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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Surface Roughness Effects on Soil Albedo

Dep. of Soil, Water and Environmental Science, Shantz Building 38, Room 429, P.O. Box 210038, 1200 E. South Campus Drive, The Univ. of Arizona, Tucson, AZ 85721-0038 USA

matthias{at}ag.arizona.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Knowledge of how surface roughness influences the reflectance of sunlight from cultivated soils is useful in various applications, such as estimating albedo values used as inputs to soil temperature models and erosion models. The albedos of two soils were studied for dry and wet surfaces with four different roughness conditions, changed from a reference smooth soil surface. The soils were the Gila fine sandy loam [coarse-loamy, mixed (calcareous), thermic Typic Torrifluvent] (Ap horizon), 10YR 6.0/3.2 dry and 10YR 4.1/3.3 wet, and the Pima clay loam [fine-silty, mixed (calcareous), thermic Typic Torrifluvent] (Ap horizon), 10YR 5.2/2.3 dry and 10YR 3.3/2.3 wet. Albedo measurements were made during selected mornings with clear skies in 1995 and 1996. The mean albedos of reference smooth surfaces (<2 mm sieved soil) were 0.279 and 0.155 for dry and wet Gila soil and 0.221 and 0.114 for dry and wet Pima soil. Four tillage conditions were studied: rough plow, disk, disk–disk, and seedbed. Tillage direction was north–south in 1995 and east–west in 1996. The goodness of fit of linear relationships determined between mean albedo and surface roughness, measured with a roughness meter and reported as the root mean square deviation, were relatively high; however, the slopes for the regression equations were different for the two soils and the two moisture conditions. The different slopes indicate that the sensitivity of albedo to surface roughness was highest for the most reflective surface (dry Gila soil) and lowest for the least reflective surface (wet Pima soil). The albedos were on average 27, 18, 10, and 8% lower for dry and wet rough-plow, disk, disk–disk, and seedbed treatments, respectively, as compared with the albedo of the reference smooth soil. These reduction percentages can be used as a general guide to estimate the albedos of tilled soils similar to the Gila and Pima soils studied here.

Abbreviations: CV, coefficient of variation • rmsd, root mean square deviation

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
SOIL ALBEDO () is the ratio between reflected (K) and incoming (K) radiation within the short wave ( 0.3–3.0 µm) portion of the solar spectrum (Coulson and Reynolds, 1971; Oke, 1987). Albedo is an important input parameter for modeling various environmental processes, such as the soil surface energy balance (e.g., Bristow, 1987) and soil erosion (e.g., Laflen et al., 1991). Albedo helps regulate soil temperature, which in turn influences soil biophysical processes, such as seed germination and microbial activity. Soil albedo can be altered to change soil temperature (e.g., Stanhill, 1965; Creswell et al., 1993), and a common way is by changing the roughness of the surface by tillage (Potter et al., 1987). This is often done in vegetable production areas of the southwestern United States where soil temperature modification early in the season can help optimize planting and harvesting dates.

In this study we addressed the question of how albedos of dry and wet soils are affected by the roughness of the surface as a function of the sun angle (). Though much has been learned in recent years about the factors affecting reflection of sunlight by soils, the influence of on albedo of rough soil surfaces is still poorly understood (Cresswell et al., 1993). Light scattering theory generally predicts that the albedos of rough soil surfaces diminish with increasing (Ogilvy, 1991; Cierniewski and Verbrugghe, 1997). The non-Lambertian behavior of soil surfaces has been observed for various soils, including dry and wet Avondale loam (fine-loamy, mixed, superactive, calcareous, hyperthermic Typic Torrifluvent) (Idso et al., 1975), dry and wet Sacramento clay (very-fine, smectitic, thermic Cumulic Vertic Endoaquoll), and wet Yolo loam (fine-silty, mixed, nonacid, thermic Typic Xerorthent) (Coulson and Reynolds, 1971). For especially rough surfaces may increase slightly as increases. Coulson and Reynolds (1971) reported a positive relationship between and for dry Yolo loam, which contrasted markedly with the inverse relationship they observed for wet Yolo loam. Coulson and Reynolds (1971) suggested that light trapping by gaps between soil particles recur more readily at high sun angles, which minimizes albedo at high sun angles. Cresswell et al. (1993) also observed that the of rough-tilled Templeton silt loam (fine-silty, isotic, isomesic Andic Dystrudept) increased slightly as increased from 41 to 76.5°. They suggested that is more likely to increase with increasing for relatively rough tilled soils where the light-trapping mechanism of the surface may function nearly equally well under both low and high . Cresswell et al. (1993) pointed out that the sun angle effect is complex and appears to be somewhat specific to a particular soil and surface condition.

The variation of albedo with roughness and may also be influenced by the relative amounts of diffuse radiation and direct beam radiation incident upon the surface (Cresswell et al., 1993). Scattering of incident light by air molecules, clouds and aerosols varies with and affects the relative contributions of direct and diffuse components in K (Rosenberg et al., 1983). At very low the diffuse contribution is relatively large, which increases .

The degree of roughness of a surface depends intrinsically on and the wavelength () of the scattered radiation (Ogilvy, 1991). Intuitively, we expect that a plowed field should appear relatively rough, with mostly diffuse scattering of light when is large at mid day. However, that same plowed field may appear smooth with a preponderance of specular reflection at small early in the morning or late in the afternoon. Whether a soil surface is smooth or rough may be defined in terms of the Rayleigh criterion (R_a) (Ogilvy, 1991)

(1)

where k is the wave number (2/) of the incident (and reflected) radiation (m^-1), and is the surface root mean square deviation (rmsd, m) from a reference smooth surface. Although it is a somewhat arbitrary criterion, a surface can be considered smooth if R_a < /4; otherwise it is rough. From Eq. [1] it is apparent that must be small ( < 10°) and must be small ( < 0.5 mm) for a soil surface to be considered a smooth scatterer of short-wave radiation (0.3 x 10^-6 < < 3 x 10^-6 m). Soil surfaces, thus, are seldom truly smooth.

If the soil surface is relatively smooth, the albedo at any will be mainly a function of the size distribution (Bowers and Hanks, 1965) and the color (Post et al., 1993, 2000) of the soil aggregates. Bowers and Hanks (1965) used laboratory studies to show that reflectance of clay particles increased exponentially with decreasing average particle size; however, Cierniewski (1987) found the effects of particle size on the spectral reflectance of undisturbed field soils to be less important than the effects of roughness due to soil aggregates and clods. Color is mainly a function of the surface water content (_m), mineral content, and organic matter composition of the soil. Increased _m and increased organic matter tends to decrease (Bowers and Hanks, 1965; Idso et al., 1975).

For a relatively rough soil surface, the will generally be less than the corresponding smooth surface due to self shadows and multiple scattering effects that decrease K (Arnfield, 1975). In addition, a decrease of soil aggregate (and clod) size and spacing generally increases bidirectional reflectance (Cierniewski, 1987; Cierniewski and Verbrugghe, 1997). Geometrical models have been developed to simulate bidirectional reflectance of light from rough soil surfaces based on the assumption that reflection is strongly correlated with the area of shadowed soil roughness as well as on the illumination and the viewing geometry (Cierniewski, 1987; Irons et al., 1992; Cierniewski and Verbrugghe, 1997). A shadowing coefficient expressing the proportion of aggregates and clods area was used by Cierniewski (1987) to convert bare soil reflectance data for smooth samples into data relating to a natural rough surface. Cierniewski (1987) found that the shadowing coefficient decreased with a decrease of soil roughness, and the model reduced the spectral level of rough soils under full shadowing conditions to one-third to one-fourth the value of the maximum for soils. Recently, Cierniewski and Verbrugghe (1997) developed a geometrical model that accounts for both diffuse and specular reflection from soil aggregates and clods (simulated by equal-sized opaque spheriods). Their model predicts that rough soil surfaces with minimal aggregate spacing can show lower variation in reflectance with view angle variation than for less rough soil surfaces.

The objective of this research was to determine how much albedos of relatively smooth reference soils change in response to changing surface roughness created by four tillage operations (categorized as rough plowed, disked, disked–disked, and seedbed). The experiment was conducted on two soils, under wet and dry conditions, and the results are reported as the percentage of change in albedo from the reference smooth surface for each soil condition.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soils and Tillage Treatments
Albedos of dry and wet Gila soil and dry and wet Pima soil were measured at 20, 30, 40, 50, 60, and 70° sun angles during the morning hours of 18 generally cloudless days. These days occurred between April and August in 1995 and 1996. The measurements were made at the University of Arizona Campus Agricultural Center in Tucson (32.25°N, 110.95°W; elevation 745 m).

The physical properties of the Gila and Pima soils are presented in Table 1 . The Gila soil was a pale brown (10YR 6.0/3.2) dry and brown (10YR 4.1/3.3) wet fine sandy loam, and the Pima soil was a grayish brown (10YR 5.2/2.3) dry and very dark grayish brown (10YR 3.3/2.3) wet clay loam.

Surface roughness conditions were not adequately characterized in 1995 and only descriptive information is available (Table 2) ; however, in 1996 the field plot s were measured with a 1-m-long, 100-pin roughness meter (Simanton et al., 1978; Kuipers, 1957; Curtis and Cole, 1972). Following Ogilvy (1991), the surface roughness was computed as the rmsd of the heights (cm) of the pins relative to a flat reference surface. The pin measurements were replicated 10 times in each plot and a mean was computed. No rmsd data were collected on the smooth soils, but it was estimated to be 0.1 cm.

Albedo Measurement
An Eppley black and white hemispherical pyranometer (Model 8-48, Eppley Instruments, Newport, RI; bandpass 0.3–2.8 µm) measured both K and K for calculation of albedo. It was attached to the end of a 1-m-long steel pipe that extended horizontally over the soil. The pipe and sensor were supported by a tripod. For K measurement the sensor was oriented upright, and for K it was inverted. The sensor was leveled to 1° of horizontal.

Tests conducted at various sun angles showed that at sensor height (h_s) 0.6 m, the shadow cast by the sensor on the soil surface had negligible effect on K for sun angles <70°. The Eppley sensor was also close enough to the surface to ensure that nearly all of the K originated from the soil under study. For the downward-viewing sensor, the relative contribution (f) to the total K from a square area of soil directly below the sensor is given by (see Siegel and Howell, 1972):

(2)

where

(3)

and b is one-half the width (m) of the square area.

For the 15 m wide by 25 m long field plots studied in 1995, with and , at least 99.3% of K was reflected from within a 15 m by 15 m area of interest directly beneath the Eppley sensor. The field plots studied in 1996 were 4 m wide by 10 m long, and the sensor height was 0.6 m. Solution of Eq. [2] indicates that 93.2% of K was reflected from a 4 by 4 m area of interest under the Eppley sensor. Of the remaining 6.8% of K received by the Eppley sensor, approximately one-half came from the plot (but outside the 4 by 4 m square) and one-half came from outside the plot. The procedure for collecting the smooth soil albedo (_s) data is described in the companion paper by Post et al. (2000).

Because only one Eppley sensor was available, K could not be measured coincident with K by the Eppley sensor. During intervals when K was measured by the Eppley sensor, the Eppley K values were calculated using K data from a Licor LI200S (Licor, Lincoln, NE) pyranometer. The albedo was then calculated from the ratio of measured Eppley K to calculated Eppley K as described by Matthias et al. (1999). Albedo measurements were made throughout the morning, first on the dry soil, followed 2 d later on the wet soil. Wetting of the plots with 5 to 10 mm of water was done by sprinkler irrigation. The albedos of the tilled soil conditions were referenced to the smooth dry and wet soil condition at the same sun angle by computing a percentage difference.

Characterization of light scattering properties of the atmosphere was made using data for direct beam radiation fluxes at normal incidence (D) and the total global solar radiation fluxes (R_s) on a horizontal surface. These fluxes were measured with an Eppley (normal incidence) pyrheliometer and an Eppley pyranometer on the roof of the Physics and Atmospheric Sciences Building at the University of Arizona campus 7 km south of the study plots. These data showed similar atmospheric conditions on all test days, with mean R_s and mean D values of 14.2 and 17.2 MJ m^-2 8 h^-1.

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Mean values of albedo (i.e., means of six albedo measurements made during morning hours) are presented in Table 3 for each tillage treatment studied in 1995 and 1996. Not surprisingly, these means indicate that albedo was generally increased by increased surface smoothness and dryness. In addition, for 10 of 12 comparisons (1995 means vs. 1996 means in Table 3), the means were greater in 1996, when tillage was east–west, than in 1995, when tillage was north–south. However, the importance of tillage direction on albedo is uncertain. Other factors, such as different clod sizes and shapes and different soil water contents between years, could have played a role in causing different albedos. It is noteworthy that the albedo values for the seedbed treatment were higher in 1996 even though the seedbed treatment both years entailed hand raking, which left no discernable ridges on the surface. Though not shown in Table 3, results from t tests (Netter and Wasserman, 1974) indicate that the treatment mean albedo values (12 pairs) for 1995 and 1996 were not statistically significantly different at the 0.05 level of significance.

View larger version (28K): [in this window] [in a new window]   Fig. 1 Mean albedo as a function of solar elevation angle for the rough-plowed, disk, and seedbed treatments (1995 and 1996 values averaged), the disk–disk treatment (1996 values only) and the smooth reference surfaces for (a) Gila soil and (b) Pima soil

(4)

Figure 2 is a histogram of the mean percentage difference (reduction) of soil albedos for the 16 soil conditions studied (two soils x two moisture levels x four tillage treatments). Each mean percentage difference for the seedbed, disk, and rough-plowed treatments was computed from the 12 albedo measurements made at six sun angles during different mornings in both 1995 and 1996. The mean percentage differences for the disk–disk treatment were computed from the six measurements made only during 1996. Values of the mean percentage reduction in albedo ranged from 35% for the dry, rough-plowed Pima to a plus 1% for the wet and dry Gila seedbeds. The standard deviations are noted in Fig. 2 for each soil condition and are quite variable. The overall mean percentage reductions (and standard deviations) of albedos of both soils for each of the four tillage treatments were 27 (6), 18 (6), 10 (5), and 8 (9) for the dry and wet rough plow, disk, disk–disk, and seedbed, respectively. The equivalent coefficients of variation (CV) were 22, 33, 50, and 112%. The relatively large CV for the seedbed condition may be an outcome of soil aggregation conditions that differed between the Gila and Pima soils. The clay loam Pima soil had more small clods than the fine sandy loam Gila soil. This may have caused the albedos of the Pima seedbed surfaces (both dry and wet) to be reduced considerably below the corresponding Pima smooth soil values. In contrast, the Gila seedbed albedos were slightly increased above the Gila smooth soil albedos. The mechanism explaining why the seedbeds of the Pima soil and Gila soil reflected relatively less and more light, respectively, than the corresponding smooth surfaces for both soils is uncertain.

View larger version (40K): [in this window] [in a new window]   Fig. 2 Overall mean percentage reductions in soil albedos (relative to the corresponding smooth reference soil albedos) for the rough-plow, disk, and seedbed treatments (1995 and 1996 values combined), and the disk–disk treatment (1996 values only)

View larger version (23K): [in this window] [in a new window]   Fig. 3 (a) Absolute albedo (mean of six measurements taken between 20 and 70°) for Gila (dry and wet) and Pima (dry and wet) soils in 1996 vs. surface roughness (increasing from the smooth reference surface to the rough-plowed surface) and (b) normalized albedo (mean albedo for a tillage treatment [r]/mean albedo of reference smooth surface [s]) and percentage reduction of albedo (100%(1 - [r/s]), Eq. [4]) vs. surface roughness

Based on all of the results in Fig. 2 and 3b, our recommendation is as follows to estimate the albedo of bare soils similar to the Gila and Pima soils. First, predict the smooth soil albedo from soil color data, specifically the color value components as discussed by Post et al. (2000), and then reduce the smoothed albedo according to the tilled condition of the soil.

Rough plow: 21 to 35% ( )

Disk: 11 to 21% ( )

Disk–Disk: 2 to 17% ( )

Seedbed: -1 to 20% ( )

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Our results and previous studies indicate that soil albedo generally decreases as sun angle and roughness increases. The changes due to sun angle were normalized by calculating the percentage change in albedo from the reference soil condition, measured at the same sun angle. The mean albedos for the rough-plow, disk, disk–disk, and seedbed tillage treatments were 27, 18, 10, and 8% below the mean smooth surface albedo.

Data from the two soils studied produced markedly different slope coefficients for linear relationships between albedo and surface roughness. These differences may be related to the different colors and textures of the two soils. Surface roughness conditions for the 1996 data regressed against the mean soil albedos were well correlated (mean r² of 0.73) for the Gila and Pima soils under dry and wet conditions. However, the linear regression slope coefficients differed, and the sensitivity of albedo to roughness varied among soils and moisture conditions. The results presented here may be used to predict the probable albedo reduction from a smooth reference condition for soils with color and texture similar to the Gila and Pima soils and for roughness conditions similar to those measured in this research. Measurements of albedo and surface roughness are needed for more soils in order to permit establishment of general empirical relationships between albedo and the surface roughness of cultivated soils.

	ACKNOWLEDGMENTS

 
We thank Sheri Musil for help in preparing figures, Lou Stevens for preparing the field plots, and Dr. William Sellers from the Atmospheric Sciences Department for providing data on the atmospheric light conditions.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Company name is included for benefit of the reader and does not imply endorsement by the University of Arizona.

Received for publication July 15, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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• Post D.F., Fimbres A., Matthias A.D., Sano E.E., Accioly L., Batchily A.K., Ferreira L.G. Predicting soil albedo from soil color and spectral reflectance data. Soil Sci. Soc. Am. J. 2000;64:1027-1034 (this issue).[Abstract/Free Full Text]
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