Climatic Change (2013) 116:71–95
DOI 10.1007/s10584-012-0600-8
The response of Lake Tahoe to climate change
G. B. Sahoo & S. G. Schladow & J. E. Reuter & R. Coats &
M. Dettinger & J. Riverson & B. Wolfe & M. Costa-Cabral
Received: 11 August 2011 / Accepted: 11 September 2012 / Published online: 11 October 2012
# Springer Science+Business Media Dordrecht 2012
Abstract Meteorology is the driving force for lake internal heating, cooling, mixing, and
circulation. Thus continued global warming will affect the lake thermal properties, water
level, internal nutrient loading, nutrient cycling, food-web characteristics, fish-habitat,
aquatic ecosystem, and other important features of lake limnology. Using a 1-D numerical
model—the Lake Clarity Model (LCM) —together with the down-scaled climatic data of the
two emissions scenarios (B1 and A2) of the Geophysical Fluid Dynamics Laboratory
(GFDL) Global Circulation Model, we found that Lake Tahoe will likely cease to mix to
the bottom after about 2060 for A2 scenario, with an annual mixing depth of less than 200 m
This article is part of a Special Issue on Climate Change and Water Resources in the Sierra Nevada edited by
Robert Coats, Iris Stewart, and Constance Millar.
G. B. Sahoo (*) : S. G. Schladow : J. E. Reuter
Tahoe Environmental Research Center, University of California Davis, One Shields Avenue, Davis, CA
95616, USA
e-mail: gbsahoo@ucdavis.edu
G. B. Sahoo : S. G. Schladow
Department of Civil and Environmental Engineering, University of California Davis,
One Shields Avenue, Davis, CA 95616, USA
J. E. Reuter : R. Coats
Department of Environmental Science and Policy, University of California Davis, One Shields Avenue,
Davis, CA 95616, USA
M. Dettinger
US Geological Survey and Scripps Institute of Oceanography, La Jolla, CA 92093, USA
J. Riverson
Tetra Tech, Inc., 10306 Eaton Place, Suite 340, Fairfax, VA 22030, USA
B. Wolfe
Northwest Hydraulic Consultants, 870 Emerald Bay Road, Suite 308, South Lake Tahoe, CA 96150,
USA
M. Costa-Cabral
Hydrology Futures, LLC, 4509 Interlake Avenue N #300, Seattle, WA 98103, USA
72
Climatic Change (2013) 116:71–95
as the most common value. Deep mixing, which currently occurs on average every 3–
4 years, will (under the GFDL B1 scenario) occur only four times during 2061 to 2098.
When the lake fails to completely mix, the bottom waters are not replenished with dissolved
oxygen and eventually dissolved oxygen at these depths will be depleted to zero. When this
occurs, soluble reactive phosphorus (SRP) and ammonium-nitrogen (both biostimulatory)
are released from the deep sediments and contribute approximately 51 % and 14 % of the
total SRP and dissolved inorganic nitrogen load, respectively. The lake model suggests that
climate change will drive the lake surface level down below the natural rim after 2085 for the
GFDL A2 but not the GFDL B1 scenario. The results indicate that continued climate
changes could pose serious threats to the characteristics of the Lake that are most highly
valued. Future water quality planning must take these results into account.
1 Introduction
Climate change at both regional and global scales is evident in the shifts of time-series trends
and patterns of long-term weather and hydrologic observations that include maximum and
minimum air temperature, snow accumulation, snow to precipitation ratio, snow melt timing,
and stream runoff (Cayan et al. 2008, 2009; Coats 2010; Dettinger and Cayan 1995; Hansen
et al. 2006). Meteorology is the driving force for lake heating, cooling, mixing, and
circulation; thus, climate change affects features of physical limnology including, but not
limited to (1) the heat budget and thermodynamic balance across the air-water interface; (2)
formation and stability of the thermocline; (3) the amount of wind-driven energy input to the
system; (4) the water budget including evaporative loss: and (5) the timing of stream
delivery into a lake or reservoir. Several authors have evaluated the impact of climate change
on the thermal behavior of lakes (Austin and Colman 2008; Coats et al. 2006; Livingstone
2003; Schneider et al. 2009).
Lake Tahoe (CA-NV, USA) is world renowned for its natural beauty and cobalt-blue
color. Observed trends in air temperature, precipitation, percent of total annual precipitation
falling as snow, and snowmelt timing indicate that the Sierra Nevada region is warming
(Sahoo et al. 2011; Schneider et al. 2009; Stewart et al. 2005), and that the Tahoe basin is
warming faster than the surrounding region (Coats 2010). Lake Tahoe is an ice free warmmonomictic lake with deep-mixing only in the winter. Lake Tahoe mixes completely to its
500 m bottom on the average once every 3 to 4 years (Tahoe Environmental Research Center
(TERC), University of California Davis 2008). A stable thermocline is established each
summer at a depth of approximately 20 m. As the lake cools in the fall, the thermocline
typically lowers and by October is at a depth of 32 m (Coats et al. 2006). It has been
hypothesized that deep-mixing could cease entirely if the warming trend continues (Coats et
al. 2006). Since deep mixing supplies dissolved oxygen from surface to bottom, reduced
mixing may result in evolution of anoxic condition near the deep-sediment interface.
Existing water quality and quantity problems at Lake Tahoe include (1) declining Secchi
depth transparency (2) increasing primary productivity rate (5 % per year), (3) pervasive
thick growths of attached algae along parts of the once-pristine shoreline, (4) increasing
volume weighted mean temperature (0.013 °C per year), (5) increasing resistance to mixing,
and (6) invasion of non-native species (Sahoo et al. 2011; Tahoe Environmental Research
Center (TERC), University of California Davis 2008). Continued climate change could
potentially exacerbate all of these issues.
The Fourth Assessment Report of the Intergovernmental Panel on Climate Change
(Intergovernmental Panel on Climate Change (IPCC) 2007) indicates that: (1) global green
Climatic Change (2013) 116:71–95
73
house gas (GHG) emissions will continue to grow over the next few decades and (2)
continued GHG emissions at or above current rates would cause further warming and induce
changes in the global climate system during the 21st century. With this global situation as a
back drop, the objective of this study was to use spatially downscaled meteorology output
(air temperature, precipitation, wind speed, longwave radiation, and solar radiation) for the
Tahoe basin obtained with the Geophysical Fluid Dynamics Laboratory Model (GFDL
CM2.1) (Delworth et al. 2006) applied to the A2 and B1 IPCC emission scenarios
(Dettinger, this issue) to estimate lake response (thermal properties, lake water level, and
lake water quality in terms of dissolved oxygen and nutrients) during the 21st Century. The
climatic response to A2 green house gas emission scenario is based on assumptions of a very
heterogeneous world economy with increasing global population, regionally oriented economic development, and more fragmented and slower technological changes. The B1
scenario is based on assumptions of a greener future with same global population that peaks
in mid-century and declines thereafter, but with rapid changes in economic structures
towards information and service, with introduction of clean and resource-efficient technology and reduction in material intensity (IPCC 2007). In addition, implications for lake
management due to changes in thermal properties in the lake are discussed. Note that the
climatic scenarios represent a range of possible future, modeled situations. These are useful
for exploring the potential implications of a changed climate.
2 Methodology
2.1 Lake clarity model
The Lake Clarity Model (LCM) (Sahoo et al. 2010) is the customized model based on the
UC Davis - Dynamic Lake Model with Water Quality (DLM-WQ) (Chung et al. 2009;
Fleenor 2001; Hamilton and Schladow 1997; Heald et al. 2005; Perez-Losada 2001). The
hydrodynamic component of the model is based on the original DYRESM (Imberger et al.
1978). Fleenor (2001) added river plunging algorithms into the hydrodynamic module. The
primary hydrodynamic model is one-dimensional (1-D) and is based on a horizontally mixed
Lagrangian layers approach (Hamilton and Schladow 1997); however, the stream inflows
and mixing due to stream turbulence are two-dimensional (2-D). All the ecological modules
are incorporated into the 1-D hydrodynamic model (Sahoo et al. 2010). The hydrodynamic
model simulates stratification, mixing, the transport of all pollutant in the vertical direction,
and determines the stream plunging depths. Lake Tahoe has 63 tributary inflows. The
ecological modules simulate transformation processes associated with algal photosynthesis
(Sahoo et al. 2010). External flows and pollutants (nutrients and fine sediment particles) into
the lake are from atmospheric deposition, streams and intervening zones (both urban and
non-urban), groundwater and shoreline erosion. Sahoo et al. (2010) calibrated and validated
the LCM using estimated (1) stream flows and associated pollutant loads (2) atmospheric
pollutant loads, (3) shoreline erosion, (4) groundwater flow and pollutant loads and (5)
5 years (i.e. year 2000 to 2004) of in-lake data. Model validation demonstrated the ability of
the LCM to capture the seasonal temperature and DO patterns.
It is evident in Fig. 1 that (1) DO concentrations continuously decrease in absence of deep
mixing and (2) the lake becomes homogenized because of the winter mixing (see March
2007 winter mixing in Fig. 1). DO concentration declines at the sediment surface (450 m
below the lake water surface) at the rate of approximately 0.1 mg/L per month as a result of
the biological and chemical processes that typically create water column biological oxygen
74
Climatic Change (2013) 116:71–95
Fig. 1 Dissolved oxygen concentrations based on SEABIRD profiles taken at approximately monthly
intervals at the mid-lake station. The open circles at the top of the figure indicate the profiling dates. Vertical
resolution is approximately 0.5 m
demand (BOD) and chemical oxygen demand (COD), and sediment oxygen demand (SOD).
At this rate DO concentration at the lake bottom would be reduced to zero in approximately
6 years in complete absence of deep mixing.
2.2 Model assumptions and LCM modification
2.2.1 Sediment release rates
As part of this study, the treatment of sediment nutrient release in LCM was modified to
account for deep-water column anoxia (Sahoo et al. 2010), a condition not currently known
to exist in Lake Tahoe. Nutrients are released from the sediments when anoxia occurs at the
sediment-water interface (Wetzel 2001). Due to the expected increase in lake stability (i.e.
reduced mixing) under future climate conditions (Winder et al. 2008); a reduction in oxygen
transfer to the sediments was expected.
Sahoo and Schladow (2008) using just the hydrodynamic model of LCM demonstrated that deep lake mixing can be reduced because of lake warming. However they
did not show how this would affect dissolved oxygen, possible impacts of nutrient
release from anoxic sediments, and the magnitude of this additional nitrogen (N) and
phosphorus (P) source relative to the complete nutrient input budget. The present
study calculated DO concentrations in the lake at each modeled depth layer. The
sediment nutrient release rates (Table 1) were assigned based on experimental results
using intact deep sediment cores and water specifically from Lake Tahoe water
(Beutel 2000, 2006). In the study, it was assumed that rates of N and P release from
the sediment remained uniform over the modeled period.
Climatic Change (2013) 116:71–95
75
Table 1 Sediment oxygen demand (SOD) and nutrient release rate of soluble reactive phosphorus (SRP),
nitrate (NO3) and ammonium (NH4) in oxic and anoxic phases (Source: Beutel 2000, 2006)
Variables
Oxic phase (DO>0.01 mg/L)
Anoxic phase (DO≤0.01 mg/L)
SOD
0.04 g-O m−2 d−1
0.00 g-O m−2 d−1
−2
SRP
0.00 mg-P m
−2
NO3-N
0.18 mg-Nm
−2
NH4-N
0.00 mg-Nm
−1
0.22 mg-P m−2 d−1
−1
0.00 mg-Nm−2 d−1
−1
0.49 mg-Nm−2 d−1
d
d
d
2.2.2 Lake water level
Water level is estimated based on the following water balance equation:
DW t ¼ DW t1 þ S t þ GW t þ Rt E t Ot Ovt
Where,
DWt
DWt-1
St
GWt
Rt
Et
Ot
Ovt
Water level at current time step t
Water level at previous time step t-1
Stream inflow contribution between time steps t-1 and t, expressed
as an equivalent height of water at the surface.
Groundwater inflow contribution between time steps t-1 and t, expressed as an
equivalent height of water at the surface. Groundwater inflow rate is from Trask
(2007). The daily value of groundwater is assumed to be the same for all years.
Direct precipitation on the lake between time steps t-1 and t, expressed as an
equivalent height of water at the surface. Isoheytal map of Lake Tahoe (Lahontan
Regional Water Quality Control Board (Lahontan) and Nevada Division of
Environmental Protection (NDEP) 2010a; Simon et al. 2003) shows that
precipitation on the lake varies nearly 50 % from the shore to the middle of the
lake, so the estimated precipitation is reduced by 35 %. This estimate was
derived from a best fit for comparing the daily estimated lake water elevation to
those of the historical measured records during calibration and validation.
Evaporation contribution between time steps t-1 and t, expressed as an
equivalent height of water at the surface.
Outflow contribution between time steps t-1 and t, expressed as an equivalent
height of water at the surface. Outflow was estimated based on the regression
equations (see Table 2).
Overflow contribution between time steps t-1 and t, expressed as an equivalent
height of water at the surface. This applies if the water level goes above the
maximum legal limit for Lake Tahoe (1898.63 m Bureau of Reclamation Datum,
or 1899.86 m NAVD) and water is spilled to the Truckee River.
The regression equations for outflow (O) were developed based on lake water depth
above the lake’s natural rim (D). The sixth order regression equations were developed to
provide the highest R2 value and match the modeled outflow as close to the measured
outflow as possible. The U.S. Geologic Survey (USGS) measures lake level at Tahoe City
(site number: USGS 10337000 LAKE TAHOE A TAHOE CITY CA). When the lake level
falls below the natural rim, there is no outflow to the Truckee River. Although data are
available since 1950, recent data 2000 to 2009 data were used in the analysis because recent
data reflects the updated gate operation at Tahoe City. While a regression was developed
83.631
140.902
0.755
c5
c6
R2
0.590
673.243
−387.968
1103.463
−655.312
c4
189.193
−537.824
257.376
−820.750
c3
−16.443
0.489
−19.913
c0
c1
c2
0.670
Feb
Jan
Regression
constants and R2
0.431
−89.320
−21.522
0.490
495.671
−1060.098
−407.378
162.213
1095.241
−563.485
−220.872
447.035
135.662
−9.913
Apr
46.138
−1.783
Mar
0.666
71.554
−398.316
868.631
−944.191
537.389
−151.695
18.688
May
0.264
−13.654
105.994
−242.317
144.110
125.547
−154.017
41.972
Jun
0.329
−127.609
823.923
−2126.939
2788.578
−1939.354
670.025
−79.781
Jul
0.765
42.749
−243.195
514.250
−480.785
165.264
12.663
−2.655
Aug
0.824
29.654
−149.478
302.391
−296.039
119.406
2.411
−0.281
Sep
0.746
−20.157
42.270
12.804
−87.201
58.184
1.745
0.078
Oct
0.958
41.874
−319.175
714.983
−657.077
239.329
−14.411
0.416
Nov
0.887
102.921
−367.620
482.213
−297.659
91.011
−2.035
0.037
Dec
Table 2 Regression equation between water depth above lake natural rim (D) and outflow (O) for the 10 years (based on the period 2000 to 2009).
O ¼ c0 þ c1 D þ c2 D2 þ c3 D3 þ c4 D4 þ c5 D5 þ c6 D6
76
Climatic Change (2013) 116:71–95
Climatic Change (2013) 116:71–95
77
between lake level and outflow for the purpose of this study, in reality the outflow rate is
governed by operating rules determined by the Federal Water Master, based on negotiated
downstream water needs (Truckee River Operating Agreement (TROA) http://www.troa.net/).
These rules change over time, and are based not only on conditions at Lake Tahoe but also
conditions of lakes, reservoirs and rivers in the Truckee River Basin (such as Lake Tahoe (and
its semi-enclosed embayment Emerald Bay), Donner Lake, Independence Lake, Stampede
Reservoir, Boca Reservoir and Pyramid Lake). Thus, the developed regression is used for
predicting future release rates, however, it is recognized that the estimated release rates using
regression equations may deviate from future actual rates.
3 Data inputs
3.1 Meteorological data input
The data used to support the two emissions scenarios (B1 and A2) of the GFDL GCM were
based on the downscaled meteorological projections (Dettinger, this issue). The details of
using the output of only one GCM (i.e. GFDL) for the lake and watershed models were
explained in (Dettinger, this issue). Briefly, the archives containing GCMs’ future projection
for the IPCC Assessment (2007) contain outputs of temperature and precipitation than other
outputs of surface variables like radiative fluxes, winds, and humidities. For these other
variables, archives often are limited to certain period or monthly statistics. Because these
other variables have significant influence on lake warming and dynamics (Sahoo et al.
2011), and we were able to obtain output of all variables for the 21st century from GFDL we
used only GFDL outputs. The uncertainties in downscaled GFDL climatic data are discussed
in the (Dettinger, this issue). Briefly, the correlation of estimated and observed daily
temperature and precipitation anomaly are above 0.9 and 0.7 for Lake Tahoe. Similarly,
the downscaled downward longwave fluxes, surface-wind speeds, and downward solar
radiation are very well correlated when aggregated to monthly time scales (correlation
>0.95, >0.9, and >0.8, respectively), giving confidence in the downscaled projections for
uses. Multiple regression equations were developed to correct biases in downscaled historical data. This was performed using measured data from the Tahoe basin (1989 to 1998) and
downscaled historical data (GFDL A2 and B1) over the same time period with algorithms
published by Woods et al. (2002, 2004) (see the details in our report http://www.fs.fed.us/psw/
partnerships/tahoescience/bmp_climate_change.shtml). The bias corrected climate data were
used in the watershed model for generation of flows and pollutant loads. The same climate data
and generated flows were used in the lake model. There are 36 grid points for the downscaled air
temperature (maximum and minimum) and precipitation data; and 81 grid points for the
downscaled shortwave radiation and wind speed on and around the lake. LCM (Sahoo et al.
2010), being a 1-D model, requires meteorological information at a single representative grid
point over the lake. That point was chosen to be the grid point which is close to the center of the
lake. In addition to downscaled precipitation, air temperature, shortwave radiation, and wind
speed, LCM requires longwave radiation and vapor pressure data. Regression equations between
air temperature and dew point were developed using the South Lake Tahoe Airport meteorological station data from 1989 to 2004. The longwave radiation was estimated using algorithms
described in Tennessee Valley Authority (TVA) (1972), with downscaled air temperature and
estimated cloud fractions data. Vapor pressure was estimated using dew point temperature.
The one-year running average of the daily meteorological data from the downscaling
exercise, over the 21st Century, along with the best fit trend lines are plotted for shortwave
Climatic Change (2013) 116:71–95
1899.0
(a)
1898.5
Estimated
Lake natural rim
Maximum legal limit
Measured
1898.0
1897.5
500
1/3/2009
1/3/2008
1/3/2007
1/2/2006
1/2/2005
1/2/2004
1/2/2003
1/1/2002
1/1/2001
(b)
USGS
400
LSPC
300
200
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
40
1991
100
0
(c)
30
20
10
0
-10
-20
1991
Relative % difference base on
mean of LSPC and USGS flows
1/1/2000
1896.5
1/1/1999
1897.0
Annual cummulative flows of
10 LTIMP streams (106 m3)
Lake surface water level (m)
78
Fig. 2 a USGS-recorded and LCM-estimated lake water surface b LSPC-estimated and USGS-recorded
stream flow of the 10 LTIMP streams, and c estimated flow percentage change to the mean of LSPC and
USGS flows
Climatic Change (2013) 116:71–95
79
radiation, longwave radiation, air temperature, wind speed, and annual precipitation. We
found that shortwave radiation remains largely unchanged while air temperature is expected
to increase approximately 4.5 °C and 2.0 °C, and longwave radiation will increase approximately 10 % and 5 % for the A2 and B1 scenario, respectively. The wind speed showed a
decline on the order of 7–10 %. Note that these types of trends help to determine the
statistics of future climate; however, extreme weather conditions over periods of days may
change lake mixing and subsequent lake ecology without significantly altering the meteorologic long-term trend.
3.2 Stream inflow and pollutant loads
2093
2097
2097
2089
2093
2085
2081
2077
2073
2069
2065
2061
2057
2053
2049
2045
2041
2037
2033
2029
2025
2021
2017
2013
2009
(a)
2005
2001
Streamflow, including tributary flow and direct runoff to the lake via intervening zones, and
associated pollutant loads through year 2100 were provided by the load simulation program
in C++ (LSPC) watershed model (Riverson et al., this issue) forced by the same downscaled
meteorological data sets. Concentrations of fine sediment particles are estimated from the
LSPC-derived stream flow based on algorithms described in Lahontan Regional Water
Quality Control Board (Lahontan) and Nevada Division of Environmental Protection
(NDEP) (2010a). The stream temperatures are estimated based on the algorithms described
in Sahoo et al. (2009). Groundwater pollutant loads are based on the estimates of USACE
(United States Army Corps of Engineers), Sacramento District (2003). However, the actual
groundwater flux was based on the estimates of Trask (2007). Estimates of atmospheric
deposition and shoreline erosion reported in Lahontan Regional Water Quality Control
Board (Lahontan) and Nevada Division of Environmental Protection (NDEP) (2010a) are
used in this study. Inputs from atmospheric deposition, groundwater and shoreline erosion
were assumed to be the same for all years (Sahoo et al. 2010) because of the lack of
adequate, long-term loading data from these sources.
100
200
300
GFDL A2
400
2089
2085
2081
2077
2073
2069
2065
2061
2057
2053
2049
2045
2041
2037
2033
2029
2025
2021
2017
2013
2009
(b)
2005
500
2001
Maximum mixing depth (m)
0
Maximum mixing depth (m)
0
100
200
300
400
500
Fig. 3 Maximum annual mixing depth for a GFDL A2 scenario and b GFDL B1 scenario
GFDL B1
80
Climatic Change (2013) 116:71–95
(a)
Simulated DO (mg/L) GFDLA2 - case
0
12
50
10
Depth from surface (m)
100
150
8
200
6
250
300
4
350
400
2
450
2097
2089
2081
2073
2065
2057
2049
2041
2033
2025
2017
2009
2001
0
Year
(b)
Simulated DO (mg/L) GFDLB1 - case
0
12
50
10
Depth from surface (m)
100
150
8
200
6
250
300
4
350
400
2
450
2097
2089
2081
2073
2065
2057
2049
2041
2033
2025
2017
2009
2001
0
Year
Fig. 4 Simulated DO concentration for a GFDLA2 and b GFDLB1 scenarios. X-axis values represent the
beginning of the year
These assumptions imply that the loads over the next 100 years will bear the same
relationship to the meteorology and stream flows as they have in the past. For example, we
did not assume the success of the Tahoe Maximum Daily Load (TMDL) program for water
quality restoration, nor take account of possible future land use changes.
Climatic Change (2013) 116:71–95
81
3.3 Lake data
Lake data are required to provide initial conditions for the LCM model runs. Vertical
profiles of temperature, chlorophyll-a, DO, biological oxygen demand (BOD), soluble
reactive phosphorous (SRP), particulate organic phosphorus (POP), dissolved organic
phosphorus (DOP), nitrate (NO3−) and nitrite (NO2−), ammonium (NH4+), particulate
organic nitrogen (PON), dissolved organic nitrogen (DON), and concentrations of
seven classes of sediment particles (0.5–1.0, 1.0–2.0, 2.0–4.0, 4.0–8.0, 8.0–16.0,
16.0–32.0, and 32.0–63.0 μm) are collected at two lake stations by UC Davis Tahoe
Environmental Research Center (TERC). Data from the mid-lake station in the deeper
part of the lake (460 m depth) were used to provide the initial conditions. Downscaled meteorological data are available starting from January 1, 2001, however, the
lake profile monitoring data used to define the initial conditions was first taken on
January 3, 2001.
The elevation of a spillway constructed at the lake outlet is approximately 1,899 m
Bureau of Reclamation Datum. Water level above 1,899 m is discharged to the
Truckee River. Bottom elevation of lake is approximately 1,400 m Bureau of Reclamation Datum. The elevation of each stream before it enters the lake was estimated
from GIS DEM and used along with stream and lake water temperature to estimate
the plunging depth of the stream discharge.
(a)
GFDLA2
10
5
0
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual SRP load (103kg)
15
(b)
GFDLB1
10
5
0
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual SRP load (103kg)
15
Fig. 5 Simulated annual average soluble reactive phosphorus release from the sediments for a GFDLA2 and
b GFDLB1 scenario
82
Climatic Change (2013) 116:71–95
4 Results and discussion
4.1 Calibration and validation
Sahoo et al. (2010) illustrates the calibration of LCM. The watershed model LSPC
was calibrated and validated for 1991 to 2008. Detailed lake data (nutrients, fine
sediments of seven bins (0.5–1 μm, 1–2 μm, 2–4 μm, 4–8 μm, 8–16 μm, 16–32 μm,
32 to <63 μm), algae, dissolved oxygen, temperature) are available since 1999. The
lake data needed for the model was not available until 1999 even though the stream
loading data was available from 1999 forward. Thus, in this study the lake water level
was calibrated and validated using measured weather and lake level records for
10 years (1999 to 2008). Figure 2a demonstrates the overall ability of LCM to
estimate the lake water level. Figure 2a shows that water level closely follows that
of USGS-recorded water level except during 2003 and 2005. Note that years 2005
and 2006 were characterized by high precipitation and LSPC overestimated streamflow by approximately 20 % to 35 % relative to the mean of LSPC and USGS 2003
to 2006 (Fig. 2b and c). Note that the relative percent difference (0(LSPC flow –
USGS flow)/mean of LSPC and USGS flow) on annual stream runoff during 1991 to
1998 for 10 LTIMP streams are only −12.0 %,–5.1 %, 7.1 %,–17.4 %, 3.0 %, 4.8 %,
14.5 %, and 4.7 %, respectively (Fig. 2c). Since the LSPC with this set up generated
streamflows of the 63 streams using downscaled meteorological data (Dettinger, this issue) for
30
(a)
GFDL A2
25
20
15
10
5
0
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual NH4–N load (103kg)
35
30
(b)
GFDL B1
25
20
15
10
5
0
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual NH4–N load (103kg)
35
Fig. 6 Simulated annual average NH4-N release from the sediments for a GFDLA2 and b GFDLB1 scenario
Climatic Change (2013) 116:71–95
(a)
83
Simulated NH -N (µg/L) GFDLA2 - case
4
450
50
45
40
460
35
465
30
470
25
475
20
480
15
485
10
2097
2089
2081
2073
2065
2057
2049
2041
2033
0
2025
495
2017
5
2009
490
2001
Depth from surface (m)
455
Year
(b)
Simulated NH -N (µg/L) GFDLB1 - case
4
450
50
45
Depth from surface (m)
455
40
460
35
465
30
470
25
475
20
480
15
485
10
490
5
2097
2089
2081
2073
2065
2057
2049
2041
2033
2025
2017
2009
2001
495
Year
Fig. 7 Close view of the bottom 45 m (450 m to 495 m) simulated NH4-N release for a GFDLA2 b GFDLB1
scenario. X-axis values represent the beginning of the year
the period 2001 to 2099, the LSPC model values are not changed in the estimation of the lake
water elevation.
84
Climatic Change (2013) 116:71–95
Fig. 8 Close view of the bottom 45 m (450 m to 495 m) simulated soluble reactive phosphorus release for a
GFDLA2 b GFDLB1 scenario. X-axis values represent the beginning of the year
4.2 Lake stratification and mixing
Lake stratification and mixing are strongly influenced by the meteorological conditions. Typically in the summer, lakes undergo thermal stratification that stabilizes the water column into an
Climatic Change (2013) 116:71–95
(a)
15
Annual SRP load (103 kg)
Fig. 9 Comparison of external
and internal annual load of 2098
and GFDL A2 scenario a soluble
reactive phosphorus (SRP) b dissolved inorganic nitrogen (DIN).
U, NU, SCE, AD, GW, SE, and
SR represent urban, non-urban,
stream channel erosion, atmospheric deposition, groundwater,
shoreline erosion and sediment
release, respectively. The symbol
‘*’ represents no data
85
10
5
0
U
NU
SCE*
AD
GW
SE*
SR
SCE*
AD
GW
SE*
SR
(b)
Annual DIN load (103 kg)
150
125
100
75
50
25
0
U
NU
upper, warmer and less dense epilimnion and a deeper, cooler and denser hypolimnion. These two zones do not readily mix, and the strength of the thermocline boundary
between these two layers intensifies with increased warming of the epilimnetic waters.
In winter, the opposite occurs when the lake cools and the thermocline deepens.
When surface and bottom density differences are reduced to zero, the lake can mix
completely from top to bottom, a process termed turnover. At Lake Tahoe complete
turnover typically occurs every 3–4 years on average (Tahoe Environmental Research
Center (TERC), University of California Davis 2008).
Lake mixing is important as it redistributes dissolved and particulate material. For
example, nutrients such as nitrate, which typically accumulates in the hypolimnion through
the summer, are reintroduced to the epilimnion when the lake mixes in the winter. Similarly,
dissolved oxygen, which is introduced across the air-water interface, is redistributed
throughout the lake when deep mixing occurs.
The maximum annual mixing depths for the period 2001 to 2098 are shown in Fig. 3, which
illustrates that mixing to the bottom will largely cease after 2060 for the GFDL A2 scenario. For
the GFDL B1 scenario, the LCM predicted deep mixing to occur only four times during the
period 2061 to 2098. For either of these emission scenarios, this would represent a very
significant change relative to historic and current conditions. There are many implications for
lake ecology based on a reduction in mixing of this magnitude (see below). The results also
indicate that deep mixing events persist for shorter periods of time than they have in the past.
86
Climatic Change (2013) 116:71–95
Insertion depth from surface (m)
2097
2089
2081
2073
2065
2057
2049
2041
2033
2025
2017
2009
(a)
2001
Year
0
50
100
150
200
250
300
350
400
GFDLA2
450
Insertion depth from surface (m)
2004/12/11
2004/09/12
2004/06/14
2004/03/16
2003/12/17
2003/09/18
2003/06/20
2003/03/22
2002/12/22
2002/09/23
2002/06/25
2002/03/27
2001/12/27
2001/09/28
2001/06/30
(b)
2001/04/01
2001/01/01
500
0
50
100
150
200
250
300
350
400
450
GFDLA2
500
Fig. 10 Daily insertion depth of Upper Truckee River a for the period 2001 to 2098 and b 2001 to 2004 for
GFDL A2 scenario. X-axis values in a represent the beginning of the year
4.3 Implication of mixing effect on DO and nutrients
Modeled DO concentration at the deep sediment-water column interface for both emission
scenarios reached zero in approximately 6 to 7 years in the absence of deep mixing (Fig. 4)
as surface water oxygen could not be transferred into deeper waters as a result of a persistent
resistance to mixing. Ammonium and SRP have been shown to be released from Lake Tahoe
sediment under these conditions (Beutel 2000, 2006). These forms of biologically available
N and P will continue to be released from the sediment at the assumed rate (Table 1) while
DO concentration is less than 0.01 mg/L (Figs. 5 and 6). It is clear from Figs. 7 and 8 that the
NH4+−N and SRP released from the sediment at the deepest part of the lake are confined in
the bottom waters because of density stratification. Due to the absence of light at that depth,
the released nutrients do not contribute to photosynthesis. That will only happen when the
released nutrients are eventually mixed to the photic zone during the period of mixing and
upwelling. The transport of nutrients by vertical eddy diffusion is significant in lake
environments (Robarts and Ward 1978; Salonen et al. 1984).
Climatic Change (2013) 116:71–95
87
Insertion depth from surface (m)
2097
2089
2081
2073
2065
2057
2049
2041
2033
2025
2017
2009
(a)
2001
Year
0
50
100
150
200
250
300
350
400
GFDL B1
450
Insertion depth from surface (m)
2004/12/11
2004/09/12
2004/03/16
2004/06/14
2003/12/17
2003/09/18
2003/06/20
2003/03/22
2002/12/22
2002/09/23
2002/06/25
2002/03/27
2001/12/27
2001/09/28
2001/06/30
(b)
2001/04/01
2001/01/01
500
0
50
100
150
200
250
300
350
400
450
GFDL B1
500
Fig. 11 Daily insertion depth of Upper Truckee River a for the period 2001 to 2098 and b 2001 to 2004 for
GFDL B1 scenario. X-axis values in a represent the beginning of the year
We recognize that future development due to population growth and socioeconomic changes could increase the fine sediment and phosphorus loads from stormwater runoff. Included in the recent Total Maximum Daily Load (TMDL) program for
Lake Tahoe (adopted in 2011), is a strategic plan to implement best management
practices and water quality improvement projects to reduce future pollutant loads to a level
below the existing the existing condition and offset the impacts of future development. Even
under a scenario of 10 % above full build-out, fine sediment loading was estimated to only
increase by 2 % relative to all sources (Coats et al. 2010; Lahontan Regional Water
Quality Control Board (Lahontan) and Nevada Division of Environmental Protection
(NDEP) 2010a).
The annual sediment release of SRP and DIN (as nitrate, nitrite and ammonium) for the
A2 scenario at the end of 21st Century are compared to other sources of the current N and P
loading budget (Lahontan Regional Water Quality Control Board (Lahontan) and Nevada
Division of Environmental Protection (NDEP) 2010a, b) in Fig. 9. When the hypolimnion is
anoxic and nutrients are released from the sediment, the lake internal SRP load contributes
approximately 51 % of the total load. Although atmospheric deposited DIN is highest among
88
Climatic Change (2013) 116:71–95
Fig. 12 Daily lake water temperature for a GFDL A2 scenario and b GFDL B1 scenario. X-axis values
represent the beginning of the year
all sources (66 %), sediment derived DIN contributes approximately 14 % to the DIN pool.
Clearly, internal nutrient load due to climate change can be significant to the lake nutrient
budget.
Climatic Change (2013) 116:71–95
89
Lake water surface elevation (m)
1899
1898
1897
GFDLA2
1896
GFDLB1
Maximum legal limit
Lake natural rim
2097
2089
2081
2073
2065
2057
2049
2041
2033
2025
2017
2009
2001
1895
Year
Fig. 13 Simulated daily lake water level for GFDLA2 and GFDLB1 scenarios. Shown are the lake maximum
legal limit and natural rim level. X-axis values represent the beginning of the year
4.4 Timing and delivery of the streams
The depth of insertion of each stream into Lake Tahoe is a complex process governed by the
density (primarily water temperature) of each stream, the stratification of the lake, the
streamflow, and the geometry of the streambed and alluvial fan. A stream inflow that
plunges into the hypolimnion of the lake has different ecological consequences than when
it is inserted closer to the water surface. The seasonal pattern of Secchi depth water clarity
will be affected by the depth at which fine sediment is delivered to the water column. The
insertion depth of the Upper Truckee River is shown in Figs. 10 and 11 for GFDL A2 and
GFDL B1 scenarios, respectively.
Figures 10b and 11b show a much more finely resolved (4 years), temporal view of the
daily insertion depth during the longer modeling period of record; while Figs. 10a and 11a
show the daily insertion depth for GFDL A2 and GFDL B1 scenarios, respectively for the
100-year model output. The river plunges deeply most of the time during January to March
(Figs. 10b and 11b); however, discharge and loads are delivered to the photic zone
(approximately 0 to 50 m) during rest of the year. Due to climate change, the lake water
will warm for the GFDLA2 scenario (Fig. 12). The lake epilimnetic temperature normally
experiences significantly seasonal warming and cooling (red during summer and blue
during winter in Fig. 12). This pattern will continue; however, the simulation shows a
progressive deepening of higher temperature contour lines (Fig. 12a). The deeper part
of the lake (>100 m) becomes warmer after 2080 for GFDL A2 while the warming
effect on the hypolimnion is less for the GFDL B1. Because stream temperature was
estimated from air temperature and shortwave radiation (Sahoo et al. 2009), stream
water temperature also increases. Since hydraulic residence time of the lake is very
long (650–700 years), lake water temperature in the upper 100 m is critical for river
insertion, since river water entrains lake water first and plunges depending on the
density of the mixed water and stratified lake. Temperature contours in Fig. 12
illustrate that the lake is strongly stratified after 2080 for GFDL A2 scenario. The
insertion depth of Upper Truckee River is below a depth 200 m for the period 2034–
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Outflow (106 m3)
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Evaporation (106 m3)
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Precipitation (106 m3)
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Stream Flows(106 m3)
90
Climatic Change (2013) 116:71–95
(a)
1000
900
800
700
600
500
400
300
200
100
0
GFDLA2
(b)
1000
900
800
700
600
500
400
300
200
100
0
GFDLA2
(c)
1000
900
800
700
600
500
400
300
200
100
0
GFDLA2
y = 0.5164x + 403.17
R² = 0.3339
(d)
1000
900
800
700
600
500
400
300
200
100
0
GFDLA2
Fig. 14 Water balance for GFDL A2 scenario
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Outflow (106 m3)
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Evaporation (106 m3)
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Precipitation (106 m3 )
2001
2004
2007
2010
2013
2016
2019
2022
2025
2028
2031
2034
2037
2040
2043
2046
2049
2052
2055
2058
2061
2064
2067
2070
2073
2076
2079
2082
2085
2088
2091
2094
2097
Annual Stream Flows(106 m3 )
Climatic Change (2013) 116:71–95
1000
900
800
700
600
500
400
300
200
100
0
1000
900
800
700
600
500
400
300
200
100
0
1000
900
800
700
600
500
400
300
200
100
0
1000
900
800
700
600
500
400
300
200
100
0
Fig. 15 Water balance for GFDL B1 scenario
91
(a)
GFDLB1
(b)
GFDLB1
(c)
GFDLB1
y = 0.2154x + 405.4
R² = 0.0964
(d)
GFDLB1
92
Climatic Change (2013) 116:71–95
2076 except 2056 and 2061, and thus out of the photic zone. By contrast, for the case
of GFDL B1 scenario more of the winter discharge occurs in the photic zone during
2077 to 2090. Stream discharge in the photic zone should have a more immediate
effect in stimulating algae growth and the associated loss of lake transparency.
4.5 Lake level
Figure 13 shows water level of the lake for both the GFDL A2 and GFDL B1 scenarios.
Note that outflows are estimated based on the lake level above its natural rim and the
regression analysis developed using 2000 to 2009 lake level and discharge data. Outflow is
zero when the lake level falls below the natural rim. The lake level dips down below the
natural rim when evaporation rate is higher than sum total of stream inflows, groundwater
contributions and on-lake precipitation over the lake. As long as the lake level is below the
rim, the effects of annual evaporation and inflow are cumulative, and cannot be influenced
by gate operation.
It is clear in Fig. 12a that that modeled lake temperature is predicted to significantly warm in the last 20 years of the 21st century for the GFDL A2 scenario. This
is due in large part to the air temperature and longwave radiation increasing at a
higher rate for GFDL A2 case (nearly at double rate) than those of the GFDL B1
case. As a result, lake evaporation is higher (Fig. 14). Figures 14 and 15 also indicate
that precipitation over the lake during 2075 to 2095 is lower for the GFDL A2 case
than for GFDL B1 so the stream inflow is lower. Due to the combination of these
factors, the lake surface level dips below the natural rim after 2086 for the GFDL A2
but not the GFDL B1 scenario.
5 Conclusions
The meteorologic and geographic conditions in the Tahoe basin combine to create a
vulnerable ecosystem. Temperatures in the Basin are increasing faster than in the surrounding region and even under historic and current conditions the lake only mixes completely to
the bottom on the average of once in 3–4 years. Processes such as climate change that warm
the surface waters will increase the resistance to deep mixing. The most significant impacts
of a future, modeled climate change at Lake Tahoe are as follows:
&
&
Under the GFDL A2 emissions scenario, the Lake Clarity Model suggests that by the
middle of the 21st Century (after about 2060) Lake Tahoe will cease to mix to the bottom,
with an annual maximum mixing depth of only less than 200 m as the most common value.
A similar, albeit not as severe outcome is seen for the GFDL B1 emissions scenario. As the
surface water heats, the resulting density difference between the warmer surface water and
the colder deeper water will be too strong for the wind energy to overcome. Indeed, this
change in density can already be seen in the measured historic data.
When the lake fails to mix completely, the bottom waters are not replenished with
oxygen and eventually dissolved oxygen at these depths will be depleted to zero. When
this occurs both soluble reactive phosphorus and ammonium-nitrogen (both biostimulatory) are released from the deep sediments resulting in an increase in nutrient loading
relative to that under the lake’s current deep mixing regime. The model shows this as a
new and significant source of nutrients, heretofore not seen in Lake Tahoe. The model
indicates that under the GFDL A2 scenario, dissolved oxygen at the lake bottom could
Climatic Change (2013) 116:71–95
&
&
&
93
reach a sustained level of zero by about 2075. At the depths below 200 m,
oxygen concentrations could be at levels inhospitable to native salmonids (<
6 mg/L) even earlier. The model also suggests that intermittent periods of anoxia
in the deepest waters could occur within the next 20 years. Under the GFDL B1
scenario, deep-water anoxia will also occur, albeit not as sustained as seen in the
GFDL A2 scenario; this results from the observation that while complete mixing
will be less frequent than historically observed, it will occur. Sensitivity analysis
on the 21st century modeled daily wind speed indicated that a 10 to 15 % increase
would be needed to maintain the historic deep mixing frequency (once in 3 to
4 years) in the late 20th century.
Based on published results for soluble phosphorus (SRP) and ammonium release from
anoxic Lake Tahoe sediments, the annual loading of SRP under sustained conditions of
lake stratification (no deep mixing) and anoxic sediments would be twice the current
load from all other sources for GFDL A2 scenario. Loading of ammonium under these
conditions would increase the amount of biological available nitrogen that enters the lake
by 14 %. This effect on the nutrient loading budgets of Lake Tahoe, in addition to the
predicted loss of functional habitat for certain species under anoxic conditions, could
have a dramatic and long-lasting impact on the food web and trophic status of the Lake.
Should the nutrients released from the bottom sediments periodically mix or otherwise
become entrained into the upper waters we expect that the impact on algal growth within
the photic zone should be significant, with an attendant impact of lake food web
dynamics and trophic status of both the pelagic and littoral regions of the lake. These
nutrients, particularly phosphorus, will be available to drive algal growth. Reducing the
load of external nutrients entering the lake in the coming decades may be the only possible
mitigation measure to reduce the impact of climate change on lake clarity and trophic status.
The lake model suggests that climate change will drive the lake surface level down
below the natural rim after 2086 for the GFDL A2 but not the GFDL B1 scenario. The
results indicate that continued climate changes could pose serious threats to the characteristics of the Lake that are most highly valued. Future water quality planning must take
these results into account.
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