From the Paddock to the Stream — Unravelling the Nitrogen Flowpaths in a New Zealand Dairying Catchment

Abstract

The Toenepi catchment (15 km 2 ) is dominated by dairying, and ranges in elevation from 40 to 130m above sea level (ASL). Most of the catchment is flat land, with some rolling and steep land occurring on the boundaries. Annual rainfall is 1132 mm and mean annual temperature is 13.3°C. Well-drained Allophanic soils dominate in the catchment in close association with granular soils of moderate permeability. Poorly drained Gley soils occur in the lowest areas adjacent to the stream and require artificial drainage. The average stocking rate is 3.0 cows ha -1 , which graze all year. The catchment export of total nitrogen through the stream had been calculated in an earlier project as 35 kg ha -1 yr -1 . The median total nitrogen concentration in the stream was 3 mg L -1 (1995/97). To better understand nitrogen flowpaths, we initially installed groundwater monitoring transects in seven subcatchments, which reflected the major site and landuse conditions. Monthly sampling indicated that the concentrations of inorganic nitrogen in the shallow groundwater were generally well below the concentrations measured in the stream. This result would not support the hypothesis that the majority of the nitrate in the stream is derived from groundwater. Monitoring of the nitrogen concentrations in drains indicated that artificial drainage may be a major conduit for nitrogen into the stream. Artificial drains bypass the deeper subsoil and riparian zones where denitrification is likely to occur. A mathematical groundwater discharge model is used to quantify the proportion of streamflow that can be explained by groundwater discharge in contrast to near-surface flowpaths (surface runoff, artificial drainage). Understanding the pathways through which nitrogen enters Toenepi Stream is considered a prerequisite for the development of the most effective and efficient measures to reduce the N contamination of the stream.

Dating of streamwater using tritium in a post nuclear bomb pulse world: continuous variation of mean transit time with streamflow

“…The baseflow fraction was 76%, using hourly flow data during the 6-year period [2004][2005][2006][2007][2008][2009]. This value is comparable with the estimates of 85% for 2003 and 71% for 2004 reported by Stenger et al (2005) and derived using a groundwater discharge model (Bidwell, 2003). While there is a year-to-year variation, it is evident that Toenepi streamflow is dominated by groundwater discharge.…”

The Molecular Pathway to ZIF‐7 Microrods Revealed by In Situ Time‐Resolved Small‐ and Wide‐Angle X‐Ray Scattering, Quick‐Scanning Extended X‐Ray Absorption Spectroscopy, and DFT Calculations

“…A mean baseflow index of 0·5 means that 50% of the rainwater reaches the stream via sub‐surface pathways. Stenger et al (2005) estimated baseflow indexes of 0·75 and 0·85 for the larger Toenepi catchment for the years 2003 and 2004 respectively. These higher baseflow indexes could be explained by the higher percentage of well‐drained Allophanic soils as compared to the Upper Toenepi catchment and its Kiwitahi sub‐catchment.…”

“…We might have underestimated the proportion of direct precipitation onto wet areas. Groundwater recharge modelling in the larger Toenepi catchment demonstrated that the groundwater flow path contributes most of the stream flow (Stenger et al , 2005).…”

Dating of streamwater using tritium in a post-bomb world: continuous variation of mean transit time with streamflow

“…The parameters are determined by convoluting the input (rainfall) tritium concentrations to simulate passage through the hydrological system in such a way as to match the output (stream) tritium concentrations. Stenger et al (2009) identified strong redox gradients in the groundwater underlying the well-drained soils that dominate in the Toenepi catchment. The uppermost oxidised groundwater was nitrate-bearing, while the deeper reduced groundwater was nearly devoid of nitrate.…”