Darwinian approaches for the Urban Critical Zone — A case study in the city of Braunschweig, Lower Saxony, Germany

The ecohydrology of the Urban Critical Zone is characterised by strong heterogeneity and the entangling of hydrological and human time scales (Sivapalan & Blöschl, 2015). This not only poses a challenge to field measurements and the transfer of insights to other urban systems, but consequently limits the development of universal theoretical approaches for urban systems. In this contribution, we propose an interdisciplinary methodology to approach this challenge. Following the school of Darwinian hydrology (Harman & Troch, 2014), we hypothesise that analog to the co-evolution of natural systems, the history of a city and its neighbourhoods is a strong control on current ecohydrological patterns and processes. Thus, we argue that field measurements must be complemented by research into the historical evolution of the urban area to provide a full description and explanation of any observations made. While we need to be careful to avoid a too deterministic or simplistic view of history, research into the historical evolution of an urban area can strengthen explanation of current urban ecohydrological behaviour and potentially enable knowledge transfer and prediction capabilities in “ungauged” cities with similar historical development, as well as to help guide measurement campaigns. Hence, we search for historical and environmental patterns that correlate to provide a testable explanation of current ecohydrological function of urban space. Similar to the "uniqueness of place" in hydrology, every society and city has a unique history that is shaped by the complex interaction among culture, environment, and political events (Berking & Löw, 2008). Thus, we want to formulate a framework for determining similarities in historical development at relevant temporal scales. This requires a strictly interdisciplinary approach, because the application of historical sciences and the interpretation of results is non-trivial and should not be attempted separately. 
 
We discuss our current progress in developing such an interdisciplinary framework in a case study of the city of Braunschweig, Germany. Braunschweig has 250,000 inhabitants, a medieval city centre with Gründerzeit–era neighbourhoods surrounding it. The former fortifications of the city have been converted into urban green spaces during the 18th century. The Oker river that surrounds the medieval city centre has been heavily modified. The built areas of the city centre show very little green space with few trees, especially compared to the surrounding neighbourhoods, where we find a multitude of street trees, smaller green spaces scattered throughout, and large parks adjacent to the built-up area. This works as an example of how the policy regarding green spaces has changed over time. In this heterogeneous environment, we hunt for urban ecohydrological units. In particular, we are interested in whether similar historical development is an indicator of similar ecohydrological function in an urban context.

References
Berking, H. & Löw, M. (2008). Die Eigenlogik der Städte, Campus Verlag, Frankfurt, Germany.
Harman, C. & Troch, P. (2014), Hydrology & Earth System Science, 18, 417–433.
Sivapalan, M. & Blöschl, G. (2015), Water Resources Research, 51, 6988–7022.