Gas exchange between the land surface and the atmosphere is becoming an increasingly important component in modelling the state and the future of the climate system for enhanced climate and weather prediction. Due to the vast inhomogeneity of the land surface and the different scale-dependent characteristics of atmospheric motions there exists a scale gap in addressing these processes in current measurement and modelling methods: No clear concept exists to bridge from the local scale where exchange processes happen close to and at the land surface, to the scales which are suitable to describe and model these transports in the atmospheric environment e.g. by eddy-covariance methods and common boundary layer models, respectively. This project will approach this problem by an integrated methodology combining a set of different local measurement techniques with boundary layer scale estimates ranging from traditional techniques up to modern remote sensing tools and a suite of modelling approaches encompassing the whole atmospheric boundary layer. A flux chamber concept especially adapted to closing the scale gap from the measurement aspect will be designed and applied.
The Okavango is a large African wetland of prime ecological importance. Its existence is in jeopardy due to upstream development and increasing water demand in combination with predicted climatic change. Less water means a reduction of the permanent and seasonal swamp areas. The project creates a computer model serving as a tool to predict the impact of upstream hydraulic measures, channel management and changes in climate on the availability of water in the delta and its spatial distribution. It also looks at the sediment transport. The availability of water e.g. expressed by the frequency of flooding at a location is essential for the type of habitat prevailing at that point. Changes of water availability will therefore also cause changes in the habitat composition of the delta. Management of channels by cutting of papyrus or dredging will change the distribution of flooded areas. The interaction of the channels of the Okavango Delta with the floodplains and the underlying groundwater is of crucial importance. Vegetation on the islands gets its water through infiltration from the river channels and the yearly flood propagates both in the channels and in the groundwater. Therefore the model couples the surface water flow - in channels and overland during flooding - with the groundwater flow. In a region with little infrastructure it is hard to find data for this modelling effort. However, nowadays many data can be found through remote sensing techniques both from satellite and airborne platforms. These include the topography (from the Shuttle Mission), the rainfall (from METOESAT 5), the evapotranspiration (from NOAA-AVHRR) and the extension of water surfaces in their temporal development (from radar satellites and others). The thickness of the groundwater body is evaluated from aeromagnetic records provided by the geological Survey of Botswana. The infiltration zones can be seen through another airborne geophysical technique, the TEM-method. This survey is financed by the Botswanan government and will take place in April 2007. All satellite and airborne information needs calibration and interpretation through ground truth which is obtained in yearly field campaigns. The water areas serve as data for the adjustment and verification of the model. The insight gained through the model will flow into the work of the Okavango River Commission (OKACOM) which has to negotiate the permissible water use in the upstream.
Wirksame Maßnahmen zum Gewässerschutz, wie sie von der EG-Wasserrahmenrichtlinie als Bestandteil des nachhaltigen Land- und Wassermanagements gefordert werden, setzen fundierte Kenntnisse zu Stoffretentions- und -umsatzprozessen in Landschaften voraus. Vergleiche von Stoffaus- und -einträgen wie auch mit den Fließgewässern ausgetragene Frachten belegen das hohe Stoffretentionspotenzial pleistozäner Einzugsgebiete und Fließgewässersysteme. Forschungsbedarf besteht zur Quantifizierung und Modellierung der dafür auf Landschaftsebene maßgeblichen Transport- und Transformationsprozesse unter den durch Wechselfeuchte und Wassermangelperioden gekennzeichneten hydrologischen Verhältnissen des pleistozänen Tieflands. Die vorliegenden pfadbezogenen Konzepte mit sehr unterschiedlicher Flächendifferenzierung unterscheiden zwischen Stofftransport auf der Landoberfläche (Oberflächenabfluss, Bodenabtrag) und im Boden/Grundwasserleiter. Problematisch gestalten sich Übertragung und Parametrisierung dieser Prozesse auf der Mesoskala (Einzugsgebiete). Weniger gut beschreibbar sind ebenso die Prozesse des Bodenabtrags, zu deren Quantifizierung auch verbesserte prozessorientierte Modelle benötigt werden, und die komplexen geo- und biogeochemischen Stofftransformationsprozesse in der nicht durchwurzelten ungesättigten und gesättigten Zone. Stofffrachten, die sich bereits auf dem unterirdischen Pfad befinden, erfahren noch vor ihrem Übertritt in die Gewässer eine Reduktion in den oft vermoorten Gewässerrandbereichen. Auch der oberirdische Stofftransfer aus dem Einzugsgebiet in das Gewässer kann in solchen, aquatische und terrestrische Ökosysteme verbindenden Landschaftselementen vermindert werden. Kenntnisse zur Quantifizierung, Bewertung und Steuerung des Stoffumsatz- und -retentionsvermögens kleinerer Fließgewässersysteme der Ober- und Mittelläufe sowie feuchter Senkenareale in Binneneinzugsgebieten werden benötigt, um Handlungsoptionen zum Gewässerschutz ableiten zu können und tatsächlich in Unterliegergewässer und -gebiete gelangende Stofffrachten abzuschätzen. Dabei zu lösende Aufgaben sind die Aufklärung der Stoffretentions- und -freisetzungsprozesse, insbesondere für die gewässergüterelevanten Stoffe N, P, C und O, die Quantifizierung von Retentionspotenzialen für geohydro- und gewässermorphologische Typen, die Ableitung von Leitprozessen und -parametern sowie Bioindikatoren und die Erarbeitung von Algorithmen zur Quantifizierung der Potenziale auf mesoskaliger Ebene. Projektziel: Entwicklung verbesserter skalen- und pfadbezogener Methoden und Modelle zur Quantifizierung der Transport- und Transformationsprozesse wassergelöster Stoffe sowie deren Wechselwirkungen in den Kompartimenten von Einzugsgebieten des pleistozänen Tieflands als Grundlage für die Beschreibung und Bewertung der Stoffretentionspotenziale sowie der Wirkung von Landnutzungsänderungen auf die Stoffbelastung kleiner Stand- und Fließgewässer.
Soil erosion is a world-wide problem with both economic and environmental effects. Consequences include loss of arable land and sediment-derived impacts on receiving water bodies. Even relatively small amounts of erosion can exceed the soil generation rate. Soil sediments are potential pollutants of receiving waters as they reduce light penetration and carry chemical pollutants such as pesticides and phosphorus. Soil erosion can be considered at local and basin scales. Rain is often the main initiator of erosion; other mechanisms include sheet erosion, rilling and gullying. These are all inherently hillslope-scale processes, the mechanisms of which involve connections between rainfall and raindrop impact, water flow, shear stress at the surface of the soil, sediment entrainment and deposition, etc. Management of soil erosion needs to be considered at the basin scale while attenuation measures are local. Physical understanding of erosion is based on local scale processes. At this scale overland flow-borne sediments and rilling (small channels that can be removed relatively easily) are the most important mechanisms. Rills have the potential to form channels under conditions of continued erosion. In addition, rills form in areas of flow concentration and thus rills are much more serious for erosion than interrill areas. The long-term goal of this fundamental research is to develop and validate process-based models of erosion-derived sediment transport at the scale of an element in a discretized catchment model, along with accompanying transport and transformations of nutrients and pollutants. This project seeks to fill one of the fundamental gaps in knowledge: mechanistic modelling of sediment transport at the local scale within a catchment. The project will further develop the mechanistic hillslope-scale Hairsine-Rose erosion model. This model includes both overland flow and sediment dynamics, and has been found to predict well erosion experimental data. However, it involves mechanistic assumptions that need to be clarified, and in addition it needs to be applied to circumstances that are more representative of reality, rather than constrained laboratory conditions. Potential mechanisms that could have significant effects on erosion modelling include the effect of infiltration/redistribution within the soil and the role of specific erosional mechanisms such as re-entrainment of previously eroded material verses transport by raindrop impact. Other factors to be investigated and modelled are multiple rainfall events and the effect of variable stone cover. Experiments and modelling will provide the basis of ascertaining the importance of such mechanisms.