Experimental observation of a hillslope-scale rank StorAge Selection function: Process controls on its functional form, time variability, and hysteresis
The recent development of the StorAge Selection (SAS) function framework provides us with unique opportunities to simulate time variable hydrologic transport in a parsimonious way. In this framework, determining the SAS function is essential as it captures system-scale hydrologic transport dynamics by relating the internal age structure and the age structure of each outflux. However, process controls on the SAS function are poorly understood.
We present the observed rank SAS function, one of the SAS functions, from a 28-days tracer experiment conducted at the Landscape Evolution Observatory – Biosphere 2, University of Arizona, USA. The observed function indicates that the hillslopes preferentially discharged older water. The older water preference was explained partly by the Hillslope number (Hi). The time variability of the function showed hysteretic behavior. Due to this hysteresis, the usual correlation-based methods used to examine processes underlying the time variability is not applicable.
Based on an extensive set of measurements and Richard’s equation-based numerical modeling, we explain the hysteresis based on: the continuous recharge to the saturated zone during the early recession period through the upslope unsaturated zone, and the flow direction change in the tension saturated zone towards the outlet after irrigation stops. Those result in the larger younger water selection of discharge from storage during the early recession period, compared to the selection at the timing of the peak storage and the peak discharge.