Journal of Hydrology 484 (2013) 45–54

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Journal of Hydrology

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Specific processes during in situ infiltration into a sandy soil with low-levelwater repellency

Christina Ganz a,⇑, Jörg Bachmann a, Axel Lamparter b, Susanne K. Woche a, Wilhelmus H.M. Duijnisveld b,Marc-O. Göbel a

a Institute of Soil Science, Leibniz University of Hannover, Herrenhäuser Str. 2, 30419 Hannover, Germanyb Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany

a r t i c l e i n f o

Article history:Received 30 July 2012Received in revised form 9 December 2012Accepted 10 January 2013Available online 24 January 2013This manuscript was handled by Peter K.Kitanidis, Editor-in-Chief, with theassistance of Ty Ferre, Associate Editor

Keywords:Stable contact anglesPonded infiltrationUnstable flowMacro-fingerWater repellency

0022-1694/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jhydrol.2013.01.009

⇑ Corresponding author. Tel.: +49 511 16577612; faE-mail address: ganz@ifbk.uni-hannover.de (C. Ga

s u m m a r y

Despite growing knowledge about water repellent soils it is not well investigated how soil water repel-lency (SWR) of a lower level influences the in situ water infiltration into soils. Hence, we investigated asandy soil where we found subcritical water repellency and unexpected stable (persistent) contact angles(CAs) in the subsoil characterized by CA measurements in the laboratory. To characterize the influence ofthis persistence on in situ water infiltration, a ponded tracer infiltration experiment was carried out. Dur-ing the infiltration, hydraulic sensors revealed a slight saturation at the wetting front which is a sign forunstable flow. The excavated dye-stained infiltration zone was conical and its lateral extent decreasedwith increasing depth, showing characteristics of a macro-finger. We suggest the subsoil to exhibit astrong hysteretic water retention characteristic governing the infiltration process, but we could not verifythis by standard laboratory measurements, as strongly persistent SWR prevented the fast wetting of thesoil samples. Close to saturation the persistence of SWR led to a wetting period of soil samples of at least56 days to overcome SWR and reaching an equilibrium state. There are three major conclusions of thisstudy: 1. In contrast to many other studies we found persistent SWR also in the subsoil and not onlyin the humic topsoil, 2. The shape of the wetting front is contrary to common expectations for homoge-neous sandy soils and 3. The wettability characteristics found might also be relevant for assessing infil-tration dynamics at other sites regarding the fact that those observations are not obvious.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Natural field soils often show soil water repellency (SWR) tosome extent. A wide range of wettability degrees can be found un-der different climate conditions all around the world (Doerr et al.,2006). Many studies dealing with SWR in natural soils were con-ducted in sandy soils (Ritsema et al., 1998; Ritsema and Dekker,1998; Bauters et al., 2000a,b). There, the effectiveness of organiccoatings on mineral grains is particularly high (Doerr et al., 2000).

The commonly accepted main origin of water repellency is thepresence of soil organic matter (SOM). This results in the commonobservations that the topsoil usually shows the highest degree ofSWR within the soil profile (Dekker and Ritsema, 1994, 2000; Mor-ley et al., 2005). Woche et al. (2005) characterized the wettabilityof different soils as function of texture, pH, SOM, land use, anddepth. For all soils, SWR in the topsoil in terms of the contact angle(CA) was found to be higher than in the subsoil. However, SWR canalso occur in the subsoil and does not have to be necessarily corre-

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x: +49 511 762 5559.nz).

lated to the total amount of SOM. This is because, additionally tothe SOM content, the composition of SOM and its distribution,i.e. bulk organic matter or thin coatings, is important for explainingthe soil’s wetting behavior (Ellerbrock et al., 2005). Moreover,Doerr (1998) stated that root residues and exudates may contrib-ute to SWR in the subsoil.

The soil–water CA is a parameter to determine the soil’s degreeof SWR and can be measured either directly or indirectly (Bach-mann et al., 2003). Direct measurements, such as the sessile dropmethod (SDM), are originally developed for smooth and homoge-neous surfaces decades ago. Furthermore, CA measurements areusually conducted under laboratory conditions on homogenized,air-dried soil samples and they cannot account for the water con-tent-dependent changes in wettability. Thus, the applicability tonatural field soil conditions is still an open question (Bachmannet al., 2000). In field studies, the water drop penetration time(WDPT) test is usually applied to classify the wettability of a soil(e.g., Bauters et al., 2000a; Bisdom et al., 1993; Wallach and Jortz-ick, 2008). This test is easy and simple to use and can also be con-ducted on field-moist samples. However, the test only allows arough classification between a wettable to subcritical and a hydro-

46 C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54

phobic soil. As a consequence, even soils which are wettableaccording to the WDPT test can show CA between 0� and 90� andthus are rated as water repellent. Bachmann et al. (2003) statedthat for sandy soils the WDPT test appears to be sensitive in a nar-row range of contact angles of around 80–90�.

There is an ongoing interest in the impact of SWR on hydraulicprocesses and its consequences for water and solute transport inthe unsaturated zone. For example, Clothier et al. (2000) showedthat infiltration into a water repellent soil is decreased duringthe initial infiltration phase, but increased with prolonged infiltra-tion time due to the breakdown of SWR. When the infiltration is re-tarded, surface runoff can occur (Crockford et al., 1991) and mayresults in soil erosion (Witter et al., 1991). Besides infiltrationretardation, SWR is known to trigger preferential flow. This causesa greater risk for groundwater contamination due to the acceler-ated transport of pollutants through these pathways. Certain wellknown phenomena of preferential flow are the so-called gravity-driven fingers, which can occur when an advancing wetting frontbecomes unstable and dissipates into fingers. A necessary condi-tion for unstable flow in homogeneous soil is the hysteretic natureof the soil water retention function. Unstable flow can further oc-cur during the following conditions: (a) when the soil is very dry,(b) when the soil is hydrophobic, (c) when the hydraulic conductiv-ity increases with depth, (d) when air is compressed ahead of thewetting front, or (e) in case of fine material layers above coarselayers.

Unstable flow and gravity-driven fingers have been investigatedin water repellent soils (e.g., Ritsema and Dekker, 1994, 1998,2000; Wallach and Jortzick, 2008; Wallach, 2010; Xiong et al.,2012) and in wettable soils (e.g., Glass et al., 1989; Selker et al.,1992; Wang et al., 1998; Geiger and Durnford, 2000; DiCarlo,2004). The gravity-driven fingers can exhibit non-monotonic satu-ration and pressure profiles (saturation and pressure overshoot)under certain boundary and initial conditions in wettable andwater repellent soil (Bauters et al., 1998, 2000b; DiCarlo, 2004; Shi-ozawa and Fujimaki, 2004; Wallach and Jortzick, 2008; Wallach,2010). In a water repellent soil, gravity-driven fingers were foundunder laboratory conditions (e.g., Carillo et al., 2000; Clothieret al., 2000; Wang et al., 2000) and in the field (e.g., Hendrickxet al., 1993; Dekker and Ritsema, 2000; Doerr et al., 2006). Wallachand Jortzick (2008) applied water as a point source to a soil withvarying degree of water repellency and found for the waterrepellent soil an unstable finger-like 2 d wetting front withsaturation overshoot in a vertical cross-section. Recently, Xionget al. (2012) pointed out the substantial differences in the shapeof wetting and redistribution plumes in relation to the variationof the CA during the contact time with water under laboratoryconditions.

It is worthwhile to mention that studies which focus on gravity-driven fingering and saturation and pressure overshoot mostly dealwith either hydrophobic or completely wettable soil. But manysoils are neither completely wettable nor hydrophobic. For soilsexhibiting CA between 0� and 90�, which are termed as subcriticalwater repellent (Tillman et al., 1989), there are rarely studies thatpoint out influences on hydraulic dynamics. In their study, Lamp-arter et al. (2006) showed for a sandy soil with a subcritical degreeof water repellency that water infiltration rates were reduced by afactor of 3–170 compared to the reference ethanol infiltrationrates, which were assumed not to be affected by SWR. These find-ings demonstrate the potential of subcritical SWR in affectinghydraulic processes.

Therefore, we hypothesize that in a subcritically water repellentsandy soil the infiltration characteristics are mainly influenced by astrong persistence of SWR in the subsoil. To prove this, we applylaboratory CA measurements to quantify the temporal change ofSWR and present results from a field ponded infiltration experi-

ment on a sandy soil to analyze the impact of SWR persistenceon the infiltration process.

2. Materials and methods

2.1. Field ponding infiltration experiment

In May 2011, we carried out a ponded infiltration experimenton a sandy soil (Gleyic Podzol), which is located 30 km north ofHannover in Fuhrberg, Germany (52.587426 N, 9.862423 E). Thestudy area has a mean annual temperature of 8.8 �C and an averageprecipitation of 680 mm (Deurer, 2000). In 1950 the grassland wasploughed up into arable land. During this period, the groundwaterlevel has been lowered from approximately 1.50 m to almost 3.0 mdue to groundwater pumping of the waterworks Fuhrberg. Since afew years, the site is under grassland use again. The main charac-teristic of the study site are presented in Table 1.

Before the infiltration started a soil pit was dug with 100 cmdistance to the infiltration source. Twelve TDR probes (EasyTestLtd., Lublin, Poland) and twelve tensiometers (T4, UMS GmbH, Mu-nich, Germany) were horizontally installed underneath the centerof the circular infiltration source (Fig. 1). The TDR probes were in-stalled at 15, 45 and 75 cm depth; the tensiometers were installedat 30, 60 and 90 cm depth. All of them were installed in four verti-cal sensor lines with different horizontal distances to the centralvertical axis of the infiltration. Thus, the temporal change in watercontent and matric potential during the infiltration could be de-tected. The outer vertical sensor line D with 30 cm horizontal dis-tance to the ponding center was installed to observe the lateralspreading of the infiltrating tracer fluid. For the insertion of thering infiltrometer, 10 cm of the topsoil was removed with the grasslayer. Then a PVC ring, 40 cm in diameter, was inserted 5 cm intothe soil. An aqueous solution consisting of 3 g L�1 potassium bro-mide (KBr), 5 g L�1 Brilliant Blue FCF was added to the ring infil-trometer. The infiltrating solution was connected to twoMariotte’s bottles supplying the solution and keeping a constanthead during the experiment. The solution infiltrated with a con-stant head upper boundary condition of h = 2 cm with an infiltra-tion rate of 26.9 cm h�1. A total amount of 175 g Brilliant BlueFCF and 105 g KBr had infiltrated with the solution. Data was re-corded until 90 min after the remaining of the tracer had disap-peared from the soil surface. The sensors were removed and avertical soil trench through the middle of the infiltration areawas dug out to characterize the shape of the dye-stained infiltra-tion plume.

2.2. Assessment of soil water repellency

For measuring the SWR, we applied different methods underlaboratory conditions. All of them are conducted on homogenizedand air-dried soil material (<2 mm). First, we measured the CAwith the modified sessile drop method (SDM) on samples from15, 40, 60, 80 and 125 cm depth according to Bachmann et al.(2000) with six water drops per sample. The SDM is a direct (opti-cal) method to determine the CA in the entire range from CA 0–180�. A drop of water is placed onto a one-grain layer of soil fixedon a glass slide using double-sided adhesive tape. The CA at the li-quid–solid–vapor contact line was measured with a CCD-equippedcontact angle microscope (OCA15, DataPhysics, Filderstadt, Ger-many). The initial CA of the drop was measured directly by evalu-ating the CA immediately after placing the drop and aftermechanical perturbations ended at both intersections of the dropcontour line with the solid surface line (baseline) by automateddrop shape analysis using the software SCA20 (DataPhysics, Fil-derstadt, Germany). The CA evaluation was also done for two sub-

Table 1Basic data for the Fuhrberg soil profile.

Depth (cm) Horizon Sand (%) Silt (%) Clay (%) SOC (%) pHCaCl2 BDa (g/cm3)

630–2000 lm 200–630 lm 63–200 lm

0–25 Ap 10.6 69.9 12.4 4.9 2.2 2.4 5.9 1.2225–60 Bsh 0.9 82.1 14.4 1.7 0.9 0.1 5.7 1.4660–90 Bhs 0.2 78.5 19.4 1.1 0.7 0.1 5.6 1.5390–130 Bhs-Go 3.5 81.4 14.5 0.5 0.1 0.1 5.5 1.52

a BD: Bulk density.

Fig. 1. Experimental setup of TDR and tensiometer sensors during the pondingexperiment with respect to the soil horizon boundaries (grey dashed lines). Sensorswere horizontally installed in the soil profile in a regular 15 cm grid.

C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54 47

sequent times of 1000 ms and 5000 ms to observe the temporalbehavior of the drop.

For the comparison of the SDM results also the capillary risemethod (CRM) and the wilhelmy plate method (WPM) were usedas indirect methods for calculating the CA. The principle of theCRM is the rise of a liquid in a capillary bundle, in which no wateris taken up for CA > 90�. Thereby, the advancing CA of slightlywater repellent or wettable soil material is measured (CA < 90�)by using air-dried samples. The CA is calculated using the Wash-burn-equation (Washburn, 1921):

h2 ¼ rcclv cosj2g

t ð1Þ

where rc is the effective radius of the capillary (m), h is the height ofthe rising liquid front (m), clv is the liquids’ surface tension (N m�1),g is the dynamic viscosity of the liquid (Pa s) and t is the time (s).Siebold et al. (1997) modified the Washburn equation according to:

m2 ¼ cq2clv cos j

gð2Þ

Thus, the mass increase of the column during capillary rise is m (g),q is the density of the liquid (Mg m�3) and c is a geometry factor(m5) that considers the pore radius and the tortuosity of the capil-laries. For CA measurements of the testing liquid water, two un-known variables occur, i.e. the CA, j, and c. The geometry factor cis determined by applying a completely wetting liquid like n-hex-ane (cosj = 1) taking a second soil column with the same propertiescompared to the column used for the water infiltration. Once c isdetermined it is then assumed that the geometry factor is constant(porosity and tortuosity of the soil packings). However, this is con-sidered as the main error source within this experimental standard

protocol beside theoretical shortcomings of the CRM (Lavi et al.,2008). To minimize this problem, an exact control of constant sam-ple in weight and filling height is essential.

The WPM is also a physically based method to determine the CAof an air-dried soil sample. Over the entire range of up to 180� thesoil’s potential WR up to extreme hydrophobicity can be distin-guished. It was introduced by Adamson and Gast (1997) forsmooth surfaces and was used by Bachmann et al. (2003) to deter-mine CA for soil material, who found a good agreement betweenWPM and CRM-determined CA. The measurement is conductedon a glass slide covered with adhesive tape on all sides. The soilmaterial is placed on the double sided tape to creating a singlegrain layer. The glass slide is attached to an electronic balanceand gradually immersed into water. Ft is the total force acting onthe plate is given by (Bachmann et al., 2003):

Ft ¼W � Fb þ Fw ¼W � Vqg þ lwclv cosðjÞ ð3Þ

where Fb (N) is the buoyancy force, Fw (N) is the wetting force, W(kg) denotes the weight of the plate, V (m3) is the volume of theplate that is immersed into water, j (�) is the contact angle, g(m s�2) denotes the acceleration caused by gravity, q (kg m�3) isthe density, lw (m) is the wetted length of the sample and (mN m�1)is the surface tension of the liquid. Buoyancy and gravity are cor-rected and the dynamic advancing contact angle j is calculated by:

cosðjÞ ¼ Fw

clvlwð4Þ

Results are calculated using a precision tensiometer (DCAT 11,software SCAT 12, Data Physics, Filderstadt, Germany). The preci-sion of the CA determination is soil dependent and approximately±5� (Bachmann et al., 2003). The WPM CA was calculated as themean of the advancing and receding CA. As pointed out by Ramí-rez-Flores et al. (2010), the mean of the advancing and the recedingWPM CA is quite close to the CA determined with SDM, indicatingthat SDM approximates the equilibrium CA in contrast to the dy-namic CA determined by WPM and CRM.

As an empirical measure of SWR we used the WDPT test to de-fine the persistence of the potential SWR of the dry soil materialaccording to Dekker and Ritsema (1994). The WDPT test is appliedin many field studies due to simplicity and easiness of use. 10 g ofthe air-dried and homogenized soil sample was put into plasticcups and a 1.5 kg steel weight was carefully placed on the surfaceof the smoothed sample to ensure equal bulk density of the sam-ple. Six droplets of 0.05 ml deionized water were randomly placedon the sample surface using a pipette with a disposable tip. Thetime for the water drop to completely infiltrate into the soil was re-corded for each droplet and the median value for each sample wasconsidered to be a representative WDPT for each soil depth.

Since the CA not only depends on the solid interfacial propertiesbut also on the liquid surface tension (SFT), we also measured theSFT clv of the tracer solution to quantify the impact of the corre-sponding tracer SFT on the CA. Different values of SFT were ad-justed with mixtures of ethanol (clv = 22.4 mN m�1) and water(clv = 73 mN m�1). These mixtures were used as testing liquids to

48 C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54

determine the SDM–CA. The SFT was determined with a wilhelmyplate using a tensiometer (DCAT11, Data Physics, Filderstadt,Germany).

2.3. Determination of wetting and drying soil water retention curves

To quantify the strength of hysteresis in the water retentioncurve (WRC) we determined the main drying and wetting waterretention curves h (h) for undisturbed field soil cores under labora-tory conditions for 15, 40, 60, 80 and 125 cm depth using conven-tional methods. First, the soil cores had been saturated from thebottom and then stepwise drained at pressure heads of �10,�63, �100 hPa with the hanging water column method. Furtherdrainage at pressure heads of �316, �1000 and �15,848 hPa wereconducted with pressure plates (Klute, 1986). The samples wereweighed to ascertain if equilibrium conditions had already beenreached. For the wetting experiment we selected soil cores that

Fig. 2. Arithmetic means of CA as a function of depth measured with SDM (n = 6),WPM (n = 3) and CRM (n = 3). For the SDM, CA at initial measuring time (t1 = 30 ms)are displayed. The error bars denote the standard deviation.

Fig. 3. Results of SDM CA as a function of soil depth for the measuring timest1 = 30 ms, t2 = 1000 ms, and t3 = 5000 ms. The error bars denote the standarddeviation (n = 6).

had been drained until �316 hPa to avoid complete drying-out ofthe sand samples.

The selected samples were placed on a ceramic plate after thelast drying pressure step of �316 hPa. Before placing the samples,the ceramic plate was immersed in water for 2 days to remove air.It was connected to a tube and drained at a pressure head of�63 hPa. The bottom of the soil cores was closed with a membranewith a pore size of 15 lm. To ensure the hydraulic contact betweenthe water in the plate and the sample we used a thin layer of siltmixed with water as a contact material. The top of the soil coreswas covered with a plastic foil against evaporation. The sampleswere weighed frequently to record the progress of the wetting pro-cess. This was also done for the next pressure steps of �10 hPa and�5 hPa. After the last wetting step the samples were dried at105 �C for 24 h to obtain the soil bulk density and the water contentof the samples at the applied pressure heads. The obtained retentiondata was fitted to the van Genuchten model (van Genuchten, 1980)using the RETC code (van Genuchten et al., 1991):

hðhÞ ¼ hr þhs � hr

ð1þ jahjnÞmð5Þ

where hr and hs are the residual and the saturated water content(cm3/cm3), a is the scaling parameter, n is the curve shape factorand m is an empirical constant that is related to n by m = 1 � 1/n.In the fitting procedure hr and hs were kept constant to values of

Table 2Results of the WDPT test to determine the SWR of the dry material depending on thesoil depth.

Sample depth(cm)

WDPTa

(s)Standarddeviation

Classificationb

15 10 1.04 Slightly waterrepellent

40 <1 – Wettable60 9 1.03 Slightly water

repellent80 19 2.08 Slightly water

repellent

a Given are the median values of six replicate measurements.b Classification according to Dekker and Ritsema (2000).

Fig. 4. Mean SDM CA as a function of the surface tension SFT (mN m�1) for soilmaterial from 15 cm depth and 125 cm depth. Measurements were conducted attimes t1 = 30 ms, t2 = 1000 ms, and t3 = 5000 ms. The error bars denote the standarddeviation (n = 6).

C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54 49

hr = 0.02, hs = 0.53 for the topsoil and hr = 0.008, hs = 0.45 for the sub-soil. Thus, only a and n were optimized.

3. Results and discussion

3.1. Effect of soil water repellency

Fig. 2 shows the CA of the investigated soil depths determinedwith the WPM, CRM and SDM method. In general, all three meth-ods show consistent results, but there were still some differences:Dynamic WPM and CRM CA increased with depth below the top-soil with WPM values ranging between 50� and about 70�. CRMCA were always close to 90�. Both methods measure the smallestCA in 40 cm depth. With the CRM only CA between 0� and 90�can be determined. As the CA measured with CRM were very closeto 90�, it should be kept in mind that the CRM method might not bevery suitable for our soil, as advancing CA measured with WPMwere higher than 90�. The SDM CA were in the range betweenthe WPM CA and the CRM CA. For comparison, we took the initialstatic SDM CA. Note that SDM as a direct method and WPM as anindirect method estimate the CA of quasi 2 d-surfaces, whereas theCRM determines the penetration of the fluid into a 3 d-pore system(Ramírez-Flores et al., 2010). As a consequence, the methods de-scribe different processes and have a different physical back-ground. Nevertheless, the results are consistent for all methods,as all results indicate an increase in water repellency with depthbelow the Ap-horizon.

To obtain more information on the temporal wetting behaviorwe evaluated the SDM CA at times 30 ms, 1000 ms and 5000 msafter placing the drop as an estimate of the stability of the wettingproperties. It is assumed that SWR exhibits a breakdown depend-

Fig. 5. Water content changes measured with TDR sensors in 15 cm, 45 cm, and 75 cm dThe vertical grey lines denote the end of the ponding when no water could be observed

ing on the time the soil particles are in contact with water. InFig. 3 the SDM CA are plotted at measuring times of t1 = 30 ms,t2 = 1000 ms, and t3 = 5000 ms. The differences between the initial(30 ms) and final measuring time (5000 ms) are highest in the top-soil (15 cm) and decrease with increasing soil depth. The smallestdifferences between initial and final measurement were found at80 cm, whereas at 125 cm the CA were stable over all times withmean values �80� and similar standard deviations. These resultsare surprising as they show relatively stable CA in the range of sub-critical SWR at a soil depth where no such values are expected. Toour knowledge there is currently no study having observedincreasing water repellency and in particular persistence in thedeeper subsoil compared to the humic topsoil. We did not furtherinvestigate this phenomenon, because the mechanisms were be-yond the scope of this study.

The results of the commonly applied WDPT test are listed in Ta-ble 2. In general, the WDPT of the samples reflected an increasingpersistence of SWR with depth on a very low level below the top-soil, which corresponded to the measured CA determined withSDM. At 80 cm depth, the persistence measured by the WDPT ishighest with 19 s. The WDPT test classified the soil only as slightlywater repellent with the exception of the 40 cm depth, which wasdetermined as wettable. The lowest persistence of SDM CA wasfound in the 15 cm topsoil sample and increased with depth. Incontrast to this, the WDPT test showed no persistence for the40 cm sample. This could not be confirmed with the SDM, CRMor WPM method. This discrepancy can be explained by the charac-teristics of the WDPT test. It only qualitatively differentiates be-tween wettable and water repellent soil. According to Bachmannet al. (2000), the WDPT test is only sensitive in a narrow rangearound CA of 90� and the WDPT test mixes the impact of initialCA and persistence for non-hydrophobic soils.

epth (A: �15 cm, B: 0 cm, C: +15 cm, D: 30 cm distance to the central vertical axis).in the infiltration ring any longer.

Fig. 6. Matric potential measured with tensiometers in 30 cm, 60 cm, and 90 cm depth (A: �15 cm, B: 0 cm, C: +15 cm, D: 30 cm distance to the central vertical axis). Thevertical grey lines denote the end of the ponding when no water could be observed in the infiltration ring any longer.

Fig. 7. Photograph of the vertical soil trench through the center of the infiltrationzone, recorded 4 h after the end of the experiment. The dark grey color illustratesthe dye-stained infiltration plume.

50 C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54

Authors investigating water repellency in natural field soils re-ported a higher degree of water repellency in the topsoil, whichwas associated with higher contents of soil organic carbon (SOC)(Dekker and Ritsema, 2000). In our case, SOC content was highestin the topsoil (Table 1). However, this does not directly implicate

that SOC is always positively related to SWR. For example, Eller-brock et al. (2005) stated, that the soil’s wetting properties canbe better understood if the composition of the organic matter isconsidered which might explain the persistence of SWR in the dee-per subsoil beside the fact that SOC was low. The statement of per-sistence is verified when considering the effect of a lower surfacetension than water on the CA (Fig. 4). The wettability differencesfor the 15 cm and the 125 cm samples are greatest as concludedfrom the SDM CA measurements in Fig. 3. For a completely wettingfluid (pure ethanol) no difference between the SDM CA measuredat 15 cm and 125 cm depth could be observed. But at the SFT ofthe in situ applied tracer fluid (40 mN m�1) differences betweenthe two samples become evident. With this SFT the 125 cm CA de-creased about 20� to 20� at t3. With a slight increase towards48 mN m�1, the CA were stable with similar standard deviationsand the CA close to 60�. Using water, the 125 cm CA depth werealso stable around 90� but with lower standard deviations. The ini-tial CA at t1 for the 15 cm depth at the SFT of water was equal tothe 125 cm depth CA at the same time step, but decreased toaround 40� at t3, which is similar to the results obtained in Fig. 3.Thus, the persistence for the deeper soil sample seems to occur al-ready at a reduced SFT. This means that the differences in wettingproperties of the 15 cm and 125 cm depth soil sample were dis-tinctive, even for a tracer solution with a smaller SFT than water.For the soil solution a reduced SFT can usually be assumed dueto dissolved organic matter (DOM) (e.g., Arye et al., 2006) and inparticular in our experiment due to the addition of Brilliant Blueand KBr. The results confirm the persistence of SWR at the125 cm depth soil.

To conclude, we state that the characteristics of SWR measuredin the Fuhrberg soil profile are different to common knowledgeabout texturally homogeneous sandy soil profiles. Thus, the follow-ing questions arise: 1. Is it possible to relate CA determined under

C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54 51

laboratory conditions and field hydraulic processes? 2. Have theobserved differences in CA stability any measurable impact on afield infiltration process? Therefore, we will look at the sensor re-sponses and the dye pattern excavated after the infiltration.

Fig. 8. Wetting and drying retention data points and fitted van Genuchtenfunctions for 15 cm, 60 cm, and 125 cm depth.

Table 3Van Genuchten parameters a and n obtained by fitting of the drying and wettingretention data points.

Depth (cm) ad nd aw nw

15 0.12 1.38 0.09 1.4160 0.13 1.64 0.20 1.93

125 0.10 1.47 0.26 1.65

The subscripts d and w refer to the drying and the wetting curve, respectively.

3.2. Hydraulic behavior in situ

Fig. 5 summarizes the time-dependent changes in water con-tent during the ponded infiltration experiment. The water contentsbefore the infiltration started were 10 vol.% in 15 cm, 6–7 vol.% in45 cm depth, and 8 vol.% at 75 cm depth. In 5B and 5C for 15 cmdepth the saturation increases faster than in 5A before the infiltra-tion has ceased, which can be interpreted as a slight saturationovershoot at this depth. Saturation overshoot can be associatedwith unstable flow conditions (e.g., DiCarlo, 2004; Bauters et al.,2000b). At the other depths, the water content peaks after or atthe time the infiltration has stopped.

In 5D, a decline in saturation could be detected only shortlyafter the end of the ponding. Thereafter, values remained nearlyconstant until the final measurement. Sensors at 45 cm depthshowed a more pronounced decline in saturation after the stopof ponding and the values returned back to initial conditions. Thesaturation decrease at 75 cm depth was weaker compared to thecorresponding values at 45 cm depth. This could be observed forthe sensor lines 5A–C. The most significant difference was foundfor 5B, where only 80 min after the start of the ponding a signifi-cant decrease of the water content values was detectable. We thinkthe divergence between these two sensor depths is a consequenceof the increased stability with depth which reveals a longer waterrepellency breakdown time at 75 cm depth than at 45 cm depth.This breakdown time has to overcome before the SWR loses itscharacteristics at the given water content during the contact timeof the soil particles and the tracer solution. A field study from theMojave Desert by Nimmo et al. (2009), who found saturation over-shoot under ponding conditions, is so far the only article in which asaturation overshoot under field conditions is reported.

In Fig. 6 the tensiometer pressure heads are depicted for thevertical distance to the ponding center as shown in Fig. 5 but fordifferent depths than the TDR sensors. The pressure declined to po-sitive pressure heads in all measured depths in 6A and 6B andstayed nearly constant with the passage of the wetting front, whichindicates that infiltration behind the front is driven by a unit gra-dient and that the hydraulic conductivity equals the downwardflux. In 6C, a positive pressure during the ponding was observedin depths of 30 cm and 60 cm. The matric potential at 90 cm depthwas lower. The tensiometers in 6D demonstrated a reaction of theponding at 15 cm depth with �20 hPa as highest values. The dee-per installed sensors only showed a weak reaction due toinfiltration.

After the infiltration has stopped, values gradually decrease,although there is a slight difference between the 60 cm and the90 cm values. The pressure head decrease is weaker for 90 cmdepth, indicating a similar difference as measured with the TDRsensors in 45 cm and 75 cm depth. The observed positive pressureheads needed to be built up to overcome the water entry value ofthe soil (Cho and de Rooij, 2002). When this threshold value isreached, the water can further penetrate into the porous medium.

However, the hydraulic sensor responses show only weak signsof unstable flow in the form of a slight saturation overshoot in thetopsoil. This non-monotonic behavior can also occur in wettablesoils and can be associated with a pressure drop directly behindthe wetting front (Bauters et al., 2000b; DiCarlo, 2007; Geigerand Durnford, 2000; Selker et al., 1992) which is regarded as thecause for gravity-driven fingering (DiCarlo et al., 2010). Except thisovershoot and the depth-dependent SWR breakdown times there

is no distinct variation from the hydraulic characteristics of a wet-table soil.

But when considering the excavated tracer pattern in Fig. 7 thedifference to an infiltration into a wettable soil is distinctive.

The excavation of the central ponding profile 4 h after the end ofthe experiment revealed a cone-shaped infiltration zone withdecreasing lateral extent of the blue-colored soil and a sharp con-trast between the stained and unstained material. The general lat-

52 C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54

eral extent of the infiltration zone was dependent on the ring sizeof the infiltration ring (£ 40 cm) with a distinctive lateral decreaseat 100 cm depth to about 50% of the PVC ring diameter. This meansa lateral reduction of 20 cm compared to the infiltration source atthe top of the soil profile and hence a reduction of the wetted areaof 75%.

Wallach and Jortzick (2008) and Xiong et al. (2012) demon-strated with packed soil columns that unstable finger-like flowcan occur in water-repellent soils showing saturation overshootduring infiltration and redistribution. The shapes of the plumes re-ported for the water repellent soils in these studies were consider-ably different compared to those of a wettable soil and in generalshowed a lesser lateral extent compared to a wettable soil as cap-illary forces are reduced. As the degree of SWR of our soil profilewas in the subcritical range (CA = 0–90�) and the WDPT test indi-cated slight SWR in the subsoil, the plume shape can be comparedto those reported in Wallach and Jortzick (2008) and Xiong et al.(2012), although we only have one snapshot of the infiltration zonein time. A main difference to the findings of Wallach and Jortzick(2008) is their positive correlation between the SOM content andthe degree of water repellency, which could not be found in ourexperiment. The wetting front of a wettable soil would proceedas a flatter, more horizontal Richards’ type wetting front (Bauterset al., 2000a), which was not the case in our experiment. Thus,we suggest the shape of the infiltration zone is a consequence ofthe special wettability features at this site. Due to the presenceof weak unstable flow signs during the infiltration we propose todefine the infiltration pattern as a macro-finger, as it was not sep-arated into individual fingers.

It was not possible to combine wm and h at the same depth forcalculating an in situ water retention curve, because of the highlydynamic transport process. Although we could not record h and wm

during the infiltration at 125 cm depth where the persistence ofSWR was greatest, we assume a more pronounced lateral reductionof the dye-stained infiltration zone with depth.

3.3. Effect of hysteresis

A hysteretic water retention characteristic function was men-tioned to be a need for unstable flow conditions and thus for grav-ity-driven fingering. We show results for the main drying andwetting of the soil cores to determine a possible hysteresis effecton the infiltration process. We selected the data of 15 cm, 60 cm,

Fig. 9. Water content increase (vol.%) of the soil core

and 125 cm depth with the fitted WRC for an overview, whichwere generated during the equilibration of the soil samples(Fig. 8). There is an obvious trend within the soil profile: At15 cm depth the fitted wetting and drying retention curves donot differ significantly, indicating no hysteresis. At 125 cm depthboth curves are more distant from each other and a hystereticbehavior is observed. The results for the 60 cm depth show anintermediate state between the depths of 15 cm and 125 cm. Thecorresponding van Genuchten parameters for the drying and wet-ting curves obtained from the curve fitting process are given inTable 3.

In fact, we did not find a strong hysteresis in the WRC. Instead,we observed a long time period for the samples until reachingequilibrium conditions. In Figs. 9 and 10 the water content increaseat the pressure heads of �10 hPa and �5 hPa is plotted againsttime. For a pressure head of �10 hPa we recorded more than50 days for the water absorption process. During this process, thetopsoil samples gained up to almost 12 vol.% of water for the100 cm3 soil cylinders. The subsoil samples took up to 4 vol.% inweight. During the next wetting step with a pressure head of�5 hPa the water content increase was reversed and thus smallerfor the topsoil than for the subsoil samples. Note the long timespan of 76 days in which a constant water content increase ofthe samples was measured. Although the subsoil samples at�5 hPa still continued to take up water, we decided to stop theexperiment to avoid intensive microbial growth. The results aresimilar to those of Ritsema and Dekker (1996) who observednon-equilibrium conditions for extremely water repellent sandfor a pressure head of �5 cm even after a time period of 150 days.

These hysteresis effects are expected to increase with decreasedequilibration time at the corresponding supply pressure. Effects ofchanged texture or density on the hysteresis observation can beexcluded as texture did not change with depth and there was notrend in bulk density observed within the samples (Table 1).

The SWR persistence in the deeper subsoil might also be recog-nized with respect to the fitted SWRC. In the topsoil, SWR brokedown quickly but from 60 cm to 125 cm depth the persistence ofSWR increased. Accordingly, the soil cores from greater depthsneeded more time to equilibrate during the rewetting of the sam-ple, which leads in tendency to a larger hysteresis.

However, we suggest the hysteresis effects being more pro-nounced during the field experiment than in the laboratory anddifferences between laboratory data and hydraulic behavior

s during wetting at a pressure head of �10 hPa.

Fig. 10. Water content increase (vol.%) of the soil cores during wetting at a pressure head of �5 hPa.

C. Ganz et al. / Journal of Hydrology 484 (2013) 45–54 53

in situ occur because of the different time-scales, temporal dynam-ics and wetting procedures. Consequently, the wetting curvesdetermined from the laboratory samples have restricted informa-tional value regarding the hydraulic behavior in situ. Nevertheless,hysteresis was shown to be greatest in water repellent sands (Nie-ber et al., 2000) and can influence wetting front saturation togetherwith small changes in pressure (Bauters et al., 2000b).

To summarize, we can conclude from our observations that theconical and sharp wetted area may be the product of persistentwettability conditions leading to an advancing infiltration frontthat is contrary to a stable and flat horizontal wetting front in awettable soil with pronounced lateral fluid spreading.

How the tracer pattern appears under non-ponding conditionsis not clear yet. Under irrigation conditions when the flux is belowthe saturated hydraulic conductivity preferential flow paths in theform of flow-fingers might be observed at this site. However, thiscannot be concluded from the observed experiments.

4. Conclusions

In this paper, we showed how persistent soil water repellencyin the subsoil can alter an advancing wetting front in a poorlystructured sandy field soil, which is not visible at first glance.The existence of SWR is not, as often assumed, restricted to the hu-mic topsoil. We found that SWR breakdown measured at the smallscale in the laboratory helps to understand some macroscopichydraulic characteristics in situ, although measurements are basedon experiments that had been carried out on homogenized and air-dried soil material. This is a new finding and evolves as a relevantadditional criterion besides the absolute degree of SWR for non-hydrophobic soils which are determined with the WDPT test. AsSWR was also shown to influence the measurement of the waterretention curve, the results gained from those standard measure-ments may not correctly reflect the hydraulic properties of the soiland underestimate the strength of the hysteretic behavior. Whenthese hydraulic parameters serve as input for water transportmodels it is likely that the predicted infiltration dynamics areunderestimated compared to the in situ conditions. The results ofthis study can be relevant for other field soils, where farming orland use change has altered subsoil wettability, for example in re-gions where effluent irrigation is practiced. Investigating the de-gree of WR also deeper in the soil profile might lead to a betterunderstanding of the site-specific infiltration behavior. We con-

clude that the persistence of SWR determined in the laboratory issignificant and can serve as additional information to improvethe effectiveness of designing drip irrigation devices in dry anddesert regions, where SWR is a problem for supplying plant-avail-able water.

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