HR: 09:45h
AN: H31F-04    [Abstracts]
TI: How Much Hillslope Does it Need to Solve the Double Paradox?
AU: * Lischeid, G
EM: gunnar.lischeid@bitoek.uni-bayreuth.de
AF: Chair for Ecological Modeling University of Bayreuth, Dr.-Hans-Frisch-Str. 1-3, Bayreuth, D-95440 Germany
AB: 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.
DE: 1860 Runoff and streamflow
DE: 1871 Surface water quality
DE: 1875 Unsaturated zone
DE: 1890 Wetlands
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting