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The effects of soil moisture on synthetic aperture radar delineation of geomorphic surfaces in the Great Basin, Nevada, USA
ARTICLE IN PRESS Journal of Arid Environments Journal of Arid Environments 56 (2004) 643–657 www.elsevier.com/locate/jnlabr/yjare
The effects of soil moisture on synthetic aperture radar delineation of geomorphic surfaces in the Great Basin, Nevada, USA N.F. Glenna,*, J.R. Carrb a b
Idaho State University-Boise Center, 12301 W. Explorer Drive Suite 102, Boise, ID 83713-1571, USA Department of Geological Sciences, Mail Stop 172, University of Nevada-Reno, Reno, NV, 89557-0138, USA Received 26 April 2002; received in revised form 17 October 2002; accepted 30 April 2003
Abstract RADARSAT-1 synthetic aperture radar (SAR) images from the western Great Basin, North America are used to map geomorphic features using environmental data (increased soil moisture), differences in incidence angles and ascending/descending satellite passes. These attributes are shown to increase the ability to delineate subtle geomorphic features along old shorelines. The change in moisture and the temporal resolution of the images provides a unique opportunity to use moisture as a geomorphic mapping tool in addition to traditional techniques such as image texture and the size and shape of the image features. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: RADARSAT; Synthetic aperture radar; Playa; Soil moisture; Geomorphology
1. Introduction and literature Synthetic aperture radar (SAR) images have wide applicability for mapping geomorphic features on both terrestrial and planetary surfaces (e.g. Farr and Chadwick, 1996; Baron et al., 1998). Geomorphic features have been mapped with SAR primarily using extraction techniques of radar shadowing and image texture (Lewis, 1998, pp. 567–630). Radar images aid geomorphic analysis through classifications of lineaments, geologic units, surface roughness, soil moisture, surface *Corresponding author. Tel.: +1-208-685-6755; fax: +1-208-658-6776. E-mail address: [email protected] (N.F. Glenn). 0140-1963/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0140-1963(03)00085-5
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textural characteristics, and vegetation. As with panchromatic aerial photographs, radar mapping is accomplished by analysing tone, texture, size, shape, and pattern variations (Lewis, 1998). Radar shadowing can also be used to map topography and slope (Lewis, 1998). Radar shadowing, including foreshortening and layover are distortions in the length of slopes due to time variations in the illumination of the foreslope relative to the backslope by the radar beam projected to the image plane. For example, layover occurs when surfaces with high relief appear to lean towards the radar platform because the top appears closer to the platform than the base. Smaller look angles (or larger depression angles) and steeper slopes result in increased layover effects (Ford et al., 1993). Foreshortening occurs when the terrain base appears closer to the platform than the top; this results in the appearance of a shorter surface in the image than on the ground (Drury, 2001). Often, the term ‘‘radar shadowing’’ describes the condition in which slopes facing the platform are strongly illuminated while those that face away from the platform are dark (due to decreased energy reaching the back side of a slope). Relationships between SAR backscatter response of geomorphic features and moisture levels in the western Great Basin, North America, are investigated herein.
Highway 447
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(a) T1 Winnemucca Lake
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(b) T2
1999 Field Soil Moisture Site 250m N Fig. 1. Location of (a) T1 and (b) T2 on west side of Winnemucca Lake, Nevada, from 1953 1:44,000 aerial photo, 9072.
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Drainages
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Fig. 2. Location of drainages on east side of Winnemucca Lake, Nevada, from 1962 1:46,000 aerial photo 1-49, GS-VAMV.
Three SAR images from the Canadian Space Agency’s (CSA) RADARSAT-1 are used to study the influence of soil moisture in delineating old shoreline features. The study area is shorelines on the west side of Winnemucca Lake, Nevada, and drainage channels and patterns on the east side of this playa (i.e. dry lake) (Figs. 1 and 2, respectively). These features are of geologic interest in studying the historical development of the playa. Up to the early 1900s, Winnemucca Lake was sustained by flow from the Truckee River during wet years when Pyramid Lake’s elevation diverted the river into an alternate channel flowing to the east. When Derby Dam was completed circa 1910 on the Truckee River, diverting some flow to Fallon, Nevada for the Newlands Irrigation and Agricultural Project, flow no longer reached Winnemucca Lake (Zones, 1961). The study herein came about when investigating the role of SAR and geostatistics in detecting soil moisture changes on the playa. During investigation of the RADARSAT-1 images and temporal changes in soil moisture, it became apparent that there was an increased ability to delineate old shoreline features in the image with increased soil moisture. The shorelines’ subtle slopes provide an ideal target for testing how changes in soil moisture can affect the ability to detect their morphology. The drainage patterns on the east side of Winnemucca Lake are included in this study to provide a comparison in the differences between soil moisture effects and orbital pattern effects on detecting morphology.
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1.1. Previous work Most studies have not taken advantage of SAR’s sensitivity to soil moisture in mapping geomorphic features. Extensive research since the 1970s (e.g. Ulaby, 1974) has been performed on the applicability of both active and passive microwave radar remote sensing systems for soil moisture characterization. Active sensors, including spaceborne SAR, have the advantage over passive airborne sensors of being most readily available to a wide-range of researchers and project application needs. Furthermore, repeat pass coverage of spaceborne systems allows many studies to focus on relative changes in soil moisture over time. Radar sensors have potential for detecting soil moisture based on the difference in dielectric properties of dry soil and liquid water (Ulaby et al., 1996). SAR systems typically use C-, K-, or L-band frequencies in order to maximize the sensitivity to the dielectric constant, and enhance the sensitivity to soil moisture (Dobson and Ulaby, 1998, pp. 407–434). Microwave wavelengths over 5 cm (such as C-band or L-band) result in dielectric constants of approximately 80 for liquid water compared to 3 to 5 for dry soil (Engman and Chauhan, 1995). As the amount of liquid water increases in the soil, the dielectric constant of the soil likewise increases. A fundamental problem with radar remote sensing of soil moisture is the sensitivity of radar backscatter to not only soil moisture, but to surface roughness and vegetation. Thus, one of the largest challenges in analysing radar backscatter is quantifying the effects of these components. Many studies have been devoted to evaluating the effects of surface roughness and vegetative cover as well as the sensitivity differences of these components between active and passive microwave sensors (e.g. Du et al., 2000). Whereas vegetative cover can enhance the radar sensitivity to soil moisture in both types of sensor systems, neither active nor passive systems can be determined to have superior sensitivity to soil moisture from vegetative effects (Du et al., 2000). A more complete discussion of vegetation and surface roughness backscatter effects and models can be found in Dobson and Ulaby (1998). Incidence angles can also affect soil moisture estimation by influencing the radar backscatter component on both smooth and rough surfaces (Sano et al., 1998). As noted previously, foreshortening and layover effects are influenced by the incidence angle, moreover these effects hinder the estimation of soil moisture from radar backscatter. The use of SAR has also been applied to the field of geomorphology, often adding information on microrelief, providing shallow penetration of very dry sand and ice, and documenting geomorphic processes in arid to tropical environments (e.g. Sugiura and Sabins, 1980, pp. 439–456; Blumberg, 1998; McHone et al., 1998; Lewis, 1998; Singhroy and Mattar, 2000). The RADARSAT-1 single frequency SAR satellite has been used for a variety of geoscience applications, including soil moisture estimation (e.g. Dempsey et al., 1998a), ocean mapping (e.g. Vainio et al., 2000), and natural hazard predictions (e.g. Singhroy and Mattar, 2000). The Application Development Research Opportunity (ADRO) program, sponsored by the CSA, National Aeronautics and Space Administration (NASA), and RADARSAT International (RSI), has supported several soil moisture studies using
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RADARSAT-1 imagery, including Soulis et al. (1998) and Dempsey et al. (1998a, b). However, RADARSAT-1’s sensitivity to surface moisture is still poorly understood. Testing this satellite’s sensitivity to soil moisture needs to be further investigated, as well as studying site-specific localities to appreciate applications to the geosciences.
2. Materials and methods 2.1. SAR images Three RADARSAT-1 SAR images were acquired from RSI and CSA. Two images were acquired during summer 1999 and one image was collected in late summer 2002. RADARSAT-1 was chosen for this study for its availability, educational pricing, and timely repeat coverage for the specified field area. RADARSAT-1 operates at C-band (5.6 cm wavelength and 5.3 GHz frequency) with HH polarization. One SAR image acquisition occurred on July 30, 1999, at 13:58 Universal Coordinated Time (UTC) in Fine 2 (F2) beam mode, descending (west looking) at an incidence angle of 40.7 (Fig. 3). The second image was acquired on August 09, 1999, at 01:53 UTC in Fine 1 Near (F1N) beam mode, ascending (east looking) at an incidence angle of 37.9 (Fig. 4). The third image was acquired on September 10, 2002, at 01:52 UTC in Fine 1 Near (F1N) beam mode, ascending (east looking) at an incidence angle of 37.8 (Fig. 5). Fine beam mode RADARSAT images provide 6.25 m pixel spacing at high incidence angles in comparison to standard beam mode at 12.5 m pixel spacing (RADARSAT International, 1999). RSI processed the images as single look (azimuth and range directions) Path Images (SGF). 2.2. Moisture and radar sensitivity The July image acquisition date corresponds to a drying period in the local climate. Previous rainfall occurred during the first week of June 1999. The August image acquisition date coincided with a wetting period, during which approximately 2 cm of rainfall occurred during the first week of August. The September 2002 image was acquired following a substantially dryer period, in which August 2002 was the second driest month on record for the State of Nevada (National Climatic Data Center, 2002). Less than 0.5 cm of rainfall occurred four days prior to the image acquisition date (Western Regional Climate Center, 2002). This small amount, coupled with the average maximum air temperature (27 C) between September 6th and 10th and the arid environment of Winnemucca Lake, could not substantially alter the background moisture levels (Western Regional Climate Center, 2002). Increased soil moisture and the consequential increase in radar reflectivity forms the basis for this study. Ford et al. (1998, pp. 511–566) note that the dielectric constant of most rocks fall within a narrow range; however, this constant varies as a function of moisture content in porous rocks, sediments, and soils. Porous surface materials with a comparatively low dielectric constant when dry have a much higher
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N 1 km Fig. 4. August 9, 1999 RADARSAT-1 F1N image and location of (a) T1; (b) T2; and (c) drainages. Field site location marker indicates area where field soil moisture sampling was performed on July 30 and August 9, 1999.
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dielectric constant when they contain moisture. This may result in considerably brighter radar returns. Though dielectric constants were not measured between radar passes, soil moisture levels were measured on the playa at a field site south of the shoreline features and south-west of the series of drainages during summer 1999 (Figs. 3 and 4). Field soil samples were collected in the upper 0–5 cm of the soil profile within 6 h of the image acquisitions. The soil moisture was measured by the oven-dried method in accordance with ASTM D 2216-90 (Bowles, 1992) in a laboratory at the University of Nevada, Reno. The moisture content is defined herein as the ratio of mass of water present in a soil mass to the mass of soil solids. The mean soil moisture in the July 30, 1999 samples was 16.2%. That for the August imaging date was substantially greater at 33.8%.
2.3. Geomorphic and vegetative features of old shorelines The old shoreline features examined in this study are topographic highs that extend into Winnemucca Lake from the west (Figs. 1, 3, 4, and 5). These topographic highs were most likely formed from deposits of the Lake Range to the west and wave action on Winnemucca Lake during Pleistocene inundation (Zones, 1961; Grose, 1988). The most northern topographic high (referred to as T1) is approximately 1 km in the north–south direction and 725 m in the east-west direction. The southern feature (referred to as T2) is approximately 600 m in the north–south direction and 725 m in the east–west direction. Both of these features have a slope of about 1 upslope to the west. The most notable feature distinguishing T1 and T2 is that T2 is topographically symmetrical whereas T1 is asymmetrical. The north slope of T1 is approximately 7 and the south slope is approximately 3.5 . The north and south slopes of T2 are approximately 4.8 . There is limited vegetation (approximately 5% cover) along the old shoreline of Winnemucca Lake. The majority of the vegetation grows in a ring-like fashion around the lake, following old shoreline levels (Figs. 6a and b). Vegetation grows at a higher density (5–10% cover) along the lower shorelines, close to the playa, whereas the upper shorelines have less vegetation (less than 5% cover). The vegetation consists of mostly sagebrush with some rabbitbrush. The soils along the old shorelines consist of graded deposits of geologically well sorted, fine sand to coarse gravel sized clasts. The lower shorelines consist of fine to medium sand, whereas the upper shorelines consist of more coarse gravels (Fig. 6c). Moisture levels were not sampled along the old shorelines during the satellite sensor image acquisitions but are assumed to be consistent with average moisture measured on the playa in 1999 (e.g. higher moisture levels during the August image acquisition date) in cases where the grain sizes are similar (i.e. fine grained). In the upper shorelines, the moisture contents cannot be assumed to be the same for July and August due to the greater particle sizes. Moisture contents were not sampled in these areas because this study was originally focused on playa soil moisture detection.
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(a)
(c)
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Fig. 6. T1: Vegetation growth on (a) south slope and (b) north slope; and (c) graded deposits on north slope.
3. Results 3.1. Image observations of geomorphic features A notable difference between the July and August 1999 images is the enhanced geomorphic expression of T1 and T2 in the August image (Figs. 7 and 8, respectively). Topographic relief is clearly expressed as patterns of linear features in the August image. This is evident along the north and south sides of T1 and the north, south, and east sides of T2. The enhanced contrast between pixels in the image along the north side of T1 reveals distinct tonal changes. Upon closer inspection of the pixels, these tonal changes are parallel bands of alternating bright and dark pixels (Fig. 7b). Based on a comparison with US Geological Survey topographic
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Fig. 7. RADARSAT-1 image of T1 on (a) July 30, 1999; (b) August 9, 1999; and (c) September 10, 2002. Boxes in (a), (b), and (c) indicate magnified area of image of T1 on right side of figure. Arrows in (b) are pointing to increased delineation of shorelines features in large August image and tonal changes of parallel bright and dark pixels in magnified August image.
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(b)
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N
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Fig. 8. RADARSAT-1 image of T2 on (a) July 30, 1999; (b) August 9, 1999; and (c) September 10, 2002. Arrows in (b) are pointing to increased delineation of shoreline features in August image.
maps (US Geological Survey, 1964), aerial photos, and field studies, the tonal changes are inferred to correspond to elevation changes and/or vegetation growth. Enhanced linear patterns, tonal changes, and textural changes in the August image provide indications as to the shape, morphology, and gradient of T2. The tonal changes are caused by the contrast of darker and lighter pixels. This contrast highlights linear patterns. Textural variation between the July and August images, also inferred to be the result of moisture variation, further distinguishes the subtle shoreline features. Because the beam modes differ for the July and August images, though, a question arises about the assertion that moisture variation is responsible for notable differences across images. As a further test of this assertion, a third image was necessarily acquired on September 10, 2002, having identical radar acquisition characteristics as the August, 1999 image. September 2002 was a substantially dryer
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period; consequently a significant moisture differential is notable between the image for this period and that for August 1999. Differences in shoreline features are substantial (Figs. 7 and 8). Given that the radar beam mode is the same for these two images, the inference that moisture variation is responsible for image differences is strengthened. 4. Discussion: the influence of soil moisture on radar images Aside from the differing appearances of shoreline features between the July and August images highlighted in this study, drainage features on the east side of Winnemucca Lake also differ in tone and texture. But, in the case of the drainage features on the east side of the playa, perhaps the different radar response between the July and August images are attributable to the different look directions and incidence angles, west versus east. This can cause differences in reflectivity due to local incidence angles between the look angle and the local topography and result in radar shadowing and enhanced topography. The old shoreline features, however, are only approximately 1 upslope towards the west and the shadowing effects are most likely minimal. The shallow slopes, coupled with the relatively high satellite incidence angles should minimize topographic effects. Other factors should be considered and include: (a) Increase in moisture levels in the bare soil causing higher reflectivity values. (b) Increase in moisture levels in the vegetation causing higher reflectivity values. (c) Grain size variations and gradations in the soil matrix resulting in varying levels of soil moisture (and thus vegetation growth) and surface roughness. (d) Increase in moisture levels accentuating subtle topographic changes. (e) A combination of all of the above. The sole influence of soil moisture causing enhanced geomorphic features (explanation (a)) is most unlikely because there would be a constant increase in radar reflectivity throughout the field sites. This is not the case. The combined influence of increased moisture levels in both the vegetation and soil is a plausible explanation because in the case of the shoreline features, the vegetation growth is limited to the ‘‘shoreline rings’’, which mimic topographic changes. If vegetation causes an increase in scattering and reflectivity, then the changes would be reflected in a ring-like fashion due to the sparseness of the vegetation growth. Gradations in the soil matrix and grain size of the shoreline features are a result of the fluctuating levels of historic Winnemucca Lake. These gradations can influence moisture levels and retain moisture for different time durations, resulting in preferred vegetation growth among the ‘‘wetter’’ contours. The gradation will also cause surface roughness changes along perpendicular transects of the shorelines. These gradation contours are then responsible for higher reflectivity of the radar response due to higher soil moisture contents, higher vegetation moisture contents, and surface roughness variations. The higher reflectivity values are expressed as tonal variations of high to low DN values.
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Increased moisture levels due to the precipitation in the August image may accentuate subtle topographic features in the shoreline. This may be due to the increased reflectivity of the soil, vegetation, and topography. The topography may affect the image reflectivity due to foreshortening, layover, and shadow effects. However, the slopes of these old shorelines are too shallow to allow for topographic effects on radar backscatter (US Geological Survey, 1964). Consequently, moisture variation may provide the best opportunity for resolving these features in microwave images.
5. Summary Subtle topographic changes are enhanced in the radar image that was acquired after several days of light precipitation. Increased moisture causes subtle topographic features to be imaged more easily through an increase in radar reflectivity. There are no known studies that have used moisture conditions as a parameter for radar mapping of geomorphic features. This technique is relatively simple, yet requires flexibility in temporal resolution when acquiring radar images.
Acknowledgements The authors would like to thank the NASA-sponsored University of Nevada System Space Grant Consortium for providing funding for this project. In addition, the authors would like to thank RSI’s Research and Development Program for providing two RADARSAT-1 images at non-commercial data prices. We are grateful for financial support from Linda Brinkley, Vice President for Research, and Jane Long, Dean, Mackay School of Mines, that enabled the acquisition of the September 10, 2002 image.
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The effects of soil moisture on synthetic aperture radar delineation of geomorphic surfaces in the Great Basin, Nevada, USA N.F. Glenna,*, J.R. Carrb a b
Idaho State University-Boise Center, 12301 W. Explorer Drive Suite 102, Boise, ID 83713-1571, USA Department of Geological Sciences, Mail Stop 172, University of Nevada-Reno, Reno, NV, 89557-0138, USA Received 26 April 2002; received in revised form 17 October 2002; accepted 30 April 2003
Abstract RADARSAT-1 synthetic aperture radar (SAR) images from the western Great Basin, North America are used to map geomorphic features using environmental data (increased soil moisture), differences in incidence angles and ascending/descending satellite passes. These attributes are shown to increase the ability to delineate subtle geomorphic features along old shorelines. The change in moisture and the temporal resolution of the images provides a unique opportunity to use moisture as a geomorphic mapping tool in addition to traditional techniques such as image texture and the size and shape of the image features. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: RADARSAT; Synthetic aperture radar; Playa; Soil moisture; Geomorphology
1. Introduction and literature Synthetic aperture radar (SAR) images have wide applicability for mapping geomorphic features on both terrestrial and planetary surfaces (e.g. Farr and Chadwick, 1996; Baron et al., 1998). Geomorphic features have been mapped with SAR primarily using extraction techniques of radar shadowing and image texture (Lewis, 1998, pp. 567–630). Radar images aid geomorphic analysis through classifications of lineaments, geologic units, surface roughness, soil moisture, surface *Corresponding author. Tel.: +1-208-685-6755; fax: +1-208-658-6776. E-mail address: [email protected] (N.F. Glenn). 0140-1963/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0140-1963(03)00085-5
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textural characteristics, and vegetation. As with panchromatic aerial photographs, radar mapping is accomplished by analysing tone, texture, size, shape, and pattern variations (Lewis, 1998). Radar shadowing can also be used to map topography and slope (Lewis, 1998). Radar shadowing, including foreshortening and layover are distortions in the length of slopes due to time variations in the illumination of the foreslope relative to the backslope by the radar beam projected to the image plane. For example, layover occurs when surfaces with high relief appear to lean towards the radar platform because the top appears closer to the platform than the base. Smaller look angles (or larger depression angles) and steeper slopes result in increased layover effects (Ford et al., 1993). Foreshortening occurs when the terrain base appears closer to the platform than the top; this results in the appearance of a shorter surface in the image than on the ground (Drury, 2001). Often, the term ‘‘radar shadowing’’ describes the condition in which slopes facing the platform are strongly illuminated while those that face away from the platform are dark (due to decreased energy reaching the back side of a slope). Relationships between SAR backscatter response of geomorphic features and moisture levels in the western Great Basin, North America, are investigated herein.
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Fig. 2. Location of drainages on east side of Winnemucca Lake, Nevada, from 1962 1:46,000 aerial photo 1-49, GS-VAMV.
Three SAR images from the Canadian Space Agency’s (CSA) RADARSAT-1 are used to study the influence of soil moisture in delineating old shoreline features. The study area is shorelines on the west side of Winnemucca Lake, Nevada, and drainage channels and patterns on the east side of this playa (i.e. dry lake) (Figs. 1 and 2, respectively). These features are of geologic interest in studying the historical development of the playa. Up to the early 1900s, Winnemucca Lake was sustained by flow from the Truckee River during wet years when Pyramid Lake’s elevation diverted the river into an alternate channel flowing to the east. When Derby Dam was completed circa 1910 on the Truckee River, diverting some flow to Fallon, Nevada for the Newlands Irrigation and Agricultural Project, flow no longer reached Winnemucca Lake (Zones, 1961). The study herein came about when investigating the role of SAR and geostatistics in detecting soil moisture changes on the playa. During investigation of the RADARSAT-1 images and temporal changes in soil moisture, it became apparent that there was an increased ability to delineate old shoreline features in the image with increased soil moisture. The shorelines’ subtle slopes provide an ideal target for testing how changes in soil moisture can affect the ability to detect their morphology. The drainage patterns on the east side of Winnemucca Lake are included in this study to provide a comparison in the differences between soil moisture effects and orbital pattern effects on detecting morphology.
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1.1. Previous work Most studies have not taken advantage of SAR’s sensitivity to soil moisture in mapping geomorphic features. Extensive research since the 1970s (e.g. Ulaby, 1974) has been performed on the applicability of both active and passive microwave radar remote sensing systems for soil moisture characterization. Active sensors, including spaceborne SAR, have the advantage over passive airborne sensors of being most readily available to a wide-range of researchers and project application needs. Furthermore, repeat pass coverage of spaceborne systems allows many studies to focus on relative changes in soil moisture over time. Radar sensors have potential for detecting soil moisture based on the difference in dielectric properties of dry soil and liquid water (Ulaby et al., 1996). SAR systems typically use C-, K-, or L-band frequencies in order to maximize the sensitivity to the dielectric constant, and enhance the sensitivity to soil moisture (Dobson and Ulaby, 1998, pp. 407–434). Microwave wavelengths over 5 cm (such as C-band or L-band) result in dielectric constants of approximately 80 for liquid water compared to 3 to 5 for dry soil (Engman and Chauhan, 1995). As the amount of liquid water increases in the soil, the dielectric constant of the soil likewise increases. A fundamental problem with radar remote sensing of soil moisture is the sensitivity of radar backscatter to not only soil moisture, but to surface roughness and vegetation. Thus, one of the largest challenges in analysing radar backscatter is quantifying the effects of these components. Many studies have been devoted to evaluating the effects of surface roughness and vegetative cover as well as the sensitivity differences of these components between active and passive microwave sensors (e.g. Du et al., 2000). Whereas vegetative cover can enhance the radar sensitivity to soil moisture in both types of sensor systems, neither active nor passive systems can be determined to have superior sensitivity to soil moisture from vegetative effects (Du et al., 2000). A more complete discussion of vegetation and surface roughness backscatter effects and models can be found in Dobson and Ulaby (1998). Incidence angles can also affect soil moisture estimation by influencing the radar backscatter component on both smooth and rough surfaces (Sano et al., 1998). As noted previously, foreshortening and layover effects are influenced by the incidence angle, moreover these effects hinder the estimation of soil moisture from radar backscatter. The use of SAR has also been applied to the field of geomorphology, often adding information on microrelief, providing shallow penetration of very dry sand and ice, and documenting geomorphic processes in arid to tropical environments (e.g. Sugiura and Sabins, 1980, pp. 439–456; Blumberg, 1998; McHone et al., 1998; Lewis, 1998; Singhroy and Mattar, 2000). The RADARSAT-1 single frequency SAR satellite has been used for a variety of geoscience applications, including soil moisture estimation (e.g. Dempsey et al., 1998a), ocean mapping (e.g. Vainio et al., 2000), and natural hazard predictions (e.g. Singhroy and Mattar, 2000). The Application Development Research Opportunity (ADRO) program, sponsored by the CSA, National Aeronautics and Space Administration (NASA), and RADARSAT International (RSI), has supported several soil moisture studies using
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RADARSAT-1 imagery, including Soulis et al. (1998) and Dempsey et al. (1998a, b). However, RADARSAT-1’s sensitivity to surface moisture is still poorly understood. Testing this satellite’s sensitivity to soil moisture needs to be further investigated, as well as studying site-specific localities to appreciate applications to the geosciences.
2. Materials and methods 2.1. SAR images Three RADARSAT-1 SAR images were acquired from RSI and CSA. Two images were acquired during summer 1999 and one image was collected in late summer 2002. RADARSAT-1 was chosen for this study for its availability, educational pricing, and timely repeat coverage for the specified field area. RADARSAT-1 operates at C-band (5.6 cm wavelength and 5.3 GHz frequency) with HH polarization. One SAR image acquisition occurred on July 30, 1999, at 13:58 Universal Coordinated Time (UTC) in Fine 2 (F2) beam mode, descending (west looking) at an incidence angle of 40.7 (Fig. 3). The second image was acquired on August 09, 1999, at 01:53 UTC in Fine 1 Near (F1N) beam mode, ascending (east looking) at an incidence angle of 37.9 (Fig. 4). The third image was acquired on September 10, 2002, at 01:52 UTC in Fine 1 Near (F1N) beam mode, ascending (east looking) at an incidence angle of 37.8 (Fig. 5). Fine beam mode RADARSAT images provide 6.25 m pixel spacing at high incidence angles in comparison to standard beam mode at 12.5 m pixel spacing (RADARSAT International, 1999). RSI processed the images as single look (azimuth and range directions) Path Images (SGF). 2.2. Moisture and radar sensitivity The July image acquisition date corresponds to a drying period in the local climate. Previous rainfall occurred during the first week of June 1999. The August image acquisition date coincided with a wetting period, during which approximately 2 cm of rainfall occurred during the first week of August. The September 2002 image was acquired following a substantially dryer period, in which August 2002 was the second driest month on record for the State of Nevada (National Climatic Data Center, 2002). Less than 0.5 cm of rainfall occurred four days prior to the image acquisition date (Western Regional Climate Center, 2002). This small amount, coupled with the average maximum air temperature (27 C) between September 6th and 10th and the arid environment of Winnemucca Lake, could not substantially alter the background moisture levels (Western Regional Climate Center, 2002). Increased soil moisture and the consequential increase in radar reflectivity forms the basis for this study. Ford et al. (1998, pp. 511–566) note that the dielectric constant of most rocks fall within a narrow range; however, this constant varies as a function of moisture content in porous rocks, sediments, and soils. Porous surface materials with a comparatively low dielectric constant when dry have a much higher
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N 1 km Fig. 4. August 9, 1999 RADARSAT-1 F1N image and location of (a) T1; (b) T2; and (c) drainages. Field site location marker indicates area where field soil moisture sampling was performed on July 30 and August 9, 1999.
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dielectric constant when they contain moisture. This may result in considerably brighter radar returns. Though dielectric constants were not measured between radar passes, soil moisture levels were measured on the playa at a field site south of the shoreline features and south-west of the series of drainages during summer 1999 (Figs. 3 and 4). Field soil samples were collected in the upper 0–5 cm of the soil profile within 6 h of the image acquisitions. The soil moisture was measured by the oven-dried method in accordance with ASTM D 2216-90 (Bowles, 1992) in a laboratory at the University of Nevada, Reno. The moisture content is defined herein as the ratio of mass of water present in a soil mass to the mass of soil solids. The mean soil moisture in the July 30, 1999 samples was 16.2%. That for the August imaging date was substantially greater at 33.8%.
2.3. Geomorphic and vegetative features of old shorelines The old shoreline features examined in this study are topographic highs that extend into Winnemucca Lake from the west (Figs. 1, 3, 4, and 5). These topographic highs were most likely formed from deposits of the Lake Range to the west and wave action on Winnemucca Lake during Pleistocene inundation (Zones, 1961; Grose, 1988). The most northern topographic high (referred to as T1) is approximately 1 km in the north–south direction and 725 m in the east-west direction. The southern feature (referred to as T2) is approximately 600 m in the north–south direction and 725 m in the east–west direction. Both of these features have a slope of about 1 upslope to the west. The most notable feature distinguishing T1 and T2 is that T2 is topographically symmetrical whereas T1 is asymmetrical. The north slope of T1 is approximately 7 and the south slope is approximately 3.5 . The north and south slopes of T2 are approximately 4.8 . There is limited vegetation (approximately 5% cover) along the old shoreline of Winnemucca Lake. The majority of the vegetation grows in a ring-like fashion around the lake, following old shoreline levels (Figs. 6a and b). Vegetation grows at a higher density (5–10% cover) along the lower shorelines, close to the playa, whereas the upper shorelines have less vegetation (less than 5% cover). The vegetation consists of mostly sagebrush with some rabbitbrush. The soils along the old shorelines consist of graded deposits of geologically well sorted, fine sand to coarse gravel sized clasts. The lower shorelines consist of fine to medium sand, whereas the upper shorelines consist of more coarse gravels (Fig. 6c). Moisture levels were not sampled along the old shorelines during the satellite sensor image acquisitions but are assumed to be consistent with average moisture measured on the playa in 1999 (e.g. higher moisture levels during the August image acquisition date) in cases where the grain sizes are similar (i.e. fine grained). In the upper shorelines, the moisture contents cannot be assumed to be the same for July and August due to the greater particle sizes. Moisture contents were not sampled in these areas because this study was originally focused on playa soil moisture detection.
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Fig. 6. T1: Vegetation growth on (a) south slope and (b) north slope; and (c) graded deposits on north slope.
3. Results 3.1. Image observations of geomorphic features A notable difference between the July and August 1999 images is the enhanced geomorphic expression of T1 and T2 in the August image (Figs. 7 and 8, respectively). Topographic relief is clearly expressed as patterns of linear features in the August image. This is evident along the north and south sides of T1 and the north, south, and east sides of T2. The enhanced contrast between pixels in the image along the north side of T1 reveals distinct tonal changes. Upon closer inspection of the pixels, these tonal changes are parallel bands of alternating bright and dark pixels (Fig. 7b). Based on a comparison with US Geological Survey topographic
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Fig. 7. RADARSAT-1 image of T1 on (a) July 30, 1999; (b) August 9, 1999; and (c) September 10, 2002. Boxes in (a), (b), and (c) indicate magnified area of image of T1 on right side of figure. Arrows in (b) are pointing to increased delineation of shorelines features in large August image and tonal changes of parallel bright and dark pixels in magnified August image.
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Fig. 8. RADARSAT-1 image of T2 on (a) July 30, 1999; (b) August 9, 1999; and (c) September 10, 2002. Arrows in (b) are pointing to increased delineation of shoreline features in August image.
maps (US Geological Survey, 1964), aerial photos, and field studies, the tonal changes are inferred to correspond to elevation changes and/or vegetation growth. Enhanced linear patterns, tonal changes, and textural changes in the August image provide indications as to the shape, morphology, and gradient of T2. The tonal changes are caused by the contrast of darker and lighter pixels. This contrast highlights linear patterns. Textural variation between the July and August images, also inferred to be the result of moisture variation, further distinguishes the subtle shoreline features. Because the beam modes differ for the July and August images, though, a question arises about the assertion that moisture variation is responsible for notable differences across images. As a further test of this assertion, a third image was necessarily acquired on September 10, 2002, having identical radar acquisition characteristics as the August, 1999 image. September 2002 was a substantially dryer
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period; consequently a significant moisture differential is notable between the image for this period and that for August 1999. Differences in shoreline features are substantial (Figs. 7 and 8). Given that the radar beam mode is the same for these two images, the inference that moisture variation is responsible for image differences is strengthened. 4. Discussion: the influence of soil moisture on radar images Aside from the differing appearances of shoreline features between the July and August images highlighted in this study, drainage features on the east side of Winnemucca Lake also differ in tone and texture. But, in the case of the drainage features on the east side of the playa, perhaps the different radar response between the July and August images are attributable to the different look directions and incidence angles, west versus east. This can cause differences in reflectivity due to local incidence angles between the look angle and the local topography and result in radar shadowing and enhanced topography. The old shoreline features, however, are only approximately 1 upslope towards the west and the shadowing effects are most likely minimal. The shallow slopes, coupled with the relatively high satellite incidence angles should minimize topographic effects. Other factors should be considered and include: (a) Increase in moisture levels in the bare soil causing higher reflectivity values. (b) Increase in moisture levels in the vegetation causing higher reflectivity values. (c) Grain size variations and gradations in the soil matrix resulting in varying levels of soil moisture (and thus vegetation growth) and surface roughness. (d) Increase in moisture levels accentuating subtle topographic changes. (e) A combination of all of the above. The sole influence of soil moisture causing enhanced geomorphic features (explanation (a)) is most unlikely because there would be a constant increase in radar reflectivity throughout the field sites. This is not the case. The combined influence of increased moisture levels in both the vegetation and soil is a plausible explanation because in the case of the shoreline features, the vegetation growth is limited to the ‘‘shoreline rings’’, which mimic topographic changes. If vegetation causes an increase in scattering and reflectivity, then the changes would be reflected in a ring-like fashion due to the sparseness of the vegetation growth. Gradations in the soil matrix and grain size of the shoreline features are a result of the fluctuating levels of historic Winnemucca Lake. These gradations can influence moisture levels and retain moisture for different time durations, resulting in preferred vegetation growth among the ‘‘wetter’’ contours. The gradation will also cause surface roughness changes along perpendicular transects of the shorelines. These gradation contours are then responsible for higher reflectivity of the radar response due to higher soil moisture contents, higher vegetation moisture contents, and surface roughness variations. The higher reflectivity values are expressed as tonal variations of high to low DN values.
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Increased moisture levels due to the precipitation in the August image may accentuate subtle topographic features in the shoreline. This may be due to the increased reflectivity of the soil, vegetation, and topography. The topography may affect the image reflectivity due to foreshortening, layover, and shadow effects. However, the slopes of these old shorelines are too shallow to allow for topographic effects on radar backscatter (US Geological Survey, 1964). Consequently, moisture variation may provide the best opportunity for resolving these features in microwave images.
5. Summary Subtle topographic changes are enhanced in the radar image that was acquired after several days of light precipitation. Increased moisture causes subtle topographic features to be imaged more easily through an increase in radar reflectivity. There are no known studies that have used moisture conditions as a parameter for radar mapping of geomorphic features. This technique is relatively simple, yet requires flexibility in temporal resolution when acquiring radar images.
Acknowledgements The authors would like to thank the NASA-sponsored University of Nevada System Space Grant Consortium for providing funding for this project. In addition, the authors would like to thank RSI’s Research and Development Program for providing two RADARSAT-1 images at non-commercial data prices. We are grateful for financial support from Linda Brinkley, Vice President for Research, and Jane Long, Dean, Mackay School of Mines, that enabled the acquisition of the September 10, 2002 image.
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