H31F-01 INVITED 09:00h
The Aquatic Systems Continuum
The Aquatic Systems Continuum is a proposed framework for interrelating the physical, chemical, and biological characteristics of aquatic ecosystems. The continuum can be represented by a three-dimensional matrix that relates aquatic ecosystems to their position within hydrologic flow paths (x-axis, a spatial dimension) and their response to climate variability (y-axis). The z-axis describes the structure of biological communities as they relate to the hydrological conditions defined by the x and y axes. The concept is an extension of the Wetland Continuum that was derived from field studies of a prairie pothole wetland complex in North Dakota. At that site, the hydrologic continuum in space is defined by ground-water flow systems. The wetlands are surface-water expressions of larger ground-water watersheds, in which wetlands serve recharge, flow-through, and discharge functions with respect to ground water. The water balance of the wetlands is dominated by precipitation and evaporation. However, the interaction of the wetlands with ground water, although a small part of their water budget, provides the primary control on delivery of major solutes to and from the wetlands. Having monitored these wetlands for more than 25 years, during which time the site had a complete range of climate conditions from drought to deluge, the response of the aquatic communities to a wide variety of climate conditions has been well documented. The Aquatic Systems Continuum extends the model provided by the Wetland Continuum to include rivers and their interaction with ground water. As a result, both ground water and surface water are used to describe terrestrial water flows for all types of aquatic ecosystems. By using the Aquatic Systems Continuum to describe the hydrologic flow paths in all types of terrain, including exchange with atmospheric water, it is possible to design studies, monitoring programs, and management plans for nearly any type of aquatic ecosystem.
H31F-02 09:15h
Linking Hydrology and Biogeochemistry in Complex Landscapes
This review seeks to examine connections between hydrology and biogeochemistry at the landscape scale. A review of research on landscape structure and organisation provides a context, and seeks to integrate work at relevant scales in ecology and geomorphology; the degree of connectivity between different landscape elements provides the key theme. Following a review of hillslope hydrology, links between runoff pathways and nutrient dynamics are then considered, with particular emphasis on riparian zones, where nutrient dynamics has direct relevance for water quality management in catchments. It is concluded that research needs to focus on the critical near-stream zone, given its importance in coupling hillslope and channel systems.
H31F-03 09:30h
Geomorphic controls of catchment scale linkages between hydrologic and nitrogen cycling
Hydrologic processes have important interactions with ecological systems through the activity of biogeochemical cycles. We investigate the space/time linkages between water, carbon and nitrogen cycling in a small Mid-Atlantic Piedmont catchment, which is part of the Baltimore LTER. We document spatial and temporal patterns of near surface and deeper profiles of soil moisture with TDR, soil nitrogen transformations, riparian groundwater depths, stream discharge and nitrate concentrations at several cross sections. We develop the geomorphic organization of these hydrologic and biogeochemical stores and fluxes at the catchment scale using LIDAR derived DEM. Plot mean soil moisture in upland areas closely followed a linear trend with the topographic wetness index, ln(a/tanĪ²) while bottomland mean soil moisture is typically decoupled from this trend, indicating deeper, more persistent sources. Plot variance of soil moisture was highest under intermediate values of soil moisture, decreasing under both wet and dry conditions. Catchment and plot level patterns in soil moisture show important, nonlinear relations with riparian groundwater levels, stream baseflow and nitrate concentrations although riparian zones appear to be the dominant source of stream nitrate. Distributed ecohydrologic simulation of water, carbon and nitrogen cycling is compared with the multivariate pattern analysis we have carried out to further explore feedbacks and controls between hydrologic and biogeochemical processes.
H31F-04 09:45h
How Much Hillslope Does it Need to Solve the Double Paradox?
Studying the connection between catchments and stream encounters a double paradox (Kirchner 2003): First, often rapid responses of stream discharge to rainstorms are observed, although usually pre-event water predominates in the stream. That would point to a large water pool being involved in stormflow runoff generation. However, stormflow stream water usually differs significantly from baseflow, pointing to two different pools of pre-event water, and a quick switch between the both. Different mechanisms have been proposed to solve that paradox, but these were usually restricted to certain subsets of catchments. Here, a more general perceptual model is presented. It is based on a literature review and on own field work in German and Swedish catchments. For this model, the unsaturated topsoil layer of the riparian zone plays a crucial role. Although saturation excess surface runoff is widely accepted as a predominating runoff generation process, a close-up to the "saturated areas" in fact reveals more a patchy pattern of saturation due to the micro relief of the soil surface. These patches are hydraulically connected by short sub-surface flowpaths through the forest floor and uppermost mineral soil. This is sufficient to mix event water with a large pool of pre-event water, that is chemically very similar to hillslope vadose zone water. On the other hand, water transport within the saturated patches is very fast, thus explaining the rapid response of the hydrograph. Moreover, mixing of event-water with pre-event water within the riparian phreatic zone was observed during heavy rainstorms and snowmelt periods. In addition, there is strong evidence that even in the saturated zone a mobile and an immobile soil water fraction can be differentiated which is clearly at odds with the homogeneity assumption in many models. Due to its short residence time, the mobile fraction tends to resemble more the vadose zone (and thus hillslope) soil water than the immobile fraction. Thus, stream water might exhibit a hillslope water signature, without any physical contribution of hillslope water during short storms. Thus, quick lateral transport from the hillslope to the stream, e.g., via interflow, pipe flow or transmissivity feedback mechanisms is not required, although it might add to these processes at single sites. According to the presented model, microscale processes in the vadose zone and in the uppermost layers of the phreatic zone play a decisive role for stormflow solute concentration in the stream. Of course, the model needs to be tested at various sites. Pathways to follow for a rigorous test of the model will be presented. Reference: Kirchner, J.W., 2003. A double paradox in catchment hydrology and geochemistry. Hydrological Processes 17: 871-874.