During periods of water deficit, growing roots may shrink, retaining only partial contact with the soil. In this study, known mathematical models were used to calculate the root–soil air gap and water flow resistance at the soil–root interface, respectively, of Robinia pseudoacacia L. under different water conditions. Using a digital camera, the root–soil air gap of R. pseudoacacia was investigated in a root growth chamber; this root–soil air gap and the model-inferred water flow resistance at the soil–root interface were compared with predictions based on a separate outdoor experiment. The results indicated progressively greater root shrinkage and loss of root–soil contact with decreasing soil water potential. The average widths of the root–soil air gap for R. pseudoacacia in open fields and in the root growth chamber were 0.24 and 0.39 mm, respectively. The resistance to water flow at the soil–root interface in both environments increased with decreasing soil water potential. Stepwise regression analysis demonstrated that soil water potential and soil temperature were the best predictors of variation in the root–soil air gap. A combination of soil water potential, soil temperature, root–air water potential difference and soil–root water potential difference best predicted the resistance to water flow at the soil–root interface.