Pressure Versus Particle Velocities From the Pedon to the Hillslope Scale.

Rapid hydrometric response to precipitation has been observed at many field sites, from the pedon to the hillslope to the catchment scale, in both forested and agricultural systems.  With velocities generally much greater than the hydraulic conductivity of the soil matrix, this rapid response has often been attributed to preferential flow via macropores and inter-pedal fractures.  However, rapid bypass of the soil matrix via preferential flow is often in conflict with evidence that the exfiltrated water is dominated by old, pre-event water.  We will discuss field experimentation designed to differentiate between hydrometric and particle velocities at both the pedon and hillslope scales, using coincident application of a tracer with a hydrometric stimulus.  With this methodology, we demonstrate that pressure response to hydraulic stimulus travels an order of magnitude faster than the tracer, over at sites with very different dominant flow regimes.  The difference in pressure vs. particle velocity demonstrated in these experiments indicates that caution must be used when attributing rapid hydrometric response to stimulus to preferential flow, as the water velocities may be much slower than the pressure response observed in the form of increases in soil water content or lateral discharge from a trenched hillslope.  These experiments demonstrate the value of incorporation of tracer applications when determining characteristic flow velocities.