Heather Hunter

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 4m and that increasing the riparian buffer width beyond 10m 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.

Management practices for control of runoff losses from cotton furrows under storm rainfall. III. Cover and wheel traffic effects on nutrients (N and P) in runoff from a black Vertosol

Transport of nitrogen (N) and phosphorus (P) in runoff was measured using a rainfall simulator on a cotton hill–furrow system with a range of on-ground cover (0–60%), each with and without prior wheel traffic in the furrow. Total N and P losses in runoff (kg/ha) from the bare plots on a whole-field basis (wheel and non-wheel tracks) were equivalent to 1.5% and 1%, respectively, of fertiliser applied that season (218 kg N/ha, 27 kg P/ha). This was for a single runoff event of 17 mm, about 1/10 of the total runoff expected in a season. Retaining surface cover and avoiding wheel traffic were both effective in reducing runoff losses of total N and P, especially when used together. Retaining cover gave considerably lower concentrations of total P, and of N and P associated with sediment, with no significant differences (P > 0.05) between wheel and non-wheel tracks. The majority of nutrients were transported with sediment, for P for all treatments, and for N from low cover plots. Concentrations of dissolved N, dominantly as NO3-N, were unaffected by cover on non-wheel tracks but increased with cover on wheel track plots where runoff occurred as shallow interflow. On a whole-field basis, N was mainly in dissolved form at higher covers, because most runoff came from wheel tracks where interflow occurred. Reducing the ratio of wheel tracks to non-wheel tracks will reduce runoff of N and P. Interflow or exfiltration above an infiltration throttle layer is a worst-case scenario for runoff transport of soluble, poorly sorbed chemicals such as NO3-N, which would otherwise leach and not enter runoff. To improve water quality, for both sorbed and dissolved forms, the combination of retaining cover and avoiding wheel traffic and subsoil compaction is needed. Similarly, land uses involving high nutrient inputs should be avoided on soils with shallow subsurface restrictions to infiltration, which are thus prone to interflow. Primary (dispersed) clay and silt were slightly enriched in sediment in runoff, while primary fine and coarse sand were depleted. However, sediments were not enriched in total N and P compared with the soil surface, and organic carbon was only slightly enriched (enrichment ratio 1.06). This is typical of the behaviour of well-aggregated soils of high clay content.