Soil Science Society of America Journal 67:365-372 (2003)
© 2003 Soil Science Society of America
DIVISION S-10—WETLAND SOILS
Hillslope Hydrology and Soil Morphology for a Wetland Basin in South-Central Minnesota
Ron J. Reuter*,a and
Jay C. Bellb
a Dep. of Rangeland Resources and Wildland Soils, Humboldt State Univ., 1 Harpst St., Arcata, CA 95521
b Dep. of Soil, Water, and Climate, Univ. of Minnesota, 439 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108
* Corresponding author (rjr11{at}humboldt.edu)
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ABSTRACT
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Knowledge of soil and landscape hydrology is essential to evaluating the processes and quality of terrestrial ecosystems. Of particular interest in Southern Minnesota is the extensive Clarion–Nicollet–Webster catena, for which little quantitative work has been done in the region. The objective of this study was to investigate the relationship between soil hydrology and morphology for an undrained catena in south-central Minnesota. Seven points along a 125-m summit-to-wetland transect were instrumented with piezometers (25, 50, 100, and 200 cm), observation wells (300 cm), Pt electrodes (25, 50, 100 cm, footslope only), and thermocouples (10, 25, 50, 100 cm). Mollic epipedons were 77-cm thick at the footslope (Cumulic Endoaquoll), thinning to 39 cm at the summit (Oxyaquic Hapludoll). The lower hillslope positions had low chroma colors below the mollic horizon, indicative of reducing conditions. The water table frequently was located in the mollic horizon for these soils and redox potentials at the footslope were frequently <200 mV. Fe content decreased from summit to footslope, indicating the reduction and removal of Fe in the lower landscape positions. With the exception of the wetland, soils had equal piezometric head with depth, indicating lateral throughflow. The wetland had alternating recharge and discharge hydrology during the study. Profile Darkness Index (PDI) had strong correlation with the duration of saturation (r = 0.88, 
= 0.05). Thickness and color of surface horizons in this landscape are strong indicators of landscape hydrology, especially when redoximorphic features associated with normal water table levels are masked by thick mollic epipedons.
Abbreviations: AERF, Agricultural Ecology Research Farm • ET, evapotranspiration • MAP, mean annual precipitation • PDI, Profile Darkness Index • PSD, particle-size distributions
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INTRODUCTION
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SEVERAL ISSUES in southern Minnesota demand a quantitative understanding of the role of soils in the regional ecosystem. Since European settlement, southern Minnesota has lost >70% of its original wetlands, mostly due to agricultural drainage (Dahl, 1990). Preservation and mitigation of remaining wetlands as well as wetland restoration requires knowledge of soil hydrology and its influence on the function and structure of wetlands. A second issue is effectiveness of site-specific management on agricultural land. A knowledge gap in the application of precision agriculture techniques has been the incorporation of landscape hydrology (Pierce et al., 1995).
An extensive soil catena of the glacial till plains of Minnesota and Iowa is Clarion–Nicollet–Webster (fine-loamy, mixed, superactive, mesic Typic Hapludolls; fine-loamy, mixed, superactive, mesic Aquic Hapludolls; and fine-loamy, mixed, superactive, mesic Typic Endoaquolls, respectively). Within the Minnesota River basin (3.9 x 106 ha), 
1.5 x 106 ha are comprised of Clarion, Nicollet, Webster, and similar soils. These soils are some of the most productive soils in the Upper Midwest and have been well studied in Iowa (James and Fenton, 1993; Khan and Fenton, 1994; Steinwand and Fenton, 1995). The studies in Iowa are spatially separated from the Minnesota River Basin by hundreds of kilometers and, for questions specific to soil hydrology this region of Minnesota, such as recharge/discharge dynamics, the characteristics of the soils should be investigated locally. From the standpoint of landscape hydrology and soil morphology, very little quantitative work has been done on this catena in Minnesota, especially in an undrained landscape. Our objectives for this study were to (i) describe the current hydrology and soils of an undrained hillslope in south-central Minnesota and (ii) investigate relationships between observed hydrology and soil morphology.
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MATERIALS AND METHODS
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Site Description
The study site was located on the University of Minnesota Southern Outreach and Education Center-Agricultural Ecology Research Farm (AERF) in Waseca (44°03'40'' N lat., 93°32'15'' W long.) (Fig. 1)
. Parent material for the soils is glacial till of the Des Moines Lobe. The Des Moines Lobe, which was active at the end of the Wisconsin glaciation (12 000 years before present), extended from west-central Minnesota to central Iowa, more than 480 km (Wright, 1972). The Minnesota River, which dissects the lobe in Minnesota, receives the majority of its flow from tributaries located in materials deposited by the Des Moines Lobe. The terrain is typical of sublimation till with low-relief hills and closed-drainage basins. Most of the area is in agricultural production, typically in a rotation of corn and soybean, and has been artificially drained. The AERF site was somewhat unique to the region in that it had limited artificial drainage. As a result, several of the wetter closed depressions had minimal tillage. The hydrology observed throughout the study may closely represent historical conditions, thereby serving as benchmark wetlands for determining predrainage hydrology.
The soils of the Des Moines lobe typically have the sequence of ablation till overlying dense and compacted basal till (Bettis et al., 1996). In northern Iowa, Bettis et al. (1996) reported an average bulk density of 1.62 Mg m-3 for the ablation till and 1.89 Mg m-3 for the basal till. The disparity in bulk densities influences the vertical and lateral transmission of water in Iowa landscapes (Steinwand and Fenton, 1995).
Thirty-year mean annual precipitation (MAP) at the Outreach Center, 1 km from the AERF site, is 82 cm (Minnesota Climatology Working Group, 1999). Mean July air temperature is 22°C and mean January temperature is -8°C.
Field Methods
During the summer of 1993, as part of the Wet Soils Monitoring Project (Lynn et al., 1996), seven monitoring stations were installed along a 125-m by 5-m hillslope transect (Fig. 2)
. The soil survey for the AERF site indicates that the transect extended through Nicollet and Webster soil map units. Stations are referred to by hillslope position as defined by Ruhe (1975) and are toeslope, footslope, backslope, shoulder, and summit (Fig. 2). The south-facing transect extended from summit to a depressional wetland at the toeslope and has an average slope of 2%. The 5-m-wide transect was grass covered and the area surrounding the transect, with the exception of the wetland basin, was cultivated in a corn–soybean rotation. The wetland basin, which includes the footslope and toeslope stations, covers 0.25 ha and contains sedges (Carex spp.), cattails (Typha latifolia L.), and reed canary grass (Phalaris arundinacea L.). The fields surrounding the 31-ha AERF site are artificially drained.
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Fig. 2. Profile of the hillslope at the Agricultural Ecology Research Farm site showing seasonal mean water levels during the study period and locations of monitoring stations (TS = toeslope; FS = footslope; BSL = lower backslope; BSU = upper backslope; SSL = lower shoulder slope; SSU = upper shoulder slope; SUM = summit).
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Each monitoring station was equipped with (i) an observation well at 300 cm, (ii) nested piezometers at 25, 50, 100, and 200 cm, and (iii) paired thermocouples at 10, 25, 50, and 100 cm, following the setup protocol described by Thompson and Bell (1996) (Table 1). At the footslope station, redox potentials were measured using in-situ Pt electrodes (2 replicates: 25, 50, and 100 cm) and a Ag-AgCl reference electrode. Readings were corrected to the standard hydrogen half-cell by adding 199 mV to the field measurement (Patrick et al., 1996). Since soil pH was 7, no adjustment for pH was made. Monitoring of the site was on a weekly to biweekly basis with more intense sampling during spring and fall to capture the effects of snow melt and plant senescence in the fall. For this study period, there were 117 sampling days across 4 yr.
Concurrent with installation of monitoring equipment, soils were described and sampled. With the exception of the toeslope position, which was inundated, soils at each of the respective hillslope positions were described using standard National Cooperative Soil Survey procedures (Soil Survey Division Staff, 1993). The physical and chemical properties of samples from described horizons were determined at the National Soil Survey Laboratory. Measured parameters include particle-size distribution (pipette method), organic carbon (acid dichromate digestion), and pH (1:1 water dilution) (Soil Survey Laboratory Staff, 1996). Fe and Al were extracted from soil samples using ammonium oxalate (Alo, Feo) and citrate-dithionate (Ald, Fed) extraction techniques (Soil Survey Laboratory Staff, 1996).
The thickness and color of surface horizons with respect to soil hydrology was evaluated using the Profile Darkness Index (PDI) of Thompson and Bell (1996). The PDI is calculated by
where Vi is the Munsell color value and Ci is the Munsell color chroma for the specified A horizon.
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RESULTS AND DISCUSSION
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Climate
When evaluating the distribution of water in the landscape and its influence on pedogenesis, it is important to consider seasonal variations in precipitation. Using the soil temperature (T) brackets of <5°C (winter), 5°C < T <10°C (fall and spring), and >10°C (summer), we defined the seasons as follows: (i) 1 December to 1 March (winter), (ii) 1 April to 31 May (spring), (iii) 1 June to 30 September (summer), and (iv) 1 October to 30 November (fall).
In 1995, 1996, and 1998, annual precipitation exceeded the MAP (82 cm) by 10% while 1997 was within 2% of MAP (Fig. 3)
. Summer precipitation makes up 40% of the MAP, spring and fall account for 25% each, and winter contributes 10%. Compared with the MAP, the springs of 1996 and 1997 were dry with wet summers while 1998 had a wet spring and a drier than normal summer.
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Fig. 3. Annual and seasonal distribution of precipitation for the Waseca Agricultural Ecology Research Farm site, including the 30-yr mean.
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In the Upper Midwest, high-energy, short-duration convective storms during spring and summer contribute large amounts of precipitation, resulting in high monthly accumulations. Such events explain the high summer values in 1997, when multiple storms occurred across a short time period between July and August.
The average maximum soil temperature at 50 cm was 20°C and minimum temperature was 0°C. Maximum and minimum temperatures of the summit and toeslope soils differed by 3°C, with the summit soil experiencing the highest and lowest temperatures. All soils on the transect had a mesic temperature regime.
Soils
Taxonomic classification (Table 2) and characterization data (Table 3) for soils attest to the high accumulation of organic matter in these soils. The most striking feature of the soils in this landscape is the thick mollic epipedon. The footslope soil has 77 cm of mollic horizon and the lower backslope and upper backslope soils have 40 and 70 cm, respectively. The three remaining soils have between 40 and 60 cm of mollic epipedon. Most of the thickness of these horizons is black (N 2/0 or 10YR 2/1) from the toeslope to the summit (Table 3) and Fe-related redoximorphic features are absent or difficult to identify.
Matsch (1972) reports clay loam textures for the Bemis Morraine and New Ulm Till of the Des Moines Lobe. The Waseca area straddles these two geomorphic features. Particle-size distributions (PSD) for soils of the AERF site are generally clay loams with little variation among the sampled horizons (Table 3, Fig. 4)
. Steinwand and Fenton (1995) reported a greater variation of PSD in the Clarion–Nicollet–Webster catena in Iowa, which indicated multiple deposition layers within the profile and across the landscape. The more homogeneous soil materials of the AERF site indicate that redistribution of materials has occurred to a lesser extent along the transect; however, examination of soils across the AERF site and surrounding agricultural land does indicate that erosive forces are redistributing materials more extensively elsewhere in the landscape. Along the transect, some horizons do deviate more from the average PSD. In the footslope and lower backslope soils, the upper horizons have between 13 and 10% lower sand content than the remaining samples. The increase in fines is likely due to accumulation of slopewash in these lower hillslope positions. The 2Bg (138–185 cm) horizon of the summit soil has a texture of silty clay with lower sand content compared with the overlying and lower horizons. Origin of the fine-textured horizon is likely related to slack-water deposition during the wasting of the Des Moines Lobe. This horizon appears to be hydraulically distinct from its bounding neighbors, remaining saturated longer, and this may explain the development of gley colors.
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Fig. 4. Particle-size distribution of the 40 sampled horizons at the Agricultural Ecology Research Farm site.
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Soil colors below the dark surface horizons of the toeslope through upper backslope are indicative of saturated, anaerobic conditions (Table 3). Most horizons have gley colors (Soil Survey Staff, 1993) indicating reduced or removed Fe. From the lower shoulder station to the summit, there is a layer of oxidized soil between the dark surface and reduced horizons. The Feo and Fed extractions show an increase of Fe with increasing distance from the wetland, indicating that the wetter soils have likely had more reduction and removal of Fe (Table 3).
Hydrology
Our discussion of hydrology involves interpretations from piezometers and wells and a brief review is appropriate. Piezometers are open only at the lower portion of the riser; therefore the water level in the piezometer is a measure of the piezometric head, or gravitational pressure of the overlying water column, at the specified depth. Changes in piezometric head with depth can be used to determine flow direction (Daniels and Hammer, 1992). Decreasing head with depth indicates recharge conditions (downward flow), increasing head with depth indicates discharge hydrology (upward flow), and equal head with depth indicates flowthrough or static conditions.
Wells are open to water entry from the base of the well up to near the soil surface. The water level in a well represents the mixed-potential reading of all layers that the well extends through. In layered soils, such as the till plains of the Midwest, well data may not accurately convey the dynamics of the water table. In these situations, a series of piezometers at different depths provide greater detail and capture the intricacies of water flow. Readings from the 300-cm well represent the mixed potential of all depths, including contributions from depths >200 cm, below the deepest piezometer. When water level in the well exceeds all piezometer readings, we conclude that deeper water table (>200 cm) has a higher piezometric head. Well readings lower than all piezometer levels indicate that the deeper water table has a lower piezometric head. A well reading that falls within the range of observed piezometric values cannot be interpreted in terms of contributions from deeper water.
Piezometric data along the hillslope, with the exception of the toeslope, are nearly identical at all depths for the individual stations. This suggests that flowthrough is prevalent in the upper parts of these soils. Because of the equal pressure gradients with depth, we will discuss hydrology with respect to the 200-cm piezometers, since this depth had the most frequent observations of saturation.
On the basis of the average seasonal readings, the water table is closest to the surface during the spring due to contributions from snowmelt and stored surface water infiltrating into thawing soils (Fig. 2). Except for a few dry periods during the study, ponded water is usually present at the toeslope position. Summer draw-downs due to evapotranspiration (ET), combined with irregular precipitation, result in a lowered water table. We anticipated an increase in water table height in the fall; however, the mean suggests the low levels achieved in summer persist through winter.
Excluding the toeslope, which is the focal point of the watershed, the summer through winter slope of the water table is 1% while the spring slope is 0.8%. The hydraulic gradient that results from this slope suggests there is potential for lateral transmission of water in the direction of the hydraulic gradient. The equal piezometric head with depth is indicative of lateral flowthrough.
Our analysis of the data indicates that several stations have similar traits and general hillslope trends can be discussed using a selection of stations, namely the toeslope, footslope, upper backslope, and summit. Our discussion of individual station hydrology begins in the wetland depression (toeslope station). The first year of complete monitoring at this station was 1996. The first and perhaps most important observation is that the hydrology of this wetland is variable on a temporal scale (Fig. 5) . All 3 yr of data suggest that piezometric head is relatively equal at all depths in the soil from winter through May/June, with the wetland receiving contributions of both near-surface water and deeper groundwater (>200 cm). With the onset of summer, ET begins to affect the balance of the groundwater and near-surface supply of water to the wetland, but this relationship varies between each of the years. In 1996, with the piezometric surface at the 100-cm depth being greater than at the 200-cm depth, the wetland appears to have recharge hydrology throughout the summer (Fig. 5a). Higher water levels in the 300-cm well during early summer of 1997 suggest an influence from deeper water sources or discharge hydrology while the upper piezometers have equal pressure gradients (Fig. 5b). The 1998 observations indicate a year where contributions from water sources at all depths are relatively equal (Fig. 5c) and neither discharge nor recharge dominates.
The annual pattern of water table fluctuations at the footslope station was similar to the toeslope during the study period. However, flowthrough conditions dominate the near-surface hydrology of the footslope (Fig. 6a)
. Water level in the 300-cm well is consistently 20 to 30 cm higher than the nested piezometers, indicating higher pressure at depths >200 cm. We speculate that between 200 and 300 cm there is a change in the hydraulic continuity of the soil, resulting in an increase in piezometric pressure that is reflected in the 300-cm well. The most likely explanation is the presence of a buried sand lens, or sand stringer, within the till, although no descriptive record was kept for the 300-cm well boring to substantiate this theory. The change in texture would restrict water flow between the till and sand layers and allow development of a higher piezometric head. The latter explanation has credence because (i) research in the Waseca area and the research of others on the Des Moines Lobe (Bettis et al., 1996; Wheeler, 1999) has discovered numerous and random sand lenses across the regional landscape that are associated with the meltwater environment of wasting glaciers, and (ii) only the footslope station exhibits this condition.
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Fig. 6. Representative piezometer readings (1997) at 25, 50, 100, and 200 cm and 300-cm well readings for the (a) footslope, (b) upper backslope, and (c) summit stations.
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The remaining hillslope positions (lower backslope–summit) have similar patterns of hydrology with respect to occurrence of peaks and depressions of the water table, although depth to the water table increases moving upslope (Fig. 6b,c). At each station, piezometric head between 25 and 200 cm is nearly equal, indicating flowthrough conditions along the length of the hillslope. The 300-cm well observations are the lowest for all depths, suggesting a discontinuity between the near-surface water table and deeper groundwater. The discontinuity is likely due to the contact between the ablation till and basal till observed by Bettis et al. (1996), which sets up the condition of a perched water table over the basal till. The slope of the contact between the two would dictate the gradient of the water table.
While we did not quantify surface runoff, we have observed that significant amounts of water derived from snowmelt are redistributed to depressional areas due to surface runoff over frozen and saturated soils. The current landscape, which is typically void of vegetation during the winter, is very different from the original prairie. How this has altered the landscape hydrology is unknown. Additionally, agricultural fields have little residue or standing biomass during the winter to trap blowing snow while snow accumulates to great depths in the wet depressional areas that have not been tilled or mowed.
A further note about the hydrology of this landscape—Calculations of average depth to water, duration of saturation, and other factors that are evaluated by both the soil survey and soil and water districts in the area rely on well readings. Comparison between well and piezometer data indicates that using well data can result in instances of over- and underestimation of the true location of the water table. The use of well data has the potential to be erroneous and misleading, potentially affecting the quality and effectiveness of management plans.
Soil Hydromorphology
At the AERF site, the thickness of the mollic horizon generally decreases from toeslope (77 cm) to summit (40 cm) (Table 3). Colors for these epipedons were generally black (chroma 
2, value 1). During the study, the mean location of the water table was within this black layer for the footslope soil, and in the two backslope soils the mean water table was very near the lower limit of the black layer. The area immediately below the black layer of the footslope and backslope soils is dominated by low chroma, high value colors, suggesting that the mean water table in these soils lies within the black layer where redoximorphic colors related to Fe are not visible.
The black surface horizons in this Mollisol landscape limit our ability to utilize standard redoximorphic features (Vepraskas, 1992) to evaluate the relationship between saturation and soil morphology. This is a recognized problem in evaluating hydric soils and hydrology in Mollisol landscapes (Bell and Richardson, 1997). Anderson (1987) demonstrated that organic matter, the primary pigmenting agent in Mollisols, is related to soil moisture content and soil temperature. Cool, moist conditions, common in the Upper Midwest, decrease the decomposition rates of organic matter, resulting in thick dark horizons. The PDI of Thompson and Bell (1996) was designed to address this issue in Mollisol landscapes. Thompson and Bell (1996) reported a significant correlation between PDI and duration of saturation at 50 cm (r = 0.69, = 0.05) and between PDI and organic carbon content (r = 0.98, = 0.001).
We calculated PDI for the described profiles at the study site (Table 4). Duration of saturation at 50 cm (determined from piezometer data) and PDI were significantly correlated (r = 0.88, = 0.02) (Fig. 7)
. Interestingly, for this site, the thickness of the black soil is better correlated with duration of saturation than PDI (r = 0.90, = 0.01). The soils evaluated by Thompson and Bell had A horizons with higher chromas and values as opposed to the black A horizons at Waseca. In either case, the results support the use of the color and thickness of the A horizon for interpreting soil hydrology in this landscape.
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Table 4. Mean water level, depth of black (value 2, chroma 1), profile darkness index (PDI), and percentage of saturation readings at 50 cm (based on 50-cm piezometer data).
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Fig. 7. Plot of actual values and the linear model for Profile Darkness Index (PDI) vs. percentage of observations with saturation (at 50 cm, based on piezometer data).
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Redox potential can also be used to evaluate the microbial activity responsible for hydromorphic feature development across a hillslope. Only the footslope station at the wetland edge was equipped with Pt electrodes. During periods of extended saturation, Eh was <400 mV, indicative of anaerobic conditions (Fig. 8)
. On several occasions, measured Eh was in the range of Fe reduction (<200 mV) (McBride, 1994). Additionally, there is a distinct increase in Eh in response to a long unsaturated period beginning July 1997. Redox potential data from all hillslope positions would help verify the relationships between soil hydromorphology and hydrology for this landscape.
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Fig. 8. Soil redox potential (Eh) values for the footslope station with saturation (measured by piezometer, shaded region) at 50-cm depth.
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CONCLUSIONS
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The soils in this landscape have similar textural classes and, with the exception of the toeslope, have similar hydrologic patterns. It is likely that the geomorphic arrangement of less dense ablation till over dense basal till has set up a hydraulically restrictive layer, resulting in a perched near-surface water table. Equal piezometric head with depth suggests a flowthrough environment with potential for lateral movement of soil water towards the wetland.
We note here that our observations included wetter than normal years and the patterns of hydrology may be significantly different in drier years. Depending on the quantity and temporal distribution of precipitation, overland flow, and subsurface flow, the wetland fluctuates between recharge and discharge hydrology.
While redoximorphic features are not visible in the over-thickened dark horizons, especially in the lower landscape positions where the mean water level is located within black soil, the color and thickness of the organic matter rich horizons can be used to evaluate soil hydrology. We saw strong correlation between PDI and duration of saturation. The index is effective in evaluating the hillslope hydrology and has the potential to serve as an alternative to the standard hydromorphic indicators. Verification of these relationships elsewhere in the region may allow use of PDI to evaluate the near-surface hydrology.
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REFERENCES
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TOP
ABSTRACT
INTRODUCTION
