M. Huang, S. L. Barbour and S. K. Carey

Abstract Multilayer covers are widely accepted reclamation designs in the oil sands region of northern Alberta, Canada, with an ultimate goal of revegetating to species characteristic of predisturbance native plant communities. To determine the optimal depth of reclamation material required to reclaim overburden shale from an oil sands mine, an evaluation was made of the long‐term performance of six reclamation soil cover depths all placed over overburden. The measured soil water contents from different cover thicknesses at South Bison Hills located at the Syncrude Mine site north of Fort McMurray, Alberta, were used to calibrate and validate a dual‐porosity model in HYDRUS‐1D. The calibrated and validated model was then used to evaluate the influence of cover thickness and climatic variability on plant available water for forest growth. The frequency distributions of actual transpiration (T r ) for six cover treatments with a range of leaf area index (LAI) cases were developed. These T r frequency distributions were then modified by coupling T r and LAI. The modified frequency distributions for annual T r for the six simulated cover thickness highlight the strong nonlinearity between the distributions of T r over a long‐term (60 years) climate cycle in that incremental increases in cover thickness do not produce proportional increases in T r . The results indicated that, once the cover thickness exceeds 100 cm, there is little incremental increase in the median value of T r over the 60‐year climate cycle. Copyright © 2014 John Wiley & Sons, Ltd.