Extent and persistence of water repellency in two Iranian soils

Abstract Soil water repellency (SWR) can affect the hydrophysical properties of soils. The objective of this study was to evaluate a new approach, which allows estimating both the extent (the modified soil water repellency index, RIm) and persistence (the water repellency cessation time, WRCT) of water repellency from a single measurement of the combined infiltration of water against time. The measurements were carried out on wettable and water repellent soil samples from 0–60 cm depth. Combined soil water repellency index, RIc, was estimated from all the water and ethanol sorptivity values. The persistence of water repellency in soil aggregates (about 20 mm × 20 mm × 20 mm in size) was measured with the water drop penetration time (WDPT) test on both the field-moist aggregates (actual WDPT, A-WDPT) and aggregates dried at 65–70°C for 24 hours (potential WDPT, P-WDPT). In comparison with the wettable soil, hydrophysical parameters of the repellent soil were significantly different at the upper part of the profile (0–40 cm, P < 0.01), what can be attributed to the differences in organic matter content in both soils. Maximum organic matter (OM) content of the repellent soil was observed at the depth of 30–40 cm. Curiously, an insignificant difference between the studied soils was found in the saturated hydraulic conductivity, Ks. The mean values of A-WDPT and P-WDPT for water repellent soil were 438- and 106-times greater than those for wettable soil, respectively. All the water and ethanol sorptivities (Sw, Se, Sww, and Swh) were significantly (P < 0.01) greater in the wettable soil than those in the water repellent soil. The repellency indices RIc and RIm in water repellent soil were about seven- and two-times higher than those in the wettable soil, respectively. Our findings pointed out the proposed method to estimate SWR can be used as a new approach. Considering that the contact angle (CA) of soil and water intrinsically depends on sorptivity state, it is suggested that the relation of CA and RIm is investigated to find reference classes for WRCT and RIm (i.e. WDPT > 5 s) and to classify water repellency states of soils.

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