things we get for free, but the cost is borne by others.

Wise governments will step in, and legislate to ensure that those gaining the benefit also pay some of the costs. This may include applying quotas, rationing, paying for a license and education programs to raise awareness. This often reduces the wastage and over-exploitation which occur when something is assumed to be "free" and "inexhaustible".

[zx16]

Doesn't it seem absurd that living on a planet where 70% of its surface is seas and oceans, we should be worried about a shortage of water?