HR: 0830h
AN: H31D-0495    [PDF]
TI: Infiltration as Bounded by the Green-Ampt and Talsma-Parlange Limits
AU: * Barry, D A
EM: d.a.barry@ed.ac.uk
AF: The University of Edinburgh, Crew Building, King's Buildings, Edinburgh, EH9 3JN United Kingdom
AB: This presentation explores some of the contributions of J.-Y. Parlange to predictive infiltration formulas. The Green-Ampt infiltration formula (1911) involves a remarkable combination of simplicity and physical realism. It is known, however, that the hydraulic functions for which this formula can be deduced from Richards' equation are reasonable only for a step change in the unsaturated hydraulic conductivity, i.e., the Green-Ampt infiltration formula is a limiting case that relies on the condition that the soil-water diffusivity varies rapidly, while the unsaturated hydraulic conductivity remains constant. Equally simple is the Talsma-Parlange formula (1972). This formula can be considered as another limiting case wherein the rapid variation in the diffusivity is mirrored by a similarly rapid variation in the conductivity (i.e., a Gardner soil). Both these formulas depend on two soil properties, the sorptivity, $S$, and the saturated hydraulic conductivity, $K_s$. Not surprisingly, actual soil behaviour lies between these limits. An interpolation leading to the ``three-parameter" infiltration equation (1982) introduced an additional parameter, $\alpha$, where 0 $\leq \alpha \leq$ 1. The former limit produces the Green-Ampt formula, and the latter gives Talsma-Parlange. Because the Gardner assumption on hydraulic properties is much more reasonable than the Green-Ampt assumption, it is, again, not surprising that infiltration is best described by $\alpha$ near 1 (e.g., $\alpha$ = 0.85). The three-parameter equation applies when surface ponding is negligible; this condition was relaxed to give a generalisation that here is, for convenience, called the ponded three-parameter equation (1985). The ambiguous interpolation parameter, $\alpha$, was given a physical basis (1990), where a new parameter, $h_{str}$, was introduced. Although the meaning of $h_{str}$ is understood as the pressure jump necessary to establish a continuous air phase in the soil, a means by which it could be easily and unambiguously determined was unavailable until very recently (2002). Thus, the journey started with Parlange's realisation that infiltration is bounded by the Green-Ampt and Talsma-Parlange limits has now been completed with a fully physically based infiltration formula.
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 3210 Modeling
SC: Hydrology [H]
MN: 2003 Fall Meeting