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Spectral Reflectance Properties of Crusted Soils under Solar Illumination

G. Eshel,* G. J. Levy, and M. J. Singer

ABSTRACT gions (Singer, 1991; Singer and Warrington, 1992). Sur-face crusts are thin (�2 mm) layers characterized byReflectance of soil crusts has been studied under artificial light butgreater density, higher shear strength, finer pores, andnot under solar illumination. Our objectives were to (i) compare reflec-

tance from crusted and noncrusted soil surfaces under solar illumina- lower saturated hydraulic conductivity than the bulk soiltion, and (ii) explore the relationship between crust permeability and below (Bradford et al., 1987). Crust formation is causedspectral signature. Two California soils were studied, Capay (fine, by two mechanisms that act simultaneously and enhancesmectitic, thermic Typic Haploxererts) and Reiff (coarse-loamy, mixed, each other: (i) a physical breakdown of soil aggregatessuperactive, nonacid, thermic Mollic Xerofluvents), with samples for caused by fast wetting of dry aggregates and the mechan-the latter taken from plots under organic (ReiffOM) and conventional ical impact of water drops; and (ii) a physicochemical(ReiffCM) management. A laboratory rainfall simulator was used to

dispersion of clay particles which migrate and clog theform crusts which were sampled at different stages of developmentconducting pores immediately beneath the surface (Mc-and permeability. A portable spectroradiometer was used to collectIntyre 1958; Agassi et al., 1981). The formation of a crustspectral data in direct sunlight for wet and dry crusted samples. Base-is commonly characterized by decreased soil IR; theline spectra (albedo) of the dry samples were, in most cases, higher

than spectra from the corresponding moist samples. Crusted samples more developed the crust, the lower the IR (Shainbergexhibited higher baseline spectra compared with the noncrusted sam- and Levy, 1995, and references cited therein).ples. The absorption feature at ≈1400 nm (related to lattice OH of Studies of crust micromorphology have revealed thatmontmorillonite) suggested accumulation of clay in the crust of the soil crusts may vary in their structure, depending on soilReiffCM, and clay depletion from the crusts of the Capay and ReiffOM. properties and conditions prevailing in the soil duringAn inverse linear relationship existed between reflectance at many crust formation. Under conditions that enhance clay dis-wavelengths and crust permeability at different stages of crust devel-

persion (i.e., soil with high sodicity, or rain water withoutopment. The reflectance at 1700 nm provided highly significant corre-electrolytes � distilled water) the crust consists of twolations with infiltration rate (IR) for all three soils, and reflectancedistinct layers (McIntyre, 1958; Gal et al., 1984; Onofiokat 2130 nm also had a highly significant correlation for the Capay.and Singer, 1984): (i) an upper layer (0.1–0.25 mm) ofOur data suggest that spectral measurements of soil surfaces under

solar illumination can differentiate between crusted and noncrusted high porosity composed predominantly of individualsurfaces and assist in evaluating the degree of crust development. sand- and silt-size grains, and (ii) a deeper layer of

considerably lower porosity composed of accumulatedfine material that was washed out from the upper layer,termed washed-in zone. Under conditions where clayReflectance of soils across the entire spectral re-dispersion was prevented, only a thin skin layer at thegion of solar illumination (400–2400 nm) carriessoil surface comprised mostly of compacted fine parti-much information about the soil (Baumgardner et al.,cles was noted with no evidence of accumulation of fine1985; Ben-Dor et al., 1998). For instance, informationparticles in the washed-in zone (Chen et al., 1980; Galregarding the nature and chemical composition of soilset al., 1984). Levy et al. (1988) noted that the structurecan be derived from measurements of their spectral re-of crusted surfaces is heterogeneous, comprising of smallflectance (Stoner et al., 1980). Reflectance data havemounds protruding from relatively smooth plains. Sur-been successfully used for prediction of numerous soilface and cross-section SEM micrographs showed thatproperties, such as soil moisture and organic matterstructure of the mounds was relatively unaffected bycontent, in laboratory studies (Dalal and Henry, 1986;raindrop impact. In the plains, a clearly compacted layerBen-Dor and Banin, 1995).with primary particles partly stripped of clay was visible.It was suggested nearly two decades ago that soil re-The differences in structure between mounds and plainsflectance measurements could assist in measuring quan-explained the significantly higher permeability mea-titative changes in soil surface conditions (Baumgardnersured in the mounds compared with that for the plainset al., 1985). Crust formation on the surface of bare soils(Levy et al., 1988).exposed to rain or overhead sprinkler irrigation is one

Spectral changes due to soil crusting have receivedcondition that may have significant adverse effects onlittle attention. Recently, Goldshleger et al. (2001) havesoil behavior, such as inducing runoff and soil erosion.noted for three Israeli soils that reflectance values inFormation of such surface crusts is a common phenome-the shortwave infrared (1100–2500 nm) from crusted sur-non in many soils, particularly in arid and semiarid re-faces were significantly higher than those from non-crusted soils. A further in-depth study of the same soilsG. Eshel and M.J. Singer, Dep. of Land, Air, and Water Resources,

Univ. of California, Davis, CA 95616; G.J. Levy, Institute of Soil, (Ben-Dor et al., 2003), has shown that (i) the magnitudeWater, and Environmental Sciences, Agricultural Research Organiza- of the difference in reflectance values between crustedtion (ARO), The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel. and noncrusted conditions among the soils tested de-Received 7 Jan. 2004. *Corresponding author (geshel@ucdavis.edu).

pended on soil texture, (ii) differences in crust micro-Published in Soil Sci. Soc. Am. J. 68:1982–1991 (2004). Soil Science Society of America677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: IFOV, instrument field of view; IR, infiltration rate.

1982

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ESHEL ET AL.: REFLECTANCE PROPERTIES OF CRUSTED SOILS 1983

fall durations. The additional column was not subjected tomorphology could be detected from their reflectancerain with energy and has been designated the zero-rainfall-spectra, and (iii) a significant relationship existed be-energy (noncrusted) sample. The IR was periodically deter-tween crust permeability and its reflectance spectra.mined during the entire rain event by measuring the volumeMost measurements of the spectral responses of soilsof water that drained from the central fitting during a knownhave been performed in the laboratory under artificial period of time. For each column, immediately after the column

illumination. A notable exception is the study of Stoner was removed from the rainfall simulator it was taken outdoorset al. (1980) that demonstrated that for wavelengths in for collection of the spectral data.the range of 520 to 1750 nm, reflectance under solarillumination for two moist soils was directly propor- Spectral Reflectance Measurementstional (about 1.5 times higher) to the reflectance ob-

An ASD Field Spec FR (Analytical Spectral Devices, Boul-tained under laboratory conditions. The objectives ofder, CO, model 6100) portable spectroradiometer with a spec-our study were to (i) compare reflectance measuredtral range of 400 to 2400 nm, fitted with an 18-degree foropticunder solar illumination from crusted surfaces to thatwas used for the spectral data collection. The spectroradiome-from noncrusted ones, and (ii) evaluate whether changes ter’s fiber optic was positioned 45 cm above and at the nadir

in the spectral response of a crust at different stages of to each sample to provide a 14.5-cm-diam. instrument fieldits development correlate with the corresponding IRs of view (IFOV).of the crust. Digital images of each sample were collected using a Nikon

(Melville, NY) Cool-Pix digital camera mounted with the fiberoptic (Fig. 1). Subsequent measurements on the dry samplesMATERIALS AND METHODSwere made with the same instrument and configuration.

All measurements were made in direct sunlight betweenSoilsthe hours 1000 and 1500. These measurements were made closeTwo California soils, the Reiff and Capay series, were sam- to the summer solstice in Davis, CA, when the sun is at itspled on the UC Davis campus. Two Reiff series samples were maximum height in the sky. Every day was cloud free andcollected, one that had been under purely organic manage- conditions were uniform for all of the digital data collection.ment for 12 yr (ReiffOM) and the other from a nearby plot Spectral data for all samples were collected in radiancethat had been under conventional management for the same mode. A reference scan of a 25- by 25-cm diffuse reflectanceperiod (ReiffCM). The Capay series soil was sampled from a panel (Spectralon, Labsphere, Inc., North Sutton, NH) andfield that has been under conventional management for many six sample scans were collected for each soil and energy sampleyears. The A horizon of each soil from the soil surface to across the range of 450 to 2400 nm in 1-nm steps. Initial200 mm below the surface was sampled from each site. The postprocessing reflectance percentages were calculated usingsamples were dry at the time of collection, and were sieved the ratio of mean sample radiance to reference panel radiance.in the field to recover �4-mm aggregates (a size fraction that Two sets of spectral data were collected for every column:generally represents the prepared seed bed in these soils). (i) immediately after completion of the rainfall simulation,After collection, the soils were stored in closed containers. A and (ii) after 48 h of oven drying at 40�C. In cases where 48 hfew characteristics of each soil are shown in Table 1. of drying were proven insufficient to fully dry the surfaces,the columns were allowed to dry for an additional period of

Rainfall Simulation 24 h to reach 72 h of drying in total. A digital image represent-ing the IFOV of the spectral scan was collected at the sameSoil was loosely packed into 185-mm-diam. by 50-mm-deeptime that spectral response was measured for each surface atcolumns by filling each column to the rim and gently tappingeach moisture content. A small soil sample (about 5 g) of theits side to settle the soil. Each column had a solid bottom withcrust was taken near the perimeter of every column after eacha central fitting where water could drain. Soil in each columnspectral measurement for determination of soil moisture con-was first saturated using zero-energy distilled-water rain ap-tent. It is important to note that because of technical problems,plied as a mist by four nozzles (Spraying Systems Company,not all samples were completely dry after 72 h in the oven. ToWheaton, IL), placed at a 0.9-m height. Water began to flowreduce the interference of water content with the reflectancethrough each column after approximately 16 min. To ensuremeasurements, only the 10 driest samples for each soil studiedthat the soil was fully saturated before energy rain was applied,were selected for the dry reflectance measurements.application of zero-energy rain continued for approximately

30 min after water began to flow out of the columns.After completion of the saturation phase, the columns were

transferred to the energy-carrying rain simulator which wasimmediately adjacent to the zero-energy one, and placed ata slope of 5% to promote runoff and reduce ponding. Thecolumns were exposed to 3.2-mm-diam. distilled-water dropsthat fell from a height of 1 m through 18-gage hypodermicneedles. Water was delivered to hypodermic needles mountedin a chamber by a variable speed electric pump. Raindropswere applied at a 70 mm h�1 rate as measured before and aftereach experiment. Different rainfall durations were applied tosubject the soil surface to different levels of energy, and thusform crusts of different permeability. Two sets of 10 columnswere packed for each soil. Nine columns from each set wereplaced randomly in the rainfall simulator. The columns wereremoved from the rainfall simulator, one at a time at predeter- Fig. 1. Schematic presentation of the reflectance measurement appa-

ratus and arrangement for collecting spectral data.mined time intervals to obtain crusts from nine different rain-

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1984 SOIL SCI. SOC. AM. J., VOL. 68, NOVEMBER–DECEMBER 2004

RESULTS AND DISCUSSION of the surface was occupied by aggregates (macro- andmicro-) and part by sand-size material (Fig. 2). WhenSurface images of dry noncrusted (zero energy) andcrusted, the surface of the Capay soil was smoothercrusted (1044 J m�2 cumulative rain energy) samples fromthan the noncrusted one, and showed some cracks thatthe three soils illustrate the differences in the untreateddeveloped during the drying phase. In the Reiff soils,and treated surfaces (Fig. 2). Surfaces of the noncrustedthe crusted surfaces showed a limited degree of cracking,samples were rough compared with the crusted samples.and they seemed smoother than the crust in the CapayIn the Capay soil, the rough surface was composed mainly

of macroaggregates; in the ReiffCM and ReiffOM soils, part soil (Fig. 2). Levy et al. (1994) demonstrated that the

Fig. 2. Images of dry (1) noncrusted and (2) crusted surfaces of (a) Capay, (b) ReiffCM, and (c) ReiffOM soils.

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ESHEL ET AL.: REFLECTANCE PROPERTIES OF CRUSTED SOILS 1985

degree of crust smoothness was inversely related to soil near saturation (42.4, 36.1, and 37.2% for Capay, ReiffCM,and ReiffOM, respectively). In all three soils, the moistclay content. The lower clay content in the Reiff soil

compared with the Capay soil may explain the observed crusted and noncrusted samples clearly showed a lowerspectral baseline than their corresponding dry equiva-smoother surfaces in the crusted Reiff soils (Table 1).

The crusted surfaces had a lighter color than the non- lents (Fig. 3). This observation was in agreement withpreviously published data (e.g., Hoffer and Johannsen,crusted surfaces (Fig. 2). The change in color suggests

that the composition of the particles at the soil surface 1969; Planet, 1970; Lobell and Asner, 2002). It has beensuggested that the primary reason for the lower reflec-changed during the crust formation process. In all three

soils, light-color spots of naked silt and sand grains (Gal tance of moist soil is the decrease in the relative refrac-tive index of the medium surrounding the soil particleset al., 1984; Onofiok and Singer, 1984), separated by

darker areas were observed. The darker areas were con- as a result of water replacing air (Twomey et al., 1986;Ishida et al., 1991).sidered to represent regions of compacted fine material

(Chen et al., 1980; Levy et al., 1988) with a limited No difference in color could be noted between thecrusted and noncrusted moist samples, yet reflectancedepletion of clay.from the crusted samples was, generally, higher thanthat from the noncrusted ones (Fig. 3). The higher re-Soil Spectral Reflectanceflectance from the crusted samples could be ascribed toField measurements of spectral curves do not contain(i) its smoother surface (Fig. 2), which decreased thedata in the 1850- to 1950-nm region because in thisshadowing effect (leading to reduced reflection) of theregion solar illumination is almost completely absorbedrougher and more aggregated surfaces of the noncrusted(Stoner et al., 1980). Therefore, only data in the 400-samples, and (ii) possible accumulation of fine materialto 1850-nm and 1950- to 2400-nm ranges, that providedin the crusted layer (Chen et al., 1980; Levy et al., 1988).useful information, were used in our analyses (Fig. 3).Although differences in clay content between the CapaySoil spectral response is comprised of a spectral baselinesoil and the Reiff soils, and differences in organic matter(albedo) and absorption features differing in position,content between the two Reiff soils were measuredshape, and intensity. Changes in the spectral baseline(Table 1), reflectance spectra for the crusted samplesare associated with physical characteristics of the soiland those for the noncrusted samples were similar insurface (aggregate size and soil moisture). Changes inthe three soils (Fig. 3).the absorption features depend on the chemical compo-

A spectral absorption feature (decrease in spectralsition of the soil (e.g., OH in clay lattice, iron oxide,reflectance) at around 1450 nm was noted in both thecarbonates, organic matter). In the range of the wave-crusted and noncrusted samples of the three soils (Fig. 3).length used in our study (400–1750 and 1950–2400 nm),This feature was assigned to OH in adsorbed water.some absorption features assigned to OH in adsorbedThe large size of the absorption feature in all the sampleswater can be found in the soil reflectance curves, includ-was ascribed to the high water content in the samples.ing a strong absorption band at 1450 nm and occasion-An additional small absorption feature at 2200 nm wasally weaker absorption bands at 970 and 1200 nmassigned to OH groups in the clay lattice and indicates(Baumgardner et al., 1985). In addition, soils containingthe presence of montmorillonite in the clay fraction ofsmectitic minerals may exhibit absorption bands atthe soils (Ben-Dor, 2002).around 1300 to 1400 and 2200 to 2500 nm. These absorp-

tion bands are assigned to OH groups in the clay mineralDry Sampleslattice because they are the only groups among the vari-

ous elements of clay minerals that provide a spectra Reflectance measurements of the dry crusted andacross the entire spectral region of solar illumination noncrusted samples (Fig. 3) were taken after the samples(Ben-Dor, 2002). were allowed to dry 48 or 72 h. Moisture contents of

the dry samples were 6.2, 2.9, and 3.5% for Capay,Moist Samples ReiffCM, and ReiffOM, respectively. Reflectance measure-ments for the crusted ReiffOM in the 1950- to 2400-nmReflectance measurements of the moist samples wereregion showed a high noise-to-signal ratio and are notmade immediately after the completion of the rainfall

simulation, thus moisture content in these samples was presented (Fig. 3). The spectral baseline of the dry sam-

Table 1. Particle size distribution, cation exchange capacity, organic matter content, CaCO3 content, and exchangeable sodium percentageof the soils studied.

Soil Particle size distribution

Series Classification Sand Silt Clay CEC‡ OM§ CaCO3 ESP¶

volume %† cmolc kg�1 g kg�1 %ReiffCM Mollic Xerofluvents 29 54 17 25.9 19.4 66.4 0.72ReiffOM Mollic Xerofluvents 27 55 18 27.8 23.0 64.2 1.17Capay Typic Haploxererts 7 62 31 41.3 14.4 77.1 0.73

† Eshel et al. (2004).‡ CEC � cation exchange capacity.§ OM � organic matter.¶ ESP � exchangeable sodium percentage.

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1986 SOIL SCI. SOC. AM. J., VOL. 68, NOVEMBER–DECEMBER 2004

Fig. 3. Spectral reflectance for moist and dry crusted (a) Capay, (b) ReiffCM, and (c) ReiffOM soils.

ples was, in most cases, higher than that of the corre- the baseline spectra of the ReiffCM and ReiffOM indicatethat the spectra of the crusted and noncrusted ReiffOMsponding moist samples, and the crusted samples were

characterized by higher baseline spectra compared with were lower than the ReiffCM (Fig. 3b, 3c). No differenceswere observed by eye between the crusted samples ofthe noncrusted samples, almost across the entire spec-

trum (Fig. 3). The higher baseline is explained by the the two soils (Fig. 2). Therefore, the lower spectra inthe ReiffOM samples compared with the ReiffCM samplessmoother surfaces and lighter color of the crusted sam-

ples compared with the noncrusted samples (Fig. 2). was ascribed to the higher organic matter in the former(Table 1). Soil organic matter plays an important, yetThe lighter color in the crusted samples suggested that

some changes in the composition of the soil surface took at times complicated, role in the spectral response ofsoils (Ben-Dor, 2002). However, in general, it has beenplace during crust formation. The possible effects of

these changes on the spectral response of the soils are observed that as organic matter content increases soilreflectance decreases, especially when organic matterdiscussed later.

Unlike the wet samples, differences in reflectance content is �2% (e.g., Hoffer and Johannsen, 1969; Stonerand Baumgardner, 1981; Baumgardner et al., 1985).were noted among the three dry soils. Comparison of

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ESHEL ET AL.: REFLECTANCE PROPERTIES OF CRUSTED SOILS 1987

Fig. 4. Ratio of noncrusted to crusted reflectance percentage of dry samples from (a) Capay, (b) ReiffCM, and (c) ReiffOM soils.

The baseline spectra of the crusted and noncrusted detection of changes in clay content between the non-crusted and crusted layer based on spectral reflec-Capay samples were also lower than those from the

ReiffCM samples (Fig. 3a, 3b). This observation is ex- tance data.The observed peak height and direction at 1400 nmplained by the rougher and darker crusted surface in

the Capay soil (Fig. 2), which is attributed to its higher varied among the soils (Fig. 4). The ratio for the ReiffCM

sample was a high upward peak with a value of �1clay content (Table 1). Goldshleger et al. (2001) alsoobserved that the baseline spectra of soils decreased (Fig. 4b), which indicated the possible accumulation of

a substantial amount of clay in the crusted layer. Thiswith an increase in soil clay content.In addition to studying changes in the baseline spectra accumulation of clay was not observed in the image of

the crusted surface of this soil (Fig. 2b). Unlike the(albedo) in the crusted samples, it is also of interestto explore the absorption features of the crusted and ReiffCM sample, the Capay and ReiffOM samples dis-

played small downward peaks in their respective rationoncrusted surfaces. Similar to the wet samples, the drycrusted and noncrusted samples from the three soils curves (Fig. 4a, 4c). These downward peaks (implying

some depletion of clay material from the crusted layer)exhibited an absorption feature at 2200 nm (Fig. 3)belonging to lattice OH in montmorillonite. The inten- were in agreement with the images of the crusted sur-

faces of these soils (Fig. 2a, 2c), which showed spots ofsity of this absorption feature in the dry samples wasgreater than in the wet ones. In addition, the dry samples lighter color belonging to silt and sand grains from which

clay was stripped. Goldshleger et al. (2001) also notedof the Capay and ReiffCM also exhibited absorption fea-tures around 1410 nm (Fig. 3a, 3b) that is related to the downward peaks in the ratio curve around 1400 nmlattice OH of montmorillonite (Ben-Dor, 2002). for three Israeli soils. Albeit our inability to explain the

To improve the ability to detect changes in the absorp- upward peak in the ratio curve of the ReiffCM sample bytion features between the crusted and noncrusted sam- the surface image, the existing agreement between (i)ples, we followed Goldshleger et al. (2001) and divided the surface images and the downward peaks in the ratiothe reflectance at each wavelength of the noncrusted curves for the Capay and ReiffOM soils, and (ii) the peakspectra by the reflectance of the crusted spectra at the direction in the ratio curve in our study and that of Gold-same wavelength (Fig. 4). Potential peaks in the ratio shleger et al. (2001), promotes further exploration ofcurve at regions of absorption features related to lattice the possible use of reflectance data for identifying mor-OH enable us to examine the possibility of accumulation phological changes in the surface layer of crusting soils.or depletion of clay from the crusted layer. A downwardpeak indicates that the height of the absorption peak Table 2. Infiltration rate (IR) at progressively greater cumulative

rain energy for the Capay, ReiffCM, and ReiffOM soils.in the noncrusted layer was greater than that in thecrusted layer; suggesting clay depletion from the crusted IRlayer compared with the noncrusted one. An upward

Cumulative rain energy Capay soil ReiffCM soil ReiffOM soilpeak indicates the opposite trend regarding the heightJ m�2 mm h�1 � SD†of the absorption peak, thus signifying the enrichment0 70 70 70of the crusted layer with clay compared with the non-18 68.5 � 2.2 62.1 � 7.4 65.4 � 6.5

crusted layer. Our samples exhibited a peak in the ratio 46 66 � 5.6 52.9 � 6.8 59.5 � 9.392 58.2 � 16.6 31.1 � 2.8 41.7 � 4.7curve around 1400 nm but not at 2200 nm. The absence138 53.3 � 13.6 29.1 � 3.8 25.2 � 5.6of a peak in the ratio curve at 2200 nm may stem from 184 19.5 � 4.1

the fact that the absorption peaks at this wavelength 276 28.4 � 9.6 16 � 4.0 13.8 � 3.4368 10.9 � 3.3were of lower intensity than those at 1400 nm (Fig. 3),414 27 � 6.9 10.6 � 0.5a phenomenon also presented in the review of Ben-Dor 552 18.5 � 2.1 10.6 � 1.5 10.4 � 1.2828 12.3 � 0.5 10.6 � 0.1 8.7 � 1.8(2002) for the data on smectitic soils of Kruse et al.1104 8.4 � 0.3 7.1 � 1.1(1991) and observed in the data of Ben-Dor et al. (2003).† SD � one standard deviation.It is postulated that this lower intensity hinders the

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1988 SOIL SCI. SOC. AM. J., VOL. 68, NOVEMBER–DECEMBER 2004

Spectral Reflectance and Infiltration cumulative rain energy and their respective spectra. Co-Rate of Crusts efficients of determination (R2) and the slopes of the

regression lines for the three soils are presented in Fig. 5.Infiltration rates of the three soils at 10 levels ofAn inverse relation (negative slope) was found betweencumulative energy are presented in Table 2. Similar toreflectance and crust permeability, that is, reflectanceother observations (Agassi et al., 1981; Betzalel et al.,decreased with an increase in IR. In all three soils, R2

1995), the increase in rain cumulative energy was accom-values � 0.81 (corresponding to correlation coefficients �panied by a decrease in IR, indicating the development0.9) were found for many different wavelengths.of a crust. Assessment of the ability to use spectral re-

From the data presented in Fig. 5, we identified forsponse to determine the degree of crust developmenteach soil the wavelength at which both the coefficientwas obtained by performing a linear regression analysis

between the IR of crusts exposed to different levels of of determination was �0.81, and the absolute value of

Fig. 5. Coefficient of determination (R2) and the slope (a ) resulting from linear regression analysis between reflectance values at every spectralwavelength and the infiltration rate for the dry samples of (a) Capay, (b) ReiffCM, and (c) ReiffOM soils.

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ESHEL ET AL.: REFLECTANCE PROPERTIES OF CRUSTED SOILS 1989

the slope was high. The latter criterion was used to en- We then plotted the reflectance at this wavelength as afunction of IR (Fig. 6) and found that logarithmic re-hance the sensitivity of the reflectance as a predictor

for changes in crust IR. It was found that the reflectance gression provided even higher coefficients of determina-tion (R2). The slope of the fitted line in the Capay andmaxima (Fig. 3) between the absorption features gave

the best correlations. The absorption maxima at 1700 ReiffOM soils was similar and more than double that inthe ReiffCM (Fig. 6). The greater slope in the Capay andnm (which is between the absorption features assigned

to the OH of the absorbed water) provided highly signif- ReiffOM soils indicated greater changes in the reflectancewith changes in the IR; that is, reflectance was more sen-icant correlations for all three soils. The absorption max-

ima at 2130 nm also provided a highly significant correla- sitive to the different stages in crust development andhence changes in the structure of the soil surface. Thetion with the clay-rich Capay. This is near the 2200-nm

adsorption feature assigned to the OH of the clay lattice. changes in the reflectance spectra between the dry crusted

Fig. 6. Reflectance at 1700 and 2130 nm as a function of infiltration rate for the dry samples of (a) Capay, and at 1700 nm for (b) ReiffCM and(c) ReiffOM soils.

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1990 SOIL SCI. SOC. AM. J., VOL. 68, NOVEMBER–DECEMBER 2004

Leavitt from the University of Nebraska for assistance withand noncrusted samples from the three soils (Fig. 3)data handling.point out that the main source of difference in the Capay

and ReiffOM were in the baseline (albedo), while in theReiffCM the notable change was in the absorption feature REFERENCESat ≈1400 nm. Changes in albedo are associated with Agassi, M., I. Shainberg, and J. Morin. 1981. Effect of electrolytechanges in the physical characteristics of the soil surface concentration and soil sodicity on infiltration rate and crust forma-

tion. Soil Sci. Soc. Am. J. 45:848–851.and are best correlated with the IR. It is suggested,Baumgardner, M.F., L.F. Silva, L.L. Biehl, and R.E. Stoner. 1985.therefore, that in the Capay and ReiffOM soils, the pro-

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which is associated with physical changes that occur in Eshel, G., G.J. Levy, U. Mingelgrin, and M.J. Singer. 2004. Criticalthe surface during the process of crust development. evaluation of the use of laser diffraction for particle-size distribu-

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changeable sodium and phosphogypsum on crust structure—depletion of fine material from the crusted layer when Scanning electron microscope observations. Soil Sci. Soc. Am. J.compared with the noncrusted samples. In the third soil, 48:872–878.the analysis implied that enrichment of the crust with Goldshleger, N., E. Ben-Dor, Y. Benyamini, M. Agassi, and D. Blum-

berg. 2001. Characterization of soil’s structural crust by spectralclay occurred.reflectance in the SWIR region (1.2–2.5 �m). Terra Nova 13:12–17.Our data suggest that detection of the development

Hoffer, R.M., and C.J. Johannsen. 1969. Ecological potentials in spec-of crusts at soil surfaces via reflectance measurements tral signature analysis. p. 1–29. In P.L. Johnson (ed.) Remote sens-is not restricted to laboratory conditions only (as has ing in ecology. Univ. of Georgia Press, Athens.

Ishida, T., H. Ando, and M. Fukuhara. 1991. Estimation of complexbeen shown in previous studies), but can also be ob-refractive index of soil particles and its dependence on soil chemicaltained when measured under natural direct sun light.properties. Rem. Sens. Environ. 38:173–182.We found that the reflectance around 1700 nm provided Kruse, F.A., M. Thiry, and P.L. Hauff. 1991. Spectral identification

a statistically significant correlation with IR for all three (1.2–2.5 �m) and characterization of Paris basin kaolinite/smectitesoils studied and around 2130 nm also provided a statisti- clays using a field spectrometer. p. 181–184. In Proc. of the 5th

Int. Colloquium, Physical Measurements and Signatures in Remotecally significant correlation with IR for the high claySensing, Courchevel, France. Vol. I. 14–18 Jan. 1991.Capay soil. Although we used just three soils, our finding

Levy, G.J., P. Berliner, H.M. du Plessis, and H.V.H. van der Watt.are supported by interpretation of other soils studied 1988. Microtopographical characteristics of artificially formed crusts.before (e.g., Lobell and Asner, 2002; Ben Dor et al., Soil Sci. Soc. Am. J. 52:784–791.

Levy, G.J., J. Levin, and I. Shainberg. 1994. Seal formation and interrill2003). The results of our study provide solid evidencesoil erosion. Soil Sci. Soc. Am. J. 58:203–209.that crust development can be examined using reflec-

Lobell, D.B., and G.P. Asner. 2002. Moisture effects on soil reflec-tance spectroscopy. Further studies, mainly under natu- tance. Soil Sci. Soc. Am. J. 66:722–727.ral conditions and at a field scale are needed to be McIntyre, D.S. 1958. Permeability measurements of soil crusts formedable to use this tool for monitoring soil conservation by raindrop impact. Soil Sci. 85:185–189.

Onofiok, O., and M.J. Singer. 1984. Scanning electron microscopeprograms or to help in mapping regions susceptible tostudies of soil surface crusts formed by simulated rainfall. Soil Sci.soil degradation.Soc. Am. J. 48:1137–1143.

Planet, W.G. 1970. Some comments on reflectance measurements ofACKNOWLEDGMENTS wet soils. Rem Sens. Environ. 1:127–129.

Shainberg, I., and G.J. Levy. 1995. Infiltration and seal formationThe work reported here was supported in part by the Bina- processes. p. 1–22. In M. Agassi (ed.) Soil erosion and rehabilita-

tional Agricultural Research and Development Fund (BARD) tion. Marcel Dekker, New York.grant IS-3111-99. We appreciate the help of Rick Perk from Singer, M.J. 1991. Physical properties of arid region soils. p. 81–109.the University of Nebraska for making the reflectance mea- In J. Skujins (ed.) Semiarid lands and deserts, soil resource and

reclamation. Marcel Dekker, New York.surements and initial data management. We also thank Bryan

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ESHEL ET AL.: REFLECTANCE PROPERTIES OF CRUSTED SOILS 1991

Singer, M.J., and D. Warrington. 1992. Crusting in the western United Stoner, E.R., M.F. Baumgardner, R.A. Weismiller, L.I. Biehl, andB.F. Robinson. 1980. Extension of laboratory measured soil spectraStates. p. 179–204. In M.E. Sumner and B.A. Stewart (ed.) Soil

crusting: Chemical and physical processes. Lewis Publ., Boca Ra- to field conditions. Soil Sci. Soc. Am. J. 44:572–574.Twomey, S.A., C.F. Bohren, and J.L. Mergenthaler. 1986. Reflec-ton, FL.

Stoner, E.R., and M.F. Baumgardner. 1981. Characteristic variations tances and albedo differences between wet and dry surfaces. Appl.Opt. 25:431–437.in reflectance of surface soils. Soil Sci. Soc. Am. J. 45:1161–1165.