Low streamflow trends at human-impacted and reference basins in the United States

Journal Pre-proofs Research papers Low streamflow trends at human-impacted and reference basins in the United States R.W. Dudley, R.M. Hirsch, S.A. Archfield, A.G. Blum, B. Renard PII: DOI: Reference:

S0022-1694(19)30989-8 https://doi.org/10.1016/j.jhydrol.2019.124254 HYDROL 124254

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Journal of Hydrology

Received Date: Revised Date: Accepted Date:

1 April 2019 6 September 2019 16 October 2019

Please cite this article as: Dudley, R.W., Hirsch, R.M., Archfield, S.A., Blum, A.G., Renard, B., Low streamflow trends at human-impacted and reference basins in the United States, Journal of Hydrology (2019), doi: https:// doi.org/10.1016/j.jhydrol.2019.124254

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Low streamflow trends at humanimpacted and reference basins in the United States R.W. Dudley*1, R.M. Hirsch2, S.A. Archfield2, A.G. Blum3, and B. Renard4 * Corresponding author: U.S. Geological Survey, 196 Whitten Road, Augusta, ME, USA 04330, [email protected]

U.S. Geological Survey, 196 Whitten Road, Augusta, ME, 04330 U.S. Geological Survey, 12201 Sunrise Valley Drive, 430 Reston VA 20192 US 3 Johns Hopkins Krieger School of Arts & Sciences Department of Earth and Planetary Sciences, 301 Olin Hall, 3400 N. Charles Street, Baltimore, MD 21218 4 Irstea, UR Riverly, Lyon, France 1 2

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Abstract We present a continent-scale exploration of trends in annual 7-day low streamflows at 2,482 U.S. Geological Survey streamgages across the conterminous United States over the past 100, 75, and 50 years (1916–2015, 1941–2015 and 1966–2015). We used basin characteristics to identify subsets of study basins representative of reference basins with streamflow relatively free from human effects (n = 259), and predominantly agricultural basins (n = 78), regulated basins (n = 220), and urban basins (n = 121). Trend significance was computed using the Mann-Kendall test considering short- and long-term persistence. Lag-one autocorrelation tests of detrended 7-day low streamflows for all gage classes show that time-series independence is not an appropriate assumption for annual low streamflow data at many basins. Among all study gages, upward trends (wetter conditions) in 7-day low streamflows outnumbered downward trends (drier conditions) approximately 2 to 1 for the 75- and 100-year trend periods—50-year trends indicated roughly equal numbers of increases and decreases. Increases in 7-day low streamflow were consistently observed for all time periods throughout much of the northeastern quadrant of the conterminous U.S. including western New England and the Mid-Atlantic, the southeastern Great Lakes basin, northern Ohio River basin, and the Upper Mississippi River and eastern Missouri River basins. Decreases in 7-day low streamflow were consistently observed for all time periods at many gages in the southeastern U.S. and in the northwestern U.S. in much of Idaho and northwestern Washington. Overall, we observed greater percentages of statistically significant trends at gages with human-induced influences than at reference gages. Low-flow trends at agricultural gages were regionally consistent with trends at reference gages. Regulated basins had many statistically significant upward trends for all three time periods tested, which may be attributed in part to substantial increases in dam-related storage prior to 1970. Urban 2

gages had the greatest percentage of significant decreases in 7-day low flows compared to all other gage classes even though most urban gages saw upward trends in mean annual flows. Urban gages also had the greatest percentage of significant increases in low flows second only to regulated gages, highlighting that urban development can increase or decrease low streamflows depending on the basin-specific development. (KEY TERMS: low streamflow; trends; climate; hydrology)

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1. Introduction Low streamflow is a seasonal phenomenon that occurs every year and defines the lowest (driest) parts of a continuous streamflow hydrograph (Smakhtin, 2001). The median annual daily streamflow provides a conservative upper bound below which streamflows may be considered ‘low’, while flows exceeded 70 to 99 percent of the time are widely used as design flows (Smakhtin, 2001). Annual mean n-day low streamflows (the lowest average flows that occur for a consecutive n-day period) are often used to define low-streamflow indices for water resources management (Smakhtin, 2001). For example, the 7-day low flow with a 10-year recurrence interval is a design flow for water quality-based effluent limitation to protect aquatic life from chronic effects (U.S. Environmental Protection Agency, 1986 and 2018). Seasonal low streamflow, the flow that occurs in the absence of precipitation, comprises storage discharge from surface water (lakes, wetlands, glaciers, dam impoundments) and groundwater from riparian aquifers (Smakhtin, 2001; Brutsaert, 2008). Relative contributions to low streamflow from these sources vary by climate, hydrogeologic setting, and human activities (Smakhtin, 2001). For example, in some parts of the United States, groundwater discharge may contribute up to 90 percent of a stream’s annual flow (Barlow and Leake, 2012). Maintaining minimum streamflows during seasonal low streamflow conditions mediates water quality including temperature and dissolved oxygen and provides volume for effluent dilution (Rolls et al., 2012). Low streamflow has great ecological importance as it defines the lowest extent of in-stream habitat (Poff and Zimmerman, 2010) and affects biota composition and distribution, and species trophic structure (Poff and Zimmerman, 2010; Rolls et al., 2012).

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1.1 Climate and human-induced influences on low streamflows Climate and human-induced influences are primary drivers of hydrologic systems, and because they have changed over time, it is useful to assess trends of hydrologic variables. Identifying and accounting for trends in observations can improve estimates of current and future low streamflow statistics, which play a crucial role in many water management and regulatory actions such as setting permit levels for wastewater discharge and water withdrawals and informing operating policies for low-streamflow releases from dams. Assessing trends can provide insight into and awareness of the many natural and human-related factors affecting low streamflows (e.g. U.S. Environmental Protection Agency, 2016). Previous studies of trends in observed annual n-day (e.g. 7-day) low streamflows, seasonal or annual minimum flows, and (or) base flow-related metrics have tended to focus on climaterelated changes by using only streamflow data from basins considered to be relatively free from human influence, sometimes referred to as “reference basins”. These studies have been done at the scale of the continental United States (U.S.; Douglas et al., 2000; McCabe and Wolock, 2002; Groisman et al., 2004; Lins and Slack, 2005; McCabe and Wolock, 2014; Ahn and Palmer, 2015) or for a region within the U.S. (e.g. Zhu and Day (2005) in Pennsylvania; Small et al. (2006) in the eastern U.S., Hodgkins and Dudley (2011) in New England; and Kormos et al. (2016), Luce and Holden (2009), and Sawaske and Fryeberg (2014) in the Pacific Northwest). Three studies have examined trends in low streamflows across the U.S. using both reference and non-reference gages (Ficklin et al., 2018, Ceylan and Lall, 2017, Rice et al., 2015). Ceylan and Lall (2017) and Rice et al. (2015) observed more significant trends in low streamflows (since about 1940) among non-reference gages relative to reference gages. Ficklin et al. (2018) examined 1981–2015 trends for a range of streamflow metrics in the U.S. and Canada and found 5

generally strong correspondence between streamflow trends at reference gages with trends at non-reference gages. They also observed weaker correspondence between those two systems for extreme low flows (1st percentile trends). Results of all three studies point to the importance of human-induced effects on low streamflows. None of those studies isolated trend results in the context of specific human impacts such as agriculture, dams, or urbanization.

1.2 Scope of this study This study is unique from previous examinations of low-streamflow trends in its continent-scale exploration of trends in annual 7-day low streamflows over the past 100, 75, and 50 years (1916– 2015, 1941–2015 and 1966–2015) and its comparison of those trends among reference basins and basins affected by agriculture, dams, and urbanization. We classified basins into reference, agricultural, regulated, and urban end-members; basins not meeting criteria for those classifications were considered to be mixed basin types. We examined trend results among these basin classes to determine whether we observed a greater number of significant trends in 7-day low streamflows among agricultural, regulated, and urban gages relative to reference gages, and determine whether trends differed among the non-reference gage classes. Given the often-substantial groundwater contribution to low streamflows, we expected that yearto-year magnitude of low streamflow may exhibit lag-one autocorrelation, similar to that observed with groundwater storage (e.g. Hodgkins et al., 2017). Significant lag-one autocorrelation indicates the presence of short and (or) long-term persistence of some kind, and trend tests that do not account for persistence may overestimate the statistical significance of the trends (Cohn and Lins, 2005). We therefore examined lag-one autocorrelation in annual 7-day low streamflows to test whether the assumption of independence is valid. We computed trend

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significance using methods that account for the time-series structure of the data including independence, short-term persistence, and long-term persistence.

2. Data and Methods 2.1. Streamflow data U.S. Geological Survey (USGS) streamflow gages considered for this study were selected from the Geospatial Attributes of Gages for Evaluating Streamflow, version II, (GAGES-II) database (Falcone, 2011; Falcone et al., 2010). The GAGES-II database comprises hundreds of basinspecific characteristics for 9,322 basins throughout the U.S., including climatic, hydrologic, topographic, land cover and use, and geologic attributes. Daily streamflow data from climatic year 1916 (beginning April 1, 1915) through climatic year 2015 (ending March 31, 2015) were downloaded from the USGS National Water Information System (NWIS) (U.S. Geological Survey, 2016) using the R package dataRetrieval (Hirsch and DeCicco, 2015; R Core Team, 2018). Downloaded data were subject to length and completeness criteria. Study gages were required to have complete data (a daily value for every day of the year) for at least 8 out of every 10 years for each decade (e.g. 1990-1999) in the time periods tested; except for the end periods of 1916–1919, 1941–1949, and 1966–1969, which were required to have 3 of 4, 7 of 9, and 3 of 4 years, respectively. Seven gages with negative streamflows (tidal influence) were omitted. The above selection criteria resulted in 2,482 study gages for the 1966–2015 period, 1,408 for 1941–2015, and 203 for 1916–2015 (Fig. 1). [Figure 1 near here]

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2.2. Classification of basin alteration Study gages were subset into one of four classifications: [1] reference (Hydroclimatic Data Network-2009 (HCDN-2009); Lins, 2012), [2] agricultural, [3] regulated, and [4] urban. High and low thresholds of basin characteristics were used to define the classifications (Table 1). Among non-reference gages, the high thresholds generally represented the highest quartile value among study gages, and low thresholds represented the lowest half among study gages. For example, study gages were classified agricultural if they did not belong to the HCDN-2009 network, had more than 20 percent cultivated crops, a normalized dam storage of less than 60 days, and developed land of less than 6 percent (Table 1). Note that due to the strict classification scheme, 73 percent of the study gages (n = 1,804) did not meet the criteria for any one of these four classifications. Study basin characteristics and classifications are in Dudley et al. (2018). Dam storage data (GAGES-II attribute STOR_NID_2009 in megaliters per square kilometer) from the GAGES-II database (Falcone, 2011) were used to quantify historical changes in storage. The GAGES-II dam storage data provide snapshots of dam storage by decade from 1940 to 1990. Changes in crop (agricultural) land use (land use code 43) and developed (urban) land use (land use codes 21-27) were computed using land use data from the U.S. conterminous wallto-wall anthropogenic land use trends (1974–2012) (NWALT) database (Falcone, 2015). The NWALT database provides five 60-meter geospatial rasters showing anthropogenic land use for the years 1974, 1982, 1992, 2002, and 2012.

2.3. Low-streamflow statistics, trend-testing, and correlation methods Annual time series of 7-day low streamflows were computed as the lowest annual value on the basis of 7-day moving averages. Mean annual streamflows also were computed for use as a basis of comparison in the Discussion section. 8

Magnitude of trends were computed using the Sen slope (Sen, 1968) (also known as the Theil– Sen estimator), which is computed as the median of all possible pairwise slopes in each temporal data set (Helsel and Hirsch, 2002). The Sen slope represents a monotonic trend result, and we acknowledge significant monotonic trends may be due to step changes (change points). Also, some changes may be best represented as non-monotonic trends. We limit the scope of work to the analysis and reporting of Sen slope for the purpose of reporting overall changes for the time periods tested. We computed the statistical significance of trends using the Mann-Kendall test (p ≤ 0.05). Significance of trends over time are sensitive to assumptions of whether the time-series data are independent, have short-term persistence, or have long-term persistence (Cohn and Lins, 2005; Koutsoyiannis and Montanari, 2007; Hamed, 2008; Khaliq et al., 2009; Kumar et al., 2009; Hodgkins and Dudley, 2011; Sagarika et al., 2014). By definition, the autocorrelation in shortterm persistence decays rapidly (exponentially) as a function of the time lag. The decay is slower for long-term persistence; processes exhibiting long-term persistence yield observations that tend to cluster in time (Koutsoyiannis and Montanari, 2007). Examples of such processes include solar forcing, volcanic activity, aquifer storage, storage in ice, and large-scale variations in linked atmospheric-oceanic processes (Koutsoyiannis and Montanari, 2007). Not accounting for persistence may overstate the statistical significance of observed trends, if present (Cohn and Lins, 2005). R-code for our general Mann-Kendall test that accounts for shortand long-term persistence in the time-series data is provided in Dudley et al. (2018). Trend significance is reported for three different null hypotheses of the serial structure of the timeseries data: independence (INDE), short-term persistence (AR1), and long-term persistence (LTP) (Hamed and Rao, 1998; Hamed, 2008). While the three versions of the Mann-Kendall test 9

all use the same test statistics, the AR1 version inflates the variance computed under an assumption of independence by a factor related to the lag-one autocorrelation coefficient, and the LTP version inflates the variance on the basis of the Hurst coefficient (Hodgkins et al. 2019). All trend directions are reported regardless of significance because they can nevertheless be informative in context with many other similar trend results on a regional basis.

3. Results 3.1. Trends in 7-day low streamflows for all streamgages Broad geographic similarities were observed in the distribution of trends in 7-day low streamflows across the three time periods (Fig. 2). In general, increases were observed in the western coastal mountains and interior mountains of California, Upper Colorado River, eastern Missouri River, Upper and Lower Mississippi River, and Ohio River basins, Great Lakes basin, northern Mid-Atlantic, and New England (Supplemental Fig. 1). Increases for the 50-year period in the Upper Mississippi River and Ohio River basins, and Mid-Atlantic and New England regions were less pronounced (fewer numbers of significant trends) and mixed with greater numbers of downward trends than for the 100- and 75-year time periods (Fig. 2). Downward trends in the southern Mid-Atlantic and South Atlantic Gulf, were evident in the 50- and 75-year periods (Fig. 2). They become more pronounced (greater number of significant trends) for the 50-year time period (Fig. 2). Trend results (slopes and significance) at each streamgage are available online in the data release by Dudley et al. (2018). [Figure 2 near here]

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3.2. Trends at reference streamgages Seven-day low streamflow trend results by gage classification (Fig. 3; Supplemental Table 1) generally indicate more upward trends than downward trends across the three time periods except for the reference gages. Few reference gages met the 100-year data length and completeness criteria (n = 10; Supplemental Fig. 2); of those few, none had significant trends in annual 7-day low streamflow. Reference gages had the smallest percentages of significant trends over the 75-year period compared to the other gage classes (Fig. 3; Supplemental Fig. 3). Nearly all significant increases over the 75-year period were in the northeast, and nearly all downward trends were in the northwest. Few reference gages in the southeast met the 75-year data length and completeness criteria, and those that did indicated downward trends over time. Fifty-year trends at reference basins predominantly indicated downward trends in low flows over time in the southeastern and western U.S. (Fig. 4). There were few 50-year trends toward higher 7-day low streamflows; some significant upward trends were observed in the northeast and Rocky Mountains. [Figure 3 near here]

3.3. Trends at agricultural streamgages Fewer than 10 agricultural gages met the 100-year data length and completeness criteria (Supplemental Fig. 2). Upward trends in annual 7-day low streamflows over the 75-year period outnumbered downward trends among agricultural gages nearly three to one (Fig. 3; Supplemental Fig. 3). For this period, significant increases were observed in and near the Upper Mississippi and some significant decreases were observed in the Lower Mississippi and South Atlantic Gulf. Fifty-year trends also predominantly indicated higher 7-day low streamflows over time in and near the Upper Mississippi River basin (Fig. 4), and while most of the trends were 11

geographically consistent with those observed for the 75-year period, there were many fewer statistically significant trends (Fig. 3). [Figure 4 near here]

3.4. Trends at regulated streamgages Upward trends in 7-day low streamflows at regulated gages outnumbered downward trends for all three time periods tested (Fig. 3). Trend results among regulated basins were less regionally coherent and more geographically mixed (Supplemental Figs. 2 and 3, and Fig. 4), which is likely a reflection of the diversity of regional climate and operational purposes of dams (e.g. hydroelectricity, flood control, drinking water, irrigation, navigation) (Poff and Hart, 2002).

3.5. Trends at urban streamgages A large majority of gages meeting criteria for classification as urban were in the eastern U.S., with the rest located near the west coast (Fig. 4d). In general, across time periods, the geographic distributions of trends at urban gages were similar to those for the reference gages where they can be seen to overlap (Fig. 4, Supplemental Figs. 2 and 3). The ratio of significant upward trends and downward trends in 7-day low flows at urban gages (15.2 percent up, 6.5 percent down, AR1 assumption) was similar to the ratio observed for reference gages (13.0 to 4.3 percent) for the 75-year period (Fig. 3b). In contrast, when we consider the 50-year period, 19662015, the proportion of urban gages with significant upward trends greatly exceeded the proportion for reference gages (Fig. 3c).

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4. Discussion 4.1. Effects of time series dependence on trend results While the presence of long-term persistence is difficult to identify without having very long records (generally greater than 100 years) (Vogel et al., 1998; Khaliq et al., 2009), we can use lag-one autocorrelation tests to identify the presence of persistence. Both short-term and longterm persistence induce lag-one autocorrelation, therefore a significant lag-one autocorrelation result only indicates the presence of persistence of some kind. Lag-one autocorrelation tests to evaluate persistence were done with Pearson’s r using detrended 7-day low streamflows. The number of gages with significant (p < 0.05) lag-one autocorrelation in detrended 7-day low streamflows was much greater than might arise by chance: 117 of 203 gages for 1916–2015, 652 of 1408 gages for 1941–2015, and 776 of 2462 gages for 1966–2015 with average Pearson’s correlation coefficients (r) of 0.44, 0.40, and 0.42, respectively. For comparison, the upper 95th percentile of the distribution of r for an independent time series is approximately 0.20 for a 100year series, 0.22 for a 75-year series, and 0.28 for a 50-year series. Percentages of significant lagone autocorrelation were lowest among reference and urban gages and highest among regulated and agricultural gages (Fig. 5). It is not surprising that autocorrelation was most prevalent among regulated gages as dams may provide storage not only between seasons but also between years. Evidence of autocorrelation in annual 7-day low streamflows among all basin classes, including the reference class considered to be relatively free from human influence, suggests data independence may not be an appropriate assumption for annual low streamflow data at many basins. [Figure 5 near here]

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We include the reporting of trend significance for the independence (INDE) assumption insofar as it is instructive to illustrate the relative percentages of significant trends in context of the short-term (ARI) and long-term (LTP) assumptions (for example, Fig. 3). The greatest number of significant trends were detected under the independence assumption, fewer significant trends were detected under the short-term persistence (AR1) assumption, and fewer still were detected under the long-term persistence (LTP) assumption.

4.2. Temporal and regional cohesion of trends There were more upward than downward trends in low streamflows over time among study gages for the two longer time periods (100 and 75 years)—increases outnumbered decreases approximately 2 to 1 (Fig. 2 and 3). In contrast, there were roughly equal numbers of upward and downward trends among study gages during the 50-year period. The large decrease in the number of significant positive trends between the more recent period, 1966–2015, and the longer time periods suggests lower low-flow conditions were likely present at many gages prior to 1966 (gages with trends toward increases during the 100- and 75-year periods and decreases during the 50-year period shown as black points in Fig. 6). These findings are consistent with findings from previous studies. McCabe and Wolock (2002) studied temporal and spatial changes in annual minimum, median, and maximum streamflows at 400 streamgages in the conterminous U.S. for the period 1941–1999. They observed that the mid-1950s was a very dry period of time in the conterminous U.S. whereas conditions were generally wetter than average after about 1970. Groisman et al. (2004) also identified the 1930s and 1950s to be particularly dry decades in the monthly precipitation record nationwide, followed by relatively wet decades from 1970–1999. McCabe and Wolock (2002) characterized observed increases in annual minimum and median daily streamflows at eastern gages as an abrupt change in streamflow around 1970 rather than a 14

gradual trend. Sadri et al. (2016) tested for step changes among trends in 7-day low flows in the eastern U.S. and found most step changes (counting both upward and downward steps) occurred during 1960-89, with the 1970s having the highest decadal count of gages with step changes. The low-flow trends computed by this study are consistent with those observations as upward trends throughout the northeastern quadrant of the U.S. had greater percentages of significant trends for the 1941-2015 period anchored in the dry 1950s, than the 1966-2015 period which begins close to the time period when the change to wetter conditions had occurred. [Figure 6 near here] Trends in 7-day low streamflows indicated 641 gages with increases for all time periods tested (gage total computed on the basis of trend direction regardless of significance) (Fig. 6). Increases were consistently observed for all time periods tested throughout much of the northeastern quadrant of the conterminous U.S., including western New England and the Mid-Atlantic approximately north of the southern border of Pennsylvania, the southeastern Great Lakes basin, northern Ohio River basin, and the Upper Mississippi River and eastern Missouri River basins. Similar trends were observed over 1940-1999 in most of these areas by Lins and Slack (2005) who tested for trends in a range of flow quantiles from annual minimums to maximums across the U.S. Easterling et al. (2017) documented greater amounts of annual precipitation (1981–2015 average compared to the 1901–1960 average) in the same regions. McCabe and Wolock (2002) noted their observed step-increases in annual streamflow statistics around 1970 were coincident with precipitation increases in the eastern U.S. Seasonally, the northeastern quadrant of the U.S. has seen relatively widespread increases in mean annual precipitation in the spring, summer, and fall (Easterling et al., 2017). Bartels et al. (2019) studied trends in the number of days with precipitation at 167 stations for the conterminous U.S. and documented increases in precipitation 15

days particularly concentrated in the Northeast, Great Lakes, and midwestern United States for the period 1951–2015. Huang et al. (2017) have also observed a 6.8% increase in annual total precipitation at 116 GHCN stations from 1901 to 2014 in the northeastern U.S. Trends in 7-day low streamflows indicated 380 gages with decreases for all time periods tested (Fig. 6). We found decreases for all time periods in the southeastern U.S. and in the northwestern U.S. throughout much of Idaho and northwestern Washington. Kormos et al. (2016) also observed that low streamflows declined from 1948 to 2013 at a majority of 42 streamgages tested on mountain streams in Washington, Oregon, Idaho, and western Montana. Using mean annual streamflow as a proxy for precipitation, and streamflow center of timing as a proxy for air temperature, Kormos et al. (2016) attributed changes in low streamflows primarily to changes in precipitation totals and secondarily to air temperature. Leppi et al. (2012) studied 1950–2008 trends in mean August flows measured at 153 snowmelt dominated streams in the central Rocky Mountains (a region overlapping with and east of the region studied by Kormos et al (2016)). They observed significant declines in stream discharge at their 65 non-regulated streams, with much smaller declines at their 88 regulated streams, and attributed the trends at non-regulated streams to changes in air temperature and its role in the accumulation and melt of mountain snowpack (Leppi et al., 2012; Pederson et al., 2011). In the southeastern U.S., Patterson et al. (2012) observed regional decreases in precipitation and streamflow during 1970-2005. The consistent decreases in low flows that we observed in the southeastern U.S. across all three time periods (Fig. 6) was likely a result of decreases that have occurred in precipitation throughout the southeastern U.S. Easterling et al. (2017) observed decreases particularly to have occurred during the spring and summer seasons.

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Kam and Sheffield (2016) observed spatial distribution of trends in 7-day low flows (1962– 2011) across the eastern U.S. from South Carolina to Maine that are consistent with results from our study; they observed increases in the northeastern U.S and decreases in the southeastern U.S. They attributed the increases in the northeast to trends in precipitation, and the decreases in the southeast to increases in potential evapotranspiration and effects of water management. A study of trends in low-flow magnitudes and timing across the eastern U.S. by Sadri et al. (2016) also observed a general pattern of increasing low streamflows in the northeast and decreasing low streamflows in the southeast over a common time period (1951– 2005). When we examined trend results in the context of drainage basin sizes, we found percentages of upward and downward trends and percentages of significance among results to be largely consistent across all basin size classes for 1966–2015 (Fig. 7). This analysis was based on segregation of basins into small (< 600 km2), medium (600-3,000 km2), and large (> 3,000 km2) drainage-area size classes, which divided our set of study basins (n = 2,482) roughly into thirds. [Figure 7 near here]

4.3. Relation of low streamflow changes to mean annual streamflow, agriculture, dams, and urbanization 4.3.1. Relation between low and mean flow changes for reference basins. We compared trends in 7-day low flows to trends in annual mean streamflow to determine how consistent these low-flow trends are relative to mean streamflow. McCabe and Wolock (2014) observed that annual and seasonal minimum, mean, and maximum daily streamflows measured at basins relatively free from human influence (reference basins) shared similar temporal variability. For example, during periods when annual mean streamflow was higher or lower than

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the long-term average, they observed annual minimum and maximum streamflows likewise were similarly higher or lower than the long-term average. They found that more than two-thirds (68 percent) of the correlations between seasonal mean flows and seasonal minimum flows were greater than 0.7. Observed trends in mean annual streamflows for the 100-, 75-, and 50-year periods (Fig. 8) generally exhibited similar spatial cohesion as those observed for 7-day low streamflows (Fig. 2) for many regions—though with many fewer statistically significant trends. Given the expected similarity between trends in mean annual streamflow and trends in 7-day low streamflows for near-natural basins (see previous paragraph), one might hypothesize that differences can be attributed in part to human influences. We evaluate this hypothesis in the following sections for regulated, agricultural and urban basins. [Figure 8 near here] 4.3.2. Regulated basins Despite the diversity of purposes for dams, their operations commonly decrease natural streamflow variability—typically attenuating the magnitude of high flows and augmenting low flows (Poff et al. 2006 and 2007). Differences between trends in 7-day low and mean annual flows are particularly notable throughout the western U.S. (compare Figs. 2 and 8) where most of the regulated gages in this study are located (Fig 4c). A comparison of 7-day low and mean annual streamflow trends among gage classes (Fig. 3, Fig. 9) show substantial differences among regulated gages for all three time periods. Few significant increases in mean annual streamflow were observed at regulated sites while many significant increases were observed for 7-day low flows for all time periods (Fig. 10).

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[Figures 9 & 10 near here] A comparison of trend magnitudes in 7-day low flows to those of mean annual flows at regulated basins (Fig. 10) shows many cases where annual flows decreased but 7-day low flows increased; there also is clustering along the zero axis where annual flows decreased but little to no change was observed in 7-day low flows. Low-flow trend magnitudes and mean annual trend magnitudes exhibited the lowest correlation at the regulated gages (Fig. 10). Sadri et al. (2016) observed that the majority of the sites they identified with step changes and upward trends in low streamflows in the eastern U.S. were regulated. Asarian and Walker (2016) examined trends in the magnitude of 7-day, 30-day, and 90-day low flows (1953–2012) in northwest California and southwest Oregon at gages that spanned a wide range of human influences. They found damrelated storage of winter and spring runoff and augmentation of summer water supplies exerted a strong influence on observed trends in streamflow and precipitation-adjusted streamflow. While about half of the unregulated gages they tested showed significant declines and very few increases, 44-48 percent of regulated gages showed increases and only 7-15 percent showed decreases (Asarian and Walker, 2016). Ficklin et al. (2018) suggested that the weaker correspondence of trends in the low 1-percentile streamflows they observed between reference and human-impacted basins was due to water management, which reduced climate-driven streamflow variability among low streamflows. Hodgkins et al. (2007) documented the magnitude of trends in annual 7-day low streamflows (1955–2004) in the Great Lakes basin; they observed larger trends at 2 of 4 regulated basins compared to the other 27 basins considered relatively free from human influence and attributed those increases in low streamflows to changes in regulation practices and addition of storage over time. The other 2 regulated basins exhibited small trends toward lower annual 7-day low streamflows. Conversely, Kam and 19

Sheffield (2016) posited that precipitation-driven downward trends in 7-day low streamflows observed over North and South Carolina (1982–2011) were likely exacerbated by regulation. Using the short-term persistence (AR1) data independence assumption, the percentage of significant increases in 7-day low flows among regulated gages for all time periods was much greater than at reference gages, and the percentage of significant decreases also was greater than reference gages except for the 50-year period. Significant trends in low flows at regulated gages (Fig. 4c) are not spatially consistent with observed changes in precipitation in the western U.S. (Easterling et al., 2017). Significant trends observed at regulated gages may be attributed to changes in dam-related storage in those basins over time. Among regulated gages from 1940 to 1990, decadal measures of dam-related storage per unit drainage area (Falcone, 2011) increased from 1940 through 1970 (Fig. 11). While dam-related storage continued to increase after 1970, most storage was built out by about 1970. This 1940–1970 growth in dam-related storage could explain, in part, the relatively high percentage of significant downward trends in mean annual flows observed over the 100-year, 1916–2015, period (Fig. 9a). Such changes may be a consequence of the increase in evaporative losses after the dam is constructed and increased consumptive use of water that may be facilitated by having the reservoir available to provide the reliable supply to sustain larger consumptive uses. Significant trends at regulated gages also may be attributed to changes in regulation rules and targets for maintenance of minimum flows over time; however, it was beyond the scope of this study to catalog changes in historical regulation rules for all regulated basins. [Figure 11 near here]

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4.3.3. Agricultural basins In contrast with the regulated gages, a comparison of 7-day low and mean annual streamflow trends among agricultural gages (Fig. 3, Fig. 9) showed agreement in overall percentages of increases and decreases. Low-flow trend magnitudes correlated with mean annual flow trend magnitudes similarly as that for reference gages (Fig. 10). Significant downward trends in 7-day low flows in the west and southeast U.S., and upward trends in the Upper Mississippi and Great Lakes regions (Fig. 4) are spatially consistent with observed changes in summer precipitation in those respective regions (Easterling et al., 2017). The percentage of significant (AR1) increases and decreases among agricultural gages was greater than at reference gages for both 75- and 50-year time periods. The percentage of significant increases among agricultural gages was much greater than at reference gages for the 75-year period. Crop land use among agricultural gages (n = 78) changed little from 1974 to 2012 (Falcone, 2015). Nearly three-quarters (73 percent) of agricultural basins had increases in crop land use, but overall changes at these basins were relatively small with a mean increase in percent crop land use of 2.3 percent. Zhang and Schilling (2006) observed trends toward increasing streamflow in the Mississippi River basin (1940–2003) that they attributed to increased precipitation mediated by land use change and agricultural practices. They observed that increases in streamflow were mainly manifested in the base-flow component of total streamflow attributed to increased infiltration of precipitation due to soil conservation practices (such as terraces, conservation tillage, and contour cropping) and the conversion of perennial vegetation to seasonal row crops that decreased evapotranspiration (Zhang and Schilling, 2006). Juracek and Eng (2017) also 21

attributed increases in low streamflows (defined as less than the 10th percentile streamflow, from 1980 to 2015) at basins in central and eastern Kansas, in part, to agricultural land-management practices related to soil conservation. A study of streamflow trends in the Great Lakes basin by Hodgkins et al. (2007) documented the magnitudes of increases in annual 7-day low streamflows (1955–2004) were generally larger at basins with greater amounts of agricultural land use. They observed that increases in 7-day low streamflow among basins with the highest amount of agricultural land use were more than 50 percent higher than increases at basins with the lowest amount, suggesting changing agricultural practices over time (e.g. irrigation, contour plowing, erosion control, etc.) have led to an increase in the lowest flows of the year. 4.3.4. Urban basins Urban gages showed somewhat greater percentages of decreases in 7-day low flows when compared to mean annual flows (Fig. 3, Fig. 9). In general, the percentages of significant increases observed among both 7-day low flows and mean annual flows were about the same while there were greater percentages of significant decreases among low flows. Low-flow trend magnitudes correlated most strongly with mean annual flow trend magnitudes at urban gages among all gage classes and showed a similar relation as the agricultural and reference gages (Fig. 10). For the 50-year period, urban gages had the greatest percentage of significant decreases in 7day low flows compared to all other gage classes even though most urban gages saw upward trends in mean annual flows (Fig 3, Fig 8, Fig. 10). Urban settings where increased impervious area results in more rapid storm runoff and higher high flows could result in higher mean annual flows while less water is available in storage for low flows. The percentage of significant (AR1) increases and decreases among urban gages was greater than at reference gages for both 75- and 50-year time periods; the percentage of significant 22

decreases among urban gages was nearly double that of reference gages for the 50-year period and was greater than the percentage at any other gage class (Fig. 3). Developed land use among urban gages (n = 121) increased in nearly all basins from 1974 to 2012 (Falcone, 2015); one basin saw no change, but the rest saw increases with a mean increase of 8.8 percent of drainage basin area. In the eastern U.S., where most of the urban gages are located, significant decreases in 7-day low flows in the southeast U.S. and increases elsewhere are generally consistent with observed changes in summer precipitation (Easterling et al., 2017), though there were exceptions. While significant trends at urban gages may be primarily driven by changes in precipitation, the specific basin-by-basin features of an urbanized landscape play important mediating roles resulting in the greatest percentages of significant decreases in low flows compared to other gage classes, as well as the greatest percentages of significant increases in low flows second only to regulated gages. The effects of urbanization on streamflow are complex. Many factors associated with urbanization can conceivably increase or decrease low streamflows (Price, 2011). While it is generally acknowledged that urban development in a basin expands drainage networks and increases impervious area which, in turn, increases surface runoff and reduces subsurface flow and groundwater recharge (Paul and Meyer, 2001), studies also indicate that a variety of other features of an urban landscape increase streamflow directly or enhance groundwater recharge leading to increased base flow such as: urban irrigation return flow or deep percolation of urban irrigation water, water transfers into the urban watershed, leaky water infrastructure, effluent discharge, storm-water detention ponds, and decreased evapotranspiration in paved or roofed areas (Allaire et al., 2015; Eng et al., 2013; Hibbs and Sharp, 2012; Wang and Cai, 2010; GarciaFresca and Sharp. 2005; Hirsch et al. 1990). Garcia-Fresca and Sharp (2005) observed that 23

groundwater recharge commonly increased in urban basins; and Brandes et al. (2005) found no evidence of decreasing base flows in response to low- to moderate-density land development in urbanizing basins in the Delaware River basin. Hodgkins et al. (2007) observed the 1955-2004 increase in annual 7-day low streamflow change at the one urban basin examined in their Great Lakes basin study was greater than increases at all of the basins relatively free of human influences; they suggested the increase may have been driven in part by water imported into the basin from the Great Lakes.

5. Conclusions This study enhances the growing body of literature on trends in low flows across the conterminous U.S. by comparing observed trends at near-natural (reference) streamgages to trends observed at agricultural, regulated, and urban end-member gages. We sought to determine whether greater percentages of significant trends in 7-day-low streamflows were observed among agricultural, regulated, and urban gages relative to reference gages, and to quantify how trends differed among the non-reference gage classes. Regulated gages had significant upward trends in low flows at much greater percentages than reference gages, and where there were downward trends for the 75- and 100-year periods, the percentage of significance also was greater than reference gages. In general, low-flow trends at regulated gages in the western U.S. were not spatially consistent with observed trends at reference gages and precipitation changes. Substantial increases in dam-related storage prior to 1970 explain, in part, many of the significant upward trends in low flows observed at regulated gages after 1940, assuming operating rules most commonly decrease natural streamflow variability and augment low flows. Likewise, the 1940–1970 growth in dam-related storage

24

could explain the relatively high percentage of significant downward trends in mean annual flows observed for the 100-year period tested. In contrast to the low-flow trends observed at regulated gages, trends at agricultural gages spatially corresponded with observed trends at reference gages and precipitation changes. Where trends were observed, the percentage of significance among agricultural gages was greater than at reference gages. Available land use data indicated the amount of crop land use among agricultural gages changed little from 1974 to 2012. Significant trends at agricultural gages may be primarily driven by regional changes in precipitation and mediated by changes in agricultural land and water-management practices. Similar to the low-flow trends observed at agricultural gages, as a group trends at urban gages broadly corresponded with observed regional trends at reference gages and precipitation changes, though there were exceptions. Urban gages had significant trends at percentages greater than at reference gages. Urban gages had the greatest percentage of significant decreases in 7-day low flows compared to all other gage classes even though most urban gages saw upward trends in mean annual flows, as well as the greatest percentages of significant increases in low flows second only to regulated gages. While significant trends at urban gages may be driven by changes in precipitation, our trend results confirm that basin-specific urban landscape features play important and complex mediating roles in low streamflow. Urbanization and urban landscape features increased and decreased low flows at different basins, resulting in changes that were more frequently statistically significant than for reference basins. We examined the presence of natural persistence in low-flow time series with lag-one autocorrelation tests of detrended annual 7-day low streamflows, for all gage classes. Evidence of significant autocorrelation at percentages much greater than expected due to chance for all 25

gage classes shows that data independence is not an appropriate assumption for annual low streamflow data at many basins. Percentages of significant lag-one autocorrelation were lowest among reference gages and highest among regulated and agricultural gages. These results provide insight into regional climate-related trends in low streamflows and changes driven, in part, by human activities on the landscape. Many basins influenced by agricultural practices, flow regulation, and urban infrastructure had greater percentages of significant trends in low streamflows than reference basins that are relatively free of human influence. Accurate assessments of low-flow statistics are important to the management of water supply, ecological conditions in streams, and protection of water quality. Resource managers and regulators are challenged to make permits for withdrawals and discharges relevant not only for the present but the near future as well; having a better understanding of how flows have changed in the past can help resource managers think about how flows may change in the future. The results presented here can be particularly useful if supplemented by regional attribution studies and compared to model hindcasts using the actual history of climate and landscape changes. Such extensions of this work will enable evaluation of the use of those models for the important task of predicting future low-flow changes.

6. Acknowledgements This article greatly benefited from reviews by Christopher Konrad, USGS; Glenn Hodgkins, USGS; and five anonymous reviewers. We thank Benjamin York, USGS, for assistance with NWALT data. This work was supported by the U.S. Environmental Protection Agency.

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TABLES Gage classification

HCDN-2009 network High land-use threshold

Reference

Yes

none

≤ 2% cultivated crops; ≤ 60 days normalized dam storage; ≤ 6% developed land

Agricultural

No

> 20% cultivated crops

≤ 60 days normalized dam storage; ≤ 6% developed land

Regulated

No

> 180 days normalized dam storage

≤ 2% cultivated crops; ≤ 6% developed land

Urban

No

> 10% developed land

≤ 2% cultivated crops; ≤ 60 days normalized dam storage

Low land-use thresholds

Table 1. Gage classification was done using four metrics derived or obtained from basin characteristics in the GAGES-II database: [1] Reference: whether the gage was part of the Hydroclimatic Data Network-2009 (Lins, 2012) considered to be relatively free from human influence; [2] Agricultural: percent basin land cover classified as cultivated crops (class 82) in the 2006 National Land Cover Database (NLCD; Fry and others, 2011); [3] Regulated: normalized dam storage derived from 2009 basin dam storage (GAGES-II attribute STOR_NID_2009, in units of megaliters per square kilometer) multiplied by the drainage area (yielding volume) and then divided by the basin mean annual streamflow (volume per unit time) on the basis of all available streamflow records from climatic years 1916 through 2015 to yield units of time (days); [4] Urban: percent basin land cover classified as developed (sum of land cover classes 21, 22, 23, and 24) in the 2006 NLCD. Basin attributes were required to meet both high and low land-use thresholds for gage classifications to be assigned. Gages that failed to meet any classification criteria above were classified as ‘Other’.

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SUPPLEMENTAL TABLE

1966 - 2015 50-year

1941 - 2015 75-year

1916 - 2015 100-year

7-Day Low Streamflow Basin class Reference Agricultural Regulated Urban Other Reference Agricultural Regulated Urban Other Reference Agricultural Regulated Urban Other

Decreases, percent Increases, percent n all INDE AR1 LTP all INDE AR1 LTP 10 50.0 50.0 4 NA NA NA NA NA NA NA NA 34 47.1 32.4 11.8 2.9 50.0 47.1 41.2 20.6 3 NA NA NA NA NA NA NA NA 152 31.6 11.8 5.9 2.6 63.2 44.7 37.5 21.1 115 53.9 8.7 4.3 4.3 41.7 13.9 13.0 6.1 37 21.6 13.5 10.8 5.4 70.3 54.1 51.4 10.8 118 21.2 11.9 6.8 0.8 72.0 50.8 37.3 19.5 46 50.0 10.9 6.5 2.2 43.5 28.3 15.2 13.0 1,092 29.9 11.7 7.5 3.3 65.2 41.2 36.0 19.3 259 59.5 16.6 7.7 4.2 30.5 6.2 4.2 1.2 78 35.9 14.1 9.0 3.8 60.3 24.4 11.5 1.3 220 35.0 10.9 4.1 0.9 60.5 44.1 29.1 14.5 121 43.0 19.0 14.9 5.0 51.2 22.3 16.5 10.7 1,804 42.5 13.5 8.3 3.7 49.8 19.8 13.4 4.7

Supplemental Table 1. Percent of streamgages with trends in 7-day low streamflows for three time periods. Significance (p ≤ 0.05) was determined for three assumptions of the time series data structure: independence (INDE), short-term persistence (AR1), and long-term persistence (LTP). [Basin classes ‘reference’, ‘agricultural’, ‘regulated’, and ‘urban’, are defined in the Data and Methods section of this article; Basin class ‘other’ includes all other study basins that did not meet criteria for the other classes; n, number of qualifying gages tested; Trend category ‘all’ includes all trends regardless of significance, NA, insufficient number (< 10) of gages; -, no trends].

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Figure 1. Study gages that met data length and completeness criteria selected from the Geospatial Attributes of Gages for Evaluating Streamflow, version II, (GAGES-II) database. Gages that qualified for 1966–2015 trend testing are shown in light gray (n = 2,482); those that qualified for 1941–2015 in dark gray (n = 1,408); and those that qualified for 1916–2015 in black (n = 203). Figure 2. Trends in annual 7-day low streamflows for (a) 100-year, (b) 75-year, and (c) 50-year time periods through 2015. Blue colors indicate increases in flows over time, brown decreases. Solid triangles indicate statistically significant trends under the three assumptions of the time series data structure: independence (lightest shade), short term persistence (medium shade), and long term persistence (darkest shade). Open triangles indicate trends that are not statistically significant under any assumption. Gray open circles indicate no trend (Sen’s slope is zero). Figure 3. Trends in annual 7-day low streamflows for (a) 100-year, (b) 75-year, and (c) 50-year time periods through 2015. The extent of the bars indicate percent of gages with trends regardless of statistical significance. Colors indicate statistically significant trends under the three assumptions of the time series data structure: independence (lightest shade), short term persistence (medium shade), and long term persistence (darkest shade). Blue colors indicate increases in flows over time, brown decreases. Figure 4. Trends in annual 7-day low streamflows, 1966–2015, for (a) reference basins in the Hydro-Climatic Data Network (HCDN), and basins representing predominantly (b) agricultural, (c) regulated, and (d) urban characteristics. See caption for figure 2 for an explanation of symbology.

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Figure 5. Autocorrelation of detrended annual 7-day low streamflows (1941–2015). Blue shaded area with dashed lines denote the bounds of the 95th percentile of the distribution of r of +/-0.22 for an independent time series of 75 years. Numbers in brackets are the total number of gages tested; numbers above them are the number of gages with significant lag-one autocorrelation. Similar results were observed for the 50- and 100-year time periods (not shown). Figure 6. Gages with consistent trends in 7-day low streamflows for all time periods tested. Increases in flows over time shown in blue (n = 641), decreases in brown (n = 380). Black points indicate gages with trends toward increases during 1916-2015 and 1941-2015 and decreases during 1966-2015 (n = 236). Figure 7. Trends in annual 7-day low streamflows for 1966-2015 among small (< 600 km2), medium (600-3,000 km2), and large (> 3,000 km2) drainage areas. Figure 8. Trends in mean annual streamflows for (a) 100-year, (b) 75-year, and (c) 50-year time periods through 2015. Blue colors indicate increases in flows over time, brown decreases. See caption for figure 2 for an explanation of symbology. Figure 9. Trends in mean annual streamflows for (a) 100-year, (b) 75-year, and (c) 50-year time periods through 2015. The extent of the bars indicates percent of gages with trends regardless of statistical significance. See caption for figure 3 for an explanation of symbology. Figure 10. 1966-2015 trends in 7-day low streamflow compared to trends in annual mean streamflow. Gray points show results for all gages. Black points highlight subsets of gages by classes. Diagonal lines indicate 1:1 line. Percent per year values used for plot clarity and computed as trends in flow units per year normalized to mean annual flow and expressed as percent. 50-year period plotted to maximize the number of gages for comparison. 40

Figure 11. Decadal measures of dam-related storage per unit basin drainage area (Falcone, 2011) among Regulated gages (n = 220). Supplemental Figure 1. Water-resources regions of the conterminous United States (from Lins and Slack, 2005). Supplemental Figure 2. Trends in annual 7-day low streamflows, 1916–2015, for (a) reference basins in the Hydro-Climatic Data Network (HCDN), and basins representing predominantly (b) agricultural, (c) regulated, and (d) urban characteristics. See caption for figure 2 for an explanation of symbology. Supplemental Figure 3. Trends in annual 7-day low streamflows, 1941–2015, for (a) reference basins in the Hydro-Climatic Data Network (HCDN), and basins representing predominantly (b) agricultural, (c) regulated, and (d) urban characteristics. See caption for figure 2 for an explanation of symbology.

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Greater percentages of significant trends at gages with human influences in the U.S. Urban gages had the greatest percentage of significant 50-year downward trends Many statistically significant upward trends in low flows at regulated basins Time-series independence often not an appropriate assumption for annual low-flows

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Figure 1

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a. 1916–2015

b. 1941–2015

c. 1966–2015

Figure 2

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a. 1916–2015

b. 1941–2015

c. 1966–2015

100

50 0 50 percent of gages with decreases | percent of gages with increases

Figure 3

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100

a. Reference

b. Agricultural

c. Regulated

d. Urban

Figure 4

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Lag-1 correlation coefficient

Reference Other

Agricultural Regulated

Urban

Gage class

Figure 5.

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Figure 6.

48

100

50 0 50 percent of gages with decreases | percent of gages with increases

Figure 7.

49

100

a. 1916–2015

b. 1941–2015

c. 1966–2015

Figure 8.

50

a. 1916–2015

b. 1941–2015

c. 1966–2015

100

50 0 50 percent of gages with decreases | percent of gages with increases

Figure 9..

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100

Trend in 7-day low runoff, in percent per year

Reference

Trend in annual mean runoff, in percent per year Figure 10.

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Figure 11.

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