Use and Hydrological Robustness of Time-to-Incipient-Ponding
- I. White ,
- M. J. Sully and
- M. D. Melville
Both the use and hydrological robustness of time-to-ponding, tp, have been questioned recently. We examine these questions here for variable rainfall rates. Using a nondimensional analytic form for tp, we show that for rainfall rates, R, greater than about 1.5 times the saturated hydraulic conductivity, Ks, tp is relatively robust. Only in the region Ks ≤ R ≤ 1.5 Ks is tp sensitive to small changes in R/Ks. We also show a comparison between field measurements of tp 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 Ks 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 tp are soundly based provided R > 1.5 Ks. Finally, we use the analytic expression for tp 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 tp, R and the steady state runoff during rainfall. These hydraulic properties agree well with those measured under a constant head. We conclude that tp is a useful and hydrologically robust property provided R(tp) > 1.5 Ks.
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