SUMMARYComputer simulation of the soil water regime was used to calculate moisture deficits for Pleistocene and fluvial Holocene clay soils for a 30‐year period. Hydraulic conductivity(K‐h)and moisture retention(h‐θ)data were derived for major horizons in both soils as well as functions to characterize bypass flow and effects of horizontal planar voids on upward unsaturated flow of water from the water‐table to the root zone. Calculated moisture deficits in Holocene soils occurred earlier during the year and were more pronounced during the summer period. Differences between the two soils could be attributed to more pronounced formation of horizontal planar voids in the Holocene soil, which was associated with more pronounced swelling and shrinkage of the undisturbed soil material. The relatively low swelling and shrinkage of the undisturbed Pleistocene soil material was attributed to very dense packing of the elementary soil p