I think this site may help you as well

Here is a link that might be useful: water needs of citrus trees

Over watering can cause leaf curl as well, so rule out watering for right now since its only effecting the one tree and they are all getting the same water.

Wind can cause leaf curl, is the tree in a windy area?

There are several pests that can cause the leaves to curl, Inspect the leaves and stems for insects. Leaf miner, aphids, and scale are common insects on citrus. Cross your fingers that its not scale, they are probably the hardest to get rid of. Check all the leaves for a sticky substance called honey dew, the scale secrets this and it attracts ants. Scale look like tiny turtles or fish scales and they don't move as adults they just suck the sap from the veins.

If it is insects I would first try a high pressure hose attachment and try to blast the pest and their eggs off the tree, if that doesn't work try a horticulture oil and insecticide soap. Apply in the evening when the sun is going down. These sprays need to actually make contact with the insect to work so you must spray the undersides of the leaves.

Good luck!

I checked tthe tree yeasterday and though the leaves are really curled, some of them have area in the edges that appear to be dead. Did not see any bugs other that one small one that scurried away from my finger when I got close. One small cluster of leaves on the north side of the tree had what looked like a smoky gray area about 1" in diameter.
The strange thing is when I first saw the leaves curling it was getting water. In fact unbeknowngst to me but the landowner also checked on it, raised the amount of water to everyday for 30 minutes. No change at all.
Now I did finf a large hole under the outer branches and wonder if some rabbits dug the hole an chewed on the roots.
Thanks for the suggestions so far.