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DIVISION S-1 - SOIL PHYSICS

Water Repellency and Critical Soil Water Content in a Dune Sand

^a Alterra, Land Use and Soil Processes Team, P.O. Box 47, 6700 AA Wageningen, The Netherlands
^b Dep. of Geography, Univ. of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
^c Dep. of Civil Engineering, Demokritus Univ. of Thrace, 67100 Xanthi, Greece

* Corresponding author (L.W.Dekker{at}Alterra.wag-ur.nl)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Assessments of water repellency of soils are commonly made on air-dried or oven-dried samples, without considering the soil water content. The objectives of this study were to examine the spatial and temporal variability of soil water content, actual water repellency over short distances, and the variations in critical soil water contents. Between 22 Apr. and 23 Nov. 1999, numerous samples were collected from a grass-covered dune sand (typic Psammaquent), at six depths, eight times in transects and two times in soil blocks. The water drop penetration time (WDPT) test was used to measure the actual water repellency of the field-moist samples and the potential water repellency after drying the samples at 25, 65, and 105°C. Highly spatial and temporal variability in water content and persistence of actual water repellency was found between the samples from all soil depths. At each depth we established an upper water content, below which samples were water repellent and a lower water content, above which samples were wettable. This water content range, called the transition zone, was different for each depth, and, for example, assessed at 0- to 2.5-cm depth between soil water contents of 18 and 23% (vol./vol.), and at 16.5- to 19-cm depth between 2 and 5% (vol./vol.). The potential water repellency of samples dried at 25 and 65°C was on some days less severe than the actual repellency of field-moist samples on other days, thus underestimating the maximal persistence of water repellency that can occur in the field. Drying of the samples at 105°C significantly increased the potential water repellency.

Abbreviations: WDPT, water drop penetration time

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE PROBLEM OF SOIL WATER REPELLENCY has been recognized in various parts of the world (Jaramillo et al., 2000) including the Netherlands (Dekker, 1998), United Kingdom (Doerr et al., 2000a), and Greece (Dekker et al., 2001) and has caused for instance serious land use problems in agriculture (Blackwell, 2000).

It has been recognized for many years that water repellency of a soil is often a function of the type of organic matter incorporated in it, and that certain organic matter induces water repellency in soils by several means. First, drying processes in organic matter can induce water repellency, for instance in the surface layers of peat soils, which are difficult to rewet after drying (e.g., Hooghoudt, 1950; Dekker and Ritsema, 1996). Second, hydrophobic plant decompositional, microbial, or fungal byproducts coating mineral soil particles may induce wetting resistance (DeBano, 2000; Doerr et al., 2000b). Third, mineral particles need not be individually coated with hydrophobic materials; intermixing of mineral soil particles with particulate organic matter, like remnants of roots, leaves, and stems, may also induce severe water repellency (Bisdom et al., 1993).

Water repellency may dramatically affect water and solute movement at the field-scale, a process which has often been underestimated (Bauters et al., 2000). Water repellency and its spatial variability have been shown to cause nonuniform wetting and preferential flow (Fig. 1) in many field soils (Dekker and Ritsema, 1994, 1996; Ritsema et al., 1998).

View larger version (150K): [in this window] [in a new window]   Fig. 1. Nonuniform wetting in the dune sand. Dark-colored areas in the soil pit indicate an actually wettable surface layer and preferential flow paths. The light colors represent the dry, actually water repellent sand areas.

The objectives of the present study are to examine (i) spatial and temporal variations in soil water content and actual water repellency over short distances in a grass-covered dune sand at six depths, and (ii) variations in the critical soil water contents between these depths.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field-Soil and Soil Sampling
The soil studied is a dune sand, located near Ouddorp in the southwestern part of the Netherlands, consisting of fine sand with <3% clay to a depth of >3 m, and is classified as mesic Typic Psammaquent (Dekker, 1998). The site is grass-covered, is in use as pasture, and has not been tilled for at least several decades. Organic matter contents of 18 and 10% (wt./wt.) were established in the surface layer at depths of 0- to 2.5- and 2.5- to 5-cm, respectively. At depths of 5- to 7.5-cm an organic matter content of 2% (wt./wt.) was found, and from 7.5 cm downwards 0.5% (wt./wt.). The soil is known to exhibit severe to extreme water repellency to a depth of >50 cm during dry periods (Dekker and Ritsema, 1994). Water repellency of this sandy soil is because of a coating of the sand grains with hydrophobic material and the presence of hydrophobic particulate organic matter (Bisdom et al., 1993).

Between 22 Apr. and 12 Oct. 1999, the spatial and temporal variability in volumetric soil water content was studied eight times in vertical transects by intensive sampling (Dekker et al., 2000). To allow a more detailed determination of the wetting patterns in the dune sand, soil blocks were sampled on 25 Oct. and 23 Nov. 1999. The soil of transects and blocks was sampled at six depths (0–2.5, 2.5–5, 7–9.5, 9.5–12, 14–16.5, and 16.5–19 cm) using steel cylinders, with a diameter of 5 cm (Dekker et al., 2000). In the transects at each depth, 35 samples were taken at a close spacing over a distance of 1.8 m. On eight occasions 210 samples were collected in the transects (1680 samples total). In the soil blocks (25 by 75 by 19 cm) at each depth, 75 samples were taken in a regular grid of 5 by 15 samples, resulting in the collection of 450 samples on two occasions. The cylinders were pressed vertically into the soil, emptied into plastic bags and used again. The plastic bags were tightly sealed to minimize evaporation of the soil. The field-moist soil in the plastic bags was weighed, the actual water repellency measured, and after drying the samples at 25°C and 65°C, potential water repellency was measured. The samples were further dried at a temperature of 105°C, and weighed again to calculate the water content and dry bulk density of the samples.

Water Drop Penetration Time Test
The persistence or stability of water repellency of the soil samples was examined using the WDPT test (e.g., King, 1981). Three drops of distilled water are placed on the smoothed surface of a soil sample, using a standard medicine dropper, and the time that elapses before the drops are absorbed is determined. We measured the soil water repellency of all 2580 samples under controlled conditions at a constant temperature of 20°C and a relative air humidity of 50%. In general, a soil is considered to be water repellent if the WDPT exceeds 5 s (Dekker, 1998). In the present study, an index was applied allowing a quantitative classification of the persistence of soil water repellency as described by Dekker and Jungerius (1990). Thus seven classes of repellency were distinguished, based on the time needed for the water drops to penetrate into the soil: Class 0, wettable, nonwater repellent (infiltration within 5 s); Class 1, slightly water repellent (5–60 s); Class 2, strongly water repellent (60–600 s); Class 3, severely water repellent (600–3600 s); and extremely water repellent (>1 h), further subdivided into Class 4, 1 to 3 h; Class 5, 3 to 6 h; and Class 6, >6 h.

We measured the actual water repellency of all field-moist samples immediately after recording their wet weight. By measuring the water content of the samples, we could assess critical soil water contents for the six layers sampled. At each depth, we established a water content below which the soil is water repellent, and a water content above which the soil is wettable.

Potential water repellency was assessed after drying the samples at 25, 65, and 105°C. The severity of the potential water repellency measured on dried soil samples, is considered to be the most appropriate parameter for comparing soils with respect to their sensivity to water repellency (Dekker and Ritsema, 1994), because differences in water content are wiped out.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil Water Content
The spatial variability of the soil water content was often found to be high in all layers sampled (Fig. 2) . The highest water contents were present in the surface layers, which have higher organic matter contents and lower dry bulk densities when compared with the deeper layers. Comparatively high, but nevertheless highly variable soil water contents were established at depths of 0- to 2.5- and 2.5- to 5-cm in April, October, and November 1999. For instance, the soil water content at depths of 0- to 2.5-cm ranged between 31.4 and 69.3%(vol./vol.), and at depths of 2.5- to 5-cm between 26.1 and 57.5% (vol./vol.) at the beginning of the study, on 22 Apr. 1999. At the end of the study, on 23 Nov. 1999, the water content in the thatch layer (0–2.5 cm) ranged between 29.7 and 47.5% (vol./vol.) (Fig. 2).

View larger version (35K): [in this window] [in a new window]   Fig. 2. Minimum (-), mean (o), and maximum (-) soil water contents of samples taken at six depths in the eight transects (n = 35) between 21 April and 12 Oct. 1999 and in the two soil blocks (n = 75) on 25 Oct. and 23 Nov. 1999.

Although the precipitation between 1 and 25 Oct. 1999 amounted to 122 mm, the soil was locally dry at depths of 7 to 19 cm, with water contents of <4% (vol./vol.). The irregular wetting patterns detected in the soil block of 25 Oct. 1999 are shown in Fig. 3 . Rainwater had moved through the wet fingers in the soil, which had water contents between 10 and 35% (vol./vol.). Figure 4 shows the actual and potential water repellency of the 25 Oct. 1999 soil block. The dry areas in this soil block were partly extremely repellent, as measured on the field-moist samples (compare Fig. 3 with the left-hand diagram of Fig. 4).

View larger version (61K): [in this window] [in a new window]   Fig. 3. Contours of the volumetric soil water content in horizontal planes (25 by 75 cm) at six depths in the 25 October soil block.

View larger version (98K): [in this window] [in a new window]   Fig. 4. Contours of the persistence of actual and potential (after drying at 25°C) water repellency in horizontal planes (25 by 75 cm) at six depths in the 25 October soil block.

Actual Water Repellency
The whole soil profile was wet at the beginning of the study on 22 Apr. 1999. All 210 samples taken at depths between 0 and 19 cm were actually wettable (nonwater repellent), and exhibited WDPT values of <5 s (Fig. 5) .

View larger version (78K): [in this window] [in a new window]   Fig. 5. Relative frequency (%) of the persistence of actual water repellency of field-moist samples (n = 35) taken at six depths in the transects on 22 April, 17 May, and 21 September, and of potential water repellency of these samples after drying at 25, 65, and 105°C (n = 35).

Water drop penetration time values varied from <5 s to >6 h for field-moist samples taken at several depths on the other sampling days (Dekker et al., 2000). However, after some rain events all 35 samples taken in the thatch layer (0- to 2.5-cm depth) were wettable with WDPT values <5 s. This was for instance the case on 22 Apr., 12 Aug., 12 Oct., 25 Oct., and 23 Nov. 1999 (Dekker et al., 2000).

The spatial variability of the persistence of actual water repellency was often remarkably high in the transects and soil blocks. Dry soil areas with extreme water repellency, and preferential flow paths (fingers) with wettable soil, were often detected over short distances. The spatial variability in severity of actual water repellency is demonstrated by the contour plots of the October soil block, with a size of only 0.75 by 0.25 by 0.19 m, or 0.036 m³ (Fig. 4). Although the severity of actual water repellency had decreased during the autumn rains, extreme repellency with WDPT values of >1 h, and locally even >6 h, still occurred in dry soil areas of the 25 Oct. 1999 soil block.

Critical Soil Water Content
All samples from the thatch (0–2.5 cm) layer taken in the transects and soil blocks between 17 May and 23 Nov. 1999, with soil water contents of >23% (vol./vol.), were found to be wettable (Fig. 6) . In contrast, all samples with a soil water content of <18%(vol./vol.) were slightly to extremely water repellent with WDPT values between 5 to 60 s (Class 1) and 3 to 6 h (Class 5). Between water contents of 18 and 23% (vol./vol.), wettable as well as water repellent conditions were encountered. This soil water content zone is introduced here as the transition zone. This means that the critical soil water content of the thatch layer was variable and ranged between 18 and 23% (vol./vol.), depending on wetting and weather history.

View larger version (45K): [in this window] [in a new window]   Fig. 6. Relationship between the soil water content and the persistence of the actual water repellency of all samples taken at six depths between 17 May and 23 Nov. 1999. The transition zone, with critical soil water contents is indicated with a grey tone.

Although there were large differences in severity of actual water repellency at specific soil moisture contents, there was a distinct increase in severity with decreasing soil water contents, as shown in the diagrams of Fig. 6.

The transition zone with critical soil water contents at depths between 0 and 19 cm in the soil profile has been given in Fig. 7 . The zone at the left-hand side of the transition zone represents the actually water repellent and the zone at the right-hand side the actually wettable zone in relation to depth and soil water content. In the transition zone soil samples can be either wettable or water repellent. The variability in critical soil water content may be because of the wetting history of the soil and the influence of drying and wetting on the severity of water repellency.

View larger version (20K): [in this window] [in a new window]   Fig. 7. Transition zone with critical soil water contents for the dune sand to a depth of about 20 cm.

Samples from the 17 May 1999 transect were almost all actually water repellent. After drying at 25°C a significant increase in persistence of potential water repellency occurred for samples from depths of 14 to 19 cm, and a slight decrease to slight increase for samples from depths of 0 to 12 cm (Fig. 5). Further drying at 65°C significantly increased the persistence of samples at depths of 7 to 16.5 cm, but slightly decreased the repellency for samples from depths of 0 to 5 cm. The persistence of the potential water repellency increased significantly at all depths after further drying of the samples at 105°C. The most dramatic increase was assessed for samples taken at depths of 0 to 2.5 and 2.5 to 5 cm (Fig. 5).

Samples taken on 21 Sept. 1999 from depths of 0 to 2.5 cm and 16 to 19 cm that were actually wettable also became water repellent by drying at 25°C (Fig. 5). The most extreme water repellent samples from depths of 2.5 to 19 cm with WDPT values of >6 h, retained their extreme water repellency after being dried at 25, 65, and 105°C. The persistence of soil water repellency of the samples from depths of 0 to 5 cm slightly decreased after drying at 65°C when compared with 25°C. However, a significant increase in persistence occurred after drying these samples at 105°C.

Variations in severity of potential water repellency occurred after drying at 25 and 65°C for samples taken at the same depths and on the same days. However, large differences in severity were also found between samples from the same depths, but taken on different days. For example, there was a significant increase in persistence at all six depths for the samples from 17 May 1999 after drying at 25 and 65°C when compared with those from 22 Apr. 1999 (Fig. 5). A further significant increase in the persistence of potential water repellency was found for the samples taken at depths of 0 to 16.5 cm on 21 Sept.1999, in comparison with the samples from 17 May 1999 (Fig. 5).

The differences in persistence of potential water repellency of samples taken on different days after drying the samples at 25°C must be because of differences in wetting history of the samples and a time-variant process of water repellency formation in the field. For example, the relatively dry soil (Fig. 2) and severe to extreme actual water repellency of the 17 May and 21 Sept. 1999 transects resulted in extreme potential water repellency (Fig. 5).

In some cases, the persistence of potential water repellency of the samples was identical to the persistence of the actual water repellency. In most cases, however, the WDPT values increased significantly after the samples had been dried at 25°C. It is worthy of note that the actual water repellency at depths of 0 to 16.5 cm on some days was more severe than the potential water repellency measured after drying at 25°C (and 65°C) on other days. For example, the samples collected at depths of 7 to 16.5 cm on 21 Sept. 1999 exhibited more extreme actual water repellency than the samples dried at 25 and 65°C at several other dates, for instance 22 Apr. and 17 May 1999 (Fig. 5). This indicates that processes which are taking place in the field during drying weather could not be artificially generated during drying in a laboratory oven over a time span of several days.

The spatial variability in persistence of potential water repellency after drying at 25°C is illustrated with the contour plots of the 25 Oct. 1999 soil block (Fig. 4). Comparison of these patterns can be made with those of the actual water repellency in the left-hand diagram of Fig. 4.

The persistence of water repellency of the soil samples increased significantly after drying at 105°C at all depths and only slight differences in severity occurred between the samples from different transects and soil blocks (see also Fig. 5).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Water repellency of soils can dramatically affect water and solute movement, because of nonuniform wetting and forming of preferential flow paths, so-called fingers, as also often occur in the dune sand studied (Fig. 1). The temporal and spatial variability of the soil water content and actual water repellency were found to be high at all depths sampled, as is illustrated with Fig. 2 to 5. The whole soil profile was moist and actually wettable at the beginning of the study on 22 Apr. 1999. Relatively dry transects were sampled on 17 May and 21 Sept. 1999. Extreme actual water repellency, with WDPT values exceeding 6 h, was present in numerous samples taken from the 21 Sept. 1999 transect. The actual water repellency of these samples was even more severe than the potential water repellency of samples dried at 25 and 65°C from moister transects as, for example the 22 Apr. 1999 transect (Fig. 5). This suggests that the maximal persistence of potential water repellency measured on samples taken in a moist status and dried at 25 or 65°C might underestimate the level of repellency as it occurs in the field during or shortly after an extremely dry period. Furthermore, this implies that seasonal changes in soil water repellency are not simply a function of variations in soil water content, but also because of other processes taking place during drying of the soil in the field, as also suggested in a study investigating highly repellent sandy-loam soils in Portugal (Doerr and Thomas, 2000). On the other hand, for the samples investigated here, oven drying at 105°C can result in WDPT values comparable with those found during dry periods in the field, as is illustrated in Fig. 5. The generally unpredictable response of the WDPT values to drying of wet samples does, however, suggest that samples should preferably be taken in a dry status in the field to best reflect the maximum water repellency that can be expected to occur. If damp samples have to be taken during less dry periods, we suggest measuring the potential water repellency after drying the samples at 105°C, as also has been recommended by Carter et al. (1994).

Numerous researchers did not measure the actual water repellency, but only the potential water repellency after air drying (e.g., Robinson, 1999; Cisar et al., 2000; Kostka, 2000) or oven drying the samples (e.g., Dekker and Jungerius, 1990; Bisdom et al., 1993; Carter et al., 1994). King (1981) recommended that water repellency tests be made on oven- or air-dry soil samples. However, we, in addition to King (1981), recommend measuring the actual water repellency on field-moist samples from different depths in the soil profile to attempt the assessment of the critical soil water contents. This knowledge aids considerably the identification of the environmental conditions at which the onset of water repellency can be expected for a given soil. The information can be of great value for land managers, allowing not only a more effective irrigation of affected soils, by keeping soil water contents above the critical threshold (Dekker, 1998; Cisar et al., 2000), but also the more accurate forecasting of conditions during which increased environmental problems can be expected in relation to water repellency development. These include enhanced hydrological response to rainstorms leading to floods and soil erosion, enhanced leaching of nutrients and agrichemicals, and reduced soil microbial activity (DeBano, 2000; Doerr et al., 2000b).

Notwithstanding the above, critical soil water content, introduced by Dekker and Ritsema (1994), appears not to be a sharp threshold above which a soil is wettable and below which a soil is water repellent, but rather a transitional range. This range of critical soil water contents for a certain depth is introduced here as the transition zone. Soil samples can be either wettable or water repellent within the transition zone. In the dune sand studied, the transition zone was assessed at depths of 0 to 2.5, 2.5 to 5, 7 to 9.5, 9.5 to 12, 14 to 16.5, and 16.5 to 19 cm, as being between soil water contents of 18 to 23, 14 to 20, 4 to 8.5, 3 to 6.5, 2.5 to 5.5, and 2 to 5% (vol./vol), respectively. The variability in critical soil water content may be because the wetting history of a soil and the influence of drying and wetting cycles on the severity of water repellency in the field. Furthermore, differences in distribution of water in and around the microaggregates of the soil might be important with this respect as well.

	ACKNOWLEDGMENTS

 
The project is carried out with financial support from the Commission of the European Community. It is financed under the work program FAIR (ref. 4027). The project does not necessarily reflect the Commission's views and in no way anticipates its future policy in this area. This study was partly supported by Aquatrols Corporation of America, Cherry Hill, NJ, USA, and by NATO Collaborative Research Grant 973169.

Received for publication December 15, 2000.

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