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Use of mineral magnetic measurements to investigate soil erosion and sediment delivery in a small agricultural catchment in limestone terrain
Catena 46 Ž2001. 15–34 www.elsevier.comrlocatercatena
Use of mineral magnetic measurements to investigate soil erosion and sediment delivery in a small agricultural catchment in limestone terrain Dan Royall ) Department of Geography, UniÕersity of Alabama, Tuscaloosa, AL 35487, USA Received 17 November 2000; received in revised form 1 May 2001; accepted 23 May 2001
Abstract Understanding sediment delivery at the hillslope scale requires information on the spatial distributions and magnitudes of erosion and deposition. Empirical models such as the RUSLE may be useful for predicting erosion, but are poorly suited for quantifying deposition. Cesium-137 ŽCs-137. is useful for quantifying both erosion and deposition, but is costly to inventory over a large area, and directly gauges soil redistribution only for recent decades. The use of rapidly acquired magnetic measurements represents a relatively new potential means of mapping and measuring soil redistribution. In this study, variations in surface magnetism are analyzed to determine patterns of erosion and sedimentation in a small agricultural catchment in northwestern Alabama ŽUSA.. Magnetic indicators of erosion are combined with published soil morphology, Cs-137 and short-term suspended sediment data from this former experimental watershed to evaluate long-term sediment delivery. All magnetic parameters measured could be related to soil erosion, although their patterns in space are not identical. Magnetic evidence suggests approximately 30 cm of soil loss on the steepest mid-slope portions of the catchment. Distributing soil loss across space according to magnetic patterns gives average long-term values lower than a prior estimate based on Cs-137 data. Long-term sediment delivery calculated using soil morphology to determine depositional volume ranges up to 45% depending on the magnetic parameter used to index soil loss, and assumptions regarding deposit geometry. These results suggest the need for continued refinement of magnetic techniques for purposes of erosion model validation and general sediment tracing applications. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Soil erosion; Magnetic susceptibility; Agricultural landscape; Sediment delivery
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Fax: q1-205-348-2278. E-mail address: [email protected] ŽD. Royall..
0341-8162r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 0 1 . 0 0 1 5 5 - 2
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1. Introduction Sediment delivery, a concept relating the quantities of erosion, deposition and sediment yield in a drainage basin, can be considered at a variety of spatial and temporal scales ŽWalling, 1983, 1990; Richards, 1993.. For drainage basins large enough to contain stream channels, downstream sediment yield data from suspended sediment monitoring can be divided by total upstream erosion measured or predicted using empirical models to calculate the sediment delivery ratio ŽWalling, 1983.. The concept is also applicable to hillslopes ŽWalling, 1990., but due to the distributed nature of soil redistribution processes and sediment yield on hillslopes, an exact quantitative accounting for delivery may be more difficult to obtain. Locating and quantifying zones of erosion and deposition, which have little visible influence on hillslope topography, are necessary but problematic steps in hillslope sediment budgeting. There are two general approaches to assessing the spatial characteristics of sediment flux on hillslopes. The first is the use of soil morphology information such as observations of buried A-horizons or profile truncation to determine net gain or loss at a point ŽBeach, 1994; Phillips et al., 1999.. This approach has been used successfully at many sites, but has as drawbacks difficulties with using auger data to fully characterize horizon boundaries, and the labor intensiveness of pit excavation or augering, which may limit the number of samples, and therefore spatial resolution. The second general approach is the use of radionuclide tracer inventories relative to known fallout, which may be used to quantify the accumulation or loss of soil to which the radionuclide is adsorbed ŽRitchie and McHenry, 1990; Walling and He, 1999.. This widely used and useful technique may also have spatial resolution limitations due to the cost of commercial analysis for large numbers of samples, and the required sampling labor. In addition, Cesium-137, the most useful and often-employed radionuclide in such applications, has been present in soils only since the 1950s, thus limiting studies of erosion to the subsequent period. In a new approach, Dearing et al. Ž1986. employed magnetic measurements to trace topsoil movement on a hillslope in Oxfordshire ŽUK.. The basis for tracing used by these authors is the magnetic differentiation of topsoil from subsoil through natural pedogenic processes ŽThompson and Oldfield, 1986; Dearing, 1994.. By looking at magnetic profiles through soil cores from multiple locations across a slope, these authors were able to determine patterns of soil movement and also discuss the probable slope processes involved. More recently, de Jong et al. Ž1998. have combined the soil magnetism approach with Cs-137 data to study soil redistribution at a site in the Canadian Prairies. The general success but limited scope of these pioneering research efforts suggests the need for continued development of mineral magnetic techniques to better understand soil redistribution and hillslope sediment delivery. In particular, the need for further research in different geological environments has been noted ŽDearing et al., 1986.. In this paper, I present the findings of soil and hillslope magnetism research at a site in the southeastern US, the Gilbert Farm in northwestern Alabama. This gently rolling agricultural land on the limestone terrain of the Tennessee River Valley has a long history of cultivation, and is a sediment source for the Tennessee River, 2 km distant. In
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addition to meeting the need for research in new environments, Gilbert Farm offers the opportunity to study soil and slope magnetism in relation to soil redistribution by water erosion and tillage displacement on eroded farmland, for which some prior monitoring data exist. These data, published by Soileau et al. Ž1990, 1994., include Cs-137 inventories for seven locations in the catchment. The specific objectives of this research are to: Ž1. determine the controls on soil and sediment magnetism at the site, particularly with regard to soil redistribution; Ž2. map the spatial variation of soil magnetism and soil redistribution; and Ž3. analyze and interpret these spatial patterns with regard to hillslope sediment dynamics and sediment delivery. Because of the desirability of both high spatial resolution and broad spatial coverage, a combination typically difficult to obtain, this research adopts an approach slightly different from that of the few similar published studies in existence. I combine rapid surface magnetic scanning, which permits larger sample numbers, with traditional soil core profiling and surface sampling to obtain greater sampling densities. 2. Study site The Gilbert Farm site Ž34845X 15Y N, 87853X 25Y W; Fig. 1. is a 3.8-ha agricultural catchment with gentle gradients in the limestone region of northwestern Alabama ŽUSA.. This catchment was the site of soil erosion and crop experiments, jointly managed by the Tennessee Valley Authority and Auburn University during the 1980s ŽSoileau et al., 1990.. Conventional tillage, planting Žcorn and cotton. and harvesting are currently employed at the site. Although the fields in this area have been farmed or grazed for a century or more ŽSoileau et al., 1990., older management methods employed here are not well known. Aerial photographs from 1937 show approximately 25% of the currently fully cleared catchment in forest. Aerial photographs from 1963 show that this forested area, which occurred around the eastern periphery of the catchment, was removed prior to 1963. Soils in the catchment are derived from Mississippian age Tuscumbia limestone beds which are nearly horizontal and do not crop out on hillslopes. Upland soils are classified as Decatur silt loam Ž2–6% slopes. which are Paleudults ŽBowen, 1994.. Soils of the bottomland, although undifferentiated on county soil survey maps, have been mapped as Emory silt loam ŽSoileau et al., 1990., and classified as Dystrochrepts possessing an Ab horizon between 51 and 91 cm below the surface. Soils developed in limestone residuum are considered to offer the greatest potential for magnetism-based studies of hillslope erosion because the low primary magnetic mineral content of the parent material allows for clear magnetic differentiation of profiles via pedogenic processes ŽDearing et al., 1985.. Mean annual precipitation is 135 cm distributed evenly throughout the year, and runoff is low at around 15% ŽSoileau et al., 1990.. 3. Methods Seven transect lines ranging from 100 to 200 m in length were established in a roughly radial pattern at the site ŽFig. 1.. Three of these ŽA, C and D; Fig. 1. were arranged to coincide with the study transects of Soileau et al. Ž1990.. The other four
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Fig. 1. Location, feature and sampling map for the Gilbert Farm Watershed Žadapted from Soileau et al., 1990..
transects between each of these were placed to further resolve spatial patterns. Surficial measurements of k lf Žlow-frequency volume magnetic susceptibility. were made every 5 m between plant rows along these transects using a Bartington search loop sensor and MS2 meter. This instrument measures surficial magnetic susceptibility to a depth of around 15 cm, with material closest to the loop Ži.e., near the surface. contributing more
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than deeper soil. Measurements of k lf are chiefly controlled by the concentrations of magnetic minerals in samples ŽDearing, 1994., which often differ between soil horizons ŽDearing et al., 1986; Thompson and Oldfield, 1986.. In addition, 68 near-surface Ž3–7 cm; below loose surficial crust. samples were collected along seven independent transects oriented roughly west-to-east across the catchment ŽFig. 1. at 20-m intervals for purposes of mapping x lf Žmass-specific susceptibility; the mass-normalized equivalent of k lf . and k fd Žfrequency dependent susceptibility.. These transects were placed parallel to crop Žcotton. rows to facilitate sample point location and access. k fd , the percentage difference between induced magnetism in low- and high-frequency fields, is related to the grain size of magnetic minerals, and its mapping was undertaken to test secondary hypotheses generated in the light of the initial search loop Ž k lf . findings. Fine superparamagnetic mineral grains often of secondary origin in soils are important contributors to k fd ŽDearing et al., 1985; Shenggao, 2000.. Detailed descriptions of these magnetic measurements have appeared in earlier works ŽDearing et al., 1986; Dearing, 1994; Thompson and Oldfield, 1986.. The basis for magnetically tracing the movement of soil on hillslopes is the magnetic differentiation of topsoil from subsoil ŽDearing et al., 1986; de Jong et al., 1998; Dearing, 1994.. Soil profile information was gathered at 11 locations in the catchment coinciding with the seven prior Cs-137 sampling locations of Soileau et al. Ž1990. Žlocations 1–4, 8, 9 and 10; Fig. 1. and four new points Žlocations 5, 6, 7 and A80; Fig. 1.. Two of the seven Cs-137 sampling locations Ž9 and 10. are located in relatively stable forested environments and together serve as a model for uneroded soils in the catchment ŽSoileau et al., 1990.. The four additional locations were selected for analytical purposes based on the initial results of surface magnetism scanning. At each location, small soil pits were excavated, described and sampled in 2-cm depth increments to 20-cm depth. This depth was selected primarily because it contains all the soil contributing to the magnetic signal at the surface. At stable location 9 and at bottomland location A80, pits were excavated to a greater depth to sample the Bt2 and Ab horizons, respectively. All soil samples collected for the project were air-dried in the laboratory, crushed and then packed into 10-cmy3 plastic pots for laboratory magnetic measurements using the Bartington MS2B dual-frequency sensor. Particle-size analysis was carried out on soil samples from the upper portions of five profiles in order to better understand the relationship between texture and erosion class, and how particle size, a secondary control of magnetic susceptibility, may be influencing susceptibility values. Study profiles were selected to be representative of high and low susceptibility areas and areas of erosion and deposition as indicated by the Cs-137 evidence reported by Soileau et al. Ž1990.. In addition, magnetic measurements were obtained from soil separates derived from location 7 Ž6–8 cm depth. using a decantation procedure similar to that of Walden and Slattery Ž1993.. Combustible organic matter content of the Ap horizon, a potential indicator of remaining topsoil component, was determined for four profiles using loss-on-ignition ŽDean, 1974.. Finally, dried and crushed samples in pots were used to determine Munsell color designations for each sample. Surfaces were interpolated for the surface magnetism data sets using Surfer ŽGolden Software, 1994.. Spatial patterns in magnetism were compared to topography, soil
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morphological attributes, Cs-137 data from Soileau et al. Ž1990. and field observations to infer their potential relationship to soil redistribution. Based on the outcome, an estimate of long-term sediment delivery was made using magnetism as a spatially distributed index of soil loss, and soil morphological information for purposes of quantifying sediment storage. Elevation was digitized from 1-ft contours originally surveyed by Soileau et al. Ž1990. to provide a useful base map for overlay and analysis.
4. Results 4.1. Surface magnetism Surficial k lf is generally lowest on gently sloping ridgecrest and bottomland areas, and highest on steep mid-slope portions of the catchment ŽFig. 2.. The same general pattern is observed in the mass-specific and frequency-dependent measurements Ž x lf and k fd ; Figs. 3 and 4.. An important exception to these trends is found in the gentle mid-slope areas on the east side of the catchment Žlocation 6 and the upper DX transect. where two discrete cells of anomalously high magnetic susceptibility Ž k lf , x lf and k fd . occur. Slope–base waterlogging, reducing conditions and attendant weak magnetism as observed in other studies ŽDearing et al., 1985, 1986; de Jong et al., 2000. are not apparent at Gilbert Farm. Although magnetism decreases downslope of steep mid-slope areas, ridgecrest and bottomland values are similar in magnitude, suggesting that factors other than wetness are controlling magnetism in the bottomland. The general pattern of high magnetism on steep mid-slope areas found at Gilbert Farm contrasts with expectations for sites having magnetically enhanced topsoil which tend to exhibit lowest magnetism at these typically eroded locations ŽDearing et al., 1986; Dearing, 1994.. 4.2. Soil profiles Soileau et al. Ž1990. found that soils near forested location 9 ŽFig. 1. contained the highest inventory of the fallout radionuclide Cs-137 of all upland locations they sampled within the catchment in 1988 ŽTable 1., demonstrating the lack of erosion at this location since 1954. The soil profile from location 9 ŽFig. 5. features three magnetic horizons: Ž1. an A1-horizon of thin organic-rich topsoil horizon having high x lf and low k fd , Ž2. an A2-horizon with low x lf and high k fd , and Ž3. high-x lf and high-k fd Bt horizons. Soil colors become yellower, lighter and lower in chroma above the A2rBt1 boundary in this profile ŽFig. 5.. Despite high values of x lf in organic-rich topsoil, k lf measured with the search loop is low Ž140 = 10y5 SI. for both forested locations, apparently due to both the sensing of the low-x lf A2 horizon just beneath the thin topsoil and lower bulk density near the surface. All currently cultivated soils Žthose other than location 9 on Fig. 5. have relatively invariant profiles of k lf , x lf , k fd and Munsell color characteristics ŽFig. 5., primarily as a result of tillage homogenization. Profiles from locations 3 and 7 are exceptions, showing coherent decrease in x lf , increase in k fd and redder hues below 18 cm. Organic matter contents of Ap horizons vary little among those sample locations analyzed ŽTable 1..
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Fig. 2. The distribution of surface k lf determined using a search loop. k lf is dimensionless but scaled here to SI units. Broad classes of magnetism were used in contouring to enable better visual assessment.
Assuming that the Cs-137 inventories from Soileau et al.’s year-1988 sampling reflect a pattern of erosion which fundamentally coincides with the long-term pattern, and that no major changes have occurred in the ensuing decade, these data can be used to relate recent magnetic measurements to erosion magnitudes. The lowest sampled Cs-137 inventory indicative of severe erosion ŽSoileau et al., 1990. occurs at location 1
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Fig. 3. The distribution of surface x lf . Sample points for these measurements are the same as those for k fd Ž k lf transect points are shown for purposes of referencing soil pit locations..
on steep slopes ŽFig. 1.. Location 1 soil has high k lf , x lf and k fd , high clay and low silt content, and red hue ŽFigs. 2–5; Table 1., properties characteristic of Bt horizons at forested location 9. Location 2 has a Cs-137 inventory greater than that of stable location 9 indicating deposition ŽTable 1.. This soil exhibits lower values of all magnetic parameters than at location 1, relatively low chromas, and higher total silt content ŽFigs. 2–5; Table 1.. Low magnetism and high silt content are consistent with magnetic
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Fig. 4. The distribution of surface k fd Ž k lf transect points are shown for purposes of referencing soil pit locations..
measurements on soil separates which show that clay fractions in these soils have double the x lf Ž298 = 10y8 m3 kgy1 . of silt and sand. With the exceptions of locations 5 and 6, all other sampled profiles have magnetic properties similar to or in between those of locations 1 and 2. Pit locations 5 and 6 ŽFig. 5. exhibit x lf values substantially lower and higher than Žrespectively. those at any other location. For location 5, low magnetism
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Table 1 Particle size, combustible organic matter and Cs-137 dataa Particle size data Sample
Sand Ž%.
Coarse silt Ž%.
Fine silt Ž%.
Total silt Ž%.
Clay Ž%.
Location 1 Ž4–6 cm. Location 2 Ž6–8 cm. Location 6 Ž6–8cm. Location 7 Ž6–8 cm. Location 8 Ž6–8 cm.
24.5 14.5 20.0 21.5 26.3
5.0 5.2 9.2 7.1 6.1
33.2 48.4 39.7 34.6 32.2
38.2 53.6 48.9 41.8 38.3
37.2 31.8 31.1 36.7 35.4
Cs-137 inventory a
Organic matter Sample
Combustible organic matter Ž%.
Location 1 Ž4–6 cm. Location 6 Ž5–7 cm. Location 7 Ž4–6 cm. Location 7 Ž18–20 cm. Location 8 Ž4–6 cm.
a
4.42 3.99 3.98 3.64 3.95
Sample
Activity ŽBq my2 .
Location 1 Location 2 Location 3 Location 4 Location 8 Location 9 Location 10
1351 6025 2807 4709 2256 5284 4435
Cs-137 data are from Soileau et al. Ž1990..
may be explained by parent material differences as suggested by the presence of siltstone fragments from local clastic layers within the limestone. The presence of mudcracks and a fine silt surface at location 6 suggest that this upland location experiences stormwater ponding and deposition in a shallow closed karst depression. Abnormally, high x lf may be attributable to this unusual topographic setting which may have favored retention of original topsoil. 4.3. InterpretiÕe model Using the uneroded profile at location 9 as a basis for examining changes in cultivated soils, a simple tillage homogenization model can be used to predict changes in surface soil magnetism with progressive soil loss ŽFig. 6.. This model was derived using the mathematical procedure 20
x dlf s
Ý xilf
20
when d s 0, otherwise,
Ž 1.
is1 lf lf x dlf s 19 Ž x dy1 . q Ž xdq20 . 20,
Ž 2.
where x dlf is the expected magnetism of the Ap horizon after erosion loss depth d, i is lf depth increment Žcm. below the surface Žregardless of erosion stage., x dy 1 is the lf lf starting average x value for the erosion stage prior to the current one, and x dq 20 is
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Fig. 5. Magnetic and Munsell color profiles from soil pit locations Žabbreviated Aloc.B on the plots. referenced on maps. Vertical axis on the left-hand diagram of each diagram pair is depth Žcm., upper horizontal axis is x lf Ž=10y8 m3 kgy1 ., represented by a solid line, and lower horizontal axis is k fd Ž%., represented by a dashed line. All depth tick intervals are 5 cm. The right-hand diagram of each diagram couplet is the Munsell color profile with horizontal axis giving hue ŽYR: solid line., value Ždashed line., or chroma Ždash–dot line..
the x lf of the 1-cm soil layer immediately below the plow depth Žregardless of erosion stage.. This model assumes complete tillage homogenization to 20-cm depth. Analogous to the preferential erosional loss of fine-particle-associated Cs-137 in soils ŽWalling and He, 1999., the preferential removal of highly magnetic fines Žclays. by erosion from the soil surface would be expected to influence surface magnetism. Particle-size selectivity is not incorporated into the above model, but would presumably result in reduced absolute values of surface magnetism at any stage of erosion, changing the average values but not the general shapes of Fig. 6 curves. Decatur soils typically exhibit approximately a 25% decrease in clay content from the Bt1 to the Ap horizons ŽBowen, 1994.. Given that clays have double the magnetism of coarser particles in these soils, a 25% decrease in clay resulting from selective erosion might produce a decrease in magnetism of about 7%. According to this simple model, erosion would gradually increase both x lf and k fd as progressively greater contributions of subsoil to surficial materials were added through tillage ŽFig. 6.. A similar, although linear relationship between erosion and k fd , is also suggested by the regression plot of k fd vs. Cs-137 ŽFig. 7.. However, the Cs-137 data are limited Žonly five non-depositional locations, two of which are forested., and
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Fig. 6. Tillage homogenization model. The left-hand diagram shows the location 9 profile on which the model Žright-hand diagram. is based. Erosion depth axis gives the expected depth of erosion corresponding to values of x lf Žsolid line. and k fd Ždashed line.. Model assumes 1-cm increments of soil loss and complete homogenization to 20-cm depth during tillage.
Fig. 7. Regression relationship between k fd and Cs-137 inventory at five upland locations. Cesium-137 inventories are from Soileau et al. Ž1990..
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despite the high correlation coefficient, the relationship must be regarded as very tentative at this time. The affinity of eroded location 1 soil properties with B-horizon is consistent with the tillage model which predicts high x lf as a result of increasing B-horizon influence with erosion. According to Fig. 6, highest magnetism is achieved after around 40 cm of soil loss. However, most of the increase occurs by around 20 cm of loss, and perhaps a reasonable estimate for long-term soil loss at location 1 in the highly magnetic steep mid-slope areas lies somewhere in between Žaround 30 cm.. This value does not account for any soil loss prior to afforestation of the currently stable location on which the model is based. A 100-year extrapolation of soil loss at location 1 based on the proportional model of Cs-137 concentration ŽSoileau et al., 1990; Walling and He, 1999. is similar at around 34 cm. Cesium-137 evidence indicates that location 8 is strongly eroded also, but not as much as location 1 ŽTable 1.. This is consistent with soil attributes at location 8 ŽFig. 5., which are like those described for location 1 except for the noteworthy exception of lower magnetism. It is important to note that at no point in the tillage model does x lf attain the maximum value recorded at location 1. Small spatial variations in parent materials represent one potential source of error in the magnetismrsoil loss relationship. Gentle gradient surfaces of the ridgecrests Žfor example, location 3. are characterized by moderate erosion Ž; 5 cm in 34 years. according to Cs-137 evidence ŽSoileau et al., 1990.. The decline in x lf and increase in k fd below 18 cm at location 3 suggest the presence of intact A2 horizon, and the low average magnetism values correspond to low soil loss in the tillage model ŽFig. 6.. If intact A2 horizon is present, long-term Ži.e., 100-year. erosion at this location might be much less than that projected from Cs-137 evidence, perhaps as little as 5 cm. Although the soil loss value is uncertain, the evidence supports the general inference that erosion has not been as severe at this location as at steeper mid-slope locations. The similar magnetic profile at location 7, a gentle upland flat, is likely to fit within this erosion category as well. Sediments in storage within the bottomland depositional zone Žroughly, elevations less than 165 m; Fig. 1., have the magnetic properties of the eroding soils upslope, minus the very fine fraction lost as sediment yield at the catchment outlet ŽSoileau et al., 1990., and minus the coarse fraction lagging behind on slopes which is concentrated in the upper soil particularly on steep gradients. Deposits at location 2 are silt-rich compared to residual materials on upland surfaces ŽTable 1.. Because magnetic minerals are concentrated in the clay fraction, magnetism would be expected to be lower in the bottomland where clays have been preferentially lost to sediment yield. The slight increases in all magnetic parameters toward the catchment outlet ŽFigs. 2, 3 and 4., may result from constriction of outflow, downstream ponding, and consequent increased deposition of fines as evidenced by mudcracks and fine silt coatings in this bowl-shaped bottomland ŽFig. 1.. Soils from the isolated cells of high magnetism in the eastern portion of the catchment have a mixture of erosional and depositional properties. Magnetism at location 6 is high like that at eroded location 1 ŽFig. 5., but clay content is low and silt dominates as at depositional location 2 ŽTable 1.. As noted earlier, field observations reveal that these locations pond stormwater, and apparently represent partially infilled
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closed karst depressions. High magnetism is characteristic of topsoil at forested location 9 ŽFig. 5. and it is possible that similar material retained in the depressions has been mixed upwards by tillage into overlying sediment. A buried A-horizon 10 m upslope from location 2 Žsite A80, 40-cm depth; Fig. 5. has higher x lf than most other surrounding soils, but is still lower in magnetism than eroded location 1, and much lower than that at location 6. However, additions of coarse silt during burial, and potential degradation of the A-horizon prior to burial may have reduced its magnetism. 4.4. Sediment deliÕery Using magnetism maps, the tillage homogenization model, published Cs-137 inventories and deep soil morphology data, it is possible to assess the particulate sediment budget and calculate a value for long-term sediment delivery. Grid math in Surfer was used to distribute soil loss depth using x lf and k fd as indices of soil loss based on the tillage homogenization model concept. However, a linear relationship rather than a curvilinear one as depicted in Fig. 6, was used for computational simplicity. The indexing accounts for 30 cm of soil loss at location 1 Žderived from the model as discussed earlier. and either 5 or 15 cm of soil loss at location 3, depending on which of the bases for estimation given in Section 4.3 ŽCs-137 or profile morphology. is used. Data points for the upland sinks on the east side of the catchment were excluded from gridding to remove their effects on surrounding magnetic contours. An important assumption of this assignment procedure is that the soil profile at location 9 used as the basis for the tillage model is representative of soils throughout the eroding uplands. Substantial variations in horizon thickness would change the shape of the curves in Fig. 6, thus changing the soil loss estimates. A second approach in which erosion was estimated using the regression relationship between k fd and Cs-137 inventory ŽFig. 7. was also tested. Values of Cs-137 inventory, spatially distributed according to k fd values, were used to calculate soil loss in depth units using the simple proportional model: wŽ B y C .rB x = Depth, where B is stable site Cs-137 inventory, C is Cs-137 inventory predicted by the regression relationship, and Depth is 15 cm, the depth of upland soil containing Cs-137 ŽSoileau et al., 1990; Walling and He, 1999.. Although the proportional model accounts for neither the Cs-137 washed away between tillage events, nor the dilution of Cs-137 concentration by tillage incorporation of Cs-137-deficient subsoil as erosion proceeds ŽWalling and He, 1999., it is used here to permit direct comparison to the results of Soileau et al. Ž1990.. All deposition areas were blanked prior to soil loss volume integration. For both approaches to erosion estimation, volumes of soil loss were converted to soil masses by assuming an average bulk density of 1.45 g cmy3 , derived from soil survey data ŽBowen, 1994. and data from Soileau et al. Ž1990.. Depositional areas spatially defined by magnetism, field observations of ponding and the observations of Soileau et al. Ž1990. were digitized and assigned estimates for soil deposition thickness based on soil survey morphological data ŽSoileau et al., 1990; Bowen, 1994. and field observations. For the bottomland, soil surveys estimate maximum depths to buried A-horizon of 90 cm in topographically similar but larger areas of
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Emory soil. Soileau et al. Ž1990. report what may be a more representative depth to Ab of 46 cm near the 164.6-m mark on the topographic map. Based on these end-member depths, two parameter sets were used for calculation of deposition volumes. The first set assumes point values of 0-cm depth at the digitized periphery, 46 cm at the 164.6-m contour, and a maximum of 90 cm in the downstream and center portion of the catchment. Set 2 assumes a maximum Ab-horizon burial depth of only 45 cm. A similar
Fig. 8. Upland soil loss distributed according to k fd . Blanked areas Ždenoting net deposition. are white.
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procedure was conducted for the upland sinks, except that values for deposition were arbitrarily set at half those of the bottomland due to the smaller size of the depressions. All depositional volumes were integrated in Surfer, and multiplied by bulk density Ž1.5 g cmy3 ; Bowen, 1994.; the resulting masses were used to calculate long-term sediment delivery, assuming that total erosion minus total deposition equals total sediment yield. The patterns of soil loss for each index used Ž x lf and k fd . are the same as the patterns of the magnetic measurements themselves. Fig. 8 depicts long-term soil loss estimated using k fd and the tillage homogenization model assuming 15 cm of soil loss at location 3 Ži.e., the Cs-137 extrapolation result.; blanked areas represent zones of net deposition. Soil loss rates based on each of the three estimation methods, and assuming 100 years of erosion Ža minimum but perhaps approximate age estimate given by Soileau et al., 1990. range from 16,546 kg hay1 yeary1 assuming minimal Ž5 cm. erosion near ridgecrests to 28,550 kg hay1 yeary1 using the k fd rCs-137 relationship ŽTable 2.. The latter value is similar to the 30,000-kg hay1 yeary1 value for the period 1954–1988 given by Soileau et al. Ž1990. based also on Cs-137. Values for total storage range from 5963 to 8465 Mg depending on choice of maximum depth to Ab. Accepting an intermediate value for sediment storage of 7214 Mg gives a range for sediment delivery ratio of 9.1–33.5% assuming 15 cm of soil loss near ridgecrests. The lower end of this range is near the 10% value calculated by Soileau et al. Ž1990. using 5 years of suspended sediment monitoring. Sediment delivery is much lower for the minimal ridgecrest soil loss scenarios and negative for assumptions of 90-cm depth of bottomland Ab burial, indicating substantial error in one or both of the associated erosion and deposition values.
Table 2 Sediment delivery data based on soil magnetism Model ID a
Total upland erosion ŽMg.
Soil loss rate Žkg hay1 yeary1 .
Total storage ŽMg.
Sediment delivery Ž%.
x lf 5r45r23 5r90r45 15r45r23 15r90r45
6287 6287 8627 8627
16,546 16,546 22,703 22,703
5963 8465 5963 8465
5.1 y34.6 30.9 1.9
k fd 5r45r23 5r90r45 15r45r23 15r90r45
6376 6376 7937 7937
16,780 16,780 20,887 20,887
5963 8465 5963 8465
6.5 y32.7 24.9 y6.6
10,849 10,849
28,550 28,550
8465 5963
22.0 45.0
k fdrCs-137 90r45 45r23
a Parameter used to distribute soil loss depth; assumptions: soil loss Žcm. assumed at location 3 Žsee text.rmaximum soil depositional thickness assumed in bottomlandrmaximum depositional thickness assumed in upland sinks.
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The magnetism-based erosion estimates for the last 100 years, which are lower than the value determined by Soileau et al. Ž1990. for the post-1954 period, imply that the average soil loss rate prior to 1954 would have had to be lower still, as low as 5085 kg hay1 yeary1 for the x lf 5r45r23 estimate. This result is contrary to the general belief that past erosion rates were higher than today’s in the southeastern US ŽTrimble, 1985.. The apparently higher forest cover along the eastern perimeter in 1937 noted on aerial photographs would account for some of the difference. However, the size of the difference suggests problems with the models as well. For example, if the original profile thickness was greater on ridgecrests than at location 9, the minimal 5-cm estimate of ridgecrest soil loss based on the magnetic profile at location 3 would be low. Likewise, the proportional model relating Cs-137 inventory and erosion potentially overestimates soil loss because it does not account for loss of surface Cs-137 to erosion prior to tillage mixing ŽWalling and He, 1999.. Alternatively, this model may underestimate soil loss because it does not account for progressive dilution of Cs-137 concentration which results from incorporation of Cs-137-deficient soil below the plow depth as erosion proceeds ŽWalling and He, 1999.. Such potential errors reduce the comparability of Cs-137-based rates with all independent estimates of soil loss.
5. Discussion and conclusions Spatial patterns of soil magnetism derived from rapid search loop scanning and surface sampling are related to soil redistribution patterns at Gilbert Farm. Patterns of magnetism are controlled by the magnetic characteristics of soil horizons exposed on or near the land surface, and the particle size characteristics of surficial materials. Without particle-size data and other supporting physical information such as profile morphology and, in this study, Cs-137 data, patterns of magnetism are difficult to interpret. The best example of this is the fact that magnetic lows are characteristic of both the high level ridgecrest areas Žweak erosion. and lowest areas Ždepositional zone. in the catchment for different reasons. Once a basis for interpretation has been established, the mapping of eroded and depositional areas might be conducted rapidly and at a high level of spatial detail using surface magnetism. Values of the three magnetic parameters measured in this study were found to have broadly similar spatial distributions. However, these were not identical, hence a choice is required regarding which best serves as an erosion index. On the eroded soils of this cultivated catchment, x lf , which clearly distinguishes between the A2 and Bt1 horizons at forested location 9, is conceptually easiest to interpret. Under less eroded conditions, k fd , which shows greatest change between topsoil and the A2 horizon of location 9, may be the best erosion indicator. Both x lf and k fd require more sampling and lab analysis labor than k lf Žmeasured with the search loop., although less so than that required for determining Cs-137 inventories or describing profile morphology at the same number of points. Although k lf measurements exhibit patterns similar to those of the other magnetic measurements and are obtainable in a fraction of the time, bulk density variation is unaccounted for. For all measurements, time savings would be greater if mapping technologies like GPS were used to locate points on an existing
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digital georeferenced site map as sampling was conducted. The additional development of a rapid scanning search loop sensor for k fd would allow even quicker assessments. A major drawback of using magnetism to define soil redistribution is the difficulty of relating magnetic values from surface measurements to specific values for depth of soil loss or deposition. A relationship between depths of soil loss and surface magnetism has been predicted for eroded areas at Gilbert Farm using a simple tillage homogenization model based on profile characteristics at an uneroded site. Modifications to account for particle-size selectivity of erosion, and other complexities such as systematic changes in original profile thickness over space would improve the model. Potential obstacles to its implementation include non-systematic Žor unrecognizable. spatial changes in the soil environment and uncertainties in land use history, such as the timing of plow technology changes in an area which would affect homogenization depth. In bottomland areas where the original surface soil may be deeply buried, upward tillage mixing would not be expected to substantially affect surface magnetism. Whatever small effect there might be would likely be obscured by particle-size influences. The inability to calibrate susceptibility measurements with regard to deposition thickness necessitates the estimation of deposition using profile morphology or perhaps Cs-137. These methods give accurate information, but the cost in time and labor, and therefore often spatial resolution, is high. The applicability of this technique in other environments depends on geological specifics. Dearing et al. Ž1985. note that studies of hillslope erosion based on profile magnetic differentiation should be easiest to implement in areas underlain by sedimentary rock, and particularly those on limestone, a rock type typically low in primary magnetic mineral content. The results from earlier published studies, the Gilbert Farm site, and an area of metamorphic terrain currently under investigation Žby this author. support the contention that the specifics of environment at a given locale require assessment on an individual site basis for purposes of linking magnetism and erosion. Metamorphic terrain may represent a particularly difficult case, due to the possibility of changes in parent material over short distances. However, presumably smaller changes still might influence magnetic patterns even in such geologically homogeneous environments as Gilbert Farm. The use of magnetic measurements may best be pursued in conjunction with Cs-137 studies, with the aim of reducing the number of Cs-137 samples required for adequate spatial coverage. Resolving the relationship between magnetism and soil erosion in different environments might be facilitated by taking advantage of the numerous sites for which Cs-137 inventories already exist as a result of prior soil erosion research. Although the use of magnetism as a suspended sediment tracer in catchments is well-established ŽWalling et al., 1979; Walden et al., 1997., the interpretation of hillslope magnetism with regard to soil redistribution remains relatively unstudied. The ability to quickly supply a more spatially distributed perspective on soil redistribution would be beneficial for assessing soil erosion models. In addition, the ability afforded by magnetism to study sediment delivery at the hillslope scale might facilitate the study of scale-dependence of delivery. Finally, an historical perspective may be gained by the sort of analysis conducted herein. Sediment budgeting based on magnetism at this site revealed potential differences in erosion and sediment delivery rates between the first
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and second halves of the 20th century. However, there are several potential sources of error which might contribute to these differences. Refining soil magnetism models to account for these sources represents a logical goal for future research.
Acknowledgements I am grateful to John Soileau Žretired, Tennessee Valley Authority. for allowing a revisit of the site and use of Cs-137 data. I also thank Forrest Wright ŽExecutive Director of the Shoals Economic Development Authority. for providing access to the site, information on its history, and other accommodations which have been important to the research effort. I also wish to acknowledge the helpful comments of two anonymous reviewers. This project was funded by a Research Advisory Committee Grant ŽNo. 2-67705. from the University of Alabama to Dan Royall.
References Beach, T., 1994. The fate of eroded soil: sediment sinks and sediment budgets of agrarian landscapes in southern Minnesota, 1851–1988. Ann. Assoc. Am. Geogr. 84 Ž1., 5–28. Bowen, C.D., 1994. Soil Survey of Colbert County, Alabama. USDA Soil Conservation Service, Washington D.C., 154 pp. Dean, W.E., 1974. Determination of carbonate and organic matter in calcareous sediments and sedimentary rocks by loss on ignition: comparison with other methods. J. Sediment. Petrol. 44 Ž1., 242–248. Dearing, J.A., 1994. Environmental Magnetic Susceptibility: using the Bartington MS2 System. Chi Publishing, Kenilworth, England. Dearing, J.A., Maher, B.A., Oldfield, F., 1985. Geomorphological linkages between soils and sediments: the role of magnetic measurements. In: Richards, K.S., Arnett, R.R., Ellis, S. ŽEds.., Geomorphology and Soils. Allen and Unwin, Boston, pp. 245–266. Dearing, J.A., Morton, R.I., Price, T.W., Foster, I.D.L., 1986. Tracing movements of topsoil by magnetic measurements: two case studies. Phys. Earth Planet. Inter. 42, 93–104. de Jong, E., Nestor, P.A., Pennock, D.J., 1998. The use of magnetic susceptibility to measure long-term soil redistribution. Catena 32, 23–35. de Jong, E., Pennock, D.J., Nestor, P.A., 2000. Magnetic susceptibility of soils in different slope positions in Saskatchewan, Canada. Catena 40, 291–305. Golden Software, 1994. SURFER for Windows. Golden, Colorado. Phillips, J.D., Golden, H., Cappiella, K., Andrews, B., Middleton, T., Downer, D., Kelli, D., Padrick, L., 1999. Soil redistribution and pedologic transformations in coastal plain croplands. Earth Surf. Processes Landforms 24, 23–39. Richards, K., 1993. Sediment delivery and the drainage network. In: Beven, K., Kirkby, M.J. ŽEds.., Channel Network Hydrology. Wiley, New York, pp. 221–254. Ritchie, J.C., McHenry, J.R., 1990. Application of radioactive fallout Cs-137 for measuring soil erosion and sediment accumulation rates and patterns: a review. J. Environ. Qual. 19, 215–233. Shenggao, L., 2000. Lithological factors affecting magnetic susceptibility of subtropical soils, Zhejiang Province, China. Catena 40, 359–373. Soileau, J.M., Hajek, B.F., Touchton, J.T., 1990. Soil erosion and deposition evidence in a small watershed using fallout Cesium-137. Soil Sci. Soc. Am. J. 54, 1712–1719. Soileau, J.M., Touchton, B.F., Kajek, B.F., Yoo, K.H., 1994. Sediment, nitrogen, and phosphorous runoff with conventional- and conservation-tillage cotton in a small watershed. J. Soil Water Conserv. 49 Ž1., 82–89. Thompson, R., Oldfield, F., 1986. Environmental Magnetism. Allen and Unwin, Boston.
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Trimble, S.W., 1985. Perspectives on the history of soil erosion control in the Eastern United States. Agric. Hist. 59, 162–180. Walden, J., Slattery, M.C., 1993. Verification of a simple gravity technique for separation of particle size fractions suitable for mineral magnetic analysis. Earth Surf. Processes Landforms 18, 829–833. Walden, J., Slattery, M.C., Burt, T.P., 1997. Use of mineral magnetic measurements to fingerprint suspended sediment sources: approaches and techniques for data analysis. J. Hydrol. 202, 353–372. Walling, D.E., 1983. The sediment delivery problem. J. Hydrol. 65, 209–237. Walling, D.E., 1990. Linking the field to the river: sediment delivery from agricultural land. In: Boardman, J., Foster, I.D.L., Dearing, J.A. ŽEds.., Soil Erosion on Agricultural Land. Wiley, Chichester, West Sussex, England, pp. 129–152. Walling, D.E., He, Q., 1999. Improved models for estimating soil erosion rates from Cs-137 measurements. J. Environ. Qual. 28 Ž2., 611–622. Walling, D.E., Peart, M.R., Oldfield, F., Thompson, R., 1979. Suspended sediment sources identified by magnetic measurements. Nature 281, 110–113.
Use of mineral magnetic measurements to investigate soil erosion and sediment delivery in a small agricultural catchment in limestone terrain Dan Royall ) Department of Geography, UniÕersity of Alabama, Tuscaloosa, AL 35487, USA Received 17 November 2000; received in revised form 1 May 2001; accepted 23 May 2001
Abstract Understanding sediment delivery at the hillslope scale requires information on the spatial distributions and magnitudes of erosion and deposition. Empirical models such as the RUSLE may be useful for predicting erosion, but are poorly suited for quantifying deposition. Cesium-137 ŽCs-137. is useful for quantifying both erosion and deposition, but is costly to inventory over a large area, and directly gauges soil redistribution only for recent decades. The use of rapidly acquired magnetic measurements represents a relatively new potential means of mapping and measuring soil redistribution. In this study, variations in surface magnetism are analyzed to determine patterns of erosion and sedimentation in a small agricultural catchment in northwestern Alabama ŽUSA.. Magnetic indicators of erosion are combined with published soil morphology, Cs-137 and short-term suspended sediment data from this former experimental watershed to evaluate long-term sediment delivery. All magnetic parameters measured could be related to soil erosion, although their patterns in space are not identical. Magnetic evidence suggests approximately 30 cm of soil loss on the steepest mid-slope portions of the catchment. Distributing soil loss across space according to magnetic patterns gives average long-term values lower than a prior estimate based on Cs-137 data. Long-term sediment delivery calculated using soil morphology to determine depositional volume ranges up to 45% depending on the magnetic parameter used to index soil loss, and assumptions regarding deposit geometry. These results suggest the need for continued refinement of magnetic techniques for purposes of erosion model validation and general sediment tracing applications. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Soil erosion; Magnetic susceptibility; Agricultural landscape; Sediment delivery
)
Fax: q1-205-348-2278. E-mail address: [email protected] ŽD. Royall..
0341-8162r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 0 1 . 0 0 1 5 5 - 2
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1. Introduction Sediment delivery, a concept relating the quantities of erosion, deposition and sediment yield in a drainage basin, can be considered at a variety of spatial and temporal scales ŽWalling, 1983, 1990; Richards, 1993.. For drainage basins large enough to contain stream channels, downstream sediment yield data from suspended sediment monitoring can be divided by total upstream erosion measured or predicted using empirical models to calculate the sediment delivery ratio ŽWalling, 1983.. The concept is also applicable to hillslopes ŽWalling, 1990., but due to the distributed nature of soil redistribution processes and sediment yield on hillslopes, an exact quantitative accounting for delivery may be more difficult to obtain. Locating and quantifying zones of erosion and deposition, which have little visible influence on hillslope topography, are necessary but problematic steps in hillslope sediment budgeting. There are two general approaches to assessing the spatial characteristics of sediment flux on hillslopes. The first is the use of soil morphology information such as observations of buried A-horizons or profile truncation to determine net gain or loss at a point ŽBeach, 1994; Phillips et al., 1999.. This approach has been used successfully at many sites, but has as drawbacks difficulties with using auger data to fully characterize horizon boundaries, and the labor intensiveness of pit excavation or augering, which may limit the number of samples, and therefore spatial resolution. The second general approach is the use of radionuclide tracer inventories relative to known fallout, which may be used to quantify the accumulation or loss of soil to which the radionuclide is adsorbed ŽRitchie and McHenry, 1990; Walling and He, 1999.. This widely used and useful technique may also have spatial resolution limitations due to the cost of commercial analysis for large numbers of samples, and the required sampling labor. In addition, Cesium-137, the most useful and often-employed radionuclide in such applications, has been present in soils only since the 1950s, thus limiting studies of erosion to the subsequent period. In a new approach, Dearing et al. Ž1986. employed magnetic measurements to trace topsoil movement on a hillslope in Oxfordshire ŽUK.. The basis for tracing used by these authors is the magnetic differentiation of topsoil from subsoil through natural pedogenic processes ŽThompson and Oldfield, 1986; Dearing, 1994.. By looking at magnetic profiles through soil cores from multiple locations across a slope, these authors were able to determine patterns of soil movement and also discuss the probable slope processes involved. More recently, de Jong et al. Ž1998. have combined the soil magnetism approach with Cs-137 data to study soil redistribution at a site in the Canadian Prairies. The general success but limited scope of these pioneering research efforts suggests the need for continued development of mineral magnetic techniques to better understand soil redistribution and hillslope sediment delivery. In particular, the need for further research in different geological environments has been noted ŽDearing et al., 1986.. In this paper, I present the findings of soil and hillslope magnetism research at a site in the southeastern US, the Gilbert Farm in northwestern Alabama. This gently rolling agricultural land on the limestone terrain of the Tennessee River Valley has a long history of cultivation, and is a sediment source for the Tennessee River, 2 km distant. In
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addition to meeting the need for research in new environments, Gilbert Farm offers the opportunity to study soil and slope magnetism in relation to soil redistribution by water erosion and tillage displacement on eroded farmland, for which some prior monitoring data exist. These data, published by Soileau et al. Ž1990, 1994., include Cs-137 inventories for seven locations in the catchment. The specific objectives of this research are to: Ž1. determine the controls on soil and sediment magnetism at the site, particularly with regard to soil redistribution; Ž2. map the spatial variation of soil magnetism and soil redistribution; and Ž3. analyze and interpret these spatial patterns with regard to hillslope sediment dynamics and sediment delivery. Because of the desirability of both high spatial resolution and broad spatial coverage, a combination typically difficult to obtain, this research adopts an approach slightly different from that of the few similar published studies in existence. I combine rapid surface magnetic scanning, which permits larger sample numbers, with traditional soil core profiling and surface sampling to obtain greater sampling densities. 2. Study site The Gilbert Farm site Ž34845X 15Y N, 87853X 25Y W; Fig. 1. is a 3.8-ha agricultural catchment with gentle gradients in the limestone region of northwestern Alabama ŽUSA.. This catchment was the site of soil erosion and crop experiments, jointly managed by the Tennessee Valley Authority and Auburn University during the 1980s ŽSoileau et al., 1990.. Conventional tillage, planting Žcorn and cotton. and harvesting are currently employed at the site. Although the fields in this area have been farmed or grazed for a century or more ŽSoileau et al., 1990., older management methods employed here are not well known. Aerial photographs from 1937 show approximately 25% of the currently fully cleared catchment in forest. Aerial photographs from 1963 show that this forested area, which occurred around the eastern periphery of the catchment, was removed prior to 1963. Soils in the catchment are derived from Mississippian age Tuscumbia limestone beds which are nearly horizontal and do not crop out on hillslopes. Upland soils are classified as Decatur silt loam Ž2–6% slopes. which are Paleudults ŽBowen, 1994.. Soils of the bottomland, although undifferentiated on county soil survey maps, have been mapped as Emory silt loam ŽSoileau et al., 1990., and classified as Dystrochrepts possessing an Ab horizon between 51 and 91 cm below the surface. Soils developed in limestone residuum are considered to offer the greatest potential for magnetism-based studies of hillslope erosion because the low primary magnetic mineral content of the parent material allows for clear magnetic differentiation of profiles via pedogenic processes ŽDearing et al., 1985.. Mean annual precipitation is 135 cm distributed evenly throughout the year, and runoff is low at around 15% ŽSoileau et al., 1990.. 3. Methods Seven transect lines ranging from 100 to 200 m in length were established in a roughly radial pattern at the site ŽFig. 1.. Three of these ŽA, C and D; Fig. 1. were arranged to coincide with the study transects of Soileau et al. Ž1990.. The other four
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Fig. 1. Location, feature and sampling map for the Gilbert Farm Watershed Žadapted from Soileau et al., 1990..
transects between each of these were placed to further resolve spatial patterns. Surficial measurements of k lf Žlow-frequency volume magnetic susceptibility. were made every 5 m between plant rows along these transects using a Bartington search loop sensor and MS2 meter. This instrument measures surficial magnetic susceptibility to a depth of around 15 cm, with material closest to the loop Ži.e., near the surface. contributing more
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than deeper soil. Measurements of k lf are chiefly controlled by the concentrations of magnetic minerals in samples ŽDearing, 1994., which often differ between soil horizons ŽDearing et al., 1986; Thompson and Oldfield, 1986.. In addition, 68 near-surface Ž3–7 cm; below loose surficial crust. samples were collected along seven independent transects oriented roughly west-to-east across the catchment ŽFig. 1. at 20-m intervals for purposes of mapping x lf Žmass-specific susceptibility; the mass-normalized equivalent of k lf . and k fd Žfrequency dependent susceptibility.. These transects were placed parallel to crop Žcotton. rows to facilitate sample point location and access. k fd , the percentage difference between induced magnetism in low- and high-frequency fields, is related to the grain size of magnetic minerals, and its mapping was undertaken to test secondary hypotheses generated in the light of the initial search loop Ž k lf . findings. Fine superparamagnetic mineral grains often of secondary origin in soils are important contributors to k fd ŽDearing et al., 1985; Shenggao, 2000.. Detailed descriptions of these magnetic measurements have appeared in earlier works ŽDearing et al., 1986; Dearing, 1994; Thompson and Oldfield, 1986.. The basis for magnetically tracing the movement of soil on hillslopes is the magnetic differentiation of topsoil from subsoil ŽDearing et al., 1986; de Jong et al., 1998; Dearing, 1994.. Soil profile information was gathered at 11 locations in the catchment coinciding with the seven prior Cs-137 sampling locations of Soileau et al. Ž1990. Žlocations 1–4, 8, 9 and 10; Fig. 1. and four new points Žlocations 5, 6, 7 and A80; Fig. 1.. Two of the seven Cs-137 sampling locations Ž9 and 10. are located in relatively stable forested environments and together serve as a model for uneroded soils in the catchment ŽSoileau et al., 1990.. The four additional locations were selected for analytical purposes based on the initial results of surface magnetism scanning. At each location, small soil pits were excavated, described and sampled in 2-cm depth increments to 20-cm depth. This depth was selected primarily because it contains all the soil contributing to the magnetic signal at the surface. At stable location 9 and at bottomland location A80, pits were excavated to a greater depth to sample the Bt2 and Ab horizons, respectively. All soil samples collected for the project were air-dried in the laboratory, crushed and then packed into 10-cmy3 plastic pots for laboratory magnetic measurements using the Bartington MS2B dual-frequency sensor. Particle-size analysis was carried out on soil samples from the upper portions of five profiles in order to better understand the relationship between texture and erosion class, and how particle size, a secondary control of magnetic susceptibility, may be influencing susceptibility values. Study profiles were selected to be representative of high and low susceptibility areas and areas of erosion and deposition as indicated by the Cs-137 evidence reported by Soileau et al. Ž1990.. In addition, magnetic measurements were obtained from soil separates derived from location 7 Ž6–8 cm depth. using a decantation procedure similar to that of Walden and Slattery Ž1993.. Combustible organic matter content of the Ap horizon, a potential indicator of remaining topsoil component, was determined for four profiles using loss-on-ignition ŽDean, 1974.. Finally, dried and crushed samples in pots were used to determine Munsell color designations for each sample. Surfaces were interpolated for the surface magnetism data sets using Surfer ŽGolden Software, 1994.. Spatial patterns in magnetism were compared to topography, soil
20
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morphological attributes, Cs-137 data from Soileau et al. Ž1990. and field observations to infer their potential relationship to soil redistribution. Based on the outcome, an estimate of long-term sediment delivery was made using magnetism as a spatially distributed index of soil loss, and soil morphological information for purposes of quantifying sediment storage. Elevation was digitized from 1-ft contours originally surveyed by Soileau et al. Ž1990. to provide a useful base map for overlay and analysis.
4. Results 4.1. Surface magnetism Surficial k lf is generally lowest on gently sloping ridgecrest and bottomland areas, and highest on steep mid-slope portions of the catchment ŽFig. 2.. The same general pattern is observed in the mass-specific and frequency-dependent measurements Ž x lf and k fd ; Figs. 3 and 4.. An important exception to these trends is found in the gentle mid-slope areas on the east side of the catchment Žlocation 6 and the upper DX transect. where two discrete cells of anomalously high magnetic susceptibility Ž k lf , x lf and k fd . occur. Slope–base waterlogging, reducing conditions and attendant weak magnetism as observed in other studies ŽDearing et al., 1985, 1986; de Jong et al., 2000. are not apparent at Gilbert Farm. Although magnetism decreases downslope of steep mid-slope areas, ridgecrest and bottomland values are similar in magnitude, suggesting that factors other than wetness are controlling magnetism in the bottomland. The general pattern of high magnetism on steep mid-slope areas found at Gilbert Farm contrasts with expectations for sites having magnetically enhanced topsoil which tend to exhibit lowest magnetism at these typically eroded locations ŽDearing et al., 1986; Dearing, 1994.. 4.2. Soil profiles Soileau et al. Ž1990. found that soils near forested location 9 ŽFig. 1. contained the highest inventory of the fallout radionuclide Cs-137 of all upland locations they sampled within the catchment in 1988 ŽTable 1., demonstrating the lack of erosion at this location since 1954. The soil profile from location 9 ŽFig. 5. features three magnetic horizons: Ž1. an A1-horizon of thin organic-rich topsoil horizon having high x lf and low k fd , Ž2. an A2-horizon with low x lf and high k fd , and Ž3. high-x lf and high-k fd Bt horizons. Soil colors become yellower, lighter and lower in chroma above the A2rBt1 boundary in this profile ŽFig. 5.. Despite high values of x lf in organic-rich topsoil, k lf measured with the search loop is low Ž140 = 10y5 SI. for both forested locations, apparently due to both the sensing of the low-x lf A2 horizon just beneath the thin topsoil and lower bulk density near the surface. All currently cultivated soils Žthose other than location 9 on Fig. 5. have relatively invariant profiles of k lf , x lf , k fd and Munsell color characteristics ŽFig. 5., primarily as a result of tillage homogenization. Profiles from locations 3 and 7 are exceptions, showing coherent decrease in x lf , increase in k fd and redder hues below 18 cm. Organic matter contents of Ap horizons vary little among those sample locations analyzed ŽTable 1..
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Fig. 2. The distribution of surface k lf determined using a search loop. k lf is dimensionless but scaled here to SI units. Broad classes of magnetism were used in contouring to enable better visual assessment.
Assuming that the Cs-137 inventories from Soileau et al.’s year-1988 sampling reflect a pattern of erosion which fundamentally coincides with the long-term pattern, and that no major changes have occurred in the ensuing decade, these data can be used to relate recent magnetic measurements to erosion magnitudes. The lowest sampled Cs-137 inventory indicative of severe erosion ŽSoileau et al., 1990. occurs at location 1
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Fig. 3. The distribution of surface x lf . Sample points for these measurements are the same as those for k fd Ž k lf transect points are shown for purposes of referencing soil pit locations..
on steep slopes ŽFig. 1.. Location 1 soil has high k lf , x lf and k fd , high clay and low silt content, and red hue ŽFigs. 2–5; Table 1., properties characteristic of Bt horizons at forested location 9. Location 2 has a Cs-137 inventory greater than that of stable location 9 indicating deposition ŽTable 1.. This soil exhibits lower values of all magnetic parameters than at location 1, relatively low chromas, and higher total silt content ŽFigs. 2–5; Table 1.. Low magnetism and high silt content are consistent with magnetic
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Fig. 4. The distribution of surface k fd Ž k lf transect points are shown for purposes of referencing soil pit locations..
measurements on soil separates which show that clay fractions in these soils have double the x lf Ž298 = 10y8 m3 kgy1 . of silt and sand. With the exceptions of locations 5 and 6, all other sampled profiles have magnetic properties similar to or in between those of locations 1 and 2. Pit locations 5 and 6 ŽFig. 5. exhibit x lf values substantially lower and higher than Žrespectively. those at any other location. For location 5, low magnetism
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Table 1 Particle size, combustible organic matter and Cs-137 dataa Particle size data Sample
Sand Ž%.
Coarse silt Ž%.
Fine silt Ž%.
Total silt Ž%.
Clay Ž%.
Location 1 Ž4–6 cm. Location 2 Ž6–8 cm. Location 6 Ž6–8cm. Location 7 Ž6–8 cm. Location 8 Ž6–8 cm.
24.5 14.5 20.0 21.5 26.3
5.0 5.2 9.2 7.1 6.1
33.2 48.4 39.7 34.6 32.2
38.2 53.6 48.9 41.8 38.3
37.2 31.8 31.1 36.7 35.4
Cs-137 inventory a
Organic matter Sample
Combustible organic matter Ž%.
Location 1 Ž4–6 cm. Location 6 Ž5–7 cm. Location 7 Ž4–6 cm. Location 7 Ž18–20 cm. Location 8 Ž4–6 cm.
a
4.42 3.99 3.98 3.64 3.95
Sample
Activity ŽBq my2 .
Location 1 Location 2 Location 3 Location 4 Location 8 Location 9 Location 10
1351 6025 2807 4709 2256 5284 4435
Cs-137 data are from Soileau et al. Ž1990..
may be explained by parent material differences as suggested by the presence of siltstone fragments from local clastic layers within the limestone. The presence of mudcracks and a fine silt surface at location 6 suggest that this upland location experiences stormwater ponding and deposition in a shallow closed karst depression. Abnormally, high x lf may be attributable to this unusual topographic setting which may have favored retention of original topsoil. 4.3. InterpretiÕe model Using the uneroded profile at location 9 as a basis for examining changes in cultivated soils, a simple tillage homogenization model can be used to predict changes in surface soil magnetism with progressive soil loss ŽFig. 6.. This model was derived using the mathematical procedure 20
x dlf s
Ý xilf
20
when d s 0, otherwise,
Ž 1.
is1 lf lf x dlf s 19 Ž x dy1 . q Ž xdq20 . 20,
Ž 2.
where x dlf is the expected magnetism of the Ap horizon after erosion loss depth d, i is lf depth increment Žcm. below the surface Žregardless of erosion stage., x dy 1 is the lf lf starting average x value for the erosion stage prior to the current one, and x dq 20 is
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Fig. 5. Magnetic and Munsell color profiles from soil pit locations Žabbreviated Aloc.B on the plots. referenced on maps. Vertical axis on the left-hand diagram of each diagram pair is depth Žcm., upper horizontal axis is x lf Ž=10y8 m3 kgy1 ., represented by a solid line, and lower horizontal axis is k fd Ž%., represented by a dashed line. All depth tick intervals are 5 cm. The right-hand diagram of each diagram couplet is the Munsell color profile with horizontal axis giving hue ŽYR: solid line., value Ždashed line., or chroma Ždash–dot line..
the x lf of the 1-cm soil layer immediately below the plow depth Žregardless of erosion stage.. This model assumes complete tillage homogenization to 20-cm depth. Analogous to the preferential erosional loss of fine-particle-associated Cs-137 in soils ŽWalling and He, 1999., the preferential removal of highly magnetic fines Žclays. by erosion from the soil surface would be expected to influence surface magnetism. Particle-size selectivity is not incorporated into the above model, but would presumably result in reduced absolute values of surface magnetism at any stage of erosion, changing the average values but not the general shapes of Fig. 6 curves. Decatur soils typically exhibit approximately a 25% decrease in clay content from the Bt1 to the Ap horizons ŽBowen, 1994.. Given that clays have double the magnetism of coarser particles in these soils, a 25% decrease in clay resulting from selective erosion might produce a decrease in magnetism of about 7%. According to this simple model, erosion would gradually increase both x lf and k fd as progressively greater contributions of subsoil to surficial materials were added through tillage ŽFig. 6.. A similar, although linear relationship between erosion and k fd , is also suggested by the regression plot of k fd vs. Cs-137 ŽFig. 7.. However, the Cs-137 data are limited Žonly five non-depositional locations, two of which are forested., and
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Fig. 6. Tillage homogenization model. The left-hand diagram shows the location 9 profile on which the model Žright-hand diagram. is based. Erosion depth axis gives the expected depth of erosion corresponding to values of x lf Žsolid line. and k fd Ždashed line.. Model assumes 1-cm increments of soil loss and complete homogenization to 20-cm depth during tillage.
Fig. 7. Regression relationship between k fd and Cs-137 inventory at five upland locations. Cesium-137 inventories are from Soileau et al. Ž1990..
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despite the high correlation coefficient, the relationship must be regarded as very tentative at this time. The affinity of eroded location 1 soil properties with B-horizon is consistent with the tillage model which predicts high x lf as a result of increasing B-horizon influence with erosion. According to Fig. 6, highest magnetism is achieved after around 40 cm of soil loss. However, most of the increase occurs by around 20 cm of loss, and perhaps a reasonable estimate for long-term soil loss at location 1 in the highly magnetic steep mid-slope areas lies somewhere in between Žaround 30 cm.. This value does not account for any soil loss prior to afforestation of the currently stable location on which the model is based. A 100-year extrapolation of soil loss at location 1 based on the proportional model of Cs-137 concentration ŽSoileau et al., 1990; Walling and He, 1999. is similar at around 34 cm. Cesium-137 evidence indicates that location 8 is strongly eroded also, but not as much as location 1 ŽTable 1.. This is consistent with soil attributes at location 8 ŽFig. 5., which are like those described for location 1 except for the noteworthy exception of lower magnetism. It is important to note that at no point in the tillage model does x lf attain the maximum value recorded at location 1. Small spatial variations in parent materials represent one potential source of error in the magnetismrsoil loss relationship. Gentle gradient surfaces of the ridgecrests Žfor example, location 3. are characterized by moderate erosion Ž; 5 cm in 34 years. according to Cs-137 evidence ŽSoileau et al., 1990.. The decline in x lf and increase in k fd below 18 cm at location 3 suggest the presence of intact A2 horizon, and the low average magnetism values correspond to low soil loss in the tillage model ŽFig. 6.. If intact A2 horizon is present, long-term Ži.e., 100-year. erosion at this location might be much less than that projected from Cs-137 evidence, perhaps as little as 5 cm. Although the soil loss value is uncertain, the evidence supports the general inference that erosion has not been as severe at this location as at steeper mid-slope locations. The similar magnetic profile at location 7, a gentle upland flat, is likely to fit within this erosion category as well. Sediments in storage within the bottomland depositional zone Žroughly, elevations less than 165 m; Fig. 1., have the magnetic properties of the eroding soils upslope, minus the very fine fraction lost as sediment yield at the catchment outlet ŽSoileau et al., 1990., and minus the coarse fraction lagging behind on slopes which is concentrated in the upper soil particularly on steep gradients. Deposits at location 2 are silt-rich compared to residual materials on upland surfaces ŽTable 1.. Because magnetic minerals are concentrated in the clay fraction, magnetism would be expected to be lower in the bottomland where clays have been preferentially lost to sediment yield. The slight increases in all magnetic parameters toward the catchment outlet ŽFigs. 2, 3 and 4., may result from constriction of outflow, downstream ponding, and consequent increased deposition of fines as evidenced by mudcracks and fine silt coatings in this bowl-shaped bottomland ŽFig. 1.. Soils from the isolated cells of high magnetism in the eastern portion of the catchment have a mixture of erosional and depositional properties. Magnetism at location 6 is high like that at eroded location 1 ŽFig. 5., but clay content is low and silt dominates as at depositional location 2 ŽTable 1.. As noted earlier, field observations reveal that these locations pond stormwater, and apparently represent partially infilled
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closed karst depressions. High magnetism is characteristic of topsoil at forested location 9 ŽFig. 5. and it is possible that similar material retained in the depressions has been mixed upwards by tillage into overlying sediment. A buried A-horizon 10 m upslope from location 2 Žsite A80, 40-cm depth; Fig. 5. has higher x lf than most other surrounding soils, but is still lower in magnetism than eroded location 1, and much lower than that at location 6. However, additions of coarse silt during burial, and potential degradation of the A-horizon prior to burial may have reduced its magnetism. 4.4. Sediment deliÕery Using magnetism maps, the tillage homogenization model, published Cs-137 inventories and deep soil morphology data, it is possible to assess the particulate sediment budget and calculate a value for long-term sediment delivery. Grid math in Surfer was used to distribute soil loss depth using x lf and k fd as indices of soil loss based on the tillage homogenization model concept. However, a linear relationship rather than a curvilinear one as depicted in Fig. 6, was used for computational simplicity. The indexing accounts for 30 cm of soil loss at location 1 Žderived from the model as discussed earlier. and either 5 or 15 cm of soil loss at location 3, depending on which of the bases for estimation given in Section 4.3 ŽCs-137 or profile morphology. is used. Data points for the upland sinks on the east side of the catchment were excluded from gridding to remove their effects on surrounding magnetic contours. An important assumption of this assignment procedure is that the soil profile at location 9 used as the basis for the tillage model is representative of soils throughout the eroding uplands. Substantial variations in horizon thickness would change the shape of the curves in Fig. 6, thus changing the soil loss estimates. A second approach in which erosion was estimated using the regression relationship between k fd and Cs-137 inventory ŽFig. 7. was also tested. Values of Cs-137 inventory, spatially distributed according to k fd values, were used to calculate soil loss in depth units using the simple proportional model: wŽ B y C .rB x = Depth, where B is stable site Cs-137 inventory, C is Cs-137 inventory predicted by the regression relationship, and Depth is 15 cm, the depth of upland soil containing Cs-137 ŽSoileau et al., 1990; Walling and He, 1999.. Although the proportional model accounts for neither the Cs-137 washed away between tillage events, nor the dilution of Cs-137 concentration by tillage incorporation of Cs-137-deficient subsoil as erosion proceeds ŽWalling and He, 1999., it is used here to permit direct comparison to the results of Soileau et al. Ž1990.. All deposition areas were blanked prior to soil loss volume integration. For both approaches to erosion estimation, volumes of soil loss were converted to soil masses by assuming an average bulk density of 1.45 g cmy3 , derived from soil survey data ŽBowen, 1994. and data from Soileau et al. Ž1990.. Depositional areas spatially defined by magnetism, field observations of ponding and the observations of Soileau et al. Ž1990. were digitized and assigned estimates for soil deposition thickness based on soil survey morphological data ŽSoileau et al., 1990; Bowen, 1994. and field observations. For the bottomland, soil surveys estimate maximum depths to buried A-horizon of 90 cm in topographically similar but larger areas of
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Emory soil. Soileau et al. Ž1990. report what may be a more representative depth to Ab of 46 cm near the 164.6-m mark on the topographic map. Based on these end-member depths, two parameter sets were used for calculation of deposition volumes. The first set assumes point values of 0-cm depth at the digitized periphery, 46 cm at the 164.6-m contour, and a maximum of 90 cm in the downstream and center portion of the catchment. Set 2 assumes a maximum Ab-horizon burial depth of only 45 cm. A similar
Fig. 8. Upland soil loss distributed according to k fd . Blanked areas Ždenoting net deposition. are white.
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procedure was conducted for the upland sinks, except that values for deposition were arbitrarily set at half those of the bottomland due to the smaller size of the depressions. All depositional volumes were integrated in Surfer, and multiplied by bulk density Ž1.5 g cmy3 ; Bowen, 1994.; the resulting masses were used to calculate long-term sediment delivery, assuming that total erosion minus total deposition equals total sediment yield. The patterns of soil loss for each index used Ž x lf and k fd . are the same as the patterns of the magnetic measurements themselves. Fig. 8 depicts long-term soil loss estimated using k fd and the tillage homogenization model assuming 15 cm of soil loss at location 3 Ži.e., the Cs-137 extrapolation result.; blanked areas represent zones of net deposition. Soil loss rates based on each of the three estimation methods, and assuming 100 years of erosion Ža minimum but perhaps approximate age estimate given by Soileau et al., 1990. range from 16,546 kg hay1 yeary1 assuming minimal Ž5 cm. erosion near ridgecrests to 28,550 kg hay1 yeary1 using the k fd rCs-137 relationship ŽTable 2.. The latter value is similar to the 30,000-kg hay1 yeary1 value for the period 1954–1988 given by Soileau et al. Ž1990. based also on Cs-137. Values for total storage range from 5963 to 8465 Mg depending on choice of maximum depth to Ab. Accepting an intermediate value for sediment storage of 7214 Mg gives a range for sediment delivery ratio of 9.1–33.5% assuming 15 cm of soil loss near ridgecrests. The lower end of this range is near the 10% value calculated by Soileau et al. Ž1990. using 5 years of suspended sediment monitoring. Sediment delivery is much lower for the minimal ridgecrest soil loss scenarios and negative for assumptions of 90-cm depth of bottomland Ab burial, indicating substantial error in one or both of the associated erosion and deposition values.
Table 2 Sediment delivery data based on soil magnetism Model ID a
Total upland erosion ŽMg.
Soil loss rate Žkg hay1 yeary1 .
Total storage ŽMg.
Sediment delivery Ž%.
x lf 5r45r23 5r90r45 15r45r23 15r90r45
6287 6287 8627 8627
16,546 16,546 22,703 22,703
5963 8465 5963 8465
5.1 y34.6 30.9 1.9
k fd 5r45r23 5r90r45 15r45r23 15r90r45
6376 6376 7937 7937
16,780 16,780 20,887 20,887
5963 8465 5963 8465
6.5 y32.7 24.9 y6.6
10,849 10,849
28,550 28,550
8465 5963
22.0 45.0
k fdrCs-137 90r45 45r23
a Parameter used to distribute soil loss depth; assumptions: soil loss Žcm. assumed at location 3 Žsee text.rmaximum soil depositional thickness assumed in bottomlandrmaximum depositional thickness assumed in upland sinks.
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The magnetism-based erosion estimates for the last 100 years, which are lower than the value determined by Soileau et al. Ž1990. for the post-1954 period, imply that the average soil loss rate prior to 1954 would have had to be lower still, as low as 5085 kg hay1 yeary1 for the x lf 5r45r23 estimate. This result is contrary to the general belief that past erosion rates were higher than today’s in the southeastern US ŽTrimble, 1985.. The apparently higher forest cover along the eastern perimeter in 1937 noted on aerial photographs would account for some of the difference. However, the size of the difference suggests problems with the models as well. For example, if the original profile thickness was greater on ridgecrests than at location 9, the minimal 5-cm estimate of ridgecrest soil loss based on the magnetic profile at location 3 would be low. Likewise, the proportional model relating Cs-137 inventory and erosion potentially overestimates soil loss because it does not account for loss of surface Cs-137 to erosion prior to tillage mixing ŽWalling and He, 1999.. Alternatively, this model may underestimate soil loss because it does not account for progressive dilution of Cs-137 concentration which results from incorporation of Cs-137-deficient soil below the plow depth as erosion proceeds ŽWalling and He, 1999.. Such potential errors reduce the comparability of Cs-137-based rates with all independent estimates of soil loss.
5. Discussion and conclusions Spatial patterns of soil magnetism derived from rapid search loop scanning and surface sampling are related to soil redistribution patterns at Gilbert Farm. Patterns of magnetism are controlled by the magnetic characteristics of soil horizons exposed on or near the land surface, and the particle size characteristics of surficial materials. Without particle-size data and other supporting physical information such as profile morphology and, in this study, Cs-137 data, patterns of magnetism are difficult to interpret. The best example of this is the fact that magnetic lows are characteristic of both the high level ridgecrest areas Žweak erosion. and lowest areas Ždepositional zone. in the catchment for different reasons. Once a basis for interpretation has been established, the mapping of eroded and depositional areas might be conducted rapidly and at a high level of spatial detail using surface magnetism. Values of the three magnetic parameters measured in this study were found to have broadly similar spatial distributions. However, these were not identical, hence a choice is required regarding which best serves as an erosion index. On the eroded soils of this cultivated catchment, x lf , which clearly distinguishes between the A2 and Bt1 horizons at forested location 9, is conceptually easiest to interpret. Under less eroded conditions, k fd , which shows greatest change between topsoil and the A2 horizon of location 9, may be the best erosion indicator. Both x lf and k fd require more sampling and lab analysis labor than k lf Žmeasured with the search loop., although less so than that required for determining Cs-137 inventories or describing profile morphology at the same number of points. Although k lf measurements exhibit patterns similar to those of the other magnetic measurements and are obtainable in a fraction of the time, bulk density variation is unaccounted for. For all measurements, time savings would be greater if mapping technologies like GPS were used to locate points on an existing
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digital georeferenced site map as sampling was conducted. The additional development of a rapid scanning search loop sensor for k fd would allow even quicker assessments. A major drawback of using magnetism to define soil redistribution is the difficulty of relating magnetic values from surface measurements to specific values for depth of soil loss or deposition. A relationship between depths of soil loss and surface magnetism has been predicted for eroded areas at Gilbert Farm using a simple tillage homogenization model based on profile characteristics at an uneroded site. Modifications to account for particle-size selectivity of erosion, and other complexities such as systematic changes in original profile thickness over space would improve the model. Potential obstacles to its implementation include non-systematic Žor unrecognizable. spatial changes in the soil environment and uncertainties in land use history, such as the timing of plow technology changes in an area which would affect homogenization depth. In bottomland areas where the original surface soil may be deeply buried, upward tillage mixing would not be expected to substantially affect surface magnetism. Whatever small effect there might be would likely be obscured by particle-size influences. The inability to calibrate susceptibility measurements with regard to deposition thickness necessitates the estimation of deposition using profile morphology or perhaps Cs-137. These methods give accurate information, but the cost in time and labor, and therefore often spatial resolution, is high. The applicability of this technique in other environments depends on geological specifics. Dearing et al. Ž1985. note that studies of hillslope erosion based on profile magnetic differentiation should be easiest to implement in areas underlain by sedimentary rock, and particularly those on limestone, a rock type typically low in primary magnetic mineral content. The results from earlier published studies, the Gilbert Farm site, and an area of metamorphic terrain currently under investigation Žby this author. support the contention that the specifics of environment at a given locale require assessment on an individual site basis for purposes of linking magnetism and erosion. Metamorphic terrain may represent a particularly difficult case, due to the possibility of changes in parent material over short distances. However, presumably smaller changes still might influence magnetic patterns even in such geologically homogeneous environments as Gilbert Farm. The use of magnetic measurements may best be pursued in conjunction with Cs-137 studies, with the aim of reducing the number of Cs-137 samples required for adequate spatial coverage. Resolving the relationship between magnetism and soil erosion in different environments might be facilitated by taking advantage of the numerous sites for which Cs-137 inventories already exist as a result of prior soil erosion research. Although the use of magnetism as a suspended sediment tracer in catchments is well-established ŽWalling et al., 1979; Walden et al., 1997., the interpretation of hillslope magnetism with regard to soil redistribution remains relatively unstudied. The ability to quickly supply a more spatially distributed perspective on soil redistribution would be beneficial for assessing soil erosion models. In addition, the ability afforded by magnetism to study sediment delivery at the hillslope scale might facilitate the study of scale-dependence of delivery. Finally, an historical perspective may be gained by the sort of analysis conducted herein. Sediment budgeting based on magnetism at this site revealed potential differences in erosion and sediment delivery rates between the first
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and second halves of the 20th century. However, there are several potential sources of error which might contribute to these differences. Refining soil magnetism models to account for these sources represents a logical goal for future research.
Acknowledgements I am grateful to John Soileau Žretired, Tennessee Valley Authority. for allowing a revisit of the site and use of Cs-137 data. I also thank Forrest Wright ŽExecutive Director of the Shoals Economic Development Authority. for providing access to the site, information on its history, and other accommodations which have been important to the research effort. I also wish to acknowledge the helpful comments of two anonymous reviewers. This project was funded by a Research Advisory Committee Grant ŽNo. 2-67705. from the University of Alabama to Dan Royall.
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