Co-location of continuous sensor data as a function of depth (top), with pore water solution samplers (bottom) provides a means to relate subsurface water fluxes with aqueous geochemical response.  Data shown are for Pedon 1 (planar hillslope location) in MC ZOB.  Soil physical data (top) temperature (T), volumetric water content (VWC), and matric potential (Ψ_m) are collected continuously for direct correlation with pore water chemistry (bottom), shown here for three depths as time series solution charge balance diagrams.

Soil surface horizon sodium mass transfer coefficient (τ_Na), a measure of the relative depletion of Na in soil relative to bedrock, for low sloping, ridgetop landscape positions relative to the humidity index defined as mean annual precipitation divided by potential evapotranspiration.  The τ_Na provides a proxy for the chemical loss of plagioclase due to chemical weathering, whereas the humidity index provides a proxy for water availability, with values greater than one indicating water in excess of evapotranspiration and values less than one indicating water limited systems.  Sodium depletion tends to increase non-linearly with increasing moisture as represented by the dashed line that is the function: y = y₀ + ax^b, fit between surface tNa and humidity index where y₀ = 0.34, a = -0.87 and b = 0.39 (r² = 0.86).  The function was fit excluding the locations with mean annual temperature (MAT) < 5 ^oC, and locations with physical erosion rate (E) < 20 g m^-2 yr^-1.  These data indicate that both erosion rate and temperature in addition to water availability may also exert significant control on Na loss.  Data from Rasmussen et al. (2011).