.|  Baltimore Ecosystem Study
Stream and Watershed Research Projects

Urban Watershed Continuum Concept
  • Sujay Kaushal - University of Maryland
  • Kenneth Belt - USDA Forest Service
Urban Watershed Continuum Concept: Urban ecosystems are constantly evolving, and they are expected to change in both space and time with active management or degradation. An urban watershed continuum framework recognizes a continuum of engineered and natural hydrologic flowpaths that expands hydrologic networks in ways that are seldom considered. It recognizes that the nature of hydrologic connectivity influences downstream fluxes and transformations of carbon, contaminants, energy, and nutrients across 4 space and time dimensions (Figure 1). Specifically, it proposes that (1) first order streams are largely replaced by urban infrastructure (e.g. storm drains, ditches, gutters, pipes) longitudinally and laterally within watersheds, (2) there is extensive longitudinal and lateral modification of organic carbon and nutrient retention in engineered headwaters (3) there are longitudinal downstream pulses in material and energy exports that are amplified by interactive land-use and hydrologic variability, (4) there are vertical interactions between leaky pipes and ground water that influence stream solute transport, (5) the urban watershed continuum is a transformer and transporter of materials and energy based on hydrologic residence times, and (6) temporally, there is an evolution of biogeochemical cycles and ecosystem functions as land use and urban infrastructure change over time. Overall, an urban watershed continuum framework proposes testable hypotheses of how transport/transformation of materials and energy vary along a continuum of engineered and natural hydrologic flowpaths in space and time. Given interest in transitioning from sanitary to sustainable cities, it is necessary to recognize the evolving relationship between infrastructure and ecosystem function along the urban watershed continuum.
Current work is also focusing on the impacts of organic carbon quantity and quality on influencing biogeochemical processes along the urban watershed continuum (Newcomer et al. 2012). Results from streamwater chemistry at two forested, two restored, and two unrestored urban streams at the BES LTER site show that there was significant relationship between daily DOC load and daily runoff in log-log scale for all types of land-use (Fig. 1). The slope and intercept of these regressions, however, differed from site to site. These changes in DOC dynamics can influence denitrification rates in streams (Newcomer et al. 2012).

Fig.1 (a) Conceptually, there are alterations in the amounts and quality of organic carbon along the urbanizing watershed continuum compared to forested watersheds. There are increases in organic carbon as water flows from low-residential areas in headwaters through progressively urbanizing areas with storm drains, sewage leaks, and algal blooms. (b) Longitudinal changes in nitrate concentrations along a 36 km transect of the Gwynns Fall. (c) Relationship between dissolved organic carbon and nitrate concentrations along longitudinal sampling of the Gwynns Falls.

Urban Watershed Continuum: The Movie The Urban Watershed Continuum is a conceptual framework for studying urban waters. This short film explains the concept to students and educators. While exploring urban waters at the Baltimore Long-Term Ecological Research site and other locations, the film explores the question: what is an urban watershed? The urban watershed continuum concept explores infrastructure as a new part of the urban stream network and examines the transport and transformation of water across four dimensions including space and time in cities. Please see the Urban Watershed Continuum film at: https://www.youtube.com/watch?v=k7Ak5rUu41I


Literature Cited:
Kaushal, S.S., and K.T. Belt. 2012. The Urban Watershed Continuum: Evolving Spatial and Temporal Dimensions. Urban Ecosystems. DOI:10.1007/s11252-012-0226-7.
Newcomer, T.A., S.S. Kaushal, P.M. Mayer, A. Shields, E.A. Canuel, P.M. Groffman, and A.J. Gold. 2012. Influence of natural and novel organic carbon sources on denitrification in forest, urban degraded, and restored streams. Ecological Monographs 82: 449-466. doi:10.1890/12-0458.1.