Representation Of Transport Processes Research Articles

Development of a national-scale framework to characterise transfers of N, P and Escherichia coli from land to water

ABSTRACT A hydrological framework encompassing nitrogen (N), phosphorus (P) and microbial (E. coli) transfer from land to water was developed to provide a consistent and rapid approach for assessing the potential impacts of land activity on water quality in New Zealand. A flow partition approach was used to route precipitation via surface and subsurface pathways from land to water. The framework included a typology-based inventory that estimates annual yields of transportable N and P from land, a regional-scale spatial layer that attenuates N in groundwater, and literature-based estimates of E. coli concentrations in surface runoff and artificial drainage. Application of the framework in four catchments highlighted the importance of local catchment knowledge of dominant hydrological processes that was needed to ensure flow partitions derived were a realistic representation of transport processes. While the approach was promising, additional refinements are needed to improve process representation (e.g. effects of groundwater lags) and ensure input data (e.g. soil attributes) have appropriate resolution to describe hydrological pathways. We contend that such a framework would provide a consistent and relatively rapid approach for identifying contaminant transfer pathways from land to water that can inform assessments of the potential consequences of land use change and intensification.

Critical source area management of agricultural phosphorus: experiences, challenges and opportunities

The concept of critical source areas of phosphorus (P) loss produced by coinciding source and transport factors has been studied since the mid 1990s. It is widely recognized that identification of such areas has led to targeting of management strategies and conservation practices that more effectively mitigate P transfers from agricultural landscapes to surface waters. Such was the purpose of P Indices and more complex nonpoint source models. Despite their widespread adoption across the U.S., a lack of water quality improvement in certain areas (e.g. Chesapeake Bay Watershed and some of its tributaries) has challenged critical source area management to be more restrictive. While the role of soil and applied P has been easy to define and quantify, representation of transport processes still remains more elusive. Even so, the release of P from land management and in-stream buffering contribute to a legacy effect that can overwhelm the benefits of critical source area management, particularly as scale increases (e.g. the Chesapeake Bay). Also, conservation tillage that reduces erosion can lead to vertical stratification of soil P and ultimately increased dissolved P loss. Clearly, complexities imparted by spatially variable landscapes, climate, and system response will require iterative monitoring and adaptation, to develop locally relevant solutions. To overcome the challenges we have outlined, critical source area management must involve development of a 'toolbox' that contains several approaches to address the underlying problem of localized excesses of P and provide both spatial and temporal management options. To a large extent, this may be facilitated with the use of GIS and digital elevation models. Irrespective of the tool used, however, there must be a two-way dialogue between science and policy to limit the softening of technically rigorous and politically difficult approaches to truly reducing P losses.

Numerical model studies of the effect of injected nitrogen oxides on stratospheric ozone

The work reported here has employed one-dimensional models, in which atmospheric transport is represented in combination with chemical kinetic mechanisms, to compute average vertical distributions of the minor constituents of the stratosphere as a function of time. Perturbation experiments simulating the effects of exhaust emissions, particularly nitrogen oxides and water vapour, from supersonic and subsonic aircraft fleets have been performed. The effect on stratospheric ozone of a possible four year variation in tropospheric N 2 O levels from 1966 to 1969 has also been investigated. Inert tracer studies from a three-dimensional tropospheric and stratospheric general circulation model have been used to examine specific limitations of the representation of transport processes in one-dimensional models. The inability of current one-dimensional models to represent counter gradient transport of minor gaseous constituents by the mean global circulation is a serious shortcoming in their use for studies of this type; the importance of mean motions in the general circulation is demonstrated by diagnostic data from a three-dimensional general circulation model, and by reference to the production of nitrogen oxides in thunderstorms. Further subjects studied were the effects on the calculated ozone reductions of using different profiles of vertical eddy diffusivity, diurnal and seasonal time dependence of the solar zenith angle in the photochemistry and temperature dependence of the chemical reaction rates. It was also found that the effects of injections from the supersonic and subsonic fleets were not linearly additive in the one-dimensional diffusive models used in this study. The optimum latitude at which to operate a one-dimensional model is discussed; for injected species with long atmospheric lifetimes a value of 34° is recommended for the Northern Hemisphere.