Understanding water flow and solute transport dynamics at scales of hillslopes and larger is challenging, with direct implications for organisms that rely on water and geochemical reactions activated by water. Detailed process understanding at the pore scale and at the patch or plot scale is of limited utility for upscaling to hillslope or catchment scales mainly because of non-linear dynamics associated with soil hydraulic properties that are not well known across scales. An alternative understanding can be gained using system-scale hydrologic signatures such as storage-discharge (S-Q) relationships and StorAge Selection (SAS) functions. The S-Q relationship explains how much water is flowing out as a function of the volume of water in the system. The SAS function explains which water ages compose the outflux. The SAS function is directly relevant to hydrogeochemical dynamics because water age affects geogenic solute concentrations. However, we lack direct observation of those signatures, and the link between those signatures and underlying hydrological processes is poorly understood. So far, those signatures are usually calibrated, assuming the structural relations a priori and neglecting hysteresis.
LEO offers unique opportunities to observe system-scale hydrologic signatures and internal hydrologic variables. The observed LEO S-Q relationship shows large hysteretic loops (due to time delay in developing the water table), and an “attractor” to which dynamics converge during flow recession. Decomposing the S-Q signature into unsaturated and saturated zones reveals simpler less hysteretic storage-discharge relationships.