Changes in Streamflow Timing in the Western United States in Recent Decades Fact Sheet March 2005

• Description: Changes in Streamflow Timing in the Western United States in Recent Decades Fact Sheet March 2005 USGS
• State: CO
• Date: 3/1/2005
• Author: Dettinger, Michael
• Title: Changes in Streamflow Timing in the Western United States in Recent Decades Fact Sheet March 2005
• Water Supply Pro - Doc Type: Publication

magnitude and timing. Streamflow
information is required for numerous
water - facility design and operational
needs, as well as for research bearing
on the long -term implications of climate
variability and human activities. The
USGS has been measuring and recording
streamflows in the western United States
since the late 19'" century. Because
USGS streamflow data (hup.- Ilivatealata.
usgs.govIusa/nkvisAw) are collected to
consistent national standards, span long
periods of time, cover the full range of
streamflow from floods to droughts, and
include observations for unregulated
natural streams, they can be analyzed to
determine when and where snowmelt and
streamflow - timing vary.
Streamflow timing can be described
by many different measures, depending
on data availability and on the aspects
of streamflow timing that are of most
concern. Roos (1991) and Dettinger
and Cayan (1995) analyzed the frac-
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02
tions of annual streamflow that occur in
spring and summer seasons, because this
fraction of total flow is, in many water -
resource systems, the most readily stored
and distributed for warm- season uses.
Cayan and others (2001) characterized
streamflow timing by the day of year
when wintertime low -flow conditions
rapidly transition to springtime high -flow
conditions with the onset of warm - season
snowmelt. These "spring -pulse dates" are
important because they indicate the tim-
ing of snowmelt and the divide between
winter and spring conditions. Stewart and
others (2004) characterized streamflow
timing by the "center of volume" of each
year's hydrograph. The center -of- volume
date is the date by which roughly half
of the streamflow for a year has passed.
These center -of- volume dates provide
direct measures of overall streamflow
timing based on runoff conditions
throughout the year.
April -July Flows as Fraction of Water -Year Total
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Changes in Streamflow Timing
Annual streamflow in most western
rivers has come progressively earlier dur-
ing the past several decades. The long-
term tendency of springtime streamflow
(that fraction of overall flow that occurs
from April to July) to decline during the
20th century in the central and northern
Sierra Nevada is shown as a fraction
of overall flow in figure 1. Regressions
of data prior to the 1945 indicate no
statistically significant trend, whereas
regression of data after 1945 show a clear
statistically significant trend toward ear-
lier streamflow. As the springtime frac-
tions of yearly flows have declined, the
fraction in winter, and especially flows
in March, have increased. These results
broadly reflect a regional trend toward
warmer winters and springs during the
same period (fig. 2).
Changes in daily streamflow variations
and center -of- volume dates of western
rivers can be illustrated by comparing
Figure 1. April - July streamflow in eight major
rivers of the western Sierra Nevada, Cali-
fornia, as a fraction of water -year (October
through September) total streamflow. Dots
indicate yearly values, blue curve is 9 -year
moving averages, and dashed line is linear
trend prior to 1945 and solid line is trend after
1945.
Figure 2. Springtime temperature trends in
North America, 1950 -1997, using gridded
1900 1920 1940 1960 1980 2000 temperature anomalies from Peterson and
Vose (1997).
70°
60°
50°
40"
30°
20°
10°
1800
_ � 0
1 1 0 0�2
—_
0 -
a' 9
4 March - May 2 01
Temperature (C) Trends 2
2 1950 -1997 1
0 0
-2 -- 0
-6
1500
1200
90°
60°