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.