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Extent and persistence of water repellency in two Iranian soils
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 S wh ) 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.
Physics of water repellent soils
2000
Although it is generally well known that water repellent soils have distinct preferential flow patterns, the physics of this phenomenon is not well understood. In this paper, we show that water repellency affects the soil water contact angle and this, in turn, has a distinct effect on the constitutive relationships during imbibing. Using these constitutive relationships, unstable flow theory developed for coarse grained soils can be used to predict the shape and water content distribution for water repellent soils. A practical result of this paper is that with a basic experimental setup, we can characterize the imbibing front behavior by measuring the water entry pressure and the imbibing soil characteristic curve from the same heat treated soil. ᭧
Water repellent soils: the case for unsaturated soil mechanics
E3S Web of Conferences, 2016
Water repellent (or "hydrophobic" or "non-wetting") soils have been studied by soil scientists for well over a century. These soils are typified by poor water infiltration, which leads to increased soil erosion and poor crop growth. However, the importance of water repellence on determining soil properties is now becoming recognised by geotechnical engineers. Water repellent soils may, for example, offer novel solutions for the design of cover systems overlying municipal or mine waste storage facilities. However, investigations into factors affecting their mechanical properties have only recently been initiated. This purpose of this paper is to introduce geotechnical engineers to the concept of water repellent soils and to discuss how their properties can be evaluated under an unsaturated soils framework. Scenarios in which water repellent properties might be relevant in geotechnical applications are presented and methods to quantify these properties in the laboratory and in the field examined.
European Journal of Soil Science, 2006
Knowledge of soil water repellency distribution, of factors affecting its occurrence and of its hydrological effects stems primarily from regions with a distinct dry season, whereas comparatively little is known about its occurrence in humid temperate regions such as typified by the UK. To address this research gap, we have examined: (i) water repellency persistence (determined by the water drop penetration time method, WDPT) and degree (determined by the critical surface tension method, CST) for soil samples (0-5, 10-15 and 20-25 cm depth) taken from 41 common soil and land-use types in the humid temperate climate of the UK; (ii) the supposed relationship of soil moisture, textural composition and organic matter content with sample repellency; and (iii) the bulk wetting behaviour of undisturbed surface core samples (0-5 cm depth) over a period of up to 1 week. Repellency was found in surface samples of all major soil textural types amongst most permanently vegetated sites, whereas tilled sites were virtually unaffected. Repellency levels reached those of the most severely affected areas elsewhere in the world, decreased in persistence and degree with depth and showed no consistent relationship with soil textural characteristics, organic matter or soil moisture contents, except that above a water content of c. 28% by volume, repellency was absent. Wetting rate assessments of 100 cm 3 intact soil cores using continuous water contact (-20 mm pressure head) over a period of up to 7 days showed that across the whole sample range and irrespective of texture, severe to extreme repellency persistence consistently reduced the maximum water content at any given time to well below that of wettable soils. For slightly to moderately repellent soils the results were more variable and thus hydrological effects of such repellency levels are more difficult to predict. The results imply that: (i) repellency is common for many land-use types with permanent vegetation cover in humid temperate climates irrespective of soil texture; (ii) supposedly influential parameters (texture, organic matter, specific water content) are poor general predictors of water repellency, whereas land use and the moisture content below which repellency can occur seem more reliable; and (iii) infiltration and water storage capacity of very repellent soils are considerably less than for comparable wettable soils.
Journal of Hydrology and Hydromechanics, 2016
This study explored the effect of soil water repellency (SWR) on soil hydrophysical properties with depth. Soils were sampled from two distinctly wettable and water repellent soil profiles at depth increments from 0-60 cm. The soils were selected because they appeared to either wet readily (wettable) or remain dry (water repellent) under field conditions. Basic soil properties (MWD, SOM, θ v) were compared to hydrophysical properties (K s , S w , S e , S ww , S wh , WDPT, RI c , RI m and WRCT) that characterise or are affected by water repellency. Our results showed both soil and depth affected basic and hydrophysical properties of the soils (p <0.001). Soil organic matter (SOM) was the major property responsible for water repellency at the selected depths (0-60). Water repellency changes affected moisture distribution and resulted in the upper layer (0-40 cm) of the repellent soil to be considerably drier compared to the wettable soil. The water repellent soil also had greater MWD dry and K s over the entire 0-60 cm depth compared to the wettable soil. Various measures of sorptivity, S w , S e , S ww , S wh , were greater through the wettable than water repellent soil profile, which was also reflected in field and dry WDPT measurements. However, the wettable soil had subcritical water repellency, so the range of data was used to compare indices of water repellency. WRCT and RI m had less variation compared to WDPT and RI c. Estimating water repellency using WRCT and RI m indicated that these indices can detect the degree of SWR and are able to better classify SWR degree of the subcritical-repellent soil from the wettable soil.
The Relationship between Soil Moisture and Soil Water Repellency Persistence in Hydrophobic Soils
Water
In this work, we modelled the response of soil water repellency (SWR) persistence to the decrease in moisture in drying soils, and we explored the implication of soil particle size distribution and specific surface area on the SWR severity and persistence. A new equation for the relationship between SWR persistence and soil moisture (θ) is described in this paper. The persistence of SWR was measured on ten different hydrophobic soils using water drop penetration time (WDPT) at decreasing levels of gravimetric water content. The actual repellency persistence showed a sigmoidal response to soil moisture decrease, where Ra(θ)=Rp/1+eδ(θ−θc). The suggested equation enables one to model the actual SWR persistence (Ra) using θ, the potential repellency (Rp) and two characteristic parameters related to the shape of the response curve. The two parameters are the critical soil moisture θc, where the Ra increase rate reaches its maximum, and the parameter δ affecting the steepness of the curve...
Effects of soil water repellency on infiltration rate and flow instability
Journal of Hydrology, 2000
Laboratory infiltration experiments were carried out to quantify the effects of soil water-repellency on infiltration rate and the wetting front instability. A two-dimensional transparent chamber (41.5 cm wide, 50 cm high and 2.8 cm thick) was constructed for infiltration experiments using three water-repellent Ouddorp sands (The Netherlands) and a wettable silicon sand. The results showed that if the water-ponding depth (h 0 ) at the soil surface was lower than the water-entry value (h we ) of repellent sands, infiltration would not start until the water drop penetration time (WDPT) is exceeded; and contrary to infiltration in wettable soils, the infiltration rate increased with time. However, infiltration could immediately start at any time when h 0 Ͼ h we : The wetting front was unconditionally unstable for h 0 Ͻ h we ; resulting in fingered flow. However, the flow was conditionally stable for h 0 Ͼ h we if the soil was not layered in a fine-over-coarse or wettable-over-repellent configuration, and if soil air was not compressed during infiltration. The occurrence of stable and unstable flow in repellent soils was consistent with the prediction based on a linear instability analysis. The findings can be used to improve irrigation efficiencies in water repellent soils, e.g. using high-ponding irrigation methods. ᭧
Alternative analysis of transient infiltration experiment to estimate soil water repellency
Hydrological Processes, 2018
The repellency index (RI) defined as the adjusted ratio between soil-ethanol, S e , and soil-water, S w , sorptivities estimated from minidisk infiltrometer experiments has been used instead of the widely used water drop penetration time and molarity of ethanol drop tests to assess soil water repellency. However, sorptivity calculated by the usual early-time infiltration equation may be overestimated as the effects of gravity and lateral capillary are neglected. With the aim to establish the best applicative procedure to assess RI, different approaches to estimate S e and S w were compared that make use of both the early-time infiltration equation (namely, the 1 min, S1, and the short-time linearization approaches), and the two-term axisymmetric infiltration equation, valid for early to intermediate times (namely, the cumulative linearization and differentiated linearization approaches). The dataset included 85 minidisk infiltrometer tests conducted in three sites in Italy and Spain under different vegetation habitats (forest of Pinus pinaster and Pinus halepensis, burned pine forest, and annual grasses), soil horizons (organic and mineral), postfire treatments, and initial soil water contents. The S1 approach was inapplicable in 42% of experiments as water infiltration did not start in the first minute. The short-time linearization approach yielded a systematic overestimation of S e and S w that resulted in an overestimation of RI by a factor of 1.57 and 1.23 as compared with the cumulative linearization and differentiated linearization approaches. A new repellency index, RI s , was proposed as the ratio between the slopes of the linearized data for the wettable and hydrophobic stages obtained by a single water infiltration test. For the experimental conditions considered, RI s was significantly correlated with RI and WDPT. Compared with RI, RI s includes information on both soil sorptivity and hydraulic conductivity and, therefore, it can be considered more physically linked to the hydrological processes affected by soil water repellency.