Water repellency as a function of soil water content or suction influenced by drying and wetting processes

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Abstract
This research aims to assess hysteresis effects on the relationships of water droplet penetration time (WDPT) against soil water contents (θ), and WDPT against soil water suction (ψ). WDPT of four wettable and artificial water-repellent (WR) soils were measured during wetting (WP) and drying processes (DP). Soil water retention curves were measured for DP and used for obtaining curves for WP. A Gaussian model (GM), a Lorentzian model (LRM) and a Lognormal model (LGM) were applied to WDPT~θ and WDPT~ψ relationships. For wettable soils, WDPT increased first, then decreased with increasing θ or ψ, but none was beyond 5s. The relative WDPT values -RWDPT, which denoted the ratio of the maximal values of WDPT in WP to WDPT of the similar θ in DP, ranged between 1.29 and 2.35 following the order silt loam > loam >sand > clay loam. There were slight hysteresis effects. For WR soils, WDPT~θ (or ψ) curves were generally unimodal, a few were bimodal. RWDPT values for the hydrophobized soils ranged between 1.3 and 138.1. There were obvious hysteresis effects. GM generally fitted better than the other two for unimodal curves of WDPT~θ (or ψ) for both DP and WP processes, but segmented models fitted better for bimodal curves. The parameter a in each model was equal to the maximal WDPT, and the corresponding θ was equal to the parameterμ. The GM model was selected as the prior one for simulating WDPT~θ (or ψ) relationship for both DP and WP processes.

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