Streamflow trends in the United States

Harry F. Lins and James R. Slack

U.S. Geological Survey, 415 National Center, Reston, VA 20192, USA

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Abstract. Secular trends in streamflow are evaluated for 395
climate-sensitive streamgaging stations in the conterminous United
States using the non-parametric Mann-Kendall test. Trends are calculated
for selected quantiles of discharge, from the 0^th^ to the 100^th^
percentile, to evaluate differences between low-, medium-, and high-flow
regimes during the twentieth century. Two general patterns emerge;
trends are most prevalent in the annual minimum (Q~0~) to median (Q~50~)
flow categories and least prevalent in the annual maximum (Q~100~)
category; and, at all but the highest quantiles, streamflow has
increased across broad sections of the United States. Decreases appear
only in parts of the Pacific Northwest and the Southeast. Systematic
patterns are less apparent in the Q~100~ flow. Hydrologically, these
results indicate that the conterminous U.S. is getting wetter, but less
extreme.

1\. Introduction

Floods and droughts cause more damage annually in the United States than
any other natural disaster. There is an increasing trend in both flood
damage and drought vulnerability (Federal Interagency Floodplain
Management Task Force, 1992; U.S. Army Corps of Engineers, 1995;
Wilhite, 1997). Most of the flood damage increase stems from continuing
urban and suburban development on floodplains and the drought
vulnerability increase is from development in regions of lower renewable
water supplies.

However, there is also a perception that extreme hydrologic events are
increasing in frequency and/or magnitude.

Most documentation of trends in surface water hydrologic conditions has
focused on monthly and annual mean discharge or the timing of monthly
mean discharge (Chiew and McMahon, 1996; Lettenmaier *et al.,* 1994;
Lins and Michaels, 1994). Since the 1940\'s, the general pattern has
been toward an increase in mean discharge in the autumn and winter
months in most regions of the conterminous United States. Systematic
analysis of trends across the spectrum of streamflows, including flood-
and low-flows, has received little treatment. Significantly, however,
the Mississippi floods of 1993 and the widespread flooding during 1997
in the West, Upper Midwest, and the Ohio Valley, as well as the
widespread 1988 and 1995-96 drought, gave rise to speculation that
floods and droughts are increasing; possibly in response to greenhouse
warming. We consider the question of any changes in hydrologic regime by
determining whether trends have occurred in streamflow over a range of
discharge quantiles.

2\. Data and Methods

A compilation of daily discharge records, relatively free of such
anthropogenic influences as watercourse regulation, diversion,
ground-water pumpage, or land use change, was developed by the U. S.
Geological Survey (USGS) to study climatically-induced variations in U.
S. surface-water conditions (Slack and Landwehr, 1992). Known as the
Hydro-Climatic Data Network (HCDN), it includes data from more than 1500
streamgages. We use a subset of the HCDN consisting of daily mean
discharge from 395 stations that provide broad spatial coverage of the
hydrology of the United States. The 395 stations represented the maximum
number furnishing continuous daily records over the 50-year period
1944-1993, with a decreasing number of stations providing data at longer
time periods; 193 at 60 years (1934-93), 70 at 70 years (1924-93), and
34 at 80 years (1914-93). Although more than 395 stations had continuous
data for periods less than 50 years, we wanted to maximize sample size
and maintain site continuity over 30-, 40-, and 50-year periods, while
not compromising spatial coverage.

Trends in the HCDN records are evaluated using the nonparametric
Mann-Kendall test. The test examines whether a random response variable
monotonically increases or decreases with time. It is a rank-based
procedure, resistant to the influence of extremes, and good for use with
skewed variables. No assumption of normality is required, although there
must be no serial correlation for the resulting p-values to be correct
(Helsel and Hirsch, 1992). These characteristics make it particularly
appropriate for use with streamflow data sampled up to one year apart.

Our approach to the assessment of trends in streamflow involves two
distinct elements. First, we test for trends in 7 quantiles of the
streamflow distribution, by selected decimals, from the annual minimum
(daily mean) (Q~0~) to the annual maximum (daily mean) (Q~100~). This
includes the 90^th^ (Q~90~), 70^th^ (Q~70~), 50^th^ (Q~50~), 30^th^
(Q~30~), and 10^th^ (Q~10~) percentiles. In so doing, we derive a more
complete picture of how the streamflow regime is changing over the
entire discharge spectrum. Second, we evaluate interdecadal streamflow
variability by calculating the quantile trends for 30-, 40-, 50-, 60-,
70-, and 80-year periods, all ending in 1993. This provides some
insights as to how the characteristics of hydrologic trends during the
century are affected by the period-of-record.

3\. Results

Trend test results, by quantile and period-of-record, are summarized in
Table 1. Moving from low- to high-flows, we first consider the annual
minimum (daily mean) discharge (Q~0~), which we use as a surrogate
measure of hydrologic drought. The Q~0~ flow represents the lowest
recorded mean daily discharge at a gage each year and generally reflects
baseflow conditions. It exhibits a strong pattern of trends through the
twentieth century. The percentage of the streamgaging stations recording
statistically significant trends ranges from a low of 28 percent at 30
years of record to a high of 49 percent at 70 years of record.
Significantly, there are many more uptrends than downtrends nationally;
that is, the broad pattern is toward increasing annual minimum
streamflow. The uptrends exceed the downtrends by 4 to 1 when averaged
over all time periods.

Progressing through the lower to middle range of streamflows (Q~10~ -
Q~50~), the pattern that emerges is quite similar to that for the annual
minimum discharge; that is, numerous statistically significant trends
(25-46 percent of the stations), with many more increases than
decreases. However, the situation changes noticeably in the upper half
of the discharge distribution. At the Q~70~ flow, the number of stations
with significant trends drops to 23 percent, averaged over the six time
periods. At Q~90~ the average percentage drops to 14 and finally, at the
annual maximum flow, only 11 percent of the stations have significant
trends. Of perhaps more note is the change in the mix of up- and
downtrends. In the low to middle flows there is a clear predominance of
uptrends. At the high flows, uptrends and downtrends are roughly equal.

This variation in trends across quantiles is illustrated in Figure 1.
The graph covers the period 1944-1993, but the same general pattern is
evident in each of the six time periods. There are two important
characteristics in the figure. First, the number of increasing
streamflow trends is high and approximately equal across the lower half
of the flow distribution, but falls sharply across the upper half.
Second, downtrends decrease in number from the Q~0~ to Q~50~ flow, but
increase from the Q~50~ to Q~100~ flow. This pattern indicates that
baseflows are increasing (which suggests that drought is decreasing),
median or average streamflow is increasing, but annual maximum flows
(including floods) are neither increasing nor decreasing.
Hydrologically, the nation appears to be getting wetter, but less
extreme.

Another aspect of the material contained in Table 1 relates to
interdecadal variations in the appearance of trends. Averaging over all
quantiles, the highest percentage of streamgaging stations having trends
is seen during the 40-year period 1954-93 (36%). High percentages are
also evident at the 60-year (1934-93; 34%) and 70-year (1924-93; 34%)
periods. However, an average of only 25 percent of the gages have
significant trends at both the 50- and 80-year periods, and the lowest
percentage of all (20%) characterizes the most recent 30-year period
(1964-93). Not surprisingly, trend analysis results are sensitive to the
variable characteristics of climatic anomalies occurring in the early
and late years of the temporal window being used. For example, between
1944 and 1993 14 stations had uptrends in the annual maximum discharge
while 21 stations had downtrends. Between 1954 and 1993, this pattern
reversed such that 31 stations were up and 22 were down. Then, between
1964 and 1993, the pattern reversed again with 12 stations recording
increases and 25 decreases. There is considerable consistency in the
pattern of trends across quantiles and time periods despite the
differences in the number of stations exhibiting trends.

We map trend direction and persistence over the six time periods for the
annual maximum, median, and minimum flows in Figure 2. There are
relatively few trends in the annual maximum flow compared to the annual
median and minimum. There are also regionally distinct patterns of
streamflow increases and decreases that are consistent across quantiles.
Finally, the annual minimum flow has more stations registering trends in
three or more time periods. Details of the salient regional features
follow.

A broad mix of up- and downtrends in the Q~100~ flow are scattered
across the eastern half of the United States, while most of the stations
with trends in the western U.S. record decreases. Regionally coherent
decreases were most notable in the Pacific Northwest and in the Southern
Plains. Considerably more regional coherence is apparent in the trends
at the median flow. This part of the discharge regime is characterized
by a broad area of uptrends that stretches from the New England to the
Lower Colorado water resources region, and that includes the
Mid-Atlantic, Ohio, Tennessee, Upper and Lower Mississippi, Texas-Gulf,
Rio Grande, and Great Basin water resources regions. Decreasing median
flows are most prevalent in the Pacific Northwest and Northern
California, and in parts of the Southeast.

Trends in annual minimum flows are nearly identical to those in the
median with respect to direction. The differences are primarily in the
number of periods when a station exhibits a trend. Many more stations,
especially in the Far West, have trends in multiple time periods at the
Q~0~ level. In the aggregate, most of the nation's water resources
regions are experiencing discharge increases at flows below the annual
maximum (and actually below the upper quartile, Q~75~). The only regions
exhibiting systematic decreases are the Pacific Northwest, Northern
California, and parts of the Southeast; and these decreases are evident
at all flow quantiles.

4\. Discussion

Although the specific causes of these variations are not simply and
immediately explained, their broad spatial consistency is sufficient to
suggest some systematic cause or causes. For example, the increases
observed in the northeastern quarter of the nation could be associated
with precipitation patterns linked with the recent and persistent high
index phase of the North Atlantic Oscillation (Hurrell, 1995; Hurrell
and van Loon, 1997). The decreases in the Pacific Northwest, especially
given the opposing increases in the Southwest, may reflect decadal-scale
variations in the tropical and North Pacific Ocean (Dettinger and Cayan,
1995; Graham, 1994; Latif and Barnett, 1994). Opposing climatic and
hydrologic anomalies between the Pacific Northwest and the Southwest are
well documented as occurring in conjunction with various North Pacific
atmospheric circulation modes (Lins, 1997; Cayan and Peterson, 1989).

Climate model simulations associated with increasing atmospheric CO~2~
have been interpreted (e.g. Houghton *et al*., 1996) as indicating an
intensification of the hydrologic cycle. This is generally translated to
mean more extreme hydrologic events such as floods and droughts. One
recent analysis of observed data appears to lend some support to these
simulations by indicating that, \"at least within the United
States\--the proportion of total precipitation contributed by extreme,
one-day events has increased significantly\" during the twentieth
century (Karl *et al*., 1995).

We suspect that our streamflow findings are consistent with the
precipitation findings of Karl and his collaborators (1995, 1998). The
reported increases in precipitation are modest, although concentrated in
the higher quantiles. Moreover, the trends described for the extreme
precipitation category (\>50.4mm per day) are not necessarily sufficent
to generate an increase in flooding. It would be useful to know if there
are trends in 24-hour precipitation in the \>100mm and larger
categories. The term \"extreme,\" in the context of these thresholds,
may have more meaning with respect to changes in flood hydrology.

What, if anything, do our results imply for the hypothesis that
increasing atmospheric CO~2~ will lead to an enhanced hydrologic cycle
and, therefore, more floods and droughts? This is a more problematic
issue, and one on which current climate modeling studies may soon be
able to shed some light.

Acknowledgements. We thank K. Wahl, K. Potter, and S. Burges for
thoughtful reviews. Special thanks also to S. Hostetler for many
critical comments and numerous insightful discussions.

References

Cayan, D. R., and D. H. Peterson, The influence of North Pacific
atmospheric circulation on streamflow in the west, in *Aspects of
Climate Variability in the Pacific and the Western Americas*, *Geophys.
Monogr. Ser.*, vol.55, edited by D. H. Peterson, pp. 365-374, AGU,
Washington, D. C., 1989.

Chiew, F.H.S., and T.A. McMahon, Trends in historical streamflow
records, in *Regional Hydrological Response to Climate Change*, edited
by J.A.A. Jones et al., pp. 63-68, Kluwer Academic Publishers,
Amsterdam, 1996.

Dettinger, M. D., and D. R. Cayan, Large-scale atmospheric forcing of
recent trends toward early snowmelt runoff in California, *J. Clim*., 8,
606-623, 1995.

Federal Interagency Floodplain Management Task Force*, Floodplain
Management in the United States: An Assessment Report*, FIA-18, Federal
Insurance Agency, Washington, D. C., 1992.

Graham, N. E., Decadal-scale climate variability in the tropical and
North Pacific during the 1970s and 1980s: observations and model
results, *Climate Dynamics*, 10, 135-162, 1994.

Helsel, D. R., and R. M. Hirsch, *Statistical Methods in Water
Resources*, Elsevier Science Publishers, Amsterdam, 1992.

Houghton, J. T., L. G. Meira Filho, B. A. Callander, N. Harris, A.
Kattenberg, and K. Maskell (Eds.), *Climate Change 1995: The Science of
Climate Change*, Cambridge University Press, Cambridge, 1996.

Hurrell, J. W., Decadal trends in the North Atlantic oscillation:
regional temperatures and precipitation, *Science*, 269, 676-679, 1995.

Hurrell, J. W., and H. van Loon, Decadal variations in climate
associated with the North Atlantic oscillation, *Climatic Change*, 36,
301-326, 1997.

Karl, T. R., and R. W. Knight, Secular trends of precipitation amount,
frequency, and intensity in the USA, *Bull. Am. Meteorol. Soc*., 79,
231-241,1998.

Karl, T. R., R. W. Knight, and N. Plummer, Trends in high-frequency
climate variability in the twentieth century, *Nature*, 377, 217-220,
1995.

Latif, M., and T. P. Barnett, Causes of decadal climate variability over
the North Pacific and North America, *Science*, 266, 634-637, 1994.

Lettenmaier, D. P., E. F. Wood, and J. R. Wallis, Hydro-climatological
trends in the continental United States, 1948-88, *J. Clim*., 7,
586-607, 1994.

Lins, H. F., Regional streamflow regimes and hydroclimatology of the
United States, *Water Resour. Res*., 33, 1655-1667, 1997.

Lins, H. F. and P. J. Michaels, Increasing U.S. streamflow linked to
greenhouse forcing, *Eos Trans. AGU*, 75, 281, 284-285, 1994.

Slack, J. R., and J. M. Landwehr, Hydro-climatic data network: a U. S.
Geological Survey streamflow data set for the United States for the
study of climate variations, 1874-1988, *U. S. Geol. Surv. Open-File
Rept. 92-129*, 1992.

U. S. Army Corps of Engineers, *Annual Flood Damage Report to Congress
for Fiscal Year 1995*, Washington, D. C., 1996.

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Am. Water Resour. Assoc*., 33, 961-968, 1997.

Received: November 6, 1998

Revised: December 2, 1998

Accepted: December 3, 1998

Table 1. Mann-Kendall trend test results for selected quantiles of
streamflow between the annual minimum and maximum.

  ----------------------- -------- --- ------ --- ------ -- ------ -- ------ -- -------
                                                                                

                          Years of                                              
                          Record                                                

                                                                                

                          30           40         50        60        70        80

                                                                                

                                                                                

  Beginning year          1964         1954       1944      1934      1924      1914

  Ending year             1993         1993       1993      1993      1993      1993

  Stations tested         395          395        395       193       70        34

                                                                                

  *Annual minimum (daily                                                        
  mean) discharge*                                                              

  No. of significant      112          177        163       85        34        13
  trends (p≤0.05)                                                               

  Percent with            28           45         41        44        49        38
  significant trends                                                            

  No. with increasing     74           145        127       76        32        10
  trend                                                                         

  No. with decreasing     38           32         36        9         2         3
  trend                                                                         

                                                                                

  *Annual 10^th^                                                                
  percentile of daily                                                           
  discharge*                                                                    

  No. of significant      101          155        143       89        32        12
  trends (p≤0.05)                                                               

  Percent with            26           39         36        46        46        35
  significant trends                                                            

  No. with increasing     63           132        117       80        30        8
  trend                                                                         

  No. with decreasing     38           23         26        9         2         4
  trend                                                                         

                                                                                

  *Annual 30^th^                                                                
  percentile of daily                                                           
  discharge*                                                                    

  No. of significant      109          160        135       81        28        9
  trends (p≤0.05)                                                               

  Percent with            28           41         34        42        40        26
  significant trends                                                            

  No. with increasing     76           148        125       79        27        8
  trend                                                                         

  No. with decreasing     33           12         10        2         1         1
  trend                                                                         

                                                                                

  *Annual 50^th^                                                                
  percentile of daily                                                           
  discharge*                                                                    

  No. of significant      98           174        116       82        27        10
  trends (p≤0.05)                                                               

  Percent with            25           44         29        42        39        29
  significant trends                                                            

  No. with increasing     76           167        113       81        26        9
  trend                                                                         

  No. with decreasing     22           7          3         1         1         1
  trend                                                                         

                                                                                

  *Annual 70^th^                                                                
  percentile of daily                                                           
  discharge*                                                                    

  No. of significant      59           130        64        58        19        6
  trends (p≤0.05)                                                               

  Percent with            15           33         16        30        27        18
  significant trends                                                            

  No. with increasing     55           124        61        58        19        6
  trend                                                                         

  No. with decreasing     4            6          3         0         0         0
  trend                                                                         

                                                                                

  *Annual 90^th^                                                                
  percentile of daily                                                           
  discharge*                                                                    

  No. of significant      43           94         40        20        10        5
  trends (p≤0.05)                                                               

  Percent with            11           24         10        10        14        15
  significant trends                                                            

  No. with increasing     18           76         23        19        10        5
  trend                                                                         

  No. with decreasing     25           18         17        1         0         0
  trend                                                                         

                                                                                

  *Annual maximum (daily                                                        
  mean) discharge*                                                              

  No. of significant      37           53         35        20        9         4
  trends (p≤0.05)                                                               

  Percent with            9            13         9         10        13        12
  significant trends                                                            

  No. with increasing     12           31         14        11        5         2
  trend                                                                         

  No. with decreasing     25           22         21        9         4         2
  trend                                                                         

                                                                                
  ----------------------- -------- --- ------ --- ------ -- ------ -- ------ -- -------

![](media/image3.wmf)

Figure 1. Number of streamgages, out of a total of 395, with
statistically significant (p≤0.05) trends for the 50-year period
1944-1993.

![](media/image4.wmf){width="6.510416666666667in"
height="7.28125in"}Figure 2. Trends (p≤0.05) in (a) annual maximum
daily, (b) annual median daily, and (c) annual minimum daily discharge
in relation to U.S. water resource regions. Upward-pointing triangles
indicate increasing discharge, downward-pointing decreasing. Solid
triangles () denote stations exhibiting a trend in 3 or more time
periods shown in Table 1; gray-shaded triangles () denote a trend in 2
time periods; open triangles (Δ) denote a trend in 1 time period; and an
open circle () denotes no trend in any time period.