Abstract
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.