gd,

How do you know how much water you are losing per time period? what is your pipe material and how large is the piping? How many zones do you have? Why such a small tank? Aloha

I'll assume that you can't see any particularly wet spots. That said, if the leak is in one location and you've dumped many gallons of water at that spot, try a prod rod. In most soils, excluding very sandy ones, the drier the soil, the more difficult it is to insert a rod (or screw driver) into it. a saturated soil can be easily distinguished from drier soils by the relatively great ease in pushing a rod into it. If you know where your lines are, follow them with the rod looking for the very soft spot. If the leak has been happening for a long time you will probably be able to put the rod in the ground easily for 1-4' or more. My prod rod is a piece of steel bar stock 4' long x 3/8" dia. with a 1/2" ball bearing welded to the soil end and a T handle on the other. The ball bearing is there to help keep me from shoving the rod through pipe, definitely a no, no! If you are looking for a leak in pipe only a foot or so deep and know where the pipe is it should be very easy to find the leak. Sometimes it can be useful to look for differences in what is growing on the ground as plants respond to vastly different moisture levels, like cattails sprouting in a cactus garden or just more growth in a small spot.

If you want to spend thousands of dollars, there are devices that are made to do just what you need that hook onto the pipe and work very well down to depths of many feet. After you find your leak maybe you could start a small business hiring out to find leaks for others and get rich :)