TWO ELECTRIC POTENTIAL SIGNATURES OF SERIAL DIURNAL FROST

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Abstract
During a 96-hour period in mid-March 1987, a series of four serial frost-thaw events occurred in a research field at the University of Michigan Matthaei Botanical Gardens. Soil temperature and electric potential were recorded at two near surface probe arrays (0, 3, 6 & 9 cm) and (0, 5, 10 & 15 cm) with a horizontal separation of approximately 1 m beneath the organic-mineral soil interface at hourly intervals. These time series revealed strong repetitive serial patterns. When the electric potential measurements were converted to a surrogate measure of electrolyte concentration, the C-index, strong coupled flow effects appeared to be present in the time series owing to the effects of surface freeze-thaw, evaporation-distillation and soil water advection. The data set shows the low frequency effects of upward soil water advection increasing the soil temperature and modulating the electrolyte concentration. These observations also demonstrate that soil freezing processes have two distinct regimes dominated by (1) early soil water advection toward the frost front from below and (2) downward expulsion of electrolyte-rich water as ice fills the pore space. These two processes of soil water advection and expulsion augmented by local evaporation produce two distinct near surface C-index signatures of serial diurnal frost. The signature depends on the direction of the vertical electrolyte gradient near the surface. [Key words: soil, frost-thaw, soil water, soil temperature.]

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