Field exploration of coupled hydrological and biogeochemical catchment responses and a unifying perceptual model

This work presents the details of a focused field experiment carried out to understand the coupled processes of runoff generation and biogeochemical cycling in an agricultural catchment in Western Australia, with a particular focus on NO3- cycling and transport. Detailed hydrometric, chemical, and geochemical data corresponding to a shallow perched aquifer along a topo-sequence in the hillslope helped identify the existence of two hydrologically and chemically distinct landscape units, i.e., upland and riparian zones. Riparian zones control the catchment storm response while upland zones can be considered as storage units, controlling the base flow component of streamflow. The biogeochemical role and functioning of these landscape units are also clear. Upland zones constitute the sources of NO3- and participate in down-slope transport, while riparian areas act as sinks for NO3- and are responsible for its depletion. The experimental results highlighted the critical role of the shallow perched aquifer whose space–time dynamics governed the periodic connection and disconnection between riparian and upland zones, which have a significant impact on the catchment’s hydrological and hydrochemical responses. The role of topography was also highlighted, as different mechanisms of NO3- attenuation in the riparian zones are possible depending on the steepness of the hillslope, with differences in slope leading to different velocities and flow rates and impacting on relative dominance of the different mechanisms of NO3- depletion. These detailed analyses have helped us to develop a “unifying perceptual model” of the coupled hydrological and biogeochemical response of Susannah Brook catchment, which will advance our ability to generalize these results to other hillslopes, and eventually to other catchments in the region.