Conceptualisation and application of models for groundwater–surface water interactions and nitrate attenuation potential in riparian zones

Riparian zones can provide a protective buffer between streams and adjacent land-based activities, by removing nitrate from shallow groundwater flowing through them. Catchment scale water quality models are useful tools for predicting catchment behaviour under various climatic conditions and land use scenarios, but most do not account for the effect of riparian buffer zones. In this paper, we present conceptual models for surface water–groundwater interactions and formulate analytical mathematical functions that describe nitrate removal in the riparian zone. We restrict nitrate attenuation capacity to potential denitrification only and present sample calculations based on the limited field data available. The models are classed into two types according to their applicability to either ephemeral or perennial streams. In ephemeral, low-order streams with the potential to form a perched water table, a simple bucket model is used. During events, stream water flows laterally into the riparian zone and may be denitrified while residing there before discharging back to the surface water system. In perennial middle-order streams, nitrate removal may occur either as base flow intercepts the root zone or when water is temporarily stored in stream banks during flood events. We incorporate these concepts within a GIS modelling framework and investigate the potential of riparian zones to reduce nitrate delivery to streams in the Maroochy catchment located in South East Queensland, Australia. The modelling results for the Maroochy catchment show that the optimum rooting depth is about 4 m and that increasing the riparian buffer width beyond 10 m yields little further reduction in nitrate. The potential nitrate removal capacity per unit length of riparian buffer in each sub-catchment is an attribute that can be used to help to prioritise riparian rehabilitation activities aimed at reducing stream nitrogen loads.