Using Soil Moisture and Matric Potential Observations to Identify Subsurface Convergent Flow Pathways Qing Zhu, Henry Lin, and Xiaobo Zhou Dept . Crop and Soil Sciences, The Pennsylvania State University. Introduction and Objective b Subsurface soil water dynamics can influence crop growth and the fate of fertilizers and pesticides. Subsurface soil horizons affect water movement, and hence plant growth, by triggering preferential funnel flow processes. Our objective is to identify subsurface convergent flow pathways at Ap-B horizon, clay layer, and soil-bedrock interfaces from the soil and topography information. 1000-m2 800-m2 500-m2 300-m2 100-m2 Materials and Methods Materials Study site: The Kepler farm. The second order soil map and digital elevation model are available (Fig. 1). Ap Horizon thickness: 145 observations (Fig. 1); map of it was generated using ordinary kriging. Depth to clay layer (clay>40%): 70 observations (Fig. 1); map of it was generated using ordinary kriging. Depth to bedrock: 77 observations (Fig. 1); map of it was generated using regression-kriging. Soil moisture content (Trime-3) and matric potential (tensiometer): 145 and 76 observations (Fig. 1), respectively; collected 11 times (3 in summer 2005, 2 in fall 2005, 2 in summer 2006, 3 in spring 2007, and 1 in summer 2007). Fig. 5. Means and 95% confidence intervals of relative values to saturation for sites on and off the simulated flow pathways at the a) clay layer and b) soil-bedrock interfaces. Mean moisture contents were calculated through the 145 observations at these 11 days. Red dash circles indicate relative values to saturation are significantly higher in sites on the simulated flow pathways. We do not provide such graphics for Ap-B horizon since those relative values between the sites on and off the simulated flow pathways are not significantly different. Fig. 2. Diagram showing the 2-D scheme of typical soil profile and soil moisture collected just above the three interfaces. Results and Discussions Methods DEMs of Ap-B horizon, clay layer, and soil-bedrock interfaces were generated by subtracting Ap thickness, depth to clay layer, and depth to bedrock from the surface DEM (3x3-m resolution). Flow pathways at these interfaces were simulated using ArcGIS hydrologic modeling tools (Fig. 3). Soil water retention curves for all sites were fitted with van Genuchten model from the field observed soil moisture and matric potential data. Moisture contents at field capacity and saturation for different soil textures and horizons were estimated (Fig. 4). Relative values to field capacity and saturation were calculated by dividing soil moisture readings with the mean field capacity and saturation in Fig. 4. For each interface, such relative values of the sites on and off the simulated flow pathways were statistically compared. Relative values just above a interface were also statistically compared with those just below or 0.2-m above it (Fig. 2). 100-m2 100-m2 500-m2 500-m2 Fig. 3. Simulated flow pathways at the soil-bedrock interface by setting thresholds of contribution areas of a cell to initiate the stream flow as a) 1000-m2, b) 800-m2, c) 500-m2, d) 300-m2, and e) 100-m2. Gish et al., 2005 also used the same tool to simulate the flow pathway above clay lens. 500-m2 500-m2 100-m2 100-m2 500-m2 500-m2 100-m2 100-m2 Fig.6. Comparison of means and 95% confidence intervals of relative values just above the interfaces vs. those just below the interfaces or 0.2 to 0.4-m above the interfaces. Red dash circles indicate relative values to saturation are significantly higher in sites on the simulated flow pathways. We only picked thresholds of 100-m2 and 500-m2 as the representatives. This figure indicates that for sites on the simulated flow pathways, relative values to saturation just above these interfaces are significantly higher than those just below or 0.2-m above them. However, such phenomenon can not be found in sites off the simulated flow pathways. Fig. 4. Estimated mean soil moisture contents and their standard deviations at field capacity and saturation for different texture classes and soil horizons at the Kepler Farm. a Summaries and Conclusions Subsurface convergent flow appears to occur at the clay layer and soil- bedrock interfaces, but not at the Ap-B horizon interface. Flow pathways simulated with threshold500-m2 are better to differentiate the relative values between sites on and off the pathways during the dry season. However, during the wet season, flow pathways simulated with thresholds300-m2 work better. 1000-m2 800-m2 500-m2 300-m2 100-m2 Threshold to initiate stream flow Fig. 1. Distribution of soil series, soil cores, tensiometers, and depth to bedrock observations at the Kepler Farm located in central Pennsylvania.