Lateral subsurface flow pathways in a semiarid ponderosa pine hillslope

The mechanisms controlling lateral subsurface flow in semiarid environments have received relatively little attention despite the fact that lateral subsurface flow can be an important runoff process in these environments. The objective of the current study is to better understand lateral subsurface flow process in semiarid environments. Natural chloride, dissolved organic carbon, and stable isotope (delta D and delta(18)O) tracers were used to investigate the lateral subsurface flow process and the chemical changes that occur as a result of lateral subsurface flow. Observed differences in chemistry between soil matrix water and lateral subsurface flow were large (for example, chloride concentrations in matrix soil water samples were >200 mg/L, compared with only 2 mg/L in lateral subsurface flow samples obtained at the same time). This difference in chemistry is indicative of a two-domain flow system in which macropores conduct lateral subsurface flow that is not in chemical or hydrological equilibrium with the soil matrix. The size of precipitation events appeared to have a strong influence on the variations in old/new water percentages, and examples of both old and new water dominated events were observed. There were also large variations in the chemistry of lateral subsurface flow with time. For example, chloride and dissolved organic carbon concentrations were 10 and 70 times greater, respectively, under saturated conditions than under unsaturated conditions.