Like many we had a very wet spring in eastern NC. We have an area of 30 oaks that were 5 years old and about a dozen died due to ground saturation (the lack of oxygen) according to NC Extension. We did irrigate 2-3 times later in the summer.
How late in the fall do we need to irrigate if things are dry?
In the Hillsborough area of NC we had the wettest summer in recent years, but things began to get dryer in the latter half of September, though we are still getting fairly frequent light rain in October. Only felt the need to irrigate one time this summer. However, the water table still seems to continue to be lower than it was ten years ago--springs dry, ponds periodically low. 2013 summer and early fall temperatures below average.
I found a study in a scientific journal regarding the amount of water that is optimal for truffle growth. The study was published in 2006. The study was conducted in Spain and looked at three watering regimens to determine which one was best.
"Treatment effects were evaluated by examining the mycorrhizal status of out-planted 1-yr-old Quercus ilex seedlings and the seedling growth parameters after 18 months in 3 distinct experimental truffle plantations."
They cited other studies with Corylus Avellana that showed the same effect. Moderate irrigation promotes fine root development and expansion of the mycorhizal colonization but overwatering leads to be "unfavorable"
The study concluded that watering is good but overwatering bad.
so that still leaves many of us with the question of how much water do I need?
of course this depends greatly on the type of soil in which you are planting.
They used a rather complicated formula looking at the "Evapotranspiration" of the soil. This takes into account the heat of the summer and the hours of sunlight as well as the general water retention of the soil.
After alot of fancy math they conclude that you can water anticipate water needs by looking at the rainfall deficit from the summer months (lowest Rainfall/highest temps) to the winter months ( higher rainfall/lower temps) and providing half of the deficit to the field in two watering sessions during during the next month.
Using this method requires you to do some estimation of rainfall and tracking your rainfall amounts and taking them into consideration before you set your irrigation system on a standard setting. ie. one gallon everynight per tree.
For example: If your average rainfall in Jan-Feb is 6 inches and July is 3 inches then only give them the equivalent of 1.5 inches in two sessions of watering in July. It is a good starting point. If you see water standing in areas then it is obviously too much.
If you wanted to do the watering like they did as a starting point. they used 55L/sqM or roughly
15 gal/sqyd divided into two application/month. at this level of watering they saw fine root development and good mycorrhizal association on those roots.
We have already drained our irrigation lines. I like the info Gretchen found ... makes sense to us. The growin area in Spain is extremely dry. We have followed a general rule to irrigate only if we have not had sufficient natural rainfall every 7-10 days, particularly in the growing period of July - September. Sufficient generally translate into about 10mm every 7 -10 days. We installed a weather station on the property to track actual rainfall among other parameters. This has been a huge help. We have had enough moisture lately that we don't anticipate any significant dry spells on into winter. We learned from our French colleagues that too much rain / water in November/December will rot the truffle. I believe it's all a fine balance. There is a lot more information available in the public domain as Gretchen has found ... much better for us all.