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United States Department of Agriculture

Agricultural Research Service

Research Project: Technologies for Managing Water and Sediment Movement in Agricultural Watersheds

Location: Watershed Physical Processes Research

Title: Hydrologic connectivity and threshold behavior of hillslopes with fragipans and soil pipe networks

Author
item Wilson, Glenn
item Nieber, John
item Fox, Garey
item Vacant,
item Ursic, Michael - Mick
item Rigby, James - Jr

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2017
Publication Date: 6/30/2017
Citation: Wilson, G.V., Nieber, J., Fox, G., Dabney, S.M., Ursic, M.E., Rigby Jr, J.R. 2017. Hydrologic connectivity and threshold behavior of hillslopes with fragipans and soil pipe networks. Hydrological Processes. 31(13): 2477-2496. DOI: 10.1002/hyp.11212.

Interpretive Summary: Many concepts have been proposed to explain the connectivity of soil water on hillslopes with streams. Water connectivity is most often defined by the presence of perched water tables over large areas of hillslopes or large areas of near saturated soils. Direct measurements of water movement from hillslopes to catchment outlets, such as perched water tables on hillslope feeding soil pipes that flow to outlets is lacking. The objectives of this study were to characterize the changes in flow through soil pipes during storm events, the relationships between perched water tables on hillslopes and pipeflows, and the threshold in precipitation (P) or soil water storage (ASI) to generate pipeflow. Hillslopes were instrumented with shallow wells adjacent to soil pipes and equipped with sensors to monitor water levels. Perched water tables developed on hillslopes during a wetting up period (October –December) and became well connected across hillslope positions throughout the high flow period (January – March). Water tables were generally not connected on hillslopes during the drying out (April-June) and low flow (July-September) periods. Even when perched water tables were not well-connected on hillslopes, water was observed to flow through soil pipes during storm events, thereby providing connectivity of upper hillslopes with catchment outlets. Correlations between soil pipeflow and perched water tables were not consistent among locations as the correlation depended upon the size and location of the soil pipes. In order for soil pipes to flow, a threshold in available soil-moisture index plus storm precipitation (ASI+P) had to be exceeded. This threshold value depended upon the season and was most appropriate during dry periods and not high flow seasons. This study suggested that percolation theory may provide a comprehensive approach for modeling hillslope water table changes with flows through soil pipes from hillslopes to streams.

Technical Abstract: Many concepts have been proposed to explain hydrologic connectivity of hillslopes with streams. Hydrologic connectivity is most often defined by qualitative assessment of spatial patterns in perched water tables or soil moisture on hillslopes without a direct linkage of the connection of water flow from hillslopes to streams. Duplex soils are known for developing perched water tables on hillslopes and fostering lateral flows, but the connectivity of localized measurements of perched water in soils with a flow through soil pipes networks is lacking. The objectives of this study were to characterize pipeflow dynamics during storm events, the relationships between perched water tables on hillslopes and pipeflows, and their threshold behavior. Two well characterized catchments in loess soil with a fragipan were selected for study because they contain multiple soil pipe networks that extend from upper hillslope positions to their catchment outlet over 100 m downslope. Hillslopes were instrumented with shallow wells adjacent to the soil pipes that were equipped with pressure transducers in pipe collapse features. Perched water tables developed on hillslopes during a wetting up period (October –December) and became well connected spatially across hillslope positions throughout the high flow period (January – March). The water table was not spatially connected on hillslopes during the drying out (April-June) and low flow (July-September) periods. Even when perched water tables were not well-connected, soil pipes exhibited flows that provided hydrologic connectivity between upper hillslopes and catchment outlets. Correlations between soil pipeflow and perched water tables were inconsistent and dependent upon the size and location of soil pipes. The threshold relationship of available soil-moisture index plus storm precipitation (ASI+P) to pipeflow was dependent upon the season and most appropriate during dry periods and not high flow seasons. This study demonstrated that soil pipes serve as a catchment backbone of preferential flow paths that provide intrinsic connectivity of upper hillslopes with streams. We propose that percolation theory may provide the comprehensive catchment hydrologic framework for integrating hillslope water table dynamics with soil pipeflows.

Last Modified: 08/20/2017
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