Despite the widespread acceptance of hydrologic importance, controls on the initiation of preferential flow in natural soil profiles and the frequency of its occurrence at different times of year remain elusive. This study determined the controls and frequency of preferential flow occurrence in the Shale Hills Critical Zone Observatory. Soil moisture profiles and precipitation were monitored at 10 sites along a topographic gradient for >3 yr, encompassing 175 precipitation events. For each event and each site, the flow regime was classified as either preferential flow, sequential flow, or nondetectable flow based on the sequence of soil moisture response at various depths within the same site. Preferential flow here specifically refers to out-of-sequence soil moisture response, with a deeper horizon responding to precipitation earlier than a shallower horizon. Indices describing antecedent precipitation, precipitation characteristics, precipitation timing, and initial soil moisture were examined to determine the characteristics of events that resulted in preferential flow vs. those that resulted in sequential flow. Analyses showed that preferential flow was common throughout the catchment, occurring during 17 to 54% of the 175 events at each of the 10 monitored sites. Preferential flow occurred in at least one site during 90% of the 175 events. While the frequency of preferential flow appeared insensitive to topographic position, the controls on preferential flow initiation varied with landscape position. Analysis of subsets of the time series data showed that while the frequency of preferential flow can be determined from 1 yr of real-time monitoring, the controls on preferential flow require much longer (≥3 yr) monitoring to be reliably identified.

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