Sources and sinks of nutrients in a New Zealand hill pasture catchment III. Nitrogen

A study was made of the nitrogen (N) inputs to, and exports from, a stream draining a pasture catchment near Hamilton, New Zealand, in order to plan measures for minimizing N losses to natural waters. An estimated 7 kg N ha^?1 was exported from the catchment during 1981 of which 86 per cent was in reduced forms (Kjeldahl-N, TKN) and the remainder as nitrate-N (NO₃-N). Virtually all of the reduced N inputs came from saturated overland flow whereas NO₃-N inputs were dominantly subsurface derived. The TKN exported by individual storm events could be predicted (R² = 0.97) from peak flow and from the peak flow rate in the seven days preceding the storm. A TKN balance for eight events showed that except for large floods (return period approximately a year) the stream system was a net sink for TKN. During large floods, scouring of the organic rich seepage areas resulted in the stream system itself being a net source of TKN. Microbial assays for nitrification and denitrification activity indicated that the main nitrate source was the well-aerated greywacke and ash soils and that the permanently saturated seepage zones were a significant nitrate sink. An in-stream nitrate addition experiment showed that up to 20mg N m^?2 h^?1 was removed from the stream. Simultaneous measurements of in situ denitrification activity demonstrated that only about 1 per cent of this removal could be accounted for by denitrification. It was inferred that plant uptake was responsible for the remainder. Retention of near-stream seepage areas is suggested as a measure for minimizing NO₃-N export, whilst removal of stock from seasonally saturated areas during periods of saturatior should reduce soil loss and hence TKN inputs to the stream.     

Estimating the Horizontal Gradient in Heterogeneous, Unconfined Aquifers: Comparison of Three-Point Schemes

At least two approaches may be used to estimate the horizontal components of the hydraulic gradient based on measured heads from three observation points. First, the gradient may be estimated by passing a plane through the measured heads (h-method). Second, if the elevation of the base of the aquifer is known to be spatially constant, an estimate of the gradient may be obtained using the squares of the measured heads (h²- method). In the present study, these methods are examined in application to a heterogeneous system. Using Monte Carlo analysis, we demonstrate that the magnitude of the gradient estimated via the h-method involved significant bias, which increased when the distance separating the wells increased. In contrast, bias in the estimated magnitude of the gradient based on the h²-method decreased with increasing separation among the wells. Estimation variances for both the magnitude and orientation of the gradient also decreased with separation distance. The variance in the orientation was observed to remain relatively high, however, even at relatively large separations among the wells (e.g., 10 integral scales). These results are Interpreted as implying that the best estimate of the gradient for steady flow in an unconfined aquifer is derived from the h²- method with the wells separated by significant distances. These results also demonstrate the uncertainty inherent in estimating the gradient based on limited field data.     

Ion microprobe analysis of O/O in authigenic and detrital quartz in the St. Peter Sandstone, Michigan Basin and Wisconsin Arch, USA: Contrasting diagenetic histories

The oxygen isotopic compositions of authigenic quartz cements in sandstones provide a monitor of the temperatures, compositions, and origins of pore-occluding fluids during diagenesis, but quartz overgrowths are too fine-grained to be amenable to conventional isotopic analysis. We have used a Cameca ims-4f ion microprobe to determine oxygen isotopic variations in authigenic and detrital quartz in four samples of the Ordovician St. Peter Sandstone from the Michigan Basin and Wisconsin Arch, midwestern USA. Ion microprobe isotopic analyses have been successfully accomplished with an internal precision of ±1‰ (1 σ) and a spatial resolution of 20–30 μm at low mass resolution using a high voltage offset technique. Repeated analyses of the quartz standard demonstrate a reproducibility of close to ±1‰ (1 sd) in good agreement with that expected from counting statistics. The four ion probe samples were chosen as representative of thirteen samples that were mechanically and chemically disaggregated, sieved, and analysed as a function of sieve size by conventional bulk-mineral techniques. Conventional and ion microprobe analyses are mutually consistent, supporting the accuracy of the ion microprobe analyses. Within-sample isotopic variations of up to 13%. and micro-scale isotopic variations of at least 4‰ over a distance of 100 μm have been measured within quartz overgrowths in a sandstone from the Wisconsin Arch. Overgrowths are uniformly higher in δ¹⁸O than detrital grains, and gradients of up to 25% exist across a few microns.

¹⁸O-enriched quartz overgrowths in sandstones from the Wisconsin Arch show complex CL zonation and reflect one of two possible processes: (1) low-temperature quartz precipitation during mixing of meteoric waters with upwelling basinal fluids; (2) higher temperature quartz precipitation during episodic gravity-driven upwelling of warm basinal fluids (of comparable isotopic composition to Michigan Basin fluids) from the Illinois Basin, related to evolution of Mississippi Valley type Pb---Zn ore-forming fluids. Quartz overgrowths in Michigan Basin sandstones, which derived their silica locally by pressure solution, are thought to have precipitated continuously over a range of temperatures from hot basinal fluids of restricted isotopic composition and circulation. Detrital quartz shows significant intea-grain and intergrain isotopic variation (5–13‰) but no significant inter-sample variation, consistent with a uniform source of granitoid and metamorphic quartz in space and time.     

The New South Wales Travelling Stock Route and Reserve (TSR) network: historical extent,spatial distribution and drivers of loss 1884–2017

ABSTRACT

The Travelling Stock Route and Reserve (TSR) network is an historical grazing reserve network under threat, where public alarm has continued over the potential sale of reserves. Present debate on the future of the TSRs is often hindered by a lack of data, as knowledge of the historical extent of TSRs is deficient. This paper documents the original extent of the New South Wales TSR network and changes during 1884–2017, and discusses the key drivers of reserve loss since the late nineteenth century. Research using archived literature showed that, in 1884, the New South Wales TSR network comprised 4,414,690?ha. Comparisons of TSR acreage from that time with present estimates revealed that the reserve network has declined by 54% since its inception. Most reductions in TSR area during 1884–2017 have occurred in the Eastern (83%) and Central (82%) regions of the state, where competing land demands are greatest. Technological changes in transport, environmental pressures and economic changes in the grazing industry were identified as key drivers of TSR loss. These findings support concerns over the efficacy of the TSR reserve network, where the extent and impacts of past government decisions to sell off TSRs has been grossly under-estimated.     

Nitrate loss and transformation in 2 vegetated headwater streams

The processes of nitrate removal from the stream waters of 2 headwater catchments were studied. In study stretches where the stream channels were vegetated with thick mats of grass (Gly‐ceria fluitans), nitrate removal processes were particularly active. In such grassed stream channels, c. 75% of the nitrate removal was attributable to plant uptake and the remainder to denitrifica‐tion. Both of these nitrate removal processes were linearly dependent on nitrate concentration, resulting in an exponential decline of nitrate level from the springs along the stream's length. Regeneration of nitrogen, by plant decay within the stream channel, results in export of dissolved organic nitrogen and paniculate nitrogen from the catchments. The overall impact of vegetated stream channels in modifying nitrogen exports from catchments is discussed.     

Blue Earth River quality monitoring using ARCHER hyperspectral data and field measurements

In past decades, remote sensing studies on water quality mapping mainly focused on lakes using medium resolution imagery. Little research utilizes hyperspectral images to assess river water quality. This study aims to assess the capability of using Airborne Real-time Cueing Hyperspectral Enhanced Reconnaissance (ARCHER) for river water quality mapping and to characterize spatial patterns of turbidity and chlorophyll in the Blue Earth River (BER) system of Minnesota. The BER was characterized by both hyperspectral imagery (HSI) and laboratory analysis of synchronously collected water quality data. The optimal bands for water quality mapping were determined using a method based on hyperspectral profiles and derivative analysis. Finally, based on the regression analysis and modelling, we mapped continuous surface water turbidity. The results revealed that the ratio of HSI band 17 to band 9 effectively determined turbidity and chlorophyll concentrations. The study also found that turbidity and chlorophyll in the river generally increases downstream.     

New light on post-main sequence stellar evolution

The 8 m class telescopes now coming on stream, the availability of new instruments on 4 m telescopes and mm/radio telescopes, are beginning to throw new light on post-main sequence stellar evolution. The February RAS discussion meeting, organized by Nye Bevan, described the impact of these facilities on our understanding of this phase. Myfanwy Bryce reports.