North Pacific Research Board Project Final Report
Update of freshwater discharge model for Gulf of
Alaska
NPRB Project 734 Final Report
Thomas C. Royer, Old Dominion University
[757 683-5547 (office), 510 318-2758 (cell)]
email: royer@ccpo.odu.edu)
Chester E. Grosch (enright@ccpo.odu.edu)
December 2007
Abstract
The coastal freshwater discharge model for the Northeast Pacific that was published in 1982
using an outdated programming language has been rewritten using Matlab. The revised model
produces monthly mean freshwater discharge rates for the northern Gulf of Alaska from 1931
though 2007 using coastal precipitation and air temperature. This coastal freshwater discharge,
that is generally in excess of 24,000 m3 s-1, helps to drive the Alaska Coastal Current, a nearshore
flow that sweeps along the coast of the Northeast Pacific Ocean from the Columbia River to the
Aleutian Island chain and into the Bering Sea through Unimak Pass. It influences the advection of
heat, salt and nutrients into the Bering Sea and Arctic Ocean. The new version of the model
incorporates discharges from glacial melting and river discharge from the coast of British
Columbia. Recent increases in the melting rate of the coastal mountain glaciers in Alaska are
accelerating this freshwater discharge. The model was tuned using the hydrographic data from the
section near Seward that was sampled by U.S. Northeast Pacific GLOBEC program from 1997
through 2004. Spectral analysis of the input parameters and the resultant discharges have been
determined. This revised model and the input data is available at www.nprb.org.
Key Words
Hydrology; coastal Gulf of Alaska; Freshwater discharge; Matlab modeling; baroclinic currents;
salinity; stratification; Alaska Coastal Current.
Citation
Royer, T.C. and C.E. Grosch. 2006. Update of a freshwater discharge model for the Gulf of
Alaska. North Pacific Research Board Final Report 734, 12 p.
Table of Contents
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Study Chronology
Introduction
The coastal mountain ranges in the Northeast Pacific have some of the highest precipitation rates
in the world (George Taylor, Oregon State University, personal communication) with annual
precipitation in excess of 3 m. However, there are no major river networks bringing this moisture
to the ocean. The narrow coastal drainage area does not allow river networks to form and the
high coastal mountains prevent hinderland rivers from entering the ocean here. The result is that
this precipitation is forced to enter the ocean via a myriad of relatively small streams. Because of
the great population of these streams, their remote locationand small flows, they are not generally
gauged. However, the summation of these streams is apparently quite large, though hidden. To
provide an estimate of this coastal discharge for early studies of the circulation in the coastal Gulf
of Alaska, a simple numerical model was constructed to use the drainage areas and precipitation
to estimate the volume of freshwater entering the Gulf of Alaska along this shoreline. This
simple model allowed year to year storage and ablation of snow and ice but did not allow any net
change in glacier volume from 1931 through 1979.
Objectives
In the development of the original discharge model (Royer, 1982), there were no measurements
of glacial ablation or accumulation available. Therefore the model run up through 1979 excluded
any net change in the volumes of the coastal glaciers though annual and interannual changes in
glacier volumes were allowed, just no net change at the end of the time span of the model (19311979). However, the work of Arendt, et al.(2002) now provides estimates of the changes in the
coastal mountain glacier volumes since 1980. From 1980 to 1995, they estimate an annual glacial
ablation of 52 km3 year-1 and 96 km3 year-1 since that time. These net losses of glacial volume
per year have been included in this new model since 1980. Of course, this presents some
difficulties when comparisons are made pre and post 1980. The net melting increased the mean
annual coastal freshwater discharge by about 1600 m3 s-1 for the earlier period and 3000 m3 s-1 for
the latter period. These glacial discharges were distributed over the year in a pattern similar to
the flow pattern of the Resurrection River near Seward.
This update also includes the addition of the upstream discharges from the British Columbia coast
and the Alaskan rivers. The object was to better represent the freshwater fluxes into the Gulf of
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Alaska. Major rivers emptying into the eastern Gulf of Alaska have a combined mean discharge
of 11,450 m3 s-1 (Table I) with the majority coming from the Columbia River. However, the
discharge from the Columbia River is transported both northward and southward depending on
the seasonal and local conditions. It is sometimes south of the bifrucation point of the North
Pacific Current. The complications from changes in ocean and wind forcing are beyond the
ability of this discharge study to resolve. Therefore, though the Columbia River is sometimes
considered as the headwaters of the Alaska Coastal Current (Hickey and Royer, 2001), it’s
contribution to the coastal flow in this region will not be included in this model.
River
Drainage Area, km2
Mean Flow, m3/s
Columbia
Fraser
Skeena
Nass
Stikine
613,800
217,000
42,200
19,200
51,500
5426
2722
918
772
1615
Ratio, Area/Flow,
km2/m3/s
113
80
46
25
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Table I. Some northeastern Gulf of Alaska drainage river characteristics
River runoffs in the prior coastal discharge model (Royer, 1982) were ignored because 1) the
narrow coastal drainage did not allow the development of major river networks, 2) most of the
rivers were not gauged, and 3) the discharge records for gauged rivers had very limited durations.
To investigate the relative influence of the rivers entering the Northeast Pacific Ocean, mean
annual discharges are compared (Fig. 1). The flows in the Columbia River are obviously
relatively large in comparison with the others. Its peak average from 1878 to 1993 was greater
than 12,000 m3/s, about twice that of the Fraser River in British Columbia, the next largest river
flow rate in the region. With damming, the peak in the Columbia rivers has been reduced to
about 8,000 m3/s, only slightly greater than the Fraser. The minimum flows have also been
increased in the spring and fall by the dams to stabilize the generation of electricity. Generally,
progressing northward along this coast, there are smaller river discharges (Table I and Fig. 1),
though the area of land necessary to generate similar flows is reduced. The exception is the
Stikine River that has a much larger drainage area than the Skeena and Nass that are farther south.
It also has a later maximum in the discharge and higher rates in fall, possibly reflecting increased
fall precipitation. The peak discharges from the Taku, Copper and Susitna rivers are all less than
1000 m3/s and their exclusion in the total coastal freshwater discharge is justified. The Alsek
River has a discharge that is comparable to Stikine but since it has a significant coastal drainage,
it is already included in the discharge as calculated from the precipitation and temperature data.
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Figure 1. Monthly mean discharges from selected Northeast Pacific Ocean drainage rivers in
Washington-Oregon, British Columbia and Alaska.
Methods
We converted the model from QuickBASIC into Matlab, a more versatile and commonly
used computer coding today. We updated the input data through 2007 and include the
new sources of freshwater discharge along the British Columbia coast. We expected that
the new discharge estimates would be at least twice the average rate of freshwater
discharge of the Mississippi River. Climate changes will probably also increase this rate.
Results
The freshwater influx from the melting coastal mountain glaciers has been added by distributing
the annual water volume (52 km3 from 1980 to 1995 and 96 km3 since 1995) over the seasonal
with above freezing temperatures. Of course, this complicates interannual and interdecadal
comparisons. For example, these ablating glaciers add about 1650 m3 s-1 from 1980 to 1995 and
about 3000 m3 s-1 since 1995.
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The contributions of the Canadian rivers have been added to the original estimate of the total
discharge and the glacial ablation. Unfortunately, access to Canadian hydrology since 1984 has
been restricted so that for this study only the long term mean monthly discharges prior to 1985
are used. That is, there are no interannual variabilities included in the Canadian flow
contributions and the same seasonal signal is added each year from 1931 to the present. The
mean freshwater flux added by the Canadian rivers (including the Stikine) is 6027 m3/s, which is
considerably less than previously thought. Estimates of flow rates comparable to those found for
Southeast and South Coast Alaska had been previously accepted, that is, about 10,000 m3 s-1. Of
course, the discharges from these rivers are still an underestimate of the flow rate since the
contributions from the streams of British Columbia are not included.
The spectra of the Southeast, South Coast and total discharges, as determined with the maximum
entropy method (MEM) reveal different significant periodicities in each of the series (Fig.2). The
South Coast discharge has significant fluctuations at about 18 years (upper panel) while the
Southeast discharge has signals in the range of 11 to 16 years (middle panel). These decadal
changes are evident in the total discharge (bottom panel) but a significant signal also appears in
the 49-50 year period. This latter signal was not highly significant (>99 C.I.) in either of the
individual components. All three discharges contain significant contributions from ENSO forcing
(2-8 years).
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Figure 2. Maximum entropy method (MEM) spectra for South Coast (top), Southeast (middle)
and total discharges (bottom).
Discussion
The coastal freshwater discharge, as estimated by this hydrology model, ranges from about
37,000 m3 s-1 in 1934 to 18,000 m3 s-1 in 1972 (Fig. 3, green curve). The filter used is a 5 year
Butterworth filter. Over the length of the entire record, the 50 year signal is evident. The glacial
ablation (Fig.3, red) adds discharges since 1980 with an additional 3000 m3 s-1 annual average
since 1995. The monthly annual signal from the Canadian rivers increases the total discharge by
about 6000 m3 s-1 (Fig. 3, blue). In the last several years, the discharges have generally been
declining, reflecting decreased precipitation and lower temperatures in this coastal region.
Though the total coastal discharge might be underestimated, it is expected that this model
captures a significant portion of the variability.
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Figure 3. Estimated coastal freshwater discharge into the Gulf of Alaska 1931-2007 (green).
Glacial ablation was added since 1980 (red). Historical mean monthly Canadian rivers are
included (blue). All series are filtered with a 5 year Butterworth filter.
The Matlab version of the Alaska coastal freshwater discharge [gulf_hydro.m] is located at
www.nprb.org. The inputs for this program are the monthly mean precipitation and temperature
averages from the NOAA Climate Summary located at www5.ncdc.noaa.gov/pdfs/cd/Alaska for
Southeast and South Coast divisions. The historical values (1931-2006) are contained in sepi.txt,
setf.txt, scpi.txt, and sctf.txt. Precipitation and temperature data since 2007 is contained in
sepi2.txt, setf2.txt, scpi2.txt and sctf2.txt. As of the date of this writing, data for only the first
eight months of 2007 are available so the September, October, November and December values
are long term monthly mean values. They should be updated and future years should be added as
they become available. The output from this model is Discharge an ascii file (7 x 924). 924 is the
number of months from Jan. 1931 through 2007. The further description of the file is:
Row 1 Decimal Year (centered at mid-month) [This should be used for
the Total Discharge and differs from the individual monthly times by
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1/2 month].
Row 2 Time for Southeast monthly mean discharge
Row 3 SE monthly mean discharge in cubic meters per month
Row 4 Time for South Coast monthly mean discharge
Row 5 SC monthly mean discharge
Row 6 Total discharge for location near Seward (SC plus SE (-1month))
Row 7 Total Discharge anomaly (departure from the long term monthly mean)
The Matlab program to add glacial melting (ablation) to the Total Discharge is
[glacial_discharge.m]. For the period 1931 through 1979 there was no net glacial ablation
included in [gulf_hydro.m]. This program adds 52 km3 year-1 from 1980 to 1995 and 96 km3
year-1 since that time (after Arendt, et al., 2002. This needs to be updated as additional
information on glacial ablation becomes available with future glacial volume studies. The output
file that is written is Discab, an ascii file with a size of 924 x 2, where the first number is the
number of months and the second is the discharge with glacial ablation.
The third Matlab program Fresh_rivers.m takes the discharges from gulf_hydro.m (Discharge)
and glacial_discharge.m (Discab). It produces line plots (1931-2007) of the freshwater
discharge without glacial or Canadian river input, its anomaly and smoothed line plots of the 1)
basic freshwater discharge (Precipitation only), 2) with glacial ablation added and 3) with glacial
ablation and Canadian river input. It produces no output files. This last plot is displayed as Fig.3.
In recent years, the discharge has been declining or holding steady, reversing the general
increasing trend that peaked in about 2000. This could help explain the ocean cooling that has
been observed in the northern Gulf of Alaska in the last year (www.ims.uaf.edu/gak1.)
Conclusions
The total freshwater discharge into the Gulf of Alaska (about 33,000 m3 s-1) far exceeds the other
freshwater system in North America, the Mississippi River (about 16.000 m3 s-1). The importance
of this potential coastal circulation driving force is further enhanced by the relatively low water
temperatures that make the coastal water density more dependent on salinity (freshwater) than
temperature. Glacial ablation contributes about 3000 m3 s-1 and the Canadian rivers add about
6,000 m3 s-1.
These freshwater sources have the potential to influence the water density structure not only in
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the Gulf of Alaska but also in the Bering Sea and Arctic Ocean. They also have the ability to
affect the poleward heat transport in the Northeast Pacific. Changes in the density structure will
alter the vertical density structure and hence can change the vertical advection of nutrient-rich
deep water into the euphotic zone. They can also change the depth of this zone. Hence, ocean
productivity can be affected changes in these freshwater sources.
The ability to estimate the input of freshwater into the Northeast Pacific is now passed on to those
who might require this information. These estimates will be improved with more sophisticated
hydrology models, improved precipitation monitoring, especially at higher elevations, continued
and improved estimates of the glacial volumes and improved monitoring of coastal ocean
conditions (hydrography) including currents.
Publications
None
Outreach
A paper was presented at the 16th annual PICES meeting in Victoria, BC in October 2007 titled
“Coastal freshwater discharge in the Northeast Pacific using an updated hydrology model” by T.
C. Royer and C. E. Grosch.
A paper will be presented at the 2008 Alaska Marine Science Meeting in Anchorage titled
“Coastal freshwater discharge in the Northeast Pacific using an updated hydrology model” by T.
C. Royer and C. E. Grosch.
Acknowledgements
The primary input data (precipitation and air temperature) were provided by the National Weather
Service though their Alaska Climate Summaries. The initial programming conversion from
QuickBASIC to Matlab was carried out by Carl Pucci, an Old Dominion University
undergraduate student.
Literature Cited
Arendt, A. A., K. A. Echelmeyer, W. D. Harrison, C. S. Lingle, and V. B. Valentine. 2002.
Rapid Wastage of Alaska Glaciers and Their Contribution to Rising Sea Level, Science, 297:
382-386.
Hickey, B. M. and T. C. Royer. 2001. California and Alaska Currents. In: Encyclopedia of
Ocean Sciences. Academic Press. London,
Royer, T. C. 1982. Coastal fresh water discharge in the Northeast Pacific. J. Geophys. Res.
87:2,017-2,021.
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Royer, T.C., C. E. Grosch, and L. A. Mysak. 2001. Interdecadal Variability of Northeast Pacific
Coastal Freshwater and its implications on Biological Productivity. Progress in Oceanography,
49:95-111.
Web site for the Alaska Temperature and Precipitation data: www5.ncdc.noaa.gov/pdfs/cd/alaska
Project Synopses
a. Introduction: The precipitation rates in the coastal region of the Gulf of Alaska are some of the
highest in the world. Combined with the relatively cold water of the gulf, the coastal freshwater
discharge significantly influences the water density. This freshwater drives the relatively large
Alaska Coastal Current in addition to influencing the water column stability. These physical
processes are expected to impact fisheries production in the region.
b. Why we did it: The primary source of information on the Gulf of Alaska coastal freshwater
discharge comes from a simple runoff model that was developed in the early 1980s and written in
a computer code that is not commonly used today. This code needed to be updated with new
knowledge of the water budgets incorporated into it such as river flows and glacial melting..
c. How we did it: The original QuickBASIC computer code was translated into C++ Matlab
language. The original program was designed to run from 1931 to the present. Glacial ablation
since 1980 is now added using information from Arendt, et al. (2002). Input from Canadian
rivers were also added though only as mean seasonal averages rather than actual monthly means.
d. What we discovered: The original estimate of the mean annual flow of freshwater into the
Alaska Coastal Current was about 24,000 m3 s-1. This compared favorably with the mean annual
flow of the Mississippi River of about 15, 000 m3 s-1. However, when we add the glacial ablation
(3,000 m3 s-1) and the Canadian river flows (6,000 m3 s-1), the total, which is still probably an
underestimate, is about 33,000 m3 s-1. Since the length of the British Columbia coastline is about
half of that of the Alaska coastline, we had thought, erroneously, that the Canadian contribution
would be about 10,000 m3 s-1. However, this still represents that largest freshwater system in
North America. Most global water budgets have not included this important source since it does
not involve large river networks exist here. This freshwater is important to the ocean dynamics
because of the ocean’s relatively low water temperature and the nonlinearity of the equation of
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state of sea water. Changes in this freshwater flux should have impacts on biological productivity
and fisheries.
e. What’s next? The Matlab programs are designed to be used by others for the foreseeable
future until more sophisticated hydrology models are developed. Better measurements of the
total freshwater discharges into the Gulf of Alaska should also be made.
f. Outreach
A paper was presented at the 16th annual PICES meeting in Victoria, BC in October 2007 titled
“Coastal freshwater discharge in the Northeast Pacific using an updated hydrology model” by T.
C. Royer and C. E. Grosch.
A paper will be presented at the 2008 Alaska Marine Science Meeting in Anchorage titled
“Coastal freshwater discharge in the Northeast Pacific using an updated hydrology model” by T.
C. Royer and C. E. Grosch.
g. The Big Picture: The rate of coastal freshwater entering the Gulf of Alaska is huge, more than
twice the rate of flow of the Mississippi River but has been ignored because of its “remoteness”
and lack of major river networks. This freshwater is important to the ocean physics, biology and
marine productivity for the Gulf of Alaska, Bering Sea and Arctic Ocean.
h. NPRB Research Interest: This work supports the NPRB mission to understand better the
physical processes affecting the fisheries production in the Gulf of Alaska and it supports the long
term monitoring of the freshwater influx into the Gulf of Alaska, Bering Sea and Arctic Ocean.
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