Fifty-four permanent monitoring sites were established in July 2016 along alternating convergent and divergent hillslopes within a 10-ha north-facing forested slope of Watershed 1 of the HJ Andrews Experimental Forest. A map of classified Topographic Position Index (TPI) was used to guide the placement of soil monitoring sites. TPI was estimated from a 1x1 m digital elevation model by subtracting the elevation of a grid cell from the mean elevation of all cells within a 30 m radius (Jenness, 2006). Positive TPI values represented divergent slope positions where the elevation of a pixel was high relative to the average surrounding locations. Conversely, negative TPI values represented convergent slope positions where the elevation of a pixel was low relative to surroundings. Locations where TPI was close to zero were relatively planar. Individual hillslopes were distinguished by a switch between negative and positive TPI along the same elevation contour.
Fourteen surveys were conducted to investigate the effect of topography on soil water content using time domain reflectometry at 0–30 cm and 0–60 cm depth. Soil moisture surveys were preformed from August–October 2016 and April–October 2017. These data include volumetric water content and date. Soil properties (bulk density, percent sand, percent clay, percent silt, gravimetric rock content, water retention, and saturated hydraulic conductivity) were measured at 15 and 45 cm depth at 13 sites during the summer of 2017. Total soil depth and resistance to penetration were measured at 38 sites using a dynamic cone penetrometer.
Soil volumetric water content (VWC) was measured at 54 sites using time domain reflectometry (TDR; model No. 1502C, Tektronix Inc., Beaverton, OR). VWC was measured over two mineral soil layers, 0–30 and 0–60 cm. Measurements points were replicated at each site so there were two VWC measurements for each depth. A measurement point consisted of a pair of TDR rods installed vertically, 5 cm apart, in mineral soil. The replicate measurement points were, on average, 2.5 m apart and the measurement volume of each point was 30 cm or 60 cm deep and 10 cm in diameter (Topp et al. 1980). The reflection trace was converted from TDR to VWC using a calibration equation developed in a nearby watershed (Gray & Spies, 1995). VWC was estimated at 30-60 cm using the VWC from adjacent 0-30 and 0-60 cm probes after accounting for differences in volume sampled: VWC(30-60cm) =2 × VWC(0-60cm) - VWC(0-30cm). The sensitivity of the instrument was 0.01 cm3/cm3 as determined from repeat measurements less than one minute apart.
Soils were collected by digging a shallow pit and exposing an undisturbed soil face. Soils overlying each sampling depth, 15 and 45 cm, were removed and a 250 cm3 metal cylinder (5 cm tall and 8 cm diameter) was pounded vertically into the soil. Saturated hydraulic conductivity (Ks) was measured in the laboratory using the falling head method on the KSAT device (METER Group Inc.). The Ks was averaged from five repeated measurements from each soil core. Soils from individual cores were subsequently dried at 105 deg C for 24 hours and the oven-dried weight was divided by the sample volume to determine the bulk density. The dried soil sample was used to quantify gravimetric coarse content and particle size. Coarse material consisted of 2-5 mm rock fragments, weathered saprolite fragments, roots, and wood. The sample was placed in a mortar and lightly tapped with a pestle to break soil aggregates but preserve saprolite fragments. The sample was then passed through a 2 mm sieve to remove coarse material, including saprolite fragments. The sieved soil was mixed, a 5-7 g subsample was collected, and then organic carbon was removed from the subsample using the hydrogen peroxide method (Mikutta et al., 2005). The subsample was sent to the Critical Zone Lab at Virginia Tech for particle size analysis using laser diffractometry on a CILAS 1190 laser particle size analyzer (Miller & Schaetzl, 2012).
Depth to bedrock was measured at 38 sites using a dynamic cone penetrometer, which is also known as a knocking pole (Shanley et al., 2003; Yoshinaga & Ohnuki, 1995). The pole consisted of 0.5 m graduated steel rod segments and a 20 mm long and 24 mm diameter cone tip, which was driven into the soil by repeated drops of a 5 kg weight onto a platform threaded on the upper segment of the pole. When the resistance to penetration became large (moving less than 1 cm in 15 or more knocks), it was assumed the cone tip had reached bedrock. Soil depth to bedrock was calculated at each site from the average of 2-3 repeat measurements taken approximately 5-10 m apart.
All soil sensors were purchased from METER Environment. Dielectric permittivity was measured at 5 and 50 cm using the 5TM water content and temperature sensor and at 100 cm using the TEROS 12 water content, temperature, and conductivity sensor. VWC was calculated from dielectric permittivity using the manufacturer’s equation, which follows Topp et al. (1980). Soil water potential was measured at 50 cm using the TEROS 21 water potential and temperature sensor. A small soil pit was dug to expose an undisturbed soil profile and sensors installed horizontally at 5 and 50 cm. To install the VWC sensor at 100 cm, a hole was augured and the TEROS borehole installation tool (METER Environment) used to insert the sensor probes horizontally into undisturbed soil at 100 cm.
Gray, Andrew N.; Spies, Thomas A. 1995. Water content measurement in forest soils and decayed wood using time domain reflectometry. Canadian Journal of Forest Research. 25: 376-385.
Miller, B. A., & Schaetzl, R. J. (2012). Precision of soil particle size analysis using laser diffractometry. Soil Science Society of America Journal, 76(5), 1719–1727. https://doi.org/10.2136/sssaj2011.0303
Mikutta et al., 2005
Shanley, J. B., Hjerdt, K. N., McDonnell, J. J., & Kendall, C. (2003). Shallow water table fluctuations in relation to soil penetration resistance. Ground Water, 41(7), 964–972. https://doi.org/10.1111/j.1745-6584.2003.tb02438.x
Topp, G. C., Davis, J. L., & Annan, A. P. (1980). Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resources Research, 16(3), 574–582. https://doi.org/10.1029/WR016i003p00574
Yoshinaga, S., & Ohnuki, Y. (1995). Estimation of soil physical properties from a handy dynamic cone penetrometer test. Journal of the Japan Society of Erosion Control Engineers, 48, 22–28.
Our study was located in Watershed 1, a 96-ha catchment at the H. J. Andrews Experimental Forest on the west slope of the central Cascade Mountains of Oregon, USA (44°12’18.8” N, 122°15’16.2” W). The average elevation of the study area is 576 m and the average slope is 37 degrees. The study catchment was 100 % clearcut from 1962–1966 and logging residues were burned in 1966 to expose a mineral soil seedbed. There were several efforts to re-establish vegetation in the watershed. The watershed was aerially seeded with Douglas-fir (Pseudotsuga menziesii) in 1967 and 10 ha were re-seeded in 1968. In 1969, 2-yr-old Douglas-fir trees were planted across the entire watershed, and in 1971, 40 ha were re-planted with 2- and 3-yr-old trees (Halpern, 1988). Forty to fifty-year-old Douglas-fir trees dominate the overstory. While much less common, both bigleaf maple (Acer macrophyllum) and western hemlock (Tsuga heterophylla) are also present. The understory includes vine maple (Acer circinatum), Oregon grape (Mahonia aquifolium), and sword fern (Polystichum munitum).
The average depth of the forest floor and organic horizon was 5 cm at our soil measurement points. Mineral soil in the top 100 cm was generally gravelly, silty clay loam with developed A and B horizons, which had gradual and poorly defined boundaries. Soils were underlain by unconsolidated, highly weathered saprolite and fractured bedrock (Gabrielli et al. 2012). Soil thickness ranged from 20 cm to more than 5 m. Parent materials primarily include tuffs and breccias, but basalts and andesites are also present (Halpern, 1988).
The regional climate is characterized by cool, wet winters and warm, dry summers. During the period of study, from August 2016 to October 2017, the site received 2,833 mm of rainfall, which was slightly more than the long-term average. The average rainfall total during the 15-month period from August to October during 1979–2015 was 2,450 mm, the maximum total rainfall was 3,512 mm and minimum was 1,607 mm (Daly et al., 2019). The spatial variability of rainfall interception can create differences in VWC if measured under the canopy or under gaps in the canopy (Gray et al., 2002). Thus, we avoided locations under large canopy gaps when establishing soil moisture monitoring sites to minimize differences in VWC associated with interception.
