DIVISION S-5-SOIL GENESIS, MORPHOLOGY& CLASSIFICATION

Temporal and Spatial Patterns of Episaturation in a Fragixeralf LandscapeP. A. McDaniel* and A. L. Falen

ABSTRACTRelatively low saturated hydraulic conductivities associated with

fragipans result in seasonal perched zones of saturation (episaturation).This study was conducted to monitor seasonal episaturation in aFragixeralf landscape of northern Idaho. Perched zones of saturationabove a fragipan in a 1.4-ha field were monitored during a 2-yr periodusing 64 piezometers arranged on a 15-m grid spacing. During thewinter of 1991-1992, relatively mild temperatures and high rainfallresulted in development of perched zones of saturation lasting fromearly December into May. Average quantities of perched water presentwithin the landscape during the winter and early spring months rangedfrom 8.4 to 15.4 cm, representing between 34 and 43% of the seasonalprecipitation that had been received at the study site. During the1992-1993 winter, colder temperatures and relatively large quantitiesof snow delayed development of episaturation. When the snow packbegan to melt in early March, an average of >20 cm of water waspresent in the saturated zone above the fragipan. This water repre-sented 58% of the seasonal precipitation that had been received.Soil morphological characteristics and elevation were correlated withquantities of perched water present on sampling dates when potentialevapotranspiration was low. For all sites, quantity of perched waterwas most strongly correlated with the thickness of the zone above thefragipan exhibiting redoximorphic features; depth to the fragipan wasless strongly correlated. Elevation was generally a poor indicator ofepisaturation. Results of this study indicate that significant periods ofepisaturation occur in fragipan-dominated landscapes under xericmoisture regimes.

EPISATURATION is defined as the condition in whicha soil is periodically saturated in one or more layers

that overlie unsaturated horizons within 2 m of the soilsurface (Vepraskas, 1992; Soil Survey Staff, 1992). Thiscondition has often been referred to in the literature asa perched water table. However, because of the difficultyin defining a water table using piezometers or tensiome-ters, the term water table is not used in the definitionof episaturation (Vepraskas, 1992). The condition ofepisaturation is more appropriately described as aperched zone of saturation. Where the perched zone ofsaturation occurs above a pedogenic horizon such as afragipan or argillic horizon, it fits the concept of second-ary pseudo- (or surface-water-) gley soils (Schlicting,1973; Fanning and Fanning, 1989).

Soils with fragipans are widely distributed throughoutthe USA and occupy =138 000 km2 of areas mappedby detailed soil survey (Witty and Knox, 1989). Although

Soil Science Division, Univ. of Idaho, Moscow, ID 83844-2339. Contribu-tion from the Soil Science Div., College of Agriculture, Univ. of Idaho.Idaho Agric. Exp. Stn. Paper no. 94704. Received 20 Oct. 1993. "Corre-sponding author (pmcdaniel@marvin.ag.uidaho.edu).

Published in Soil Sci. Soc. Am. J. 58:1451-1457 (1994).

the genesis of fragipans is not completely understood,their strong influence on hydrological processes has longbeen recognized (Soil Survey Staff, 1975). The presenceof a slowly or very slowly permeable fragipan createsstrongly anisotropic soil profile characteristics with re-spect to water movement. Franzmeier et al. (1989) re-ported an approximate 100-fold decrease in the saturatedhydraulic conductivity of a fragipan relative to overlyinghorizons in a soil of southeastern Indiana. This sharpdecrease hi hydraulic conductivity with depth causes waterto accumulate above a fragipan during periods of highrainfall and low evapotranspiration, creating a perchedzone of saturation. Within landscapes, water accumulatesabove a fragipan and then moves downslope in responseto the lateral component of the hydraulic gradient (Dan-iels and Hammer, 1992). Soil morphological evidenceof episaturation and associated lateral movement of waterwithin a landscape includes the presence of E horizonsoverlying the more slowly permeable horizons (Milleret al., 1971; Simonson and Boersma, 1972; Barker,1981).

Although a great deal of research has focused on recogni-tion, genesis, and morphology of fragipans (Grossman andCarlisle, 1969; Smalley and Davin, 1982; Smeck andCiolkosz, 1989), limited research has been directed atquantifying their role in landscape hydrology. Miller etal. (1971) estimated that «50% of the January-to-Juneprecipitation was accounted for as subsurface lateral flowabove a fragipan in Ohio under a udic moisture regime.They also reported that the subsurface lateral drainageaverage for February was >80% of the monthly precipita-tion. These data indicate that fragipans influence thehydrological behavior of substantial proportions of pre-cipitation received within a landscape during periods oflow potential evapotranspiration.

There is little published information about the role offragipans on field-scale hydrological processes underxeric moisture regimes. Soil survey data compiled byWitty and Knox (1989) indicates the majority of soilsthat have fragipans are found in areas with aquic andudic moisture regimes, and this may help explain thelack of information on fragipans that have xeric regimes.Because much of the precipitation received under a xericmoisture regime comes during the months when potentialevapotranspiration is low, a high proportion of the totalprecipitation is able to move into the soil (Soil SurveyStaff, 1975). As a result, we believe that developmentof perched zones of saturation above fragipans is animportant landscape process in these areas despite thefact that total precipitation may be considerably less thanthat received under udic and aquic regimes. Accordingly,

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1452 SOIL SCI. SOC. AM. J., VOL. 58, SEPTEMBER-OCTOBER 1994

one objective of this study was to monitor the temporaland spatial patterns of episaturation in a Fragixeralflandscape of northern Idaho. A second objective was torelate these patterns to soil, landscape, and climaticinfluences.

MATERIALS AND METHODSEnvironmental Setting

The study area consists of a 1.4-ha field in the Palouseregion of northern Idaho, =40 km east of Moscow (Fig. 1).The field was enrolled in the Conservation Reserve Programin 1988 and is under permanent pasture. Previously, it hadbeen used for small grain production. The landscape is rollingwith moderate relief and is mantled with Pleistocene loess(Busacca, 1989). Miocene basalts of the Columbia River BasaltGroup underlie the loess (Bush and Seward, 1992). Averageelevation is = 860 m above sea level.

Soils of the area receive = 70 cm of precipitation annuallyand have xeric moisture and frigid temperature regimes(Barker, 1981). Uplands of the study area are mapped as Santa(coarse-silty, mixed, frigid Ochreptic Fragixeralf) silt loamunits with 5 to 20% slopes (Barker, 1981). A small valleyfloor composed of alluvial soils (Xerofluvents) and a narrowband of colluvial slopes graded to the stream course occupythe lower-lying landscape positions. These nearly level togently sloping soils do not have fragipans.

Santa soils have formed under grand fir [Abies grandis(Douglas ex. D. Don) Lindley] forest and are one of themost extensive fragipan-containing soils in the eastern Palouseregion of northern Idaho (Weisel, 1980; Barker, 1981). Thesesoils are characterized by silt loam textures and typically havean ochric epipedon and a cambic Bw horizon overlying an Ehorizon and a fragipan Btxb horizon (Barker, 1981). The Ehorizons form an abrupt boundary with the underlying fragipanand contain redoximorphic features such as Fe/Mn concretions,

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1180 CANADA 115°-49°

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Fe masses (high-chroma mottles), and Fe/clay depletions (Ve-praskas, 1992).

Field MethodsSixty-four positions were sampled on a 15-m grid spacing

within the study area using a cat-mounted hydraulic probewith a 5-cm-diam. soil sampling tube (Fig. 2). Samples weretaken to just below the depth at which a fragipan was present,usually 70 to 80 cm. Depth to the fragipan was recorded andgenetic horizons were described. Thickness of the zone abovethe fragipan exhibiting redoximorphic features (Vepraskas,1992; Soil Survey Staff, 1992) was also noted. Piezometersconstructed of perforated, 5-cm-diam. aluminum tubing werethen installed in each of the 64 core holes. Moistened soil wasrepacked around the tops of the piezometers to prevent waterfrom running down the sides of the tubes. Piezometers werethen covered with a loose-fitting cap to keep out rain andsnow. A topographic survey of the area was made using a rodand transit and taking elevation readings on a 5-m grid spacing.Two additional pedons were described and sampled for bulkdensity and particle-size analyses. Bulk densities were mea-sured using the clod method of Blake and Hartge (1986).Particle-size distribution was determined using pipette andcentrifugation procedures: samples were treated with H2O2 todestroy organic matter and dispersed using Sodium-hexa-metaphosphate and sonication (Gee and Bauder, 1986).

Following onset of winter rains in late October, perchedzones of saturation were monitored as weather conditionsdictated. Depth to the free water surface was measured bylowering a calibrated tube into each piezometer and gentlyblowing until bubbling occurred. The thickness of the perchedzone of saturation above the fragipan was calculated by usingthe depth to the free water surface and the depth to the fragipan.To allow comparison with precipitation data, the saturatedzone thickness was then converted to centimeters of water usingporosity values calculated from bulk density measurements.

To help illustrate spatial relationships, a geostatistical soft-ware package was used to help generate various contour maps

Fig. 1. Location of the Palouse Region and the study site.Fig. 2. Elevation in meters. Black circles indicate location of 64 sam-

pling sites and piezometers.

.MCDANIEL & FALEN: EPISATURATION IN FRAGIXERALF LANDSCAPES 1453

Table 1. Selected morphological and physical properties of a Santa pedon (fine-siltyt, mixed, frigid Ochreptic Fragixeralf).Color Particle-size distribution

Horizon Depth moist dry sand silt clay Bulk density Redoximorphic features

ApABBwBEE

Btxbl

Btxbl

cm0-10

10-1919-3939-5151-68

68-95

95-120f

% of <2 mm fraction10YR 3/310YR 4/37.5YR 4/310YR 4/310YR 6/3

7.5YR 4/4

7.5YR 4/3

10YR 5/310YR 5/310YR 6/410YR 6/310YR 7/2

10YR 5/4

10YR 5/4

10.27.87.97.3

10.9

7.1

7.0

70.071.373.172.366.7

67.0

64.3

19.820.919.020.422.4

25.8

28.7

g cm"3

-t—

1.551.631.64

1.75

1.75

nonenonenonenone

many fine & medium 7. SYR 5/6masses

10YR 6/2 ped faces; common Femasses in ped matrix

10YR 6/2 ped faces; common Feand Mn masses in ped matrix

t Santa series is officially classified as coarse-silty.| not determined.

of the study site showing elevation, depth to fragipan, thicknessof soil zone exhibiting redoximorphic features, and thickness ofthe perched zone of saturation. Contours of these data weregenerated by a moving least squares gridding method availablewith MacGRIDZO1 software (RockWare, Inc, Wheat Ridge,CO).

Weather data used in this study was obtained from a weatherstation located 2.2 km ENE of the study site. Data wererecorded at this station by an independent weather observerand include maximum and minimum daily temperature anddaily precipitation.

RESULTS AND DISCUSSIONThe landscape unit selected for use in this study con-

sists of two distinct slope segments, a slightly convexslope occupies the northern half and a smaller concaveslope area is located in the southern half of the field (Fig.2). The western boundary of the study area generallycorresponds to a change in landform and soil type. Eleva-tions within the study area range from 870 m at the

1 Trade names and company names are included for the benefit of thereader and do not imply any endorsement or preferential treatment of theproduct by the Idaho Agric. Exp. Stn.

summit position in northeast corner to 855 m at thelower slope positions.

Morphological and physical characteristics associatedwith fragipans and epiaquic conditions are present in theSanta soils (Table 1). The Btxb horizons have largeprismatic structural units that average 15 cm in diameter,bulk densities of 1.75 g cm"3, and no root penetrationinto ped interiors. Flattened roots are only present alongped exteriors and apparently provide the energy sourcerequired for reduction of Fe and subsequent formationof bleached (10YR 6/2) ped faces (Vepraskas, 1992).Iron and Mn accumulations are found within ped interi-ors. High color values and low chromas are associatedwith the E horizons overlying the fragipans. The presenceof Fe accumulations (high-chroma mottles) suggests thatE horizon genesis may include redox processes associatedwith perched zones of saturation.

Depth to the fragipan for the 64 sites averaged 72 cmand ranged from 42 to 103 cm. Because we felt thatfragipan depth .might be an important factor in determin-ing patterns of perched zones of saturation, a contourmap of the study site showing depth-to-fragipan wasgenerated (Fig. 3a). Fragipan depth is greatest in the

v® J -n • ̂•\jiTJj) /. r -

(a)Fig. 3. (a) Depth to fragipan in centimeters and (b) thickness in centimeters of soil zone above fragipan exhibiting redoximorphic features.

1454 SOIL SCI. SOC. AM. J., VOL. 58, SEPTEMBER-OCTOBER 1994

20 -i

5-

r 20

- 15

10 |

£0 c

3

0 ' , i i i i i i i i i i i i i i i i i ! i I ' -10O N DIJ F M A M J J A S O N DIJ F M A

Month (1991-1993)Fig. 4. Monthly precipitation and average temperature data recorded

2 km ENE of the study site.

lower positions of the concave slope area. This is likelydue to greater accumulation of hillslope sediments inthese positions, thereby increasing the thickness of thesurface horizons overlying the fragipan. However, theredoes not appear to be any consistent relationship betweenlandscape position and fragipan depth throughout therest of the field. In fact, most of the fragipan depthcontours tend to run perpendicular to the elevationalcontours. For the 64 sampling sites, fragipan depth isnot significantly correlated with elevation (r = -0.028).This lack of a systematic change in fragipan depth withrelative position along the slope segments suggests thatprocesses of hillslope erosion and deposition played alimited role in determining fragipan depth in these soils.We feel the stratigraphy of the Palouse loess depositsmay be more important than local landscape position indetermining fragipan depth. Fragipans of the easternPalouse region have been interpreted as features that areassociated with paleosol argillic horizons formed in olderloess (Barker, 1981; Busacca, 1989). Observation of

11 Dec. 1991

numerous road cuts in the area indicated that orientationof paleosol layers is not accordant with modern-daysurface topography. Daniels and Hammer (1992) pointedout that the Palouse loess has a more complicated deposi-tional environment than the thick deposits associatedwith valley outwash in the midwestern USA. As a result,we conclude that depth to fragipan is controlled to alarge extent by the orientation of the underlying paleosolB horizon; this would explain the lack of a systematicpattern of fragipan depth and the poor correlation withelevation. Thus, with the exception of the concave, lowerslope positions, it appears that surface topography is oflittle use in determining the spatial patterns of fragipandepth in these landscapes.

Thickness of the soil zone above the fragipan exhibitingredoximorphic features was also measured at each site,and the spatial relationships for these data are shown inFig. 3b. Thickness of this zone averaged 25.3 cm andranged from 0 to 52 cm. Describing this zone provedto be less subjective than determining thickness of theE horizons overlying the fragipan; the zone was definedby the presence of high-chroma Fe accumulations (Ve-praskas, 1992). In most cases, this zone corresponds tothe E horizon(s), further suggesting that redox-relatedprocesses may play an important role in the genesis ofthe E horizons (Simonson and Boersma, 1972). As shownin Fig. 3b, the redoximorphic zone tends to be thickerin the concave slope area where convergent throughflowwould be maximized (Hugget, 1976).

Climatic data indicate that seasonal precipitation washigher for 1992-1993 than for 1991-92 (Fig. 4), withamounts of October to April precipitation totaling 56.5and 46.8 cm, respectively. Other differences were alsonoted between the two seasons. The 1991-1992 winterwas significantly milder and the majority of precipitationfell as rain. In 1992-93, however, colder temperaturesresulted in most of the December to February precipita-tion falling as snow, and the snowpack remained on the

(a)

20 Apr. 1992

(b)Fig. 5. Thickness (centimeters) of perched zone of saturation on (a) 11 Dec. 1991 and (b) 20 Apr. 1992.

MCDANIEL & FALEN: EPISATURATION IN FRAGIXERALF LANDSCAPES 1455

Table 2. Water table and precipitation data for study area on 1991-1992 and 1992-1993 sampling dates.

Cumulative Average thicknessseasonal of perched zone of

Sampling date precipitation! saturation

Averagequantity of

perched H2Ot%ofppt§

11 Dec. 199122 Jan. 19922 Mar. 199220 Apr. 19924 June 199214 Dec. 19929 Mar. 199320 Apr. 199318 May 1993

20.9 21.225.7 28.038.8 36.545.5 38.849.9 <117.7 035.0 51.150.7 35.557.2 2.0

8.411.114.515.4trace

020.214.10.8

40433734<1

058281

Table 3. Correlation coefficients between thickness of the perchedzone of saturation above the fragipan at each of the 64 sites ondifferent sampling dates and various soil and landscape charac-teristics.

t Includes rain and snow since 1 Oct.$ Based on soil bulk density of 1.60 g cm~3.§ Percentage of total seasonal precipitation present as perched water.

ground from November until March. In December 1992and January 1993, »185 cm of snow fall was recordedand relatively colder temperatures helped delay peaksnow melt until March. These climatic differences areprobably responsible for the differences observed in thepatterns of development of perched zones of saturationbetween the 2 yr.

Episaturation was observed in early December 1991as saturated soil zones developed above the fragipanprimarily in the lower slope positions of both the convexand concave slope areas (Fig. 5a). For the entire field,the thickness of the perched zone of saturation averaged21.2 cm, which represented 8.4 cm of perched waterand =40% of the precipitation recorded since 1 Oct.(Table 2). Hammermeister et al. (1982) reported thatperched water tables develop most rapidly in concaveregions of slopes in western Oregon and suggested thatpreferential subsurface flow may be a significant contrib-utor to the rapid development. At our study site, quanti-ties of perched water continued to increase into Aprilwhen an average of 38.8 cm of saturated soil was ob-served. The thickness of saturated soil zone was generally

20 Apr. 1993

(a)

Sampling date

22 Jan. 19912 Mar. 199220 Apr. 19929 Mar. 199320 Apr. 1993

Thickness of soilzone exhibiting

redoximorphic features

0.608***0.701***0.687***0.559***0.6%***

Depth tofragipan

0.402**0.450***0.521***0.605***0.505***

Elevation

-0.068-0.092-0.030-0.226-0.107

**, *** Significant at the 0.01 and 0.001 probability levels, respectively.

greatest in the concave slope area, with five samplinglocations having >50 cm of saturated soil above thefragipan (Fig. 5b). The zone of episaturation containedan average of 15.4 cm of water, which represents = 34%of the total seasonal precipitation received at that time.

No perched water was observed hi December 1992.We were unable to determine the tune of initial develop-ment of episaturation because snow cover prevented fieldmonitoring until early March 1993. At that time, theaverage thickness of the saturated zone above the fragipanwas 51.1 cm (Table 1), the maximum amount of episatu-ration observed during this study. This perched zone ofsaturation represented 20.2 cm of water, which accountedfor 58% of the seasonal precipitation that had beenreceived at that tune. Because most of the precipitationhad been received as snow and a snow pack of « 60 cmwas still present, a considerable amount of the recordedprecipitation (snow) had not yet melted and moved intothe soil. Based on this, we conclude that a very highpercentage of the seasonal precipitation received at thesite would be accounted for in the perched zone ofsaturation, the overlying unsaturated soil, and the snow-pack. Spatial patterns of episaturation on this date (Fig.6a) were similar to those observed during the wettest

18 May 1993

(b)Fig. 6. Thickness (centimeters) of perched zone of saturation on (a) 20 Apr. 1993 and (b) 18 May 1993.

1456 SOIL SCI. SOC. AM. J., VOL. 58, SEPTEMBER-OCTOBER 1994

part of the previous winter. The greatest quantities ofwater were concentrated in the lower portions of theconcave slope. The amount of episaturation was substan-tially less in the upper portion of the convex slope andthe extreme northwest corner of the study area.

Following sampling on 20 Apr. 1993, unseasonablywarm, dry weather resulted in rapid depletion of perchedwater. By 18 May, the majority of piezometers containedno free water and, for the entire field, the perched zoneof saturation averaged only 2.0 cm in thickness (Table2). The landscape pattern of episaturation was very local-ized and primarily confined to the lower-lying portionsof the concave slope (Fig. 6b). Hammermeister et al.(1982) and Evans and Franzmeier (1986) have reportedsimilar geomorphic distributions of late-season perchedwater tables.

To better discern relationships between episaturationand soil-site characteristics, thickness of perched zonesof saturation on various sampling dates were correlatedwith soil morphological properties and elevation (Table3). Strongest correlations exist between thickness of theperched zone of saturation and thickness of the zoneexhibiting redoximorphic features. This suggests thatepisaturation is or has been accompanied by sufficientdecreases in redox potential to cause reduction of bothFe and Mn and is consistent with our hypothesis thatredox processes are important in the genesis of these Ehorizons. The relationships between episaturation andassociated changes in soil redox status in these soils willbe the subject of a future paper.

Correlations between thickness of the perched zoneof saturation and depth to fragipan indicate that greaterquantities of perched water are generally associated withareas of the landscape where the fragipan is deeper.We suspect that the areas of deeper fragipans representsubsurface low spots where lateral throughflow accumu-lates. Other explanations for this association, are lessclear in view of die fact that we were not able to recognizeany systematic relationship between fragipan depth andlandscape position. Elevation, an indicator of landscapeposition within this landscape, provides a much weakernegative correlation with thickness of the perched zoneof saturation and is not significant (P < 0.01). Thus,relative slope position is not a good indicator of thequantity of perched water present in these soils. Thissuggests that the hydrological processes associated withthese slopes are complex and may be strongly influencedby the stratigraphy of the Palouse loess.

Our results indicate that episaturation does occur forsignificant periods of time under a xeric moisture regime.This condition may develop as early as December andpersist into May, with maximum episaturation occurringin March and April. During this time, quantities ofwater contained in the perched zones of saturation mayrepresent as much as 58% of the seasonal precipitationreceived. As a result, the potential for saturated, lateralthroughflow above the fragipan in these strongly slopinglandscapes appears to be high. Furthermore, morphologi-cal characteristics of the E horizons overlying the fragi-pans indicate that redox status has been sufficiently lowto mobilize and redistribute Fe. Given the proximity

of fragipans to the soil surface in these landscapes,interactions between climatic fluxes and patterns of epi-saturation may have significant management implica-tions.

ACKNOWLEDGMENTSWe gratefully acknowledge the University of Idaho Research

Office for their support of this project. We also thank GeorgeHatley for the gracious use of his property, Donald Gustinfor collection of weather data, and Drs. M.J. Vepraskas andW.R. Guertal for their helpful suggestions and discussion.

KHAN & FENTON: MOLLISOL SATURATED ZONES AND MORPHOLOGY 1457