Mike -

I would have to say that the HID light in itself will be creating enough heat for your plants to use that much water. On average, I am sure that the 12 tomato plants use far more water than we would like to think. How strong are your nutrients by the way? The weaker your nutrients, the more water your plants will be consuming.

Also, what does fairly good size mean? Depending on the developmental stage of the plant, there will be large differences in the quantity of water the plants consume. For instance, if your plants are now fruiting then you can assume water will be taken up at a drastic rate to keep those 'maters growing plump and juicy.

They are fruiting and have roots that are longer than 15". My ppm is about 2200.

Mike