Urban Watersheds

Site: 
BES LTER scientist sampling water at a long-term suburban watershed site in Baltimore County, MD.
Joseph Bushey

A watershed is the area of land where all of the water that is under it or drains off of it goes into the same place. The watershed approach, where the quantity and quality of water leaving a watershed is sampled is like urinalysis, where doctors monitor chemicals in the urine to assess a patient's health. The watershed approach has been applied very successfully in many LTER sites to understand the structure and function of unmanaged ecosystems and to evaluate the long-term effects of forest management, air pollution and other human-associated activities on ecosystems. BES researchers hypothesized that the watershed approach would be a strong platform for comparing urban ecosystems with the more natural ecosystems in the LTER network. It was expected to be a powerful tool for understanding the importance of natural and human processes in these ecosystems. Furthermore, it should also be useful for evaluating long-term changes in urban ecosystem function in response to climate change and human activities such as sewage and stormwater infrastructure improvements.

Every week, BES scientists sample a network of urban, suburban, agricultural and forested watersheds. The USGS collects data on the flow of water at these sites and chemistry and hydrology data are combined to calculate nutrient input-output budgets for different watersheds.

Early results from BES watershed monitoring showed surprisingly high nitrogen retention in urban and suburban watersheds, i.e. nitrogen inputs from fertilizer and rainfall were much higher than outputs in the stream. While we had expected retention to be quite low (e.g, 20% of inputs), it was actually much higher (~70%). For comparison, our forested reference watershed had retention > 90%. This was an important result because it suggested that urban and suburban ecosystems were performing an important ecosystem service -- preventing the movement of nitrogen to receiving waters such as the Chesapeake Bay. This result motivated extensive followup research to determine just where this retention was occurring -- in soils and/or plants in riparian (streamside) zones, or in upland forests and lawns, or streams. BES research also aimed to discover how nitrogen retention might be increased even further through improvements in land management.

Continued long-term monitoring of BES watersheds through severe drought (2002) and high rainfall (2003, 2004) periods showed that the high nitrogen retention of urban watersheds was vulnerable to climate variation and change. Nitrogen retention dropped markedly in some urban watersheds during the high rainfall periods (e.g., from 70 down to 30%), while retention dropped much less in the forested watersheds. These results suggested that the resilience of urban watershed nitrogen retention functions may be low and has motivated new research on the factors that control this resilience.

Long-term watershed data has also been useful for evaluating the effects of sewage infrastructure improvements on urban ecosystem function. The City of Baltimore is in the process of spending $900 million to fix sanitary sewer infrastructure that leaks into streams. BES data has shown that fixing big leaks on small streams results in marked improvements in water quality but that effects on larger streams are harder to see and will require longer-term (> 10 years) monitoring.

For further reading: 
Groffman, P.M., C.O. Williams, R.V. Pouyat, L.E. Band and I.C. Yesilonis. 2009. Nitrate leaching and nitrous oxide flux in urban forests and grasslands. Journal of Environmental Quality 38:1848-1860.
Kaushal, S.S., P M. Groffman, L.E. Band, C.A. Shields, R.P. Morgan, M.A. Palmer, K.T. Belt, C.M. Swan, S.E.G.Findlay and G.T. Fisher. 2008. Interaction between climate and land use amplifies watershed nitrogen export. Environmental Science & Technology 42:5872-5878. DOI: 10.1021/es800264f
Groffman, P.M., N.L. Law, K.T. Belt, L.E. Band and G.T. Fisher. 2004. Nitrogen fluxes and retention in urban watershed ecosystems. Ecosystems 7:393-403.
For further information: 
Holly Beyar, Project Facilitator, Baltimore Ecosystem Study, LTER
Contact email: 
Feedback

Theme by Danetsoft and Danang Probo Sayekti inspired by Maksimer