2.2 Data collections
At the Shale Hills Catchment, a total of 106 manual soil moisture
measurement locations were recorded on about a weekly basis over a 6-yr
period (2004 to 2010; Fig. 1). A portable TRIME-FM Time Domain
Reflectomery (TDR) Tube Probe (IMKO, Ettlingen, Germany) was used to
determine soil moisture contents while being placed at specific depth
(i.e., 0.1-, 0.2-, 0.4-, 0.6-, 0.8-, and 1.0-m) intervals using a PVC
access tube installed at each sampling site. The number of locations
measured on each measurement day varied due to the number of actual
locations, personnel availability, and also weather conditions. The
subsequent analysis includes 36 days where at least 65 soil moisture
annual measurement locations were measured, as a trade-off between
having a sufficient number of measuring sites to adequately represent
spatial coverage and sufficient sampling days to have a more complete
temporal coverage. Among these 36 measurement days, it can be classified
as13 wet (>22%), 14 moist (<22%,
>15%), and 9 dry (<15%) times based on the
field averaged wetness condition, respectively.
At each sampling site of soil water content, we also measured other
properties that were anticipated to influence the water transport and
storage. The intact soil cores were collected throughout the catchment
during the installation of soil monitoring tubes. Each soil core was
first described using standard soil survey procedures, including horizon
thickness, color, texture, structure, roots, and amount of redoximorphic
features (Fig. 2). A digital elevation model (DEM) of the catchment was
interpolated from light detecting and ranging (LiDAR) elevation point
clouds collected by an airplane flown over the catchment in 2006 (PAMAP,
PA Department of Conservation and Natural Resources). Approximately
40,000 LiDAR elevation points were converted into a 1x1 m DEM using
ArcGIS 9.2 (ESRI Inc., Redland, CA, USA). The smoothed DEM was then use
to derive the four primary topographic attributes (elevation, slope,
curvature, and upslope contributing area) and one composite topographic
attribute (topographic wetness index). Terrain attributes were then
extracted at all soil moisture monitoring locations using the
coordinates obtained from the total station survey.
In addition to spatially extensive manual measurement, soil moisture has
been monitored in real-time at 5 representative sites, with 4-10 depths
at each site depending on soil thickness and horizonation. These sites
are located from the ridge top (site 74) to the valley bottom (site 61),
and both planar (site 51)/convex (site 53) hillslopes and concave swales
(site 15) (Fig. 1). At each long-term monitoring site, a pit was
excavated, and capacitance-type probes of Decagon Devices, Pullman WA
(EC10 or EC5) were used to monitor the profiled soil moisture at 10 min
intervals. A Pluvio load cell rain gauge (OTT Hydrometry, Kempten,
Germany; precision ) was located in a clearing on the north ridge of the
catchment and automatically recorded precipitation every 10 minutes
automatically. Further details on the soil moisture probe installation
can be found in Lin and Zhou (2008).