Climate Change Indicators in the United States Technical Documentation River Flooding

River Flooding

Identification

1. Indicator Description

This indicator presents the changes in magnitude and frequency of days with large water discharge in
rivers and streams from 1965 to 2015. These flooding events are largely influenced by recent local and
upstream precipitation. Differences in the size of large events over time (magnitude) or distribution of
events over time (frequency) could indicate changes in regional climate.

Components of this indicator include:

• Change in the magnitude of river flooding in the United States from 1965 to 2015 (Figure 1).

• Change in the frequency of river flooding in the United States from 1965 to 2015 (Figure 2).

2. Revision History

August 2016: Indicator published.

Data Sources

3. Data Sources

This indicator is based on instantaneous peak and daily discharge data from stream gauges maintained
by the U.S. Geological Survey (USGS). The analysis was first developed by Iman Mallakpour and Gabriele
Villarini at the University of Iowa, who published results related to these trends for the north-central
United States (Mallakpour and Villarini, 2015). These analyses were then updated and expanded
nationwide by Louise Slater and Gabriele Villarini at the University of Iowa. Daily mean and
instantaneous peak streamflow data are housed in the USGS National Water Information System
(NWIS). The set of stations presented in the indicator derives from the Hydro-Climatic Data Network
(HCDN-2009) subset of the Geospatial Attributes of Gages for Evaluating Streamflow (GAGES-II)
database, which was developed by USGS and is described in Lins (2012).

4. Data Availability

Trend data for this indicator were obtained directly from Drs. Gabriele Villarini and Louise Slater at the
University of Iowa. Underlying streamflow data from individual stations are publicly available online
through the surface water section of NWIS at: http://waterdata.usgs.gov/nwis/sw. Sites were narrowed
down based on site characteristics, which are available for each stream gauge in the GAGES-II database
at: http://water.usgs.gov/GIS/metadata/usgswrd/XML/gagesll Sept2011.xml. A list of the HCDN-2009
subset of stations is available online at: http://water.usgs.gov/osw/hcdn-2009.

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Methodology

5. Data Collection

Flooding events featured in this indicator are determined by examining daily water discharge values for
each given station. Discharge is defined as the total volume of water that passes through a stream gauge
site within a given window of time. Discharge values are determined from data collected at stream
gauging stations by devices that record the elevation (or stage) of a river or stream at regular intervals
each day. USGS maintains a network of more than 25,000 stream gauging stations throughout the
United States (http://waterdata.usgs.gov/nwis/sw). USGS has been collecting stream gauge data since
the late 1800s at some locations. Gauges generally are sited to record flows for specific management or
legal issues, typically in cooperation with municipal, state, and federal agencies. Stream surface
elevation is recorded at regular intervals that vary from station to station—typically every 15 minutes to
one hour.

Streamflow (or discharge) is measured at regular intervals by USGS personnel (typically every four to
eight weeks). The relation between stream stage and discharge is determined and a stage-discharge
relation (rating) is developed to calculate streamflow for each recorded stream stage (Rantz et al.,
1982). These data are used to calculate the daily mean discharge for each day at each site. This indicator
uses these daily mean discharge values as inputs. All measurements are made according to standard
USGS procedures (Rantz et al., 1982; Sauer and Turnipseed, 2010; Turnipseed and Sauer, 2010).

This indicator uses data from a subset of USGS stream gauges that have been designated as HCDN-2009
"reference gauges" (Lins, 2012). These reference gauges have been carefully selected to reflect minimal
interference from human activities such as dam construction, reservoir management, wastewater
treatment discharge, water withdrawal, and changes in land cover and land use that might influence
runoff. The subset of reference gauges was further winnowed to meet the following criteria:

• At least 30 years of data during the period of interest (1965-2015).

• No more than four consecutive years of missing data at the beginning or end of the period of
interest. Thus, this indicator excludes stations that start in 1970 or later and stations that end in
2010 or earlier.

• No gaps longer than two consecutive years during the rest of the period.

A total of 526 sites met these criteria for Figure 1, and 481 sites for Figure 2. The year 1965 was selected
as a starting point to maximize the number of years and sites available for a national-scale analysis. All
of the selected stations and their corresponding basins are relatively independent—that is, the analysis
does not include gauges with substantially overlapping watershed areas.

6. Indicator Derivation

Both the magnitude and frequency of river flooding presented in this indicator are based on discharge
measurements from stream gauges, measured in cubic feet or cubic meters per day.

Figure 1. Change in the Magnitude of River Flooding in the United States, 1965-2015

Figure 1 shows how the magnitude of floods has changed over the period of study. It is based on an
analysis of the annual maximum instantaneous peak discharge values at each site. Calculation of the

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magnitude trend uses a block approach, whereby the largest instantaneous discharge value for each
calendar year is identified. A Mann-Kendall test was used to calculate whether the sizes of these annual
maximum flood events have a discernable trend over the period of record. The Mann-Kendall approach
is a widely used non-parametric test of whether a variable is statistically trending upward or downward.

Figure 2. Change in the Frequency of River Flooding in the United States, 1965-2015

Figure 2 shows how the frequency of river flooding events has changed over the period of study. The
analysis uses a "peaks-over-threshold" approach, which sets a baseline daily discharge value for which
events are considered to be "flooding." This threshold value is defined as the value that produces an
average of two flood events per year. During a 50-year study period, this approach essentially involves
identifying the 100 largest days of discharge at each station. By analyzing when these 100 largest
discharge events fall during the period of study, this indicator is able to identify whether such large
events have become more or less frequent over time. Trends and their significance were determined
through Poisson regression, which is a widely used method to assess trends in count data—in this case,
the number of large flooding events per year. For the calculation of frequency trends, flood events were
only considered discrete events when separated by at least 15 days.

7. Quality Assurance and Quality Control

Quality assurance and quality control (QA/QC) procedures are documented for measuring stream stage
(Sauer and Turnipseed, 2010), measuring stream discharge (Turnipseed and Sauer, 2010), and
computing stream discharge (Sauer, 2002; Rantz et al., 1982). Stream discharge is typically measured
and equipment is inspected at each gauging station every four to eight weeks. The relation between
stream stage and stream discharge is evaluated following each discharge measurement at each site, and
shifts to the relation are made if necessary. Additional QA/QC procedures are documented in
Mallakpour and Villarini (2015).

The GAGES-II database incorporated a QC procedure for delineating watershed boundaries acquired
from the National Hydrography Dataset Plus. The dataset was cross-checked against information from
USGS's National Water-Quality Assessment Program. Basin boundaries that were inconsistent across
sources were visually compared and manually delineated based on geographical information provided in
USGS's Elevation Derivatives for National Applications. Other screening and data quality issues are
addressed in the GAGES-II metadata available at:

http://water.usgs.gov/GIS/metadata/usgswrd/XML/gagesll Sept2011.xml.

Analysis

8. Comparability Over Time and Space

All USGS streamflow and discharge data have been collected and extensively quality-assured by USGS
since the start of data collection. Consistent and well-documented procedures have been used for the
entire periods of recorded discharge at all gauges (Corbett et al., 1943; Rantz et al., 1982; Sauer, 2002).
Trends in stream discharge overtime can be heavily influenced by human activities upstream, such as
the construction and operation of dams, flow diversions and abstractions, and land-use change. To
remove these artificial influences to the extent possible, this indicator relies on a set of reference gauges
that were chosen because they represent least-disturbed (though not necessarily completely
undisturbed) watersheds. The criteria for selecting reference gauges vary from region to region based

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on land use characteristics. This inconsistency means that a modestly impacted gauge in one part of the
country (e.g., an area with agricultural land use) might not have met the data quality standards for
another less impacted region. The reference gauge screening process is described in Lins (2012) and is
available in the GAGES-II metadata at:

http://water.usgs.gov/GIS/metadata/usgswrd/XML/gagesll Sept2011.xml.

Analytical methods for this indicator have also been applied consistently over time and space.

9. Data Limitations

Factors that may impact the confidence, application, or conclusions drawn from this indicator are as
follows:

1. This analysis is restricted to locations where streamflow is not highly disturbed by human
influences, including reservoir regulation, diversions, and land cover change. However, changes
in agricultural practices, land cover, and land use over time could still influence trends in the
magnitude and frequency of flooding events at some sites.

2. In calculating changes in frequency over time, truly discrete flood events may have fallen within
a window smaller than 15 days, thereby masking suitably distinct events as if they were part of a
single event.

3. Large daily discharges do not necessarily correlate to the risk posed to river communities and
surrounding areas. Protective infrastructure, such as levees and seawalls, can provide a measure
of safety to vulnerable areas.

4. Reference gauges used for this indicator are not evenly distributed throughout the United
States, nor are they evenly distributed with respect to topography, geology, elevation, or land
cover.

10. Sources of Uncertainty

Uncertainty estimates are not available for this indicator as a whole. As for the underlying data, the
precision of individual stream gauges varies from site to site. Accuracy depends primarily on the stability
of the stage-discharge relationship, the frequency and reliability of stage and discharge measurements,
and the presence of special conditions such as ice (Novak, 1985). Accuracy classifications for all USGS
gauges for each year of record are available in USGS annual state water data reports. USGS has
published a general online reference devoted to the calculation of error in individual stream discharge
measurements (Sauerand Meyer, 1992).

11. Sources of Variability

Streamflow and discharge naturally vary from day to day. This indicator intentionally captures some of
this variability by focusing on the magnitude and timing of daily peaks. Peak streamflow and discharge
also vary from year to year as a result of variation in precipitation, air temperature, and other factors.
This indicator focuses on long-term trends over a 50-year period to reduce the "noise" associated with
interannual or decadal-scale climate variability.

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Some sites may be more affected by direct human influences (such as development and land-use
changes) than others. Other sources of variability include localized factors such as topography, geology,
elevation, and natural land cover.

12. Statistical/Trend Analysis

A Mann-Kendall test and a Poisson regression were used for data shown in Figures 1 and 2, respectively,
to assess trends and their significance. Mallakpour and Villarini (2015) document these methods in more
detail. Of the 2,997 sites associated with Figure 1, 569 (19 percent) had significant trends in flood
magnitude: 202 with increases and 367 with decreases. Of the 2,337 sites shown in Figure 2, 553 (24
percent) had significant trends in flood frequency: 237 with increases and 316 with decreases. Figures 1
and 2 differentiate between significant trends (larger, solid-color triangles) and insignificant trends
(smaller, outlined triangles). In both cases, significance refers to a 95 percent level (p < 0.05).

References

Corbett, D.M., et al. 1943. Stream-gaging procedure: A manual describing methods and practices of the
Geological Survey. U.S. Geological Survey Water-Supply Paper 888.
https://pubs.er.usgs.gov/publication/wsp888.

Lins, H.F. 2012. USGS Hydro-Climatic Data Network 2009 (HCDN-2009). U.S. Geological Survey Fact Sheet
2012-3047. http://pubs.usgs.gov/fs/2012/3047.

Mallakpour, I., and G. Villarini. 2015. The changing nature of flooding across the central United States.
Nature Climate Change 5:250-254.

Novak, C.E. 1985. WRD data reports preparation guide. U.S. Geological Survey Open-File Report 85-480.
https://pubs.er.usgs.gov/publication/ofr85480.

Rantz, S.E., et al. 1982. Measurement and computation of streamflow. Volume 1: Measurement of stage
and discharge. Volume 2: Computation of discharge. U.S. Geological Survey Water Supply Paper 2175.
http://pubs.usgs.gov/wsp/wsp2175.

Sauer, V.B. 2002. Standards for the analysis and processing of surface-water data and information using
electronic methods. U.S. Geological Survey Water-Resources Investigations Report 01-4044.
https://pubs.er.usgs.gov/publication/wri20014044.

Sauer, V.B., and R.W. Meyer. 1992. Determination of error in individual discharge measurements. U.S.
Geological Survey Open-File Report 92-144. http://pubs.usgs.gov/of/1992/ofr92-144.

Sauer, V.B., and D.P. Turnipseed. 2010. Stage measurement at gaging stations. U.S. Geological Survey
Techniques and Methods book 3. Chap. A7. U.S. Geological Survey, http://pubs.usgs.gov/tm/tm3-a7.

Turnipseed, D.P., and V.P. Sauer. 2010. Discharge measurements at gaging stations. U.S. Geological
Survey Techniques and Methods book 3. Chap. A8. U.S. Geological Survey.
http://pubs.usgs.gov/tm/tm3-a8.

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