With reference to a more compacted and less conductive upper soil layer overlying a less compacted and more conductive subsoil, a simple three-dimensional (3D) infiltration run is expected to yield more representative results of the upper layer than the subsoil. However, there is the need to quantitatively establish what is meant by more representativeness. At this aim, numerically simulated infiltration was investigated for a theoretically unconfined process under a null ponded head of water (d0H0 setup, with d = depth of ring insertion and H = ponded depth of water) and a practical beerkan run (d1H1 setup, d = H = 1 cm). The considered layered soils differed by both the layering degree (from weak to strong) and the thickness of the upper soil layer (0.5-3 cm). It was confirmed that water infiltration should be expected to be more representative of the upper soil layer when this layer is the less permeable since, for a 2-h experiment, the instantaneous infiltration rates for the layered soil were 1.0-2.1 times greater than those of the homogeneous low permeable soil and 1.3-20.7 smaller than those of the homogeneous coarser soil that constituted the subsoil. Similarity with the homogeneous fine soil increased as expected as the upper layer became thicker. For a weak layering condition, the layered soil yielded an intermediate infiltration as compared with that of the two homogeneous soils forming the layered system. For a strong layering degree, the layered soil was more similar to the homogeneous fine soil than to the homogeneous coarse soil. Using the practical setup instead of the theoretical one should have a small to moderate effect on the instantaneous infiltration rates since all the calculated percentage differences between the d1H1 and d0H0 setups fell into the relatively narrow range of -18.8% to +17.4%. A sequential analysis procedure appeared usable to detect layering conditions but with some modifications as compared with the originally proposed procedure. The practical setup enhanced the possibility to recognize the time at which the characteristics of the subsoil start to influence the infiltration process. In conclusion, this investigation contributed to better interpret both the theoretical and the practically established 3D infiltration process in a soil composed of a less conductive upper soil layer overlying a more conductive subsoil and it also demonstrated that modifying a recently proposed procedure only using infiltration data could be advisable to determine the time when layering starts to influence the process.

In his seminal paper on solution of the infiltration equation, Philip (1957) proposed a gravity time, tgrav, to estimate practical convergence time of his infinite time series expansion, TSE. The parameter tgrav refers to a point in time where infiltration is dominated equally by capillarity and gravity derived from the first two (dominant) terms of the TSE expansion. Evidence that higher order TSE terms describe the infiltration process better for longer times. Since the conceptual definition of tgrav is valid regardless of the infiltration model used, we opted to reformulate tgrav using the analytic approximation proposed by Parlange et al. (1982) valid for all times. In addition to the roles of soil sorptivity (S) and saturated (Ks) and initial (Ki) hydraulic conductivities, we explored effects of a soil specific shape parameter β on the behavior of tgrav. We show that the reformulated tgrav (notably tgrav= F(β) S^2/(Ks - Ki)^2 where F(β) is a β-dependent function) is about 3 times larger than the classical tgrav given by tgrav, Philip= S^2/(Ks - Ki)^2. The differences between original tgrav, Philip and the revised tgrav increase for fine textured soils. Results show that the proposed tgrav is a better indicator for convergence time than tgrav, Philip. For attainment of the steady-state infiltration, both time parameters are suitable for coarse-textured soils, but not for fine-textured soils for which tgrav is too conservative and tgrav, Philip too short. Using tgrav will improve predictions of the soil hydraulic parameters (particularly Ks) from infiltration data as compared to tgrav, Philip.