Site Physical Description

The PMRW is located in the Panola Mountain State Conservation Park, in the Piedmont of Georgia, USA (84°10’W, 33°37’N) (Figure 1) located 25 km southeast of Atlanta. Studies began at the PMRW in 1985 as part of the USGS Acid Rain Thrust Program (Huntington et al., 1993). In 1991, the 41-ha forested watershed became one of five Water, Energy and Biogeochemical Budgets (WEBB) sites, focusing research on the movement of water and solutes within small forested watersheds and the effects of anthropogenic and environmental change (Lins, 1994).

Climate at the PMRW is humid continental to subtropical. A long growing season, warm temperatures, and many sunny days result in high rates of evapotranspiration (ET), particularly during summer. Air temperature averages 15.2°C annually and the average monthly temperatures range from 5.5°C during January to 25.2°C during July (NOAA, 1991). Peters et al. (2003) report that during water years (October through September) 1986 to 2001, annual precipitation averaged 1220 mm and ranged from ~760 to 1580 mm; less than 1% of the precipitation occurred as snow; annual runoff averaged 377 mm and ranged from ~150 to 700 mm; and annual water yield averaged 30% and ranged from 16% to 50%. Winter frontal systems provide long, typically low intensity rainstorms in contrast to short intense convective thunderstorms in spring and summer (from April through September). During the first 6 months of 2002, i.e., the period of this datanote, PMRW was drier than average with slightly more frequent but smaller rainstorms compared to the 1989-2001 period.

PMRW is covered with a mixed deciduous/coniferous forest; the oldest deciduous trees are ~130 years old and the oldest coniferous trees are ~70 years old (Carter, 1978; Cappellato, 1991). The dominant deciduous tree species are Carya tomentosa (mockernut hickory), Carya glabara (pignut hickory), Quercus rubra (northern red oak), Quercus alba (white oak), and Lirodendron tulipifera (tulip poplar). The dominant coniferous tree species is Pinus taeda (loblolly pine). Land-use records indicate that the original forest was cut in the early 1800s and was farmed until the early 1900s (Huntington, 1995). The trees on the PMRW hillslope consists primarily of hickory and oak; some of the largest trees are oaks.

The PMRW hillslope was established in 1995 with the excavation of a 1.5 m-wide, 20-m-long trench, at the base of a 48-50 m-long hillslope to examine active hydrologic pathways delivering water to the ephemeral stream channel 30 m downslope (McDonnell et al., 1996). The PMRW hillslope is bounded at the top by a small granitic bedrock outcrop, which extends ~10 m to the watershed divide (Figure 1). The PMRW hillslope faces south with an average slope of 13º. The surface of the PMRW hillslope, surveyed on a 2 x 2 m grid with a total station (Burns et al, 1998; Freer et al., 1997, 2002) and interpolated to a 1 x 1 m resolution, is planar with a linear zone of depression in the middle of the hillslope (Figure 2). Soil depth was determined on the same 2 x 2 m grid using a soil corer and hand-auger, where the distinction of the overlying soil from saprolite and weathered bedrock below was defined by corer or auger refusal (Zumbuhl, 1998; Freer et al., 2002). In contrast to the planar surface, the subsurface topography (referred to herein as bedrock topography) is highly irregular. Soil depth ranges from 0 to 1.86 m. Eleven ground penetrating radar (GPR) profiles of the PMRW hillslope provide additional data to which surveyed depths to bedrock can be compared (Zumbuhl, 1998).

Soils on the PMRW hillslope and adjacent hillslopes are sandy-loams of the Ashlar-Wake complex (Zumbuhl, 1998; McIntosh et al., 1999) and are underlain by 320 M yr old Panola Granite, a biotite-oligioclase-quartz microcline granodiorite (Higgins et al, 1988; Crawford et al., 1999). Description of soil profiles include color-based horizons of sandy-loam: a dark, grayish brown surface layer, a yellowish brown subsoil, and a brown soil in some locations (McIntosh et al, 1999) or a single layer of yellowish brown subsoil extending the length of the profile (Tromp-van Meerveld, unpublished field notes, 2002).

Data characterizing the sandy-loam soils have been collected and/or estimated at varying scales. Detailed analysis of four small cores (5 cm diameter by 30 cm long) and one large (38 cm diameter by 38 cm long) soil core from nearby PMRW hillslope sites provide bulk density, porosity and saturated hydraulic conductivity (Ksat) (McIntosh et al., 1999). Average attributes of the four small soil cores at the middle and bottom horizons are 1.41 and 1.38 g cm-3 for bulk density and 0.47 and 0.48 for porosity, respectively. Porosities estimated from in-situ time domain reflectometry measurements of the large undisturbed soil core are 0.65, 0.58, and 0.51 for top, middle, and bottom horizons, respectively. Similar estimates of soil porosity were obtained from oisture retention curves for soil cores taken directly from the PMRW hillslope (Tromp-van Meerveld, unpublished data, 2003). Porosities range from 0.64 to 0.52 for 15, 40 and 70 cm depths below land surface, with the highest porosity near the surface and the lowest at 40 cm. Vertical Ksat using the constant head method on the large soil core is 64 cm hr-1 at a depth of 10 cm (McIntosh et al., 1999). Also, laboratory moisture retention curves using a tension table are available for the sandy-loam hillslope soils for depths of 15, 40 and 70 cm below land surface (Tromp-van Meerveld, unpublished data, 2003).

The Panola Granite regolith is described in McIntosh et al. (1999) as a soft layer of highly weathered bedrock above a harder bedrock layer. During the installation of wells and soil moisture access tubes across the experimental hillslope, saprolite was found intermittently and predominantly in the deepest soil sections of the hillslope. Bedrock in the trench is competent. Typical values of Ksat for weathered granite range from 0.36 to 7.2 cm hr-1 (Morris and Johnson, 1967). Although direct measurements of Ksat for the bedrock underlying the experimental hillslope do not exist, indirect evidence and nearby estimates are available. Bedrock permeability estimates, derived from sprinkler experiments conducted on lower sections of the hillslope adjacent to the trench, range from 0.5 to 3.6 cm hr-1 (Tromp-van Meerveld et al., 2007; C.B. Graham, unpublished data, 2006). Direct measurements of Ksat were made on cores collected from a nearby ridge top profile using a falling head permeameter (White et al., 2002). Ksat of the B horizon from a core drilled from a nearby ridge top at 4 depths (0.78 to 1.75 m below land surface) range from 1.0 x 10-2 to 5.0 x 10-4 cm hr-1, with the lowest Ksat near the surface. At the transition between the B horizon and the underlying saprolite (~ 1.75 m), a duripan, consisting of a 10-cm thick clay layer, was a confining unit. Below this, an increase in Ksat was measured, with B horizon/saprolite (at 1.85 cm) and saprolite (at 2.35 cm) Ksat values of 0.03 and 1.83 cm hr-1, respectively (White et al., 2002). In contrast, unsaturated conductivities Kunsat decrease exponentially with decreases in regolith water content (Stonestrom et al., 1998). Therefore, Kunsat can be several orders of magnitude lower than Ksat.

Five individual macropores (Figure 2), 10 to 60 mm in diameter, were identified during the excavation of the trench to intersect the trench face and to deliver water (Burns et al., 1998; Freer et al., 2002; Uchida et al., 2005). The development and delivery of subsurface stormflow from both the soil matrix and the five individual macropores was monitored using custom tipping bucket gauges (Burns et al., 1998; Freer et al., 1997; 2002; Tromp-van Meerveld 2006b). The gauges were plumbed to 2-m sections of the trench using plastic walls (10 cm high) sealed to the rock surface with epoxy and to the individual macropores (Freer et al., 2002).

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