He told us to be sure to water the trees by filling the pipe every two weeks. Here in New Mexico, in all but the sandiest soils, irrigating an established tree every two weeks in the summer and once a month in the winter should be adequate. However, the frequency of the irrigation is not the important factor in your case. You should be concerned about where you apply the irrigation water. The pipe in the ground will place the water below the level of the majority of the absorbing roots, those responsible for absorbing the water and mineral nutrients needed by the trees.
Most people are surprised at the shallow depth of tree roots.
The majority of the absorbing roots of trees are in the top six to twelve inches of the soil. Some absorbing roots will extend down to a depth of about three feet, but the pipes placed at the base of your trees start irrigating at three feet, below the level of the ash tree roots. The second problem I see in what you described is the fact that you are watering at only one point for each tree. A tree has an extensive, spreading root system which is necessary to extract sufficient water and mineral nutrients from the soil. A wide spreading roots system is also necessary to support the tree in strong winds, which we experience here in New Mexico.