Critical pathways programme: Unravelling sub-catchment scale nitrogen delivery to waterways

Abstract

To be effective and efficient, decision making on land use, land management, mitigation measures, as well as policy, need to be based on a clear understanding of cause-effect relationships. Present practice is to link activities on the land and water quality outcomes at spatial scales of 100s to 1000s of km². However, such large catchments are inevitably heterogeneous. Consequently, it is exceptionally difficult to link an observed contaminant flux at the catchment outlet to the many past and present activities within the large catchment that collectively have caused it. The need to focus on the sub-catchment scale (10s of km²), i.e. the local streams that feed the large rivers that are typically being monitored has therefore recently been emphasised internationally. To unravel sub-catchment scale nitrogen delivery to waterways, we are introducing an innovative multi-scale measurement, data analysis and modelling approach that allows to coherently link transect, sub-catchment and catchment scale hydrogeophysical information. Three key innovations will collectively enable us to achieve this. Firstly, we will introduce a novel geophysical measurement suite (airborne and ground-based) to gain information on structural, hydrological, and chemical characteristics controlling N transport and attenuation, particularly in the shallow subsurface zone (top 20m). Secondly, innovative Environmental Data Analytics (EDA) techniques will be used to integrate the information from the ‘Big Data’ created by the new geophysical measurements. Thirdly, we will use the hydrogeophysical units, identified by EDA together with Lidar to conceptualise and develop a numerical structure for catchment-scale flow models. To simulate the sub-catchment scale flow, transport, and attenuation, we will nest finer resolution models within the coarser catchment models using information gathered at the sub-catchment scale. Two intensively farmed catchments with contrasting hydrological and biogeochemical conditions provide our case studies. The Waiotapu Stream catchment (≈ 312 km²) on the North Island’s Central Plateau represents a baseflow-dominated upland catchment with a 2 large groundwater reservoir in young volcanic deposits. In contrast, the Piako River headwater catchment is a lowland catchment (≈ 104 km² ) in the upper Hauraki Plains with aquifer deposits of lower transmissivity and a high quickflow fraction in the flow hydrograph.