Direct observation of hillslope scale StorAge Selection functions in an
experimental hydrologic system: Geomorphologic structure and the
preferential discharge of old water
Abstract
Spatially-integrated water transport dynamics at the hillslope scale
have rarely been observed directly, and underlying physical mechanisms
of those dynamics are poorly understood. We present time-variable
transit time distributions (TTDs) and StorAge Selection (SAS) functions
observed during a 28 days tracer experiment conducted at the Landscape
Evolution Observatory (LEO), Biosphere 2, University of Arizona, AZ,
USA. The observed form of the SAS functions is concave, meaning that
older water in the hillslope was discharged more than younger water. The
concavity is, in part, explained by the relative importance of advective
and diffusive water dynamics and by the geomorphologic structure of the
hillslopes. A simple numerical examination illustrates that, for
straight plan shape hillslopes, the saturated zone SAS function is
concave when hillslope Peclet (Pe) number is large. We also investigated
the effect of hillslope planform geometry on the SAS function: The more
convergent the plan shape is, the more concave the SAS function is. A
numerical examination also indicates that the unsaturated zone SAS
function is concave for straight and convergent hillslopes, when the
soil thickness is constant. The concavity of those sub-component SAS
functions signifies that the hillslope scale SAS function is concave for
straight or convergent plan shape hillslopes when the hillslope Pe
number is high.