Soil moisture patterns in a forested catchment: A hydropedological perspective

To connect pedon and landscape scales of soil moisture, a key lies in the distribution of different soils over the landscape. Mapping the fabric of soils over a watershed helps optimal sampling design and appropriate modeling of landscape hydrology. Such a hydropedological perspective is examined in a 7.9-ha forested catchment in central Pennsylvania in order to understand the spatio-temporal organization of soil moisture and its relationships with soil-landscape features. Soil moisture changes at four depth intervals (0-0.06, 0.11-0.29, 0.51-0.69, and 0.91-1.09 m) were monitored at 30 sites in the catchment for over 3 months in 2004. In addition, a reconnaissance campaign was conducted at 189 points on ten days during a 2.5-month period in 2003. Soil distribution and topographic metrics were correlated with the observed soil moisture patterns to reveal their relationships with soil type, depth to bedrock, topographic wetness index, slope, precipitation, and stream discharge. Among the five soil series identified in the catchment, the Ernest soil series (fine-loamy, mixed, superactive, mesic Aquic Fragiudults) remained the wettest throughout the monitoring periods, which was consistent with its morphology and topographic position (e.g., many redox features and a fragipan-like layer starting at 0.3-0.5 m depth). The Weikert soil series (loamy-skeletal, mixed, active, mesic Lithic Dystrudepts) had the driest condition because of its shallow depth to bedrock and steep slopes. Cluster analysis based on soil depth, topographic wetness index, and local slope showed that the 30 monitoring sites could be grouped into wet, moderately wet, moderately dry, and dry locations that exhibited different spatio-temporal patterns of subsurface soil moisture. Such a grouping correlated with the soil series plus local slopes. Because of complex interplays between soils and topography, the individual contributions from soils and topography to the soil moisture grouping were hard to separate. Time series data showed a quick stream flow response to precipitation forcing, indicating the rapid movement of water within the catchment into the stream channel. A conceptual model of hillslope hydrology in this catchment was developed to elaborate the patterns of soil moisture distribution along the hillslope and within soil profiles, their relations to the soil series, and four main flow pathways downslope (i.e., subsurface macropore flow, subsurface lateral flow at A-B horizon interface, return flow at footslope and toeslope, and flow at the soil-bedrock interface). This conceptualization enhances the understanding and modeling of preferential flow dynamics at the small catchment scale, particularly with regard to the role of detailed soil mapping and lateral flow in hillslope hydrology.