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).