All the manual observations at Cranbrook of the depth to the perched water table over 3 growing seasons in 1997, 1998, and 1999 and the automated observations in the 2000 season are presented as cumulative frequency distributions in Fig.
From the distributions it is clear that the control has a higher occurrence of a shallow perched water table than the raised beds.
In the control as well as the raised beds the perched water table rapidly rose during rainfall events but there was a difference between the treatments in the height of rise and the rate of fall of the perched water table.
From the convex shape of the perched water table in Fig.
In 1988, however, rainfall exceeded pan evaporation almost every week from mid-May until late August; throughout this period the perched water table on top of the mottled zone remained quite high (Fig.
As the rootholes extend far upward, the higher the perched water table, the more water can flow into, and down, the rootholes penetrating into the mottled zone, and hence on to the restricting layer at the top of the pallid zone, causing water to perch there (cf.
This is demonstrated by the fact that, in the winter of 1987, which was not consistently wet, in nearby ploys that had been deep-ripped, build-up of the perched water table at 1.0 m was less, but at 3.0 m was more, than in the conventionally tilled plots.