Published in Soil Sci. Soc. Am. J. 68:168-174 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-5—PEDOLOGY
Perched Water Table Responses to Forest Clearing in Northern Idaho
S. L. Rockefellera,
P. A. McDaniel*,b and
A. L. Falenb
a USDI-Bureau of Land Management, Vale, OR 97918
b Soil and Land Resources Division, P.O. Box 442339, Univ. of Idaho, Moscow, ID 83844-2339
* Corresponding author (pmcdaniel{at}uidaho.edu).
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ABSTRACT
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Many forested soils of northern Idaho containing fragipans and seasonal perched water tables (PWTs) have been cleared for agricultural use. However, little information exists on the direct effect of canopy removal on PWTs in these soils. We compared PWTs in soils where forest canopy had been removed with those in adjacent soils with intact canopy. Study sites were selected such that both cleared and forested treatments occurred within the same soil map unit. Perched water table levels in shallow wells were monitored weekly or biweekly from November to June or July for 3 yr. Results indicate that canopy removal substantially affects both average height and duration of seasonal PWTs. Average PWT levels were 6 to 107% higher under cleared treatments, with greatest increases observed when seasonal precipitation was close to long-term averages. Seasonal PWTs developed 2 to 8 wk sooner under cleared treatments compared with forested treatments. Additionally, it took as much as four months before PWTs in the forested treatments reached an equivalent height as those in the cleared treatments. At one study site, the average volume of perched water in the cleared treatment was 7.5 cm greater than that in the forested treatment over the period of episaturation. Results suggest that land-use interpretations based on duration and proximity of a seasonal PWT to the soil surface may need to be adjusted when vegetation cover is altered. It may also be appropriate to distinguish between cleared and forested phases of fragipan-containing soil series when developing hydrologic interpretations.
Abbreviations: ABGR, Abies grandis • CLUN, Clintonia uniflora • MAP, mean annual precipitation • MAT, mean annual temperature • PWT, perched water table • THPL, Thuja plicata
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INTRODUCTION
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EXTENSIVE AREAS OF NORTHERN IDAHO have soils with fragipans or fragipan-like horizons within 1 m of the soil surface. A seasonal PWT, also known as episaturation, is typically present in these soils between November and the end of May, when approximately 75% of the annual precipitation is received. Many of these soils have developed under forest vegetation, commonly grand fir [Abies grandis (Douglas ex D. Don) Lindl.] (ABGR) and/or western red-cedar (Thuja plicata Donn ex D. Don) (THPL) (Barker, 1981). Since settlement of the region in the late 1800s, many of the lower-elevation forested areas have been cleared for agricultural use, creating a mosaic of agricultural land and forest. Although several studies have examined PWT dynamics in soils of the region (McDaniel and Falen, 1994; Reuter et al., 1998; McDaniel et al., 2001), little is known about the effects of these changes in vegetation cover on the behavior of seasonal PWTs.
Considerable research has demonstrated that removal of forest canopy affects the hydrologic balance of watersheds. Clearing of forests results in increased catchment water yields (Hibbert, 1967; Williamson et al., 1987; Swank et al., 1988) and soil-water content (Hibbert, 1967; Sharma et al., 1987). Increases in soil-water content result from two major changes associated with replacement of trees by grasses: reduced transpiration losses resulting from less annual water uptake by the replacement species (Lassoie et al., 1983; Eastham et al., 1994); and decreased evaporation losses resulting from less aboveground interception of annual precipitation (USDA, 1940; Calder, 1979). Studies have also shown that ground water table levels rise in response to forest canopy removal (Heikuranen, 1966; Peck and Williamson, 1987). These studies also demonstrate that increases in soil water are more pronounced in higher rainfall areas.
Given the amount of literature demonstrating the effect of vegetation on the soil-water balance, it follows that seasonal PWTs would be affected by forest canopy removal. In southern Australia, the extent of near-surface PWTs has increased in response to replacement of native tree vegetation with pasture over the past 125 yr (Cox et al., 1996). Studies in southwestern Australia have also demonstrated the importance of these shallow, seasonal perched water systems in generation of stream flow and deep ground water recharge (Turner et al., 1987), which have, in turn, contributed to dryland salinization (Cox and McFarlane, 1995).
There is little information regarding the direct impact of land-use conversion on episaturation. The NRCS does not distinguish between forested and cleared land when developing land-use suitability ratings for soil map units (Barker, 1981; Soil Survey Division Staff, 1993). However, a substantial difference in depth and duration of seasonal PWTs after forest clearing could change the suitability of a soil for specific land uses, notably septic drainfields, dwellings with basements, and local roads and streets. Additionally, subsurface lateral transport of solutes via PWTs could be enhanced with increases in perched water volume (Hammermeister et al., 1982; Kladivko et al., 1991; Mallawatantri et al., 1996; Reuter et al., 1998). Therefore, the goal of this study was to characterize changes in seasonal PWTs resulting from forest canopy removal in fragipan-containing soils of northern Idaho. The specific objective was to compare the duration and quantity of seasonal episaturation under adjacent forested and cleared units occurring within single soil map units.
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MATERIALS AND METHODS
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Site Selection
Three sites with fragipan-containing soils in northern Idaho were selected for this study (Fig. 1)
. Using recent aerial photography and the assistance of NRCS personnel, we identified sites in which the forested and cleared treatments were contained within the same soil map unit delineation, and consequently had similar landscape position, slope, and aspect. All three map units selected for use represent consociations, and are referred to hereafter by the dominant soil taxon occurring in each. The boundary between forested and cleared treatments was oriented in a predominantly downslope direction to minimize the effect of hydrological conditions associated with one treatment on those of the adjacent treatment (Fig. 2)
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Fragixeralf Site
One site was chosen to represent drier forest soils of the region having xeric moisture regimes (Soil Survey Staff, 1999). This site is located 18 km east northeast of Moscow, ID (Fig. 1) and is dominated by soils of the Santa series of coarse-silty, mixed, superactive, frigid Vitrandic Fragixeralfs (Barker, 1981; Soil Survey Division, 2002). This site is referred to hereafter as the Fragixeralf site. Vegetation on the forested treatment represents a mature ABGR/ninebark (Physocarpus malvaceus) (PHMA) habitat type (Cooper et al., 1991), with canopy closure estimated to be approximately 55%. Average tree age was estimated to be 50 to 100 yr. The adjacent treatment was cleared for farming in the late 1930s and at the time of sampling had been placed in permanent pasture. Mean annual precipitation (MAP) is estimated to be approximately 800 mm and temperature (MAT) is approximately 6°C for the Fragixeralf site, based on climatological data for soils with similar vegetation (Barker, 1981) and nearby weather observations. Daily weather observations for the 1995-1997 period and long-term (30-yr) data were collected at a station located approximately 23 km east northeast of the Fragixeralf study site (D. Gustin, unpublished data, 1997). Thirty-year data indicate MAP is 830 mm at this site.
Fragiudalf Sites
Two sites were selected near Weippe, ID to represent some of the more productive forests of the region that have been influenced by volcanic ash (Fig. 1). These sites receive more precipitation than the Fragixeralf site, especially in the form of snow, and have udic soil moisture regimes (Soil Survey Staff, 1999). Data from the Pierce weather station 15 to 20 km north northeast of these sites indicate a MAP of 1050 mm and MAT of 5.6°C (Idaho State Climate Services, 1998). The Andic Fragiudalf site is located 15 km south southwest of Pierce and is dominated by soils of the Kauder series (fine-silty, mixed, active, frigid Andic Fragiudalfs) (Soil Survey Division, 2002). Habitat type for the site is THPL/quencup beadlily (Clintonia uniflora) (CLUN) (Cooper et al., 1991), with approximately 90% canopy closure and an estimated average tree age of 50 to 100 yr. In 1991, part of the site was clear cut and current vegetation consists of a few young conifers and an abundance of native shrubs and annual grasses.
The second site, the Vitrandic Fragiudalf site, is 20 km south southwest of Pierce. Soils are mapped as a consociation of the Reggear series (fine-silty, mixed, active, frigid Vitrandic Fragiudalfs) (Soil Survey Division, 2002). The site represents a ABGR/CLUN habitat type (Cooper et al., 1991). Canopy closure is approximately 80% with little understory growth; estimated average tree age is 50 to 100 yr. The cleared treatment, which consists mostly of native shrubs and annual grasses, has been used for pasture for at least 25 yr.
Field Methods
A soil pit was excavated and genetic horizons were sampled in both cleared and forested treatments at each of the three sites. Genetic horizons were described using standard designations (Soil Survey Division Staff, 1993). Five cores were then extracted using a volume-core sampler (340 cm3) from the genetic horizons in each pit. A cluster of nine monitoring wells was installed in a 3 by 3 pattern on approximately 0.2 to 0.4 ha within each treatment to monitor the seasonal PWTs (Fig. 2). A 15- to 25-m wide buffer zone was left between monitoring wells on the adjacent forested and cleared treatments to minimize edge effects on the two treatments. Fragipan depth at each monitoring-well location was determined using a 9-cm diam. hand auger. Sampling wells were constructed from 6.4-cm diam. polyvinyl chloride (PVC) well casing slotted along the bottom 50 cm; the bottom of each well was placed just below the top of the fragipan (Fig. 3)
. Fine sand was used to backfill the boreholes and cover the slots. Bentonite was used to seal the wells at ground surface to prevent preferential water flow downward along the tubes and each well was loosely capped. Following the onset of fall rains in 1995, depth to the seasonal PWT in the 54 wells was monitored manually weekly to biweekly for 3 yr. Because of timber harvesting, the Fragixeralf site was not monitored during the 1997-1998 season. Perched water table levels above the hydraulically restrictive horizon for the nine monitoring wells comprising each treatment were calculated and averaged for each date. Significant differences between the treatments were determined using a nonparametric Wilcoxon rank sum. In those cases where one of the two treatment averages was zero, a confidence interval for the non-zero average was constructed and used to assess the significance difference of that treatment average from zero. All tests and intervals were performed at the 95% level of significance using the SAS software system (SAS Institute, 1999).
Soil-moisture monitoring began in June 1996 after the first season of episaturation to relate the formation and disappearance of seasonal PWTs with soil-water content. Three neutron hydroprobe access tubes were installed in each treatment at the Fragixeralf site. Tubes were not installed at the Fragiudalf sites because of limited accessibility. Boreholes for the 90-cm long aluminum access tubes were augered with a 9-cm-diam. auger. The boreholes were backfilled with soil and tamped to ensure good contact with the surrounding soil. The tubes were loosely capped to prevent precipitation inputs. Soil-water contents were measured throughout the summer and early fall of 1996 using a neutron hydroprobe (503 DR, CPN Corp., Martinez, CA).
Laboratory Methods
Saturated hydraulic conductivities (Ksat) were determined using a falling-head permeameter; cores with Ksat values >14 cm d–1 were rerun using a constant-head permeameter (Klute and Dirksen, 1986). Four or five cores were used for each horizon, and each core was run a minimum of three times. Because Ksat values were assumed to be log-normally distributed, a geometric mean Ksat value was then calculated for each horizon (Biggar and Nielsen, 1976). After Ksat measurements were completed, cores were used for bulk density determinations using a volumetric method (Blake and Hartge, 1986). Particle-size distribution was determined using a combination of sieving and pipette methods (Gee and Bauder, 1986).
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RESULTS AND DISCUSSION
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Morphology and Hydraulic Characteristics
Soils at each of the study sites exhibit similar morphological characteristics between forested and cleared treatments that fall within the dominant series' range in characteristics (Tables 1 and 2) (Soil Survey Division, 2002). All soils contain fragipans formed in either loess or silty alluvium and overlain by loess and/or volcanic ash. Fragipans were identified by a combination of properties including high soil strength, brittleness, very hard or extremely hard dry rupture resistance, coarse or very coarse structural unit size, and exclusion of roots (Soil Survey Staff, 1999). In addition to roots there was only slight variation in depth to the fragipan at each site (Tables 1 and 2). This variation appeared to be random and not related to treatment. Redoximorphic features such as Fe and Mn concentrations and Fe depletions were common in the upper fragipan and in the overlying E, EB, and/or E/B horizons, indicating the presence of a seasonal PWT and associated changes in soil redox status.
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Table 1. Selected study site characteristics. The dominant soil in the map unit delineation at each site is shown in parentheses; average depth to fragipan represents the mean of nine observations.
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Trends in bulk density (Db) and Ksat were similar to those reported for other soils of the region (Reuter et al., 1998; McDaniel et al., 2001) (Table 2). Measured Ksat generally decreases with depth and is lowest in fragipans. The Ksat values for the upper horizons at each site ranged from 30 to 372 cm d–1 and were approximately two to three orders of magnitude greater than those measured for the fragipans. The average Ksat of the fragipan horizons was 0.67 cm d–1. Bulk densities are similar among the treatments and generally increased with depth, with the fragipans being the densest horizons (Table 2).
Seasonal Perched Water Tables
To analyze monitoring data, we prepared seasonal PWT hydrographs for both forested and cleared treatments (Fig. 4 and 5)
. Because of the importance that the timing and abundance of precipitation play in PWT dynamics (Palkovics et al., 1975; McDaniel et al., 2001), nearby climate data are also included for comparison. The hydrographs reflect the mean of all nine wells in each treatment and express PWT levels as height of the free-water surface above the fragipan. Considerable year-to-year variation in PWT behavior can be seen, resulting largely from variation in the timing and abundance of precipitation. However, differences in patterns of episaturation between forested and cleared treatments were apparent at all sites.
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Fig. 4. Hydrographs showing mean perched water table height above the fragipan and precipitation for 1995-1997 at the Fragixeralf site; asterisks indicate dates with significant treatment differences (0.05 level) in perched water table heights. Climate data are taken from the Helmer weather station (D. Gustin, unpublished data, 1997).
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Fig. 5. Hydrographs showing mean perched water table height above the fragipan and precipitation for 1995-1998 at the Fragiudalf sites; asterisks indicate dates with significant treatment differences (0.05 level) in perched water table heights. Climate data are taken from the Pierce weather station (Idaho State Climate Services, 1998).
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Seasonal PWT data for the Fragixeralf site represent two relatively wet years. Precipitation during the 1995-1996 water year (October through May) was 173% of normal, and 155% of normal during the 1996-1997 water year (D. Gustin, unpublished data, 1997). Perched water tables formed at approximately the same time, late November to early December, under both treatments for both years (Fig. 4). An analysis of precipitation data indicates that development of the PWTs under the forested treatment required an additional 7 cm of precipitation, primarily as rain, in 1995, and 6 cm in 1996, compared with the cleared treatment. Although initial formation of PWTs was similar between treatments, there was significantly more perched water under the cleared treatment for 3 to 4 wk at the start of the PWT season. Beginning around the first part of Jan., PWT heights under both treatments were not significantly different (with a few exceptions in February and March 1997) and showed the same general patterns for the rest of the PWT season. We refer to this point as the equivalent water storage point, and define it as the point at which both treatments have PWT table heights that are statistically similar and, after which, the PWTs behave in a similar manner for most of the remainder of the season. The reason for significantly more perched water in the cleared treatment observed on sampling dates in February and March 1997 appears to be related to snowpack and snow melt. More snowpack was present on the cleared treatment for most of the PWT season. In fact, there were times when snow was present on the cleared treatment but not under the canopy. This resulted in more water infiltrating into the cleared soil profiles during periods of warmer temperatures and snow melt.
Similar PWT differences between cleared and forested treatments were also observed at the Fragiudalf sites during the three monitoring years (Fig. 5). During the 1995-1996 water year, precipitation was 159% of normal (Idaho State Climate Services, 1998). Perched water tables formed early in November under both forested and cleared treatments, in response to 265 mm of precipitation received during the month of November (vs. long-term average of 120 mm). Although monitoring began after PWTs initially formed in 1995, we still were able to observe a 2- to 3-wk period when there was significantly less perched water under the forested treatment. This lag in formation of PWTs under forest was 3 to 5 wk during 1996-1997, when precipitation was 125% of normal. During the 1997-1998 water year when precipitation was 96% of normal, this lag was more pronounced. Seasonal PWTs did not form under the forested treatments until the middle of January, approximately 2 mo later than formation of PWTs under the cleared treatments. It took 8 to 12 wk for soils under the forested treatments to reach the equivalent water storage point. At the Fragiudalf sites, late-season rains in 1998 resulted in PWTs reforming under the cleared treatment in late June and early July. This increased PWT response is attributed to greater interception and evaporation of this precipitation by the forest canopy and presumably greater transpiration from the tree leaf surfaces.
In addition to looking at the duration of PWTs under forested and cleared treatments, we also examined the quantities of perched water present for all treatments. In all cases, average PWT heights under cleared treatments were greater than those of the corresponding forested treatments (Table 3). The average quantity of perched water present was calculated for the period during which a seasonal PWT was observed. By using the average porosity of horizons overlying the fragipans, these means were converted to centimeters of perched water. For example, a 30-cm high PWT in a horizon with 50% porosity would contain 15 cm of water. These data demonstrate that during the 1997-1998 season, the cleared treatment at the Andic Fragiudalf site contained 7.5 cm more perched water than the forested treatment over the 5- to 7-mo period that PWTs are present. This suggests that greater contribution to local stream flow can be expected from these cleared landscapes. Furthermore, our data show that the biggest treatment differences in perched water yield occur during water years with close-to-average rather than above-average precipitation.
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Table 3. Average perched water table (PWT) height and quantity of perched water. Data reflect means of nine monitoring wells in each treatment during the period in which perched water was present at a site.
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Soil Moisture
Soil-water contents at the Fragixeralf site were monitored after seasonal PWTs were no longer present and until they reformed in November. Soils under both treatments were driest on 7 September. At this time, soil-water content of the cleared treatment was lower than the forested treatment (Fig. 6)
. Following some fall rains and before the development of PWTs, it can be seen that the cleared soils began to wet up faster than did the forested soils. This occurred when grass growth had ceased, suggesting that the fall rains are more effective in wetting up soils of the cleared treatment. We also observed that during some of these rain events, the forest canopy appeared to intercept a considerable proportion of the precipitation and much of it did not reach the forest floor. The amount of interception and its effect on stand water balance depends on several factors, but intercepted water can evaporate at several times the rate of transpiration from unwetted foliage (Pallardy et al., 1995). Thus, canopy evaporation appears to be an important mechanism by which soil moisture recharge is delayed under the forested treatment relative to the cleared treatment. It is also possible that the trees are able to transpire additional water in the fall when moisture becomes available and temperatures are still relatively warm, while transpiration by grasses and forbs has effectively ceased until the following spring.
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Fig. 6. Selected seasonal soil-water contents for the Fragixeralf site during 1996. Each point represents the mean of three measurements.
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SUMMARY
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Based on the results of this study, we offer the following explanation for the seasonal PWT dynamics in fragipan-containing soils under cleared and forested conditions. As the water year begins in the fall, soil-water content of both cleared and forested treatments is roughly equivalent and at the annual minimum. With the onset of fall rains, precipitation is more effectively added to soils of the cleared treatment. This occurs as a result of greater interception and subsequent evaporation losses from the forest canopy compared with the cleared areas. Transpirational water losses are presumably negligible from senescent grasses and forbs of the cleared areas when compared with the forest. Thus, as precipitation continues and temperatures decrease, soils become saturated more quickly under the cleared treatment. Although there is a lag in development of saturated conditions under the canopy, this lag may be minimized by abnormally high quantities of precipitation. After sufficient precipitation has been received, an equivalent water storage point is reached. It represents the point at which the soils have similar PWTs, and after which, the PWTs respond generally in a similar manner. It also marks the point at which vegetation differences have a relatively minor effect on PWT dynamics until later in the spring when temperatures warm up. Relatively little difference is seen in the timing of PWT disappearance in late spring, suggesting that the increasing evapotranspiration rates in both treatments are sufficiently high to cause rapid PWT dry down.
Data from this study demonstrate that PWT dynamics and, therefore, suitability of fragipan-containing soils for certain land uses can be altered by removal of the forest canopy. Seasonal PWTs under cleared areas may develop up to two months earlier compared with those under forest canopy and have significantly higher PWT levels for up to 3 mo. The increased duration of episaturation and greater yield of water can increase the potential for lateral transport to occur in these landscapes as well as affect soil suitability for roads, waste disposal, and building-site development. Establishing cleared phases of these fragipan-containing forest soils may represent a means by which land-use interpretations could be improved.
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ACKNOWLEDGMENTS
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The authors gratefully acknowledge the financial support of the Stillinger Trust for forestry research at the University of Idaho. We are also grateful to Glenn Hoffmann and Brian Gardner of the USDA-NRCS for their invaluable field assistance.
Received for publication December 13, 2002.
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REFERENCES
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ABSTRACT
INTRODUCTION
