Field and Laboratory Measurements of Unsaturated Zone Hydrologic Response After Wildfire.

One of Glendon Gee’s most important contributions to the field of soil physics is the dewpoint potentiameter for measuring matric potentials in porous media under dry conditions. An example where extremely dry conditions exist is after wildfire. Here we report measurements of matric potential using a dewpoint potentiameter for characterization of soil conditions in the first year after a wildfire including the post-fire hydrologic response to precipitation events. The field site is in an area of Colorado, USA burned by the Fourmile Canyon Fire in September, 2010. Field samples analyzed for matric potentials coupled to thermogravimetric soil-water content estimates allow the capture of the initial conditions prior to rainfall, which is a critical control on infiltration and runoff generation. Dewpoint potentiameter measurements on controlled wetting experiments in the laboratory are used to estimate soil-water retention curves and investigate thresholds of matric potential and soil-water content that demarcate regions of elevated water repellency, which also substantially impacts infiltration in burned areas.  The unsaturated zone hydrologic response is further characterized using automated frequency-domain dielectric constant sensors, heat dissipation probes, tensiometers, raingages, and Parshall flumes that capture runoff. Our findings illustrate the value of Glendon’s contributions to soil physics for investigating post-wildfire hydrology and to decipher the complex processes that increase the potential for damaging floods and debris flows.