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Temporal Stability of Soil Moisture Spatial Pattern and Subsurface Preferential Flow Pathways in the Shale Hills Catchment

Dep. of Crop and Soil Sciences, 116 ASI Building, The Pennsylvania State Univ., University Park, PA 16802

View larger version (82K): [in a new window]   Fig. 1. The V-shaped Shale Hills Catchment (rendered three-dimensionally) located in central Pennsylvania. The soil map depicts the five soil series identified in the catchment, with four soil series colored differently and the Weikert series covering the rest of the catchment (not colored in order to show the elevation variation). The inset photo shows a landscape view downslope from a point indicated by the blue star. Note the scale bar indicated is only an approximation because the three-dimensional rendering distorted the actual scale.

View larger version (83K): [in a new window]   Fig. 2. The 77 monitoring sites established in this study. The numbers correspond to the site numbers listed in Table 2. The subsoil wetness clusters (wet, moderately wet, moderately dry, and dry) were based on a combined consideration of soil thickness (depth to bedrock), topographic wetness index, and local slope. The background map is the topographic wetness index calculated with Eq. [2]. The red dashed polygons are topographic depressions (swales). Note the scale bar indicated is only an approximation because the three-dimensional rendering distorted the actual scale.

View larger version (44K): [in a new window]   Fig. 3. Daily precipitation and stream discharge at the outlet of the catchment for the period of this study (24 Mar. 2004–19 Mar. 2005). Daily soil moisture data were collected for two periods from 14 to 30 June and from 13 July to 15 Aug. 2004. Detailed precipitation data were collected using 16 rain gauges from 29 Sept. to 8 Dec. 2004.

View larger version (31K): [in a new window]   Fig. 4. Time average plots of ranked relative soil moisture content deviated from the catchment-wide mean value at four monitoring depths. Vertical bars correspond to the associated temporal standard deviation for the 1-yr monitoring period. Site numbers refer to the monitoring locations indicated in Fig. 2 and Table 2. A few sites with relatively large temporal standard deviation are identified using soil moisture clusters of dry (D1), moderately dry (D2), moderately wet (W2), or wet (W1) groups.

View larger version (29K): [in a new window]   Fig. 5. Temporal change of mean soil volumetric moisture content as a function of depth in each of the five soil series. Note that the lines connecting the measurement points are intended for visualization, which do not represent actual soil moisture change between the measurement dates.

View larger version (27K): [in a new window]   Fig. 6. Time average plot of ranked relative soil moisture storage deviated from the catchment-wide mean. Vertical bars correspond to the associated temporal standard deviation over the 1-yr monitoring period. Solid dots and open circles are minimum and maximum values, respectively. Site numbers refer to the monitoring locations indicated in Fig. 2 and Table 2. Soil moisture clusters of dry (D1), moderately dry (D2), moderately wet (W2), and wet (W1) groups are indicated next to each site number in the parentheses.

View larger version (27K): [in a new window]   Fig. 7. Averaged soil moisture storage within 1.1-m solum from 22 July 2004 to 19 Mar. 2005: (A) grouped by the five soil series and (B) comparison of landform units between the north- and south-facing slopes. Note that the lines connecting the measurement points are intended for visualization, which do not represent actual soil moisture change in between the measurement dates.

View larger version (43K): [in a new window]   Fig. 8. Averaged daily soil volumetric moisture content at two monitoring depths from 14 June to 15 Aug. 2004 for the five soil series: (A) 0.11 to 0.29 m and (B) the deepest monitoring depth (0.91–1.09 m or shallower depending on the depth to bedrock). Also shown for comparison are daily precipitation and stream discharge at the outlet of the catchment. Note that the lines linking the measurement points are intended for facilitating visual comparison, but do not imply the actual soil moisture change in between the measurement dates.

View larger version (42K): [in a new window]   Fig. 9. Comparison of 16 rain gauges' data collected from 29 Sept. to 8 Dec. 2004 between the north- (in blue) and south-facing (in red) slopes. The cumulative rainfall curves showed each tipping (0.2 mm of rain) recorded in the rain gauges. The black dots are stream discharge at 15-min intervals as recorded in stream gauging station at the outlet of the catchment.

View larger version (117K): [in a new window]   Fig. 10. Illustrations of observed preferential flow pathways at the Shale Hills Catchment along a hillslope of Weikert–Berks–Ernest soil catena: (A) fractured shale in the upslope area (the Weikert soil), (B) a chipmunk burrow about 5 cm in diameter in the midslope area (the Berks soil) when it is dry vs. when it is saturated, and (C) return flow at the footslope and toeslope area (the Ernest soil) showing macropore bubbling seepage and surface runoff near the stream bank.

View larger version (143K): [in a new window]   Fig. 11. Illustrations of observed preferential flow pathways at the Shale Hills Catchment: (A) subsurface lateral flow occurred as seepage at the interface between the A and B horizons in the Blairton soil pit during Hurricane Ivan vs. when the soil was dry and (B) macropore bubbling seepage that occurred as uplifting at the beginning of the perennial stream in the catchment (near the sediment fence). These two preferential flow pathways were particularly prominent during storm events (e.g., Hurricane Francis and Ivan in September 2004). See Fig. 2 regarding the locations of these two sites within the catchment.