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Hydroclimatic flood trends in the northeastern UnitedStates and linkages with large-scale atmosphericcirculation patternsWilliam H. Armstronga, Mathias J. Collinsb & Noah P. Snyderca INSTAAR and Department of Geological Sciences, University of Colorado at Boulder,Boulder, Colorado 80309, USAb National Oceanic and Atmospheric Administration, Restoration Center, Gloucester,Massachusetts, USAc Department of Earth and Environmental Sciences, Boston College, Chestnut Hill,Massachusetts, USAAccepted author version posted online: 12 Nov 2013.Published online: 19 Aug 2014.

To cite this article: William H. Armstrong, Mathias J. Collins & Noah P. Snyder (2014): Hydroclimatic flood trends in thenortheastern United States and linkages with large-scale atmospheric circulation patterns, Hydrological Sciences Journal,DOI: 10.1080/02626667.2013.862339

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Hydroclimatic flood trends in the northeastern United Statesand linkages with large-scale atmospheric circulation patterns

William H. Armstrong1, Mathias J. Collins2 and Noah P. Snyder3

1INSTAAR and Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado 80309, USAwilliam.armstrong@colorado.edu2National Oceanic and Atmospheric Administration, Restoration Center, Gloucester, Massachusetts, USA3Department of Earth and Environmental Sciences, Boston College, Chestnut Hill, Massachusetts, USA

Received 22 May 2013; accepted 2 September 2013; open for discussion until 1 March 2015

Editor Z.W. Kundzewicz; Associate editor H. Lins

Abstract We evaluate flood magnitude and frequency trends across the Mid-Atlantic USA at stream gaugesselected for long record lengths and climate sensitivity, and find field significant increases. Fifty-three of 75 studygauges show upward trends in annual flood magnitude, with 12 showing increases at p < 0.05. We investigatetrends in flood frequency using partial duration series data and document upward trends at 75% of gauges, with27% increasing at p < 0.05. Many study gauges show evidence for step increases in flood magnitude and/orfrequency around 1970. Expanding our study area to include New England, we find evidence for lagged positiverelationships between the winter North Atlantic Oscillation phase and flood magnitude and frequency. Our resultssuggest hydroclimatic changes in regional flood response that are related to a combination of factors, includingcyclic atmospheric variability and secular trends related to climate warming affecting both antecedent conditionsand event-scale processes.

Key words flooding; hydroclimatology; Mid-Atlantic USA; northeastern USA; partial duration series

Tendances hydroclimatiques dans les inondations du Nord-Est des Etats-Unis et liens avec lesstructures de la circulation atmosphrique grande chelleRsum Nous avons valu les tendances de lintensit et de la frquence des crues travers les Etats amricainsdu Mid-Atlantique, au niveau de stations de jaugeage slectionnes pour leurs longues sries denregistrement etleur sensibilit au climat. Nous avons mis en vidence des augmentations significatives sur le terrain. 53 des 75stations de ltude montrent des tendances la hausse de lintensit des crues annuelles, dont 12 prsentent uneaugmentation avec une probabilit de rejet p < 0,05. Nous avons tudi les tendances dans la frquence des crues partir de sries de dures partielles et avons dcrit des tendances la hausse pour 75% des stations, 27%augmentant avec une probabilit de rejet p < 0,05. De nombreuses stations tudies indiquent des augmentationspar palier dans lintensit et / ou la frquence des crues autour de lanne 1970. En tendant notre zone dtude la Nouvelle-Angleterre, nous avons mis en vidence des relations dcales positives entre la phase hivernale deloscillation nord-atlantique et lintensit et la frquence des crues. Nos rsultats suggrent des changementshydroclimatiques dans les crues lchelle rgionales, qui sont lis une combinaison de facteurs, incluant lavariabilit atmosphrique cyclique et des tendances sculaires lies au rchauffement climatique, qui affectent lafois les conditions antrieures et les processus lchelle vnementielle.

Mots clefs inondations ; hydroclimatologie ; Etats-Unis Mid-Atlantique ; Nord-Est des Etats-Unis ; srie de dures partielles

1 INTRODUCTION

There has been considerable interest in evaluatingUnited States (US) streamflow trends because ofanticipated changes in the hydrologic cycle due toanthropogenic climate change. Lins and Slack (1999,2005) showed systematic increases in minimum and

median flow quantiles on streams selected for climatesensitivity throughout the eastern and central US,while the western US showed few trends in eitherdirection. Other studies corroborated these findings(Douglas et al. 2000, McCabe and Wolock 2002,Hodgkins and Dudley 2005). Trends in high flowquantiles have been less clear. Groisman et al.

Hydrological Sciences Journal Journal des Sciences Hydrologiques, 2014http://dx.doi.org/10.1080/02626667.2013.862339

1

This material is published by permission of the U.S. National Oceanic and Atmospheric Administration Restoration Center under Contract No. WC133F-10-BU-0036/01.The US Government retains for itself, and others acting on its behalf, a paid up, non-exclusive, and irrevocable worldwide license in said article toreproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

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(2001) documented increasing trends in monthlymean streamflow during the month of maximumstreamflow in the eastern US. Vogel et al. (2011)found increases in annual maximum instantaneouspeak flow at 13% of 1588 climate-sensitive streamgauges across the conterminous US. However, sev-eral other studies found no clear trend in high stream-flow variables such as: 90th percentile daily mean(Lins and Slack 1999, 2005), annual maximum dailymean (Lins and Slack 1999, 2005, Douglas et al.2000, McCabe and Wolock 2002, Rice and Hirsch2012), annual high flow (Small et al. 2006), andannual maximum instantaneous peak (Villarini et al.2009, Villarini and Smith 2010).

On a regional scale in New England, however, aless ambiguous picture of hydroclimatic trends inhigh flows is emerging. Collins (2009) found increas-ing trends in annual maximum instantaneous peakdischarge, while Armstrong et al. (2012) foundincreasing trends in flood frequency. Hodgkins(2010) corroborated the findings of Collins (2009)and further showed the importance of using recentflood records to obtain conservative statistical floodfrequency estimates. These increasing flood trendscoincide with a pronounced increase in annual pre-cipitation over the 20th century throughout much ofthe US (Karl and Knight 1998). The precipitationtrend is especially strong in the northeastern US andhas occurred through a disproportionate increase inthe frequency and intensity of heavy precipitationevents (Karl and Knight 1998, Groisman et al.2001, 2004, Madsen and Figdor 2007, Spierre andWake 2010, Douglas and Fairbank 2011).Interestingly, the northeastern US is also a regionwhere Hirsch and Ryberg (2012) show results thatsuggest a positive relationship between global meancarbon dioxide concentration and flood magnitude.

We believe these New England results moreclearly show upward hydroclimatic flood trends thatare congruent with the observed precipitation trendsfor a number of reasons. First, our New England inves-tigations (Collins 2009, Armstrong et al. 2012) andthose of Hodgkins (2010) used more gauges than theearlier investigations that had larger spatial domains(i.e. Lins and Slack 1999, 2005, Small et al. 2006,Villarini et al. 2009). Second, these studies were ableto make use of data through 2006, which in some casesadded more than 10 years of recent data. Douglas andFairbank (2011) recently showed the importance of themost recent years in the record for their analyses ofprecipitation extremes in the region. Finally, the NewEngland investigations made use of stream gauges

carefully selected for having flood regimes that are asnatural as possible. In contrast, Villarini et al. (2009),Villarini and Smith (2010), and Rice and Hirsch (2012)investigated flooding trends in the eastern US usingrecord length as their primary criterion for selectinggauges. As a result, many of their gauges were affectedby flood flow regulation, which limited their ability todraw conclusions about the influence of hydroclimaticvariability (natural variability and/or anthropogenic cli-mate change) in observed trends. We recognize that wecannot rule out all land-use and flow manipulationeffects on floods at our New England stream gaugeswithout highly detailed historical watershed analysesfor each. However, we believe the methods weemployed in that region, and extend to this study,minimize the risk of confusing anthropogenic changesin watershed runoff properties and regulation withhydroclimatic changes (see also a similar discussion inHirsch and Ryberg 2012).

In this study, we investigate the Mid-Atlanticregion (MAR) of the US, which, like New England,has been characterized by increases in total annual-and heavy precipitation (Groisman et al. 2001, 2004,Madsen and Figdor 2007, Seager et al. 2012). Thisextension substantially increases the number of cli-mate sensitive gauges with long records that we cananalyse in the northeastern United Statesa regionwhere any observed hydroclimatic changes in floodmagnitudes and frequencies will have important con-sequences for human communitiesand enables newanalyses, interpretations, and comparisons with otherrecent national and regional investigations looking atclimateflood relationships via different metrics andusing different criteria to select stream gauges foranalyses (e.g. Smith et al. 2010, Villarini andSmith 2010, Hirsch and Ryberg 2012). For example,our expanded regional dataset better enables us toanalyse the potential influences of large-scale atmo-spheric circulation patterns and we do so using anapproach employed by Tootle et al. (2005). Hirschand Ryberg (2012) encourage a wide range of empiri-cal approaches to hydroclimatic analyses of floodtrends, and recommend detailed regional analyseslike those presented here to potentially identify pat-terns not evident via other analytical approaches.

We first investigate MAR trends in flood magni-tude using the annual maximum series (AMS; i.e.time series of the largest instantaneous discharge ofthe water year). We then investigate trends in floodfrequency using partial duration series (PDS) data.The PDS includes all floods over a specified thresh-old discharge and thus contains more data than the

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annual series, especially for low-magnitude floods,which are important for channel morphology andaquatic habitat (Leopold and Wolman 1960, Poff2002, Armstrong et al. 2012). By summing the num-ber of threshold exceedances annually, we have adirect measure of flood frequency for trend analyses.

In addition to investigating monotonic trends, wealso evaluate step changes in flood magnitude and/orfrequency around 1970a date around which manyhydrologic studies have found step increases instreamflow and precipitation in the eastern US(McCabe and Wolock 2002, Mauget 2003, Collins2009, Villarini and Smith 2010, Armstrong et al.2012, Rice and Hirsch 2012). We then expand ourstudy region to include the entire northeastern USdraining to the Atlantic Ocean by adding NewEngland flood data from previous research (Collins2009, Armstrong et al. 2012) to investigate potentiallinkages between large-scale atmospheric circulationpatterns and observed regional flooding trends on alarger spatial scale. Previous studies show some evi-dence of linkages between the phase of the NorthAtlantic Oscillation (NAO) and northeastern UShydroclimate (Bradbury et al. 2002a, 2002b, Tootleet al. 2005, Kingston et al. 2007, Smith et al. 2011),including lagged relationships with flood magnitudeand frequency (Collins 2009, Armstrong et al. 2012).There is also some work that suggests an El Nino-Southern Oscillation (ENSO) influence in the north-eastern US: Tootle et al. (2005) documented ENSOeffects on annual streamflow medians in the regionwhile Smith et al. (2011) found ENSO linkages withheavy rainfall.

2 METHODS

2.1 Flood series

The AMS records the largest instantaneous peak dis-charge in each water year (WY). It is the most widelyused time series for floods because it typically satis-fies the independence assumptions necessary formany statistical tests and it is well suited to estimat-ing the discharges of infrequent, high magnitudeflood events, which are important for design andrisk assessments. It is also well suited for the purposewe employ it: investigating flood magnitude trends.

A PDS, on the other hand, includes all dis-charges over a specified threshold discharge (TD).The TD can be established a posteriori by the dataanalyst or may be set by a data collection and dis-semination entity. For this study, we use PDS

available from the US Geological Survey (USGS)that are defined by the TDs they established foreach gauge. The USGS typically sets the TD for astream at a value expected to be exceeded 34 timesper WY (Langbein 1949). Any time the TD isexceeded, the event is identified as a partial peakand included in the PDS after satisfying USGS dataquality reviews. The PDS typically includes manymore events per year than the AMS, and the numberof annual TD exceedances per year can be used as adirect measure of flood frequency. Hereafter, we referto each TD exceedance as a peak over threshold, orPOT, and we use the number of POT occurring in aWY (POT/WY) to investigate flood frequencytrends.

2.2 Gauge selection

In this study we define the MAR by the boundariesof the USGS Hydrologic Unit Code 02 (HUC 02),which includes all of New Jersey, Delaware,Maryland and the District of Columbia, and parts ofVermont, New York, Virginia, West Virginia,Massachusetts and Connecticut (Fig. 1). We carefullyselect stream gauges with flood regimes that are asnatural as possible and are thus sensitive to changesin hydroclimate. To do so, we begin with a pool ofgauges from the USGS Hydro-Climatic DataNetwork (HCDN; accessible at http://pubs.usgs.gov/wri/wri934076/), which the USGS identifies as beingrelatively free of human influence (Slack andLandwehr 1992). HCDN gauges are located in water-sheds with minimal land-use change, extremegroundwater withdrawal, and flow regulation duringthe periods of record Slack and Landwehr (1992)analysed. We impose additional criteria to ensure agauge set with minimal anthropogenic alteration offlood flows, adequate record lengths to support ourquestions of interest and the statistical methods weuse to investigate them, and overall data quality.

We review gauge metadata in detail, includingUSGS annual water data reports and peak dischargequalification codes, and remove any gauge recordsthat have (1) evidence of peak flow regulation and/ordiversion (e.g. peak discharge qualification codes 5or 6); (2) fewer than 59 years of record (i.e. allrecords start in WY 1951 or earlier to ensure ade-quate record length before 1970 for our step changeinvestigation); (3) records ending before WY 2009;or (4) more than 5% missing data (i.e. no more than 5missing annual peaks in a 100 year record). Ourlongest span of consecutive missing years is three

Hydroclimatic flood trends in the northeastern US 3

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http://pubs.usgs.gov/wri/wri934076/http://pubs.usgs.gov/wri/wri934076/

years, which occurs only at one gauge. Ninety-fivepercent of our gauges have one missing year orfewer. Additionally, we remove several gauges forunique disqualifiers. For example, we exclude aNew Jersey gauge record because dredging and chan-nel realignment around 1960, reported in the stateannual water data reports, could have affected peakdischarges.

Seventy-five MAR gauges have AMS recordsthat satisfy these criteria (Table 1). The study gaugesare fairly well distributed throughout the studyregion, though southeastern Pennsylvania, easternMaryland, and Delaware are unrepresented, likelydue to significant urbanization in these areas. Weobtain AMS discharge data (AMSQ) for the studygauges from the National Water Information System(NWIS; accessible at http://nwis.waterdata.usgs.gov/usa/nwis/peak/). Eight gauges did not maintain POTdata for significant portions of their periods of recordor had TD changes that affected data quality, leaving67 gauges for POT/WY trend analyses. POT data are

not available through NWIS, so we obtain these datadirectly from USGS state Water Science Centers. Theshortest period of record we use is 61 years, and thelongest is 102 years, with an average record length ofaround 80 years (Table 1). The rivers drain a widerange of watershed sizes, from 62 to 10 550 km2,with a median basin size of about 450 km2.

2.3 Trend analyses

Before formal statistical testing, we conduct explora-tory data analyses using scatter plots and other datavisualizations to detect potential errors in the dataand ensure that they satisfy the assumptions of thetests we employ (Helsel and Hirsch 2002). Flood dataare frequently non-normally distributed, so we havechosen non-parametric tests that require no distribu-tional assumptions.

We use the Mann-Kendall trend test (MK) todetect monotonic trends in AMSQ and POT/WY.The MK is a non-parametric, rank-based test that

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Mid-Atlantic AMSQ TrendsPercent change (%)

< 25

> 25 to 10

10 to 10

>10 to 25

>25 to 50

> 50

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Fig. 1 Spatial distribution of trends in flood magnitude (AMSQ), represented as percent change over the period of recordfor each gauge. Darker colored symbols are new data presented in this study. Lighter colored symbols are HUC 01 (NewEngland) streams from Collins (2009) and are presented here to provide regional context. New England symbol sizes arescaled with the same criteria as Mid-Atlantic symbols.

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Tab

le1Trendsin

Mid-A

tlantic

floodmagnitude

(AMSQ)andfrequency(POT/W

Y).Boldfont

indicatesstatistical

testswith

p-values