The human altered landscape is a complex hierarchical system with a large diversity of landscape elements, which influences the emission, transport, and export of diffuse pollution. Selective simplification of this complexity is necessary to focus both research and management activity. Framing river basins and watersheds as landscape systems can aid in this process simplification by providing an organized way to approach conceptual model formation.
In the workshop on “Landscape controls on diffuse nutrient transfers in agriculture catchments” organizers and keynote speakers suggest using a landscape level framework to think about prioritizing programs of research and implementation for diffuse pollution management.
The workshop started with two short presentations, the first by Jean-Marcel Dorioz (Hydrobiologie Lacustre, INRA-CARRTEL, Thonon, France) entitled: “Mechanisms of phosphorus diffuse transfer and transformation: case study of the Lake Geneva region.” The second presentation was by Mark Dubin (Chesapeake Bay Program Office, University of Maryland, U.S.A) and was entitled: “Mechanisms of nitrogen diffuse transfer to coastal environment: case study of Chesapeake Bay, USA.”
Dr. Dorioz emphasized the various roles of landscape variables in controlling P emission and transfer. For example, in their work in France the connecting landscape elements and their specific role in pollution attenuation is a very important factor in P emission. In addition, their experimental work altering field relationships (i.e. changing the position of tilled fields and hay fields) resulted in important changes in P flux. The intensity of agriculture (traditional vs. semi-intensive vs. intensive dairy) strongly interacted with landscape position contributing to the overall complexity of the landscape system. Understanding this diversity is important to strategic P reduction action plans.
Dr. Dubin provided a good overview of the Chesapeake Bay Watershed, a very large and diverse landscape. He focused on problems of low dissolved oxygen and decreasing chlorophyll A (an indicator of toxic blooms) in the waters of the Bay. While agriculture is the largest single nutrient pollutant source, the sources are another aspect of landscape complexity. For example, in the case of nitrogen, 25% of N comes from atmospheric sources, somewhat unevenly deposited across the watershed. Given this landscape-level complexity, BMPs include a wide variety of strategies (e.g., nutrient management, enhanced nutrient management, waste management for animal facilities, feed management, tillage management, cover crops, vegetative buffers, and land conversions). Planners and managers assume that BMPs are additive, but their relationship in space and time alter their efficacy. The complexity in management is highlighted by the 92 impaired hydrological segments. Action at the level of smaller jurisdictions is required to achieve cumulative reductions at the level of the whole watershed.
Given the demonstrated complexity of the Lac Leman and Chesapeake Bay examples, how can we focus research and action to optimize results? Aubert Michaud initiated the workshop discussion with several thematic questions targeting landscape concepts:
Role of spatial relationships. Does arrangement make a difference?
Role of specific connection types. How does arrangement (pattern) affect process?
Role of landscape element diversity. What aspects of diversity effect process?
Role of scale. At what scales should we understand the landscape system to provide management insights?
The audience had many different perspectives on how to rank relative importance of these factors. Undoubtedly some of these differences reflect the differences in environment (climate, seasons, soil, topography, loads, agricultural practices, etc.) among the places that participants work.
Research in the Netherlands and Belgium highlight the importance of subsurface landscape processes in flat areas. This is an area that has not been studied very much from a landscape perspective. The diversity of soil types and slopes within watershed is also very challenging to manage (e.g. in the Chesapeake Bay watershed). Topography is also important as it affects hydrologic accumulation and flow paths.
A general consensus was that spatial organizations matters, but there was some concern that management programs do not really take this into account. Also, while diversity clearly exists in the landscape, how much of this diversity needs to be described in order to make strategic decisions about P reductions (BMPs etc.). In addition, there was a question about how much detail about individual farming systems is useful. Perhaps we just need to know what fields are in excess.
The suggestion was made that over the long term there may be a much simpler way to understand the problem. A mass balance perspective points to the inevitable conclusion that land conversion may be the only alternative to modern agriculture. Rearranging fields and hydrologic connections, creating temporary buffers, storing nutrients in wetlands and riparian areas, moving intensively managed fields further away from hydrologic connections — may just buy time, but not solve the problem in a sustainable way.
In the mean time, as the P the stock has no strong relationship to the flux at the local scale and short time scale, we probably can buy more time by considering landscape controls on rates of diffuse pollution emission through appropriate BMPs. It was pointed out that understanding regional complexity by having a good sense of the diversity of sites and specific farming practices is critical to effective management.
It was also mentioned that subsurface connectivity is an important issue in some areas, down to the level of knowing the direction of tillage and effect of local specific ditches on nutrient transport. Mapping the location of preferential flows may also be needed to optimize activities at the field scale. Dr. Dubin supported this need for specific detail in discussing declining buffer effectiveness. They are revising their estimates down from 80% as these buffers age and are not harvested. Build up of nutrients in buffer areas may result in less removal over time.
Other participants indicated that detailed topographic information may also be needed to understand and manage diffuse pollution. Lower topographic positions in hill and valley area of Pennsylvania are really important seasonal sources of diffuse pollution. Connectivity of these areas to hill-slope areas exacerbates the problem. What causes these surface and subsurface connections?
Others pointed out that local knowledge of “hot spots” can be critical to targeting the worst problems and thus the most effective solutions. Dr. Dorioz related that small differences in his field can make a big difference. Do we need to track the farmer and his shovel? Can we use stream order to prioritize where we should be looking for this detail of hydrologic connectivity?
Others suggested that the critical source area concept can help organize all this specific detail. All fields are not created equal and the concept helps prioritize areas of concern (including aspects such as p index – to identify risk via source-transport understanding; hydrologic variability; hydrologic activity; differential sources from differential application; proximity to surface water, etc.).
As the discussion came to a close it was re-emphasized that perhaps we don’t need complicated tools and models to address the problem. We can keep it simple, the big problem is nutrient surplus. No specific conclusion can be drawn from the workshop discussion, but the theme of complexity at the landscape level was well accepted and while this doesn’t point to specific strategies for research and management, perhaps the useful conclusion is that we haven’t yet found a general approach to either characterizing or prioritizing areas to target management. Many aspects are potentially important (surface and subsurface connection, presence of buffers, hydrologic activity, micro and macro topography and flow paths, relative position, nutrient load, etc.).
* We welcome further general comments from participants, including any conclusions that your draw from the collective experience of those working on these issue of diffuse pollution. Just comment below by clicking on the “comment” field. If you would like to participate in the “survey,” please comment on the “Participant Experience” posts above.