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Use and Hydrological Robustness of Time-to-Incipient-Ponding

CSIRO Centre for Environmental Mechanics, GPO Box 821, Canberra, ACT 2601, Australia

M. D. Melville

School of Geography, Univ. of New South Wales, PO Box 1, Kensington, NSW 2033, Australia

*Corresponding author.

ABSTRACT

Both the use and hydrological robustness of time-to-ponding, t_p, have been questioned recently. We examine these questions here for variable rainfall rates. Using a nondimensional analytic form for t_p, we show that for rainfall rates, R, greater than about 1.5 times the saturated hydraulic conductivity, K_s, t_p is relatively robust. Only in the region K_s R 1.5 K_s is t_p sensitive to small changes in R/K_s. We also show a comparison between field measurements of t_p using a rainfall simulator on a structurally unstable, sodic and saline gravelly silty clay loam and those predicted using the analytic expression and measured values of K_s and sorptivity determined under ponded conditions with disc permeameters. The good agreement found, together with the ease and reproducibility of the field measurements, indicate that both the determination and prediction of t_p are soundly based provided R > 1.5 K_s. Finally, we use the analytic expression for t_p in the inverse sense to determine the soil's sorptivity, hydraulic conductivity and characteristic mean pore size, a measure of soil pore structure, from values of t_p, R and the steady state runoff during rainfall. These hydraulic properties agree well with those measured under a constant head. We conclude that t_p is a useful and hydrologically robust property provided R(t_p) > 1.5 K_s.

Contribution from CSIRO, Australia.

Received for publication September 14, 1988.