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Soil Science Society of America Journal 65:805-810 (2001)
© 2001 Soil Science Society of America

DIVISION S-5 - PEDOLOGY

Perched Water Tables on Argixeroll and Fragixeralf Hillslopes

P.A. McDaniela, R.W. Gabehartb, A.L. Falena, J.E. Hammela and R.J. Reuterc

a Soil Science Division, Dep. of Plant, Soil, & Entomological Sciences, Univ. of Idaho, Moscow, ID 83844-2339
b Idaho Dep. of Agriculture, 629 C Washington St. North, Twin Falls, ID 83301
c Dep. of Rangeland Resources & Wildland Soils, Humboldt State Univ., Arcata, CA 95521

Corresponding author (pmcdaniel{at}uidaho.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Seasonally perched water tables (PWTs) are common in loess-derived Argixerolls and Fragixeralfs of the Palouse region of northern Idaho and eastern Washington. However, little is known about the short-term PWT dynamics in these rolling to hilly landscapes and how they are influenced by a regional climatic gradient. In this study, PWTs on an Argixeroll hillslope receiving 700 mm of mean annual precipitation (MAP) and a Fragixeralf hillslope receiving 830 mm of MAP were monitored hourly for four seasons. Results demonstrate that timing of PWT formation may vary considerably from year to year, and may occur up to 3 wk earlier in Fragixeralfs than in Argixerolls. Once formed, the PWTs respond rapidly to precipitation and snowmelt in both soils, with PWT levels increasing as much as 60 cm within a period of <24 h. Water table levels are at or near the soil surface numerous times during the season following periods of rainfall or snowmelt. Perched water table dynamics are remarkably consistent across the region, with similar responses observed in hillslopes located 28 km apart. Relatively dense, light-colored E horizons overlying the restrictive horizons remain continuously saturated for up to 6 to 7 mo yr-1 and develop redox potentials sufficiently low for Fe reduction to occur. Results suggest that seasonal PWTs drive the processes of ferrolysis and hydroconsolidation, and these processes are responsible for many of the E horizon properties common to Argixerolls and Fragixeralfs of the region.

Abbreviations: Ksat, saturated hydraulic conductivity • MAP, mean annual precipitation • PWT, perched water table


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
SOILS WITH PERCHED WATER TABLES (PWTs) occupy {approx}82 million ha in the continental USA (unpublished data from USDA-NRCS STATSGO database). In the eastern Palouse region of northern Idaho (Fig. 1) , many loessial soils contain hydraulically restrictive subsurface horizons. These horizons, in combination with relatively high winter precipitation, create shallow, seasonal perched zones of saturation that are continuous across most upland areas receiving more than {approx}600 mm of annual precipitation.



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  		Fig. 1. Location map of the northwestern USA. Inset shows location of study sites; approximate extent of the core of the Palouse region is indicated by shading

 
Several pedologic studies have focused on PWT dynamics in soils with xeric moisture regimes. In western Oregon, PWT levels measured daily in soils of low-lying landscape positions were found to remain near the soil surface for much of the winter (Boersma et al., 1972). The sequence of bleached E horizons overlying relatively impermeable Bt horizons in these soils was specifically related to the PWTs (Simonson and Boersma, 1972). In southwestern Australia, perched water tables in upland soils were shown to respond rapidly to precipitation, although the magnitude of response was extremely variable within a hillslope (Cox and McFarlane, 1995). In Fragixeralfs of northern Idaho, McDaniel and Falen (1994) reported a relationship between patterns of episaturation and E-horizon morphology. Changes in PWT levels were only reported for 4- to 6-wk intervals, but results showed a strong correlation between the height of perched water and E-horizon thickness above a fragipan.

Under udic soil moisture conditions, continuous monitoring of PWT levels over a fragipan in Pennsylvania demonstrated that the most rapid fluctuations occur in late spring and summer, and these were attributed predominantly to increased evapotranspiration (Palkovics et al., 1975). More recently, Calmon et al. (1998) obtained hourly or 15-min water table levels along a hillslope in Pennsylvania under a udic soil moisture regime. These data illustrate the magnitude of some of the short-term variation in perched water table levels and allow quantification of the frequency and duration of episaturation. Numerous, short-duration periods of saturation were observed above horizons where redoximorphic features were present, suggesting that a clear understanding of PWT dynamics cannot be derived from morphological features alone.

Regional water quality concerns have fueled a need to better understand behavior and characteristics of near-surface water resources; in the Palouse region of northern Idaho, this includes extensive upland perched water table systems. Perched water tables at or near the surface are able to interact with applied agrichemicals and provide a pathway for rapid transport of solutes on strongly sloping to steep slopes (Mallawatantri et al., 1996; Reuter et al., 1998). Therefore, the purpose of this study was to monitor seasonal PWTs in hillslopes dominated by two extensive soils of the region that exist along a precipitation gradient. Our objectives were (i) to examine and compare PWT dynamics on Argixeroll and Fragixeralf hillslopes receiving approximately 700 and 830 mm of mean annual precipitation and (ii) to relate these dynamics to observed soil morphology.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Environmental Setting
The Palouse region of eastern Washington and northern Idaho is part of a 20000-km2 area of deep loessial soils in the northwestern USA (Fig. 1) (Busacca, 1989). The rolling to hilly landscape is mantled with a sequence of soils formed in Pleistocene and Holocene loess. In the eastern Palouse region of Idaho, surface soils have formed in Holocene loess and typically include part of a late Pleistocene paleosol (Kemp et al., 1998). This paleosol is expressed as an argillic (Btb) or fragipan (Btxb) horizon, depending on the MAP. Both the argillic and fragipan horizons associated with the paleosol are hydraulically restrictive; saturated hydraulic conductivity, Ksat, values as low as 0.01 cm d-1 have been measured for these horizons (Rockefeller, 1997; Reuter et al., 1998).

A dry to moist climatic gradient exists across the Palouse region in northern Idaho; MAP ranges from {approx}550 mm in the west to >800 mm along the eastern margin. With {approx}550 mm of MAP, soils are classified as Argixerolls and have somewhat brittle, clay-enriched argillic subsoils (Barker, 1981; Soil Survey Staff, 1998). As MAP increases, soils have argillic horizons with well-developed fragipan morphology and are classified as Fragixeralfs (Soil Survey Staff, 1998). Together, Argixerolls and Fragixeralfs comprise more than one-third of the Latah County survey area (Barker, 1981).

Approximately 70% of the annual precipitation in the Palouse region is received during the period between 1 November and 30 May when evapotranspiration is low. This, coupled with the presence of hydraulically restrictive argillic or fragipan horizons, results in the formation of seasonal PWTs that may exist from approximately November through May (Barker, 1981; McDaniel and Falen, 1994; Reuter et al., 1998).

Field Methods
Using published soil surveys and field reconnaissance, two study sites were chosen to represent the most extensive soils of the region, Argixerolls and Fragixeralfs, and the associated differences in MAP (Fig. 1). During Fall 1994, three monitoring wells were installed at each site on a hillslope having an average slope gradient of 9%. The three wells were installed on the upper third, middle third, and lower third of the backslope segment of the hillslope; approximate distance between adjacent wells was 25 m. An 8.9-cm-diam. core was extracted to a depth just below the top of the hydraulically restrictive argillic or fragipan horizon. Core samples were saved for chemical and physical analyses. A 6.3-cm-diam. polyvinyl chloride pipe was placed in the borehole with slotted (0.3 mm) well screen on the lower end of the well. The annular space was packed with sand around the slotted pipe and with bentonite at the top to provide a hydraulic seal. Wells were then equipped with pressure transducers connected to data loggers in order to capture the short-term changes in water table levels. A pressure transducer (Model PX 160, Electronic Engineering Innovations, Las Cruces, NM) was placed inside the well at a depth corresponding to the top of the hydraulically restrictive horizon. Transducers were calibrated to measure hydrostatic head and then wired to a data logger (Model 422, Tumut Gadara Corp., Whitehall, OH) that was programmed to obtain and store hourly readings. Data were collected for four water years (approximately Nov. through May), and hydrographs illustrating the dynamics of PWTs were prepared by plotting the height of the saturated zone above the hydraulically restrictive argillic or fragipan horizon as a function of time.

Approximately 10 volume cores (340 cm3) were collected from genetic horizons in soil pits at each site to determine Ksat. Cores were obtained in both vertical and horizontal orientations. Platinum electrodes were used to monitor soil redox status. The tips of five Pt electrodes were placed near the bottom of the E horizons of soils adjacent to the middle- and lower-slope monitoring wells at the Argixeroll and Fragixeralf sites. Redox potentials were measured every 1 to 3 wk for three perched water seasons and reported relative to a standard H2 electrode.

Daily weather observations for the 1994 through 1998 period and long-term (30-yr) data were collected at a station located {approx}2 km east-northeast of the Fragixeralf study site (D. Gustin, 1999, unpublished data). Thirty-year data indicate MAP is 830 mm. Long-term precipitation data are not available for the Argixeroll site located 28 km away, but have been extrapolated from work by Reuter et al. (1998) and a nearby weather station located at Moscow, ID. Based on this, MAP at the Argixeroll site is estimated to be 700 cm.

Laboratory Methods
Particle-size distribution of bulk samples was determined by a combination of sieving and centrifugation procedures following digestion of organic matter with NaOCl (pH 9.5) and dispersion of soil particles using Na hexametaphosphate (Gee and Bauder, 1986). Saturated hydraulic conductivity of cores was measured using either a falling- or constant-head permeameter (Klute and Dirksen, 1986).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Morphology and Physical Properties
Despite differences in MAP, soils at the Argixeroll and Fragixeralf sites exhibit similar horizonation. Both soils are dominated by silt loam textures and have Ap, Bw, and E horizons overlying Btb or Btxb horizons (Table 1). Soils at the Argixeroll site have higher quantities of organic C expressed as a mollic epipedon; the E horizons have high Munsell value and qualify as albic horizons (Soil Survey Staff, 1998). Although the Btb horizons are brittle, they do not exhibit the typical morphology associated with fragipans. Structural units within these horizons are coarse angular blocks and there are no gray seams associated with ped faces. Nevertheless, bulk densities are relatively high ({approx}1.65 Mg m-3) and roots are generally excluded from ped interiors (Reuter et al., 1998). Similarly, saturated hydraulic conductivity values are lowest in the Btb horizon (Table 1). Little difference was observed between Ksat values measured on cores collected with vertical versus horizontal orientation, so only vertical values are presented. The low Ksat values are primarily attributed to the absence of a continuous macropore network (Hammel et al., 1994; Reuter et al., 1998).


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  		Table 1. Selected soil morphological and physical characteristics at Argixeroll and Fragixeralf sites. Data for each site represent the average of two sampled pedons

 
At the Fragixeralf site, the main morphological difference is the presence of an ochric rather than mollic epipedon. The fragipan has very coarse prismatic structure, with interpedal planar voids containing flattened roots. Adjacent ped faces are light gray (10YR 7/2 dry) and are coated by thick (0.5–1.0 mm) argillans. Saturated hydraulic conductivity of the fragipan is similar to that of the Argixeroll Btb horizon (Table 1).

Redoximorphic features are absent in upper horizons (Ap, Bw) at both the Argixeroll and Fragixeralf sites. The primary expression of seasonal reducing conditions is the high-value, low-chroma matrix colors of the E horizons. In addition, few to common Fe or Mn concretions are present in the E and upper Btb and Btxb horizons. Many of the Argixeroll and Fragixeralf E horizons are depleted in clay relative to over- and underlying horizons (Table 1) (Barker, 1981).

Perched Water Tables
Perched water table hydrographs for the midslope wells at the Argixeroll and Fragixeralf sites illustrate the characteristics of episaturation in soils of the region (Fig. 2) . It is apparent that PWTs are not static and respond rapidly to weather patterns. Periods of heavy rainfall or snowmelt result in rapid increases of PWT levels. Water tables rise as much as 60 cm within a period of <24 h, such as in the Fragixeralf in first part of January 1996 (Fig. 2). These rapid increases are observed at both Fragixeralf and Argixeroll sites. Ongoing research has shown that the rise and fall of PWTs in these soils is accompanied by only a 4 to 5% change in volumetric water content (Regan, 2000). Thus, relatively small inputs of precipitation and/or snowmelt are able to cause the large, rapid increases in PWT levels observed during the winter and spring months. Similarly, drainage of perched water from the more permeable Ap and Bw horizons, presumably as lateral throughflow, results in the relatively rapid declines of PWT levels that coincide with the intervals between precipitation and snowmelt events.



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  		Fig. 2. Hydrographs for middle slope well positions at Argixeroll and Fragixeralf sites for 1994 through 1998. Genetic horizon depths are indicated at the right of each hydrograph sequence. Weather data are from a location 2 km east-northeast of Fragixeralf site

 
A second feature of the hydrographs is that they illustrate considerable variation in the initial formation of PWTs. Timing of initial PWT formation ranged from early November (1995-1996) to January (1997-1998). 1995-1996 was a wet year, with November through May precipitation 145% of normal; 1997-1998 was a relatively dry year, with 90% of normal precipitation. Comparison of hydrographs from the two sites shows that PWTs do not necessarily form earlier at the higher-precipitation Fragixeralf site. In 1994-1995 and 1996-1997, PWTs at the two sites formed simultaneously. In 1995-1996 and 1997-1998, PWTs formed 1 to 3 wk earlier at the Fragixeralf site. Based on our data and nearby precipitation records, {approx}150 mm of precipitation are required to initiate formation of seasonal PWTs at the Fragixeralf site. When much of the precipitation is received as snow, PWT formation is delayed. This was the case in 1997-1998 when PWTs did not form at the Fragixeralf site until mid December.

The PWTs disappeared in May or June at both the Argixeroll and Fragixeralf sites (Fig. 2). Data from the four monitoring seasons indicate that there is very little or no difference in timing of PWT disappearance between the two sites. Increased evapotranspiration associated with warmer temperatures and the accompanying increase in plant growth appears to drive the disappearance of PWTs. A period of late-season wet weather can cause PWTs to reform, as was observed in middle to late May and early June in 1996 and 1998. These data illustrate the degree to which short-term quantity and distribution of precipitation control the variability in seasonal initiation and duration of PWTs in this climatic regime.

The hydrographs also illustrate how patterns of saturated conditions vary by genetic horizon in these soils. Saturation of surface Ap horizons occurs as numerous, short-duration events. In contrast, episaturation of E horizons occurs for long, uninterrupted periods. The middle slope position hydrograph at the Argixeroll site shows PWT levels rose into the Ap horizon 12 separate times during the 1995-1996 season (Fig. 2). On these occasions, water table levels rose all the way to the soil surface, but did not remain there more than 12 to 24 h. In contrast, E horizons of both the Argixeroll and Fragixeralf remained continuously saturated for most of the 5- to 6-mo period in which PWTs were present.

Comparison of the PWT dynamics within the Fragixeralf and Argixeroll hillslopes demonstrates that all hillslope positions respond similarly (Fig. 3) . The major difference within a hillslope is that the lower slope position maintains a higher level of perched water—this is especially true at the Argixeroll site. The PWTs remain at the surface and in upper horizons for longer periods of time relative to the upper hillslope position. This pattern probably reflects the larger contributing area and lateral throughflow that is associated with the lower slope positions and matches the general pattern observed along a xeric hillslope in southwestern Australia (Cox et al., 1996). The similarity of PWT dynamics at the different hillslope locations observed in this study indicates that there is little soil variability. This is in contrast to results in southwestern Australia where PWT response to rainfall events was extremely variable within a distance of only 10 m along a hillslope (Cox and McFarlane, 1995). The similarity in PWT dynamics that we observed across hillslopes coupled with previous tracer studies (Reuter et al., 1998) indicate that these PWTs are fairly large, hydrologically continuous features of upland landscapes of the region.



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  		Fig. 3. Hydrographs for hillslope transects at Argixeroll and Fragixeralf sites during the 1994-1995 season. Genetic horizon depths are indicated at the right of each hydrograph

 
Relationships between Perched Water Tables and E Horizon Morphology
Extended periods of episaturation in these soils are associated with changes in soil redox status. Using an average E-horizon pH value of 5.5, we estimated 235 mV (expressed relative to a standard H electrode) to be the Fe(II)-Fe(III) phase transition in this soil (Hudnall and Wilding, 1992). Data from the E horizons at the Argixeroll site show Eh values are sufficiently low to reduce and mobilize Fe (and Mn) (Fig. 4) . Values dropped below this threshold for periods of a few weeks to {approx}3 mo during each of the three perched water seasons when soil redox status was monitored. Iron reduction appeared to first occur in March or April, when soil temperatures and biological activity begin to increase substantially. Reducing conditions appeared to persist through May and, in some cases, into June, despite the fact that perched water tables had normally dried up by this time. Thus, there is an apparent 2- to 4-wk lag period between cessation of episaturation and the seasonal transition from reducing to oxidizing conditions. At the Fragixeralf site, mean Eh readings were more variable (higher standard deviation), but similar seasonal trends were observed.



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  		Fig. 4. Eh potentials for E horizons of middle and lower slope positions at Argixeroll site. Each data point represents average of five Pt electrode readings corrected to a standard H2 electrode. Horizontal dashed line indicates Fe(III)-Fe(II) transition based on soil pH of 5.5

 
Low measured Ksat values and previous tracer studies indicate that E horizons of these Argixerolls and Fragixeralfs do not support rapid flow of soil water (Table 1) (Reuter et al., 1998). It therefore seems unlikely that these horizons resulted from the rapid eluviation of clays and organic compounds. Perched water table hydrographs illustrate that the E horizons of these soils experienced long, continuous periods of saturation. This, coupled with Eh data obtained from this study suggest that formation of E horizons is linked to ferrolysis, whereby oxidation–reduction cycles involving Fe (oxy)hydroxides result in development of low-chroma matrix colors and silicate clay destruction (Fanning and Fanning, 1989). Furthermore, the seasonal saturation, relatively high bulk density, and weak structure associated with E horizons suggest that hydroconsolidation may also be an important operative process in their formation. Hydroconsolidation is a physical process in which the packing arrangement of mineral particles is collapsed by loading and wetting (Assallay et al., 1998; Bryant, 1989). Loading stresses of 5 kPA are sufficient to cause significant collapse in silty materials, and these stresses can be achieved with as little as 0.35 m of saturated overburden (Assallay et al., 1998). With E horizons occurring at an average depth of {approx}0.5 m, PWT levels appear high enough in these soils to generate sufficient overburden pressure for hydroconsolidation. Thus, seasonal PWTs coupled with dry summers characteristic of a xeric moisture regime provide a likely mechanism by which hydroconsolidation can occur in these soils.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Results of this study illustrate dynamics of seasonal PWTs under xeric soil moisture regimes on hillslopes of the Palouse region of northern Idaho. Seasonal PWTs at the Fragixeralf site (830 mm MAP) form either at the same time or 1 to 3 wk earlier than those at the Argixeroll site (700 mm), depending on precipitation patterns. Despite differences in MAP, there appears to be no difference between the two hillslopes in the timing of PWT disappearance in late spring. At both sites, PWT levels respond rapidly to precipitation and snow melt, and these responses are very consistent in both timing and magnitude. Surface horizons are subject to numerous short periods of saturation, and PWTs are at or close to the soil surface several times during the year. The morphological similarities between Fragixeralfs and Argixerolls appear to reflect the observed similarities in PWT dynamics. The E horizons show the effects of nearly continuous saturation for periods of up to 7 mo; Eh measurements indicate Fe reduction occurs during a period ranging from a few weeks to {approx}3 mo during the spring. Results of this study clearly demonstrate the close linkage between regional weather patterns and seasonal perched water table dynamics in the Fragixeralfs and Argixerolls of this region.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the financial support of the USDA-National Research Initiative Competitive Grants Program and the USGS-Water Resources Program.

Received for publication June 8, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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