Keeping Wet by Resistance Sensitive Relay

A simple way to do this is to use a negative temperature coefficient thermistor.

More on thermistors here.

How about cutting a notch in the edge containment, trickling in enough water to compensate for evaporation, and allowing the excess to drain through the notch?

Do a search on Global Spec (our gracious sponsors) for "Leak Detection Relay". The logic is the opposite, but the technology is what you are after, logic can be manipulated to your needs. It's typically done with what's called a capacitive sensor. Resistive are too easily fooled by condesnation and such.

Any of the above, or a simple level switch might be all that is needed.

What about a simple photo electric switch with a captive float between the sensors to either obstruct or clear the light path (dependent on the mode of operation required) when water is getting low?

This could easily be set up to be very sensitive to level change.

I believe posts 1-5 are all correct and appropriate responses, and are all better than the solution you propose.

To answer your question, it is certainly possible to use the electrical conductivity of the water to cause a circuit to switch, but for safety the switching circuit would have to run on low voltage, and would either require a logic inversion (the pins would conduct when water was present, and you want the pump to run when water is absent), or control a relay whose contacts were normally closed, and would open when the relay was activated. Also, the water would not conduct sufficient current to activate most relays, so some form of amplification would likely be required. Thus go back and choose between the possibilities of 1-5.

There will be losses due to perculation and evoporation. Acertain the quantum of losses. perculation losses can be reduced considerably by covering the entitre bed with black cotton soil.Evoporation will be mostly in summer,topup to make good the losses at suitable intervals.Since the quantum of losses will not be much a temporary arrangement will serve the purpose.

Take your pick! Or call them for advice.

http://www.gemssensors.com/en/Products/Level

Global Spec contains many hi-tech solutions to this, but if it were my problem, I'd consider using a toilet float and valve. They have proven their reliability under pretty crappy conditions.

I like this idea. I guess you could set it up in some kind of mount. And you'd probably have to extend the rod on the float. It might not even be a bad idea to construct a foam float in some fashion to replace the bulb.

Use a 12 volt bilge pump with a switch on it for a small boat take the pump off the assembly and put it in a drum of water. You can buy them at a lot of stores for around $15< $30 USD...