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a Dep. of Natural Resources Science, 210B Woodward Hall, Univ. of Rhode Island, Kingston, RI 02881
b Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545
* Corresponding author (agold{at}uri.edu)
Received for publication December 18, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: PD, poorly drained soil • SSURGO, Soil Survey Geographic database • STATSGO, State Soil Geographic database • USGS, U.S. Geological Survey • VPD, very poorly drained soil
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Riparian zones can serve as critical control points for nitrogen flux within a watershed (Hill, 1996; Gilliam et al., 1997; Hedin et al., 1998). In many watersheds, ground water recharge from upland areas moves through riparian zones before discharging as baseflow to streams (Correll, 1997; Winter et al., 1998). Dramatic changes in riparian zone ground water quality have been observed over short distances, particularly if the water table within the riparian zone approaches the surface and encounters anaerobic soils with high accumulations of organic carbon. There is great interest in incorporating riparian zone N processing in watershed-scale models of N dynamics. Riparian zones display great variability in their water table dynamics and patterns of subsurface carbon and nitrogen removal capacity (Simmons et al., 1992; Nelson et al., 1995; Correll, 1997; Lowrance, 1998). Watershed-scale models of nitrogen dynamics may benefit from the inclusion of data on the extent and variation of characteristics that affect nitrogen retention by riparian zones. As with all proposed enhancements to watershed modeling, this endeavor requires an assessment of the accuracy of the spatial data available to depict key properties (Wagenet and Hutson, 1996; Whittemore and Beebe, 2000). Currently, a number of watershed scale models use the State Soil Geographic Database (STATSGO) as a source of input data for total maximum daily load (TMDL) assessments (USEPA, 2000). STATSGO has a scale of 1:250000 with minimum line widths that equate with field widths in excess of 60 m and minimum mappable polygons (i.e., the minimum size of a soil unit that can be delineated on a map) greater than 600 ha (Soil Survey Staff, 1997). This scale is quite coarse relative to the size of many riparian zone features.
There is great interest in developing relationships between riparian nitrogen removal capacity and visually obvious (i.e., mappable) field attributes. Pinay et al. (1995) suggest that geomorphic features can provide insight into riparian zone denitrification capacity. Lowrance et al. (1997) used STATSGO to classify riparian nutrient retention functions in different geomorphic regions of the Chesapeake Basin and the Delmarva Peninsula. In that area, certain geomorphic regions are dominated by level riparian zones with high water tables, conditions that have been found to generate substantial ground water nitrate removal (Correll, 1997). In previous work, we have found substantial ground water nitrate removal in riparian zones with hydric soils (Simmons et al., 1992; Nelson et al., 1995; Gold et al., 1998). The hydric soil designation was developed to enhance wetland classification and is now linked to specific soil series with high water tables (Soil Survey Staff, 1993). Dramatic declines in dissolved oxygen concentrations and substantial increases in ground water nitrate removal capacity often occur within the first several meters of hydric soils (Simmons et al., 1992; Nelson et al., 1995). We have also observed little ground water nitrate removal in nonhydric riparian soils that develop in response to deep water tables and often occur in settings adjacent to deeply incised streams.
The presence of hydric soils in riparian zones does not assure retention of N from upland sources. Another source of variation in riparian ground water nitrate removal occurs where ground water emerges in surface seeps at the upland boundary of hydric soils and traverses the riparian zone as surface runoff. In this type of setting, Hill (1996) found minimal ground water nitrate removal due to insufficient retention and interaction of nitrate-laden ground water with biologically active portions of the root zone. Unlike surface seeps, upwelling of deep ground water directly into the stream is difficult to recognize from visual observations. Areas dominated by upwelling may have limited riparian ground water nitrate removal potential, since ground water flowpaths traverse riparian zones at depths well below the biologically active, upper zone of hydric soils (i.e., <1–2 m) (Devito et al., 2000; Hill et al., 2000). Seeps and upwelling are a function of structure and may be related to other mappable features, such as geomorphology of the site.
Riparian attributes conducive to substantial ground water N removal often occur in narrow strips less than 30 m from the stream (Nelson et al., 1995). Several studies have found that most reduction of ground water nitrate concentration occurs within the first 10 to 15 m of a riparian forest (Lowrance et al., 1984; Peterjohn and Correll, 1984; Jacobs and Gilliam, 1985). Thus, accurate assessment of the role of riparian zones in the flux of nitrogen at the watershed scale may require high resolution data that may not be displayed at the STATSGO map scale. The Soil Survey Geographic (SSURGO) digital, georeferenced soil survey (Soil Survey Staff, 1997) database is at much finer scales (1:15840–1:31680) than STATSGO. In a previous study (Gold et al., 1989), we found that soil survey data at a scale of 1:15840 provided an accurate assessment of surficial geology. Line widths on SSURGO maps equate with actual field widths of 4 to 8 m. Both STATSGO and SSURGO soil databases contain spatially explicit information on hydric soil status and geomorphology; however, the SSURGO data set affords greater detail for identifying these attributes at the riparian scale.
Here we present the results of an intensive field reconnaissance study that examined the relationship between selected field (i.e., groundtruthed) characteristics of riparian zones (i.e., hydric soil widths, presence of surface seeps) and SSURGO map classifications. Our goal was to determine the efficacy of using SSURGO data to improve our ability to classify the ground water N removal capacity of riparian zones for use in watershed modeling, protection, and restoration efforts.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Glacial tills are unstratified deposits of sand, silt, clay, and boulders that are located on gently sloping uplands and higher-gradient drumlins (Wright and Sautter, 1988). Tills can be either ablation (loose) or basal (dense). Hydraulic conductivity values for Rhode Island tills range from 4 x 10-3 to 1 x 10-2 cm s-1 in ablation tills to <1 x 10-4 cm s-1 in basal tills (Rector, 1981). Soils developed from dense basal till generally have higher hydraulic conductivity in the upper horizons (above the dense till layer) resulting in perched water table systems and shallow ground water flow. Streams in till areas usually have a high gradient due to the relative position of till on the landscape. Floodplains are uncommon in till settings. Glacial outwash deposits are stratified layers of sand and gravel deposited by glacial meltwater streams. They typically occur in valleys commonly referred to as outwash plains (Rector, 1981). Hydraulic conductivity values of Rhode Island outwash soils can range from 1 x 10-3 to 1 x 10-2 cm s-1 (Rector, 1981). Streams in outwash are at low gradients and often have low flow velocities. Organic soils have extremely high water holding capacities and high water tables for a significant portion of the year. Alluvial soils occur as floodplains where sand and silt is deposited by streams during flooding. Both organic and alluvial deposits are usually found on flat areas of the landscape having low gradients (Wright and Sautter, 1988).
Site Selection
We randomly located approximately 1500 orthogonal site lines on the 362 km of first- and second-order streams in the watershed using a digitized, georeferenced USGS 1:24000 topographic hydrography coverage (Rhode Island Geographic Information System, 1988) that meets Federal Geographic Data Committee standards. The orthogonal site lines were added graphically to all first- and second-order streams on the digital stream coverage by on-screen digitizing, at approximately 250-m intervals. Each site line was assigned a unique identification number. Using a random number generator, sites were randomly selected. Less than 10% of potential sites were discarded after screening for landowner permission to enter private property. In total, 100 randomly generated sites were selected and used for field data collection.
Field reconnaissance maps were developed for each of the 100 sampling sites using ArcView 3.1 software (Environmental Systems Research Institute, 1998). These maps incorporated 1995 digital orthophotography (1:12000 scale) as a backdrop to the orthogonal site lines. Using the reconnaissance maps, sites were pinpointed through orienting methods using obvious landscape features such as road intersections and farm fields as points of reference. A measuring tape and compass were used to determine exact site locations.
We used the SSURGO digital spatial database for Rhode Island developed in 1996 (USDA, 1998) to compare soil map unit attributes with field results. We corrected the Rhode Island SSURGO data set to parallel current requirements for SSURGO certification standards. Since 1998, all new SSURGO databases contain a hydrography data layer (hydrolines) digitized from the original soil survey report. Rhode Island SSURGO does not have a separate hydrography layer. To overcome this limitation, we used the USGS digitized hydrolines during site location. Sites were then visually matched to the hydrolines in the original soil survey report to determine the exact SSURGO riparian soil map unit for use in determining a site's SSURGO categorization in analysis.
The SSURGO database classified the 100 study sites into 25 different soil map units. We compared the distribution of soil parent material and hydric status of the SSURGO designated map units at the study sites with the distribution of those attributes in the soil map units found along the entire length of riparian corridors of all lower-order streams of the study area. Hydric soils formed in till, outwash, or organic/alluvium comprised 69% of the undisturbed soils based on SSURGO data of a 30-m buffer of the lower-order streams throughout the entire study area. Within the 100 sites selected for this study, the SSURGO database classified 73% as hydric soils, suggesting that the sites were reasonably representative of the overall watershed.
Site Vegetation and Stream Classification
The riparian sites were forested and displayed no recent signs of cutting or disturbance. In most hydric soils the dominant overstory tree was red maple (Acer rubrum L.), with sweet pepperbush (Clethra alnifolia L.), highbush blueberry (Vaccinium corymbosum L.), sphagnum moss (Sphagnum sp.), skunk cabbage [Symplocarpus foetidus (L.) Salisb. ex W.P.C. Barton], and cinnamon fern (Osmunda cinnamomea L.) commonly comprising understory species. In the upland portions of the riparian sites, white pine (Pinus strobus L.), red oak (Quercus rubra L.), white oak (Quercus alba L), and red maple were the dominant tree species, with bullbrier (Smilax rotundifolia L.), cinnamon fern, sweet pepperbush, and Lycopodium sp. as the common understory species. All sites met the Riverine Lower Perennial system and subsystem classification as defined for the National Wetlands Inventory (Cowardin et al., 1979).
Site Characterization
At each site, six transect lines were delineated into the riparian forest. These transects were perpendicular to the stream channel with three on each side of the stream (specified as left and right banks determined from the direction of stream flow). On each bank, the three transects were placed 7.5 m apart and extended in parallel 30 m away from the stream channel (Fig. 1). Thus, each site encompassed an area 15 m (along the stream) x 60 m (perpendicular to the stream). The top of the stream bank was used to define the stream channel edge.
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On each stream bank the width of each soil drainage class and the hydric soil widths from the three transects were averaged to create a set of mean drainage class widths and mean hydric soil widths per bank. Transect averages were used for all direct comparisons with the digitally derived SSURGO data for the same cross-section. Transects were located along straight stream reaches, rather than meanders, in order to minimize intrasite variability. By averaging the three transects per bank we hoped to account for some of the spatial error associated with the width of the digitized site line.
For each site, we recorded the presence or absence of seeps. Ground water surface seeps, also referred to as lateral springs (Downes et al., 1997), occur at the base of sloping lands within certain riparian zones of the study area. The upslope edge of such seeps is easily identified by distinct vertical faces usually 15 to 30 cm in height where mineral soil materials are exposed. Immediately upslope of the seepage face, the soils are mineral and nonhydric, while organic soil materials typically occur at the base of the seepage face. These seeps generate surface water at the bottom of the exposed mineral face for either the entire year or during the spring and fall, when water tables are elevated. Defined rivulets often emanate from the seepage face, but do not exist above the seep, suggesting that the surface water observed is not the result of upland runoff, but rather from ground water emerging at the seepage face.
Statistical Analyses
We examined the symmetry of left and right banks of the sites using a t test and found that within a site, the hydric soil widths of adjacent banks were not independent at the p < 0.05 level. As a result, a data set was created that used only one bank per site by selecting a random bank from each site for all statistical comparisons. This was done by assigning a random number to each of the 100 sites. If the random number was even, then the right bank was used, and if the random number was odd, then the left bank was used in all statistical comparisons.
For the following attribute categories, each field site was placed into a particular class based on the characteristics of the SSURGO soil map unit identified through the digital site line:
Within each SSURGO category we used analysis of variance (ANOVA) to test the equality of means of field-measured hydric soil width between category classes. Where there was significance, Tukey's Honest Significant Difference (HSD) test was used to test for differences between classes (Ott, 1993). In addition, within each SSURGO category, we used Chi Square analyses to examine differences in the occurrence of observed surface seeps between category classes. All statistical analyses were performed on Statistica for Windows (StatSoft, 1999). Significant differences for all statistical analyses are reported at p < 0.05.
Several of the classes found in different SSURGO categories are interrelated. Within the soil units of the study area all organic/alluvium soil units are also classified as hydric soils. In addition, within the 100 study sites, all soil map units classified as hydric also were classified as SSURGO Slope Class A (0–3%), and 96% of all A slope classes were mapped hydric.
High Capacity Ground Water Nitrate Removal Riparian Sites
Based on past research findings (Lowrance et al., 1984; Peterjohn and Correll, 1984; Jacobs and Gilliam, 1985; Hill, 1991, 1996) we defined "high capacity ground water nitrate removal riparian sites" based on the co-occurrence of >10 m of groundtruthed hydric soil width and an absence of surface seeps. We examined the proportion of sites with those features within each soil survey category class. We then explored the effects of map scale on the depiction of soil map units in the riparian zone with high capacity for ground water nitrate removal. We compared the STATSGO and SSURGO databases within the entire 64000 ha of the study area. We used the attribute files of each database to depict selected soil map units that bordered the entire stream network of the digitized, georeferenced USGS 1:24000 topographic hydrography coverage (Rhode Island Geographic Information System, 1988). The extent and pattern of stream reaches bordered by these queried soil map units were then obtained.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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SSURGO Slope Classes
The mean hydric width of the A slope class was significantly higher than the other three slope classes (Table 1). Hydric soil widths >10 m were found on more than 65% of the A slope class compared with less than 10% on each of the other slope classes.
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Riparian Classes with High Ground Water Nitrate Removal Capacity
The hydrogeomorphic classes stratified the field data into two distinct groups. Most sites classified as hydric–outwash and organic/alluvium contained both >10 m hydric soils and a lack of seeps (Table 4). In contrast, most sites classified as nonhydric or as hydric till had either minimal hydric widths or contained ground water surface seeps. None of the other categorization schemes separated the field sites into such widely divergent groups based on a co-occurrence of attributes that suggest high ground water nitrate removal capacity.
Depicting High Ground Water Nitrate Removal Capacity Riparian Zones
Figure 3 compares STATSGO with SSURGO to illustrate the effects of different map scales on the identification of stream reaches bordered by riparian zones with either the outwash–hydric or organic/alluvium soil survey classes. Figure 3 depicts the drainage network of streams within the watershed of the Chipuxet River, a fourth-order stream that flows into the Pawcatuck River. Using the SSURGO database (Fig. 3b), we found riparian zone soils with potentially high ground water nitrate removal capacity throughout the drainage network. In contrast, with the STATSGO database (Fig. 3a), riparian zones with potentially high ground water nitrate removal capacity only occurred along higher-order streams at the most downgradient portions of the stream network. We found striking differences between STATSGO and SSURGO at the watershed scale. Whereas soil map units with potentially high ground water nitrate removal bordered 39.2% of the entire drainage network of the Pawcatuck Watershed using the SSURGO database, only 2% of the drainage network was bordered by STATSGO map units with potentially high ground water nitrate removal capacity.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Based on our research, SSURGO can be a valuable tool to watershed managers and modelers in the identification of riparian corridors with high potential for ground water nitrate removal. At the scale of SSURGO, the hydric soil classification was capable of resolving much of the variation in hydric soil width noted in riparian sites, while the geomorphic classification separated sites based on the occurrence or absence of surface seeps. By aggregating individual map units into functional soil classes, we enhance our ability to extend insights from the site scale to the watershed scale (Bouma et al., 1996).
Classifying riparian soils based on a combination of geomorphology and hydric soil status created two functionally distinct sets of riparian soil classes:
(i) Those soil classes that predominately represent sites with conditions that suggest ground water flow through a functionally significant width of hydric soils. These classes include soil map units classified as outwash–hydric and organic/alluvium.
(ii) Those soil classes where ground water nitrate removal may be constrained by either minimal hydric soil width (till–nonhydric or outwash–nonhydric classes) or by surface seepage and rapid flow across the riparian zone (till–hydric class).
Riparian sites classified by SSURGO as organic/alluvium–hydric or as outwash–hydric soil classes have a high potential for ground water nitrate removal. A substantial majority of these sites have groundtruthed hydric soil widths > 10 m and no surface seeps. Given the slow rate of ground water flow along stream reaches bordered by these hydrogeomorphic riparian classes, ground water from neighboring uplands will be subjected to lengthy retention times in soils that have been found to have high nitrate transformation rates (Simmons et al., 1992; Nelson et al., 1995). Riparian sites with soils classified as nonhydric (i.e., till–nonhydric and outwash–nonhydric) are expected to have low potential for ground water nitrate removal. More than 85% of these nonhydric sites have groundtruthed hydric soil widths of <5 m (median <1 m). Given these narrow widths of hydric soils, ground water retention in soils with high nitrate transformation rates would be limited, minimizing the capacity for removal.
Our data suggest that sites classified as till–hydric may have lower ground water nitrate removal potential than other riparian sites classified as hydric soils. Less than 40% of the till–hydric sites had hydric soil widths >10 m and many sites contained ground water seeps, which can short-circuit ground water retention in riparian hydric soils (Hill, 1991). These seeps reflect the subsurface flow processes generated by the physical geography of till settings. Till often contains fragipans or highly compact layers that restrict vertical infiltration and generate perched water tables and lateral subsurface flowpaths (Veneman and Bodine, 1982; Pickering and Veneman, 1984; Stolt et al., 2001). In addition, riparian zones in till settings are usually located at toeslope landscape positions and the change in slope associated with hydric riparian sites in till reduces the hydraulic gradient of the ground water. The combination of shallow restrictive layers and marked change in hydraulic gradient induces surface seepage (Winter et al., 1998) and minimizes subsurface flow and nitrate transformation within the anaerobic, carbon-rich hydric soils that border the stream.
The differences we observed in drainage class patterns within the hydric soils of different geomorphic classes may also have implications for ground water nitrate removal capacity. Because PD soils dominated the hydric soils of till sites, we expect these locations to be subject to lower water tables or shorter periods of high water tables than the hydric soils of organic/alluvium sites that are largely composed of VPD soils. Accumulations of organic matter and anaerobic conditions within PD till hydric soils are more likely to be limited to the upper portions of the soil, whereas the VPD organic/alluvium soils appear to generate anaerobic conditions and accumulation of organic carbon deeper into the profile and may contain a more extensive zone of favorable conditions for ground water denitrification. The observed agreement between SSURGO drainage class and the field-observed drainage class of hydric soils suggests that SSURGO could be also be a useful database to parameterize more site-specific riparian biogeochemical models such as the Riparian Ecosystem Management Model (REMM) (Lowrance et al., 2000). In addition, the differences in saturation between hydric soils of different hydrogeomorphic classes should have implications for other riparian functions, such as habitat restoration and conservation biology.
Obtaining spatial data at scales relevant to the functional patterns of ecosystem processes is a major challenge in watershed modeling (Groffman and Wagenet, 1994; Wagenet and Hutson, 1996; Addiscott and Tuck, 1996). Our field data demonstrate that the SSURGO spatial database can capture much of the pattern and extent of riparian zones with characteristics that indicate a high capacity to serve as sinks for watershed nitrate. Equally important, our field data demonstrate that STATSGO data can substantially misrepresent the structure and ground water nitrate removal capacity of riparian zones in lower-order streams of heterogeneous landscapes, such as the glaciated northeastern USA. STATSGO failed to display the pattern of critical soil features along streams. Clearly, much of this disparity results because the STATSGO database is compiled by generalizing more detailed soil survey maps. Over the entire 64000-ha study area, STATSGO distributed the soils among 11 map units and only one of those units is dominated by hydric soils. In contrast, SSURGO uses 79 soil map units to describe the same area, of which six are either hydric outwash or organic/alluvium classes.
Our study demonstrates the value of field-validated SSURGO riparian classification schemes for watersheds with diverse riparian zone structure. The SSURGO database is increasingly being used for county scale decision-making (Fernandez et al., 1993) and is expected to be available for more than half the USA in the next several years.
Although a SSURGO-based classification scheme advances our ability to identify riparian zones with high ground water nitrate removal capacity, our study also points out important research gaps. Given the sizeable proportion of hydric riparian zones with surface seeps, we need further work to clarify the fate of nitrate in these riparian settings. Hill (1991), working in Canada, reported minimal riparian nitrate removal where surface seeps fostered overland flow across wetland soils to the stream. In contrast, Downes et al. (1997) reported substantial riparian nitrate removal in a similarly described setting in New Zealand.
Approximately 50% of the sites we classified as high ground water nitrate removal capacity riparian zones (>10 m hydric soil width and no seeps) were characterized by hydric soils in outwash. Along higher-order streams, deep (>5 m) outwash deposits occur in glacial meltwater valleys (Johnston and Dickerman, 1985) and nitrate-laden ground water may move below the upper, biologically active portions of the saturated zone, limiting ground water nitrate removal (Devito et al., 2000). However, within the headwaters of glaciated watersheds, where most lower-order streams occur, outwash deposits are likely to be shallow. To advance the usefulness of the SSURGO database for classifying riparian ground water nitrate removal capacity, additional work is needed to determine the extent of interaction between ground water flowpaths and the biologically active zones in the ground water of outwash–hydric riparian sites (Hill et al., 2000).
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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