Tom Whitaker, Hydrologist
Carolyn Hunsaker, Research Ecologist
USDA Forest Service
Pacific Southwest Research Station
Sierra Nevada Research Center
Fresno, CA
twhitaker@fs.fed.us
chunsaker@fs.fed.us
The Influence of Climate Change on Headwater Stream
Discharge in the Southern Sierra Nevada (1957-2007)
Paper No: GC41A-0107
Jason Adair, Hydrologic Technician
Brent Lignell, Physical Sciences Technician
Kevin Mazzocco, Hydrologic Technician
Kurt Weidich, Hydrologic Technician
Funded in part by the National Fire Plan, the Federal Interagency
Joint Fire Science Program, and California Bay-Delta Program
Introduction
Long-Term Trends at Teakettle Creek
The Kings River Experimental Watershed (KREW) was established in 2001 in the Sierra National Forest to better understand the effects of
forest thinning and prescribed fire on headwater streams in the southern Sierra Nevada. KREW consists of eight watersheds located on two
clustered sites (Figure 1). Each site consists of a control watershed as well as ones that will receive the mechanical thinning, control burn, and
combined thin and burn treatments. The watersheds included in the study vary in elevation from 1500 to 2500 m, with the four lower elevation
basins located at the Providence Creek Site and the remaining, higher elevation, watersheds found at the Bull Creek Site. After 5 years of
collecting baseline data, the treatments are scheduled to begin in 2008.
Discharge data from the Teakettle Experimental Forest spans back to 1957. To look for trends or changes in the headwater systems as a result of climate change, the
long-term discharge record at Teakettle 003 was analyzed for changes in the following characteristics.
1.
2.
3.
4.
Stream discharge data has been collected on KREW since 2002. Stream base flows range from less than 1 lps to 10 lps. However, during
periods of spring snowmelt the streams experience continuous flows as high as 400 lps lasting a month or more. A single winter rain event on
December 31, 2005 produced peak flows of more than 1,000 lps. Because of this large variance in discharge, KREW uses a double flume
system to accurately measure flows of all magnitudes (Figure 2). Backup stream gaging devices are installed on the larger flumes to capture
high flow events during storm or spring snowmelt periods when the smaller flumes may be overtopped or blocked with debris.
Unfortunately, with extended gaps in both the 1970’s and the 1990’s of the annual record a trend
analysis becomes very difficult (Figure 7). However, it is still possible to compare the discharge
records from the 1950’s and 1960’s with that collected by KREW since 2003 to see if any noticeable
differences are evident. Results of this analysis are shown in Table 2.
Figure 2. KREW’s double flume system (left) accurately captures periods of both low (center) and high (right) flows.
The Teakettle Experimental Forest was established in the early 1940's and is one of the few remaining examples of mid-elevation, old growth
forest in the southern Sierra Nevada. Weirs were constructed (Figure 3) and gaging stations have been operated periodically on four
watersheds in Teakettle since the 1950's. Teakettle 003 stream is just over the ridge from, and serves as the control for, the KREW Bull Creek
Site.
As a result of climate change, a 2º-5º C temperature rise is expected for the Sierra Nevada over the next century (Mitchell et al., 1990), with an
approximate 1º-2º C rise having already occurred since the 1940's (Dettinger et al., 1995) For the Sierra Nevada, this will result in more
precipitation falling as rain instead of snow as well as an earlier spring snowmelt period making less water available to California during the dry
summer months.
By comparing the annual discharge values of the high (Bull Creek) and low (Providence Creek) elevation sites of KREW we begin to look at the
effects the warmer temperatures and a greater fraction of precipitation as rain instead of snow may have on the long-term water supply from
these small headwater streams. In addition, the historical discharge data available from Teakettle Creek provide an interesting opportunity to
evaluate whether the hydrological effects of climate change are already evident on small, headwater systems in the southern Sierra Nevada.
The onset of spring runoff
The size of the spring discharge pulse
Fraction of discharge available during the summer months
Frequency of large winter rain events
Discharge Variability Within KREW
The elevation range in which winter precipitation may fall as either rain or snow is known as the rain-snow interface boundary. In the southern Sierra
Nevada this range is typically 1250 to 2450 m. Both the Providence Creek (1485-2115 m) and Bull Creek (2050-2490 m) Sites of KREW fall within this
boundary. Although the sites are primarily snow dominated, it is common for the watersheds to be exposed to one to five large rain events each winter.
In order to better understand the effects of elevation and the rain-snow interface on stream discharge, three KREW watersheds are analyzed.
Attributes for the Providence 303, Bull 203, and Teakettle 003 watersheds are shown in Table 1. The mean annual flow rate over a four-year period is
shown in Figure 4. The discharge at Teakettle and Bull for any given year is significantly higher than Providence. By taking into account watershed
area and looking at the fraction of precipitation to become discharge (Figure 5) we see that the three watersheds act distinctly different. Although at
similar elevations and locations, 10-25% more precipitation is converted to discharge at Bull than at Teakettle. For Providence 303 it is considerably
less.
Comparing Bull 203 to Providence 303 over a single year highlights these
differences (Figure 6). From the hydrograph, it is clearly evident that
Providence (184 cm) receives much less snow than Bull (382 cm). This
results in a much more pronounced spring snowmelt pulse on Bull,
accounting for more than 80% of the total annual discharge over the April
through July period. In addition, the onset of spring snowmelt occurs
earlier on Providence and there is an increased frequency of winter rain
Table 1. Differences in the physical attributes of three KREW watersheds.
events. Although these two watersheds are about the same size and
receive approximately the same amount of precipitation, the Bull
watershed has more than double the discharge of Providence. Two factors play a role in this difference. The first is the increased canopy cover on
Providence and the greater precipitation interception experienced as a result. Second, a higher fraction of precipitation falls as rain, and periods of
intermittent snowmelt occur on Providence due to warmer, mid-winter temperatures. These factors provide more opportunity for soils and vegetation to
take in water that would otherwise leave the watershed as discharge during extended periods of spring snowmelt.
Using the method developed by Lundquist (2004) the day of the onset of the spring runoff period
was determined for each year. As would be expected with rising temperatures, the beginning of the
spring snowmelt period is approximately four days earlier when comparing the two periods.
Table 2. Comparative analysis of potential climate change
effects on Teakettle Creek 003.
The comparison also shows that the size of the spring
snowmelt pulse has increased. However, this is primarily the
result of two above normal precipitation years in the 20042007 period, and it is unlikely a change would be evident with
the onset of the spring melt period only increasing by four
days.
Of even more importance to California’s water supply is the
amount of discharge left available during the summer months.
It would be expected that with higher temperatures causing
an earlier spring thaw and increasing the rate of melt that less
water would be present at the end of the year. A comparison
of the discharge data from 1958-1969 to that of 2004-2007
shows a decrease of 4% in the amount of discharge after
June 1.
Finally, by analyzing the historic discharge record we are able
to determine the number of rain events occurring each winter.
A slight increase in the number of large storm events, in
which the discharge increases by at least 100%, during the
winter months is observed between the two periods.
Figure 7. Available discharge and precipitation records for Teakettle Experimental Forest watersheds.
Future Work
KREW will continue to monitor discharge on the eight headwater streams included in the study for at least five years after the treatment phase scheduled to begin
next summer. This includes the Teakettle 003 stream which will create a larger data set with which to further evaluate the effects of climate change on this
watershed. In addition, we would like to quantify the effects different parameters such as onset of spring runoff, winter temperatures, snow depth and snow water
equivalence, the size and frequency of winter rain events, canopy cover, and evapotranspiration have on the variability in annual discharge. This is best illustrated in
Figure 8 where two water years from Teakettle 003 with similar annual precipitation show considerably different values in discharge.
A better understanding of the impact of interception, evapotranspiration, and the
precipitation regime on annual discharge becomes increasingly important when
you consider the long term effects of climate change. It is expected that as
temperatures continue to rise over the next century the rain-snow interface
boundary will increase in elevation. As a result of climate change, it is likely that
the distribution of rain and snow at the Bull Site may become similar to that
currently found at Providence -- a reduced snow pack and a greater frequency of
winter rain events. How much of an effect this will have on annual discharge is
of critical importance to California’s water supply.
In 2007, KREW was selected to host the Southern Sierra Critical Zone Observatory
by the National Science Foundation. The addition of measurements like
evapotranspiration rates and soil moisture, as well as watershed modeling
exercises will enable us to better understand the mechanisms contributing to
watershed-scale discharge variability and possible effects from climate change.
Figure 3. Teakettle 003 historic weir,
then (1956) and now (2004).
References
Dettinger, M.D., Ghil, M. , and Keppenne, C.L., 1995. Interannual and Interdecadal
Variability in United States Surface-Air Temperatures, 1910-87. Climatic Change,
31, 35-66.
Lundquist, J.D., Cayan, D.R., and Dettinger, M.D., 2004. Spring Onset in the Sierra
Nevada: When is Snowmelt Independent of Elevation?. Journal of
Hydrometeorology, 5, 327-342.
Mitchell, J.F.B., Manabe, S., Meleshko, V., and Tokioka, T., 1990. Equilibrium
Climate Change - and its Implications for the Future. Climate Change-The IPCC
Scientific Assessment. Cambridge University Press. New york, pp. 131-174.
On the Web
KREW: http://www.fs.fed.us/psw/programs/snrc/water/kingsriver
Figure1. The KREW is located on headwater tributaries of the Kings River in the Sierra Nevada
of California. KREW consists of two sites approximately 15 km apart: the Providence Creek
(top) and Bull Creek (bottom) watershed clusters.
Figures 4 and 5. Annual flow rate (left) and discharge to precipitation ratio (right) for three KREW watersheds.
Souther Sierra CZO: http://www.czen.org/Southern_Sierra
Figure 6. Comparison of discharge and snow depth of Bull and
Providence sites for WY 2005.
Figure 8. A comparison of two water years from Teakettle 003.
Historic T003 Discharge Data: http://waterdata.usgs.gov/nwis
KREW Discharge Data: http://www.fsl.orst.edu/climhy/harvest