Jean:

I have investigated the influence of soil depth on simulated soybean yields. It is published in the journal Agricultural Systems 63:19-32. 2000.

Our results indicated a nonlinear decrease in yield when the soil profile depth was reduced from 90 cm. A greater reduction seems to happen when soil depth is reduced below 67 cm.  The publication also gives the cumulative probability function plot for soybean yield in various soil depths. Perhaps this would give you reasons to believe soil depth is critical in plant available water supply.

Alagarswamy