Urban Hydrology: What the USGS Studies Where Cities Meet Water

Urbanisation changes how water moves: streamflow, coastal flooding and groundwater recharge. Paved surfaces, diverted streams and groundwater pumping all alter the natural water cycle. Understanding how these interact with changing land use helps water managers with stormwater, and with a city's exposure to flood and drought. Scientists of the U.S. Geological Survey monitor and model urban water systems; their streamflow and groundwater data are in national databases, and their analyses, including long-term streamflow trends and how well management practices work, are published in USGS reports.

A cement-lined stream in Queens County, New York. USGS.

The Urban Landscapes Capability Team

The USGS Northeast Region's Urban Landscapes Capability Team pools expertise to provide and interpret data on urban issues at local, regional and national scales, using up-to-date monitoring, modelling and laboratory methods to understand urban hydrology and its effects on aquatic life and people. Examples:

• continuous streamflow stations with alert systems in areas prone to flash floods, giving the public real-time flood warnings;
• data for better inundation and storm-surge forecasts and maps, and early warning of coastal flooding for emergency crews;
• studies of how surface water and groundwater interact, to inform planners.

Urban hydrology describes and measures the effects of A, impervious surfaces; B, groundwater sources; C, stream response time and sewage overflow; and D, waterway impairment. USGS.

Urban streams

The USGS runs a network of more than 8,000 streamgages with local, state and federal partners. Runoff from roads, parking lots and roofs raises peak flows in storms, which can destabilise stream channels.

• Non-contact radar stage sensors and acoustic Doppler velocimeters track fast-changing conditions in streams and estuaries in real time.
• The Stochastic Empirical Loading and Dilution Model (SELDM), built with the Federal Highway Administration, assesses the risk of harm from storm runoff, whether mitigation is needed, and how well it works.
• Knowing how land use affects stream and coastal flooding helps officials manage their flood risk, now and later.
• USGS scientists also work with planners and community groups to study stream restoration, such as daylighting (uncovering buried streams) and dam removal.

How paving a watershed changes a stream's hydrograph. Water runs off streets and roofs faster than off vegetated land, and parking lots, artificial channels and storm sewers shorten the time it takes to reach streams. The shorter lag brings faster currents, more erosion, higher peak discharge and a greater risk of flash floods. Modified from Leopold (1968). USGS.

Groundwater in cities

The USGS also runs an extensive network of groundwater wells for measuring groundwater levels, quality and aquifer properties. Cities affect groundwater in many ways:

• paved surfaces change recharge, and sewers and water mains change natural water budgets;
• pumping for public supply takes water from storage and can reduce the base flow that feeds streams;
• heavy rain, rising sea level and climate change may worsen flooding of infrastructure by raising groundwater;
• droughts and pumping can lower groundwater, which can lead to land subsidence or saltwater intrusion.

To understand complex aquifers and how they respond to changes in rainfall, urbanisation, wastewater recharge or pumping, the USGS has built many groundwater flow models of urban areas with programs such as MODFLOW.

The share of streamflow coming from groundwater (base flow) in two Long Island streams. In urbanised East Meadow Brook, at Freeport, sewering is blamed for the fall in streamflow, which made low flows worse in the 1960s drought; the rural, unsewered Carmans River, at Yaphank, was sustained by base flow throughout. USGS.

Examples of USGS urban studies

• Scientific Investigations Report 2014–5058
• Fact Sheet 076–03
• Scientific Investigations Report 2014–5036
• Open-File Report 2014–1184
• Circular 554
• Water-Resources Investigations Report 96–4187

Working with the USGS

The USGS works with local, state, federal and Tribal agencies, nonprofits, universities and watershed groups, and the team aims to continue and expand urban water studies. Jointly funded work is possible through the Cooperative Water Program.

Sources

• Joseph M. Bell, Amy E. Simonson and Irene J. Fisher, Urban Hydrology—Science Capabilities of the U.S. Geological Survey, USGS Fact Sheet 2016–3023, April 2016. https://doi.org/10.3133/fs20163023
• The photograph and figures are taken from the fact sheet's PDF; its banner photograph, under a Creative Commons licence, is not reproduced here.