Search

To better understand flow pathways and patterns in the subsurface, a stable isotope monitoring network was established at the Susquehanna-Shale Hills Critical Zone Observatory (SSHCZO). Soil water samples were collected approximately biweekly using suction-cup lysimeters installed at multiple depths along four different transects in the catchment. Groundwater and stream water were collected daily in the valley using automatic samplers, while precipitation samples were collected automatically on an event basis. The 3+ years (2008–2012) of monitoring data showed strong seasonal precipitation isotope compositions, which were imprinted in seasonal patterns of soil water at different spatial locations and depths. The groundwater isotope composition remained relatively constant throughout the year and closely matched the yearly amount-weighted precipitation average, suggesting groundwater received recharge water in each season, although recharge mechanisms differed between growing and nongrowing seasons. Soil water samples showed clear attenuation with depth, with the largest variability in the shallow soil water (≤30 cm) mirroring precipitation inputs, moderate variability in the intermediate depths (40–100 cm), and the least variability in the deep soil water (≥120 cm) where the average remained near the groundwater average. Soil water isotope composition profiles also provided clear evidence for preferential flow occurring both laterally and vertically in different seasons and at various soil depths in the catchment. Putting all together, the extensive dataset of soil water isotopic compositions obtained in this study have provided a number of insights into complex subsurface hydrologic processes that are transferable to other similar landscapes.

European Union policy on soil threats and soil protection has prioritized new research to address global soil threats. This research draws on the methodology of Critical Zone Observatories (CZOs) to focus a critical mass of international, multidisciplinary expertise at specific field sites. These CZOs were selected as part of an experimental design to study soil processes and ecosystem function along a hypothesized soil life cycle—from incipient soil formation where new parent material is being deposited, to highly degraded soils that have experienced millennia of intensive land use. Further CZOs have been selected to broaden the range of soil environments and data sets to test soil process models that represent the stages of the soil life cycle. The scientific methodology for this research focuses on the central role of soil structure and soil aggregate formation and stability in soil processes. Research methods include detailed analysis and mathematical modeling of soil properties related to aggregate formation and their relation to key processes of reactive transport, nutrient transformation, and C and food web dynamics in soil ecosystems. Within this program of research, quantification of soil processes across an international network of CZOs is focused on understanding soil ecosystem services including their quantitative monetary valuation within the soil life cycle. Further experimental design at the global scale is enabled by this type of international CZO network. One example is a proposed experiment to study soil ecosystem services along planetary-scale environmental gradients. This would allow scientists to gain insight into the responses of soil processes to increasing human pressures on Earth's critical zone that arise through rapidly changing land use and climate.

Surface water, the vadose zone, and groundwater are linked components of a hydrologic conƟ nuum. In order to capture the interacƟ on between diff erent components of a hydrologic conƟ nuum and to use this understanding in water management situaƟ ons, an accurate numerical model is needed. The quality of model results depends on accurate representaƟ on of the physical processes and the data describing the area of interest, as well as performance of the numerical formulaƟ on implemented. Here we present a physics-based, distributed, fully coupled, second-order accurate, upwind cell-centered, constrained unstructured mesh based fi nite-volume modeling framework (FIHM) that simultaneously solves two-dimensional unsteady overland fl ow and three-dimensional variably saturated subsurface fl ow in heterogeneous, anisotropic domains. A mulƟ dimensional linear reconstrucƟ on of the hydraulic gradients (surface and subsurface) is used to achieve second-order accuracy. Accuracy and effi ciency in raster data and vector-boundary representaƟ ons are facilitated through the use of constrained Delaunay meshes in domain discreƟ zaƟ on. The experiments presented here (i) explore the infl uence of iniƟ al moisture condiƟ ons, soil properƟ es, anisotropy, and heterogeneity in determining the pressure head distribuƟ ons in the vadose and saturated zones, (ii) show the existence of localized “fl ux rotaƟ on” phenomenon due to heterogeneous anisotropy, leading to the creaƟ on of convergence–divergence zones, (iii) show the infl uence of verƟ cal drainage from unsaturated zone on the response of an unconfi ned aquifer to pumping, and (iv) show the eff ects of capillarity, saturaƟ on excess, infi ltraƟ on excess, and iniƟ al water table locaƟ on on determining the overland fl ow generaƟ on.