Adaptations for Hydroclimate Variability:
Revising Water Year Classification in
Non-stationary Climates
Sarah E. Null, Utah State University
David E. Rheinheimer, U.C. Merced
Josh H. Viers, U.C. Merced
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September 9, 2014
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Water Year Indices Simplify Complex
Hydrology
 Unimpaired
precipitation/runoff
compared to historical
averages
 Single, numerical metric
 Threshold-based
 Is there ‘enough’ water
 Framework for water
allocations
 Tied to water resource
management
 Used in rule-based decisionmaking
 Examples:





Palmer Drought Severity Index
Standard Precipitation Index
Surface Water Supply Index
Reclamation Drought Index
Deciles
 Study Indices:
 Sacramento Valley Index (SVI)
 San Joaquin Valley Index (SJI)
 Yuba River Index
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Research Questions
 How will anticipated
hydroclimatic
changes affect
water allocation?
 How will stationary
water management
frameworks
function with
climate change?
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Climate change is
altering California’s
hydrology
 Precipitation
shifting from
snowfall to rainfall
 Snowpack stores
water
 Earlier runoff
 More winter flooding
 More summer
drought
California’s Sacramento-San Joaquin Bay Delta
 ~ 43% of CA’s
average annual
surface runoff
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Who Depends on the Sacramento and
San Joaquin Valley Indices?
Delta smelt (endangered)
Banks Pumping Plant (SWP)
Water Supply
 CVP and SWP
pumps
 Source of drinking
water for ~ 24
million Californians
 Agricultural water
Delta islands
Delta Farmers
 Farming on Delta
islands
 Consumptive uses
Environment
 Bay-Delta outflow
 Salinity and water
temperature
objectives
 River instream flows
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 Drive water policy
Sacramento and San Joaquin Valley Indices
SVI = (0.4*current Apr-Jul runoff) +
(0.3*current Oct-Mar runoff) +
(0.3*previous year’s index)
SJI = (0.6*current Apr-Jul runoff) +
(0.2*current Oct-Mar runoff) +
(0.2*previous year’s index)
Historical Water Year Type Thresholds
Water Year Type
Sacramento Valley
Index (maf)
San Joaquin Valley
Index (maf)
≥9.2
≥3.8
Above Normal
>7.8 and <9.2
>3.1 and <3.8
Below Normal
>6.5 and ≤7.8
>2.5 and ≤3.1
Dry
>5.4 and ≤6.5
>2.1 and ≤2.5
≤5.4
≤2.1
Wet
Critical
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Historical Water Year Type
Distribution
1905-2000
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Methods – multiple model, multiple emissions
scenario approach
Long-term (2051-2099)
Near-term (2001-2050)
Time Periods
Historical (1951-2000)
CNRM CM3
GFDL CM2.1
SRESA2
CCSR MIROC 3.2
medium
Emissions 
GCMs 
SRESB1
MPI-OM ECHAM5
SVI
VIC 
WYTs
SJI
NCAR CCSM3.0
NCAR PCM1
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Percentage of Years
60
50
40
30
20
10
0
Percentage of Years
Critical
60
50
40
30
20
10
0
Percentage of Years
Dry
Below
Normal
Above
Normal
Wet
Dry
Below
Normal
Above
Normal
Wet
Dry
Below
Normal
Above
Normal
Wet
- more even distribution
of year types
b) 2001-2050
Critical
60
50
40
30
20
10
0
Relative Frequency
of Sacramento
Valley Water Years
a) 1951-2000
c) 2051-2099
Critical
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Percentage of Years
90
80
70
60
50
40
30
20
10
0
a) 1951-2000
Percentage of Years
Critical
Below
Normal
90
80
70
60
50
40
30
20
10
0
Above
Normal
Wet
- distribution skewed
toward dry year types
b) 2001-2050
Critical
Percentage of Years
Dry
Relative Frequency
of San Joaquin
Valley Water Years
Dry
Below
Normal
90
80
70
60
50
40
30
20
10
0
Above
Normal
Wet
c) 2051-2099
Critical
Dry
Below
Normal
Above
Normal
Wet
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Distribution of Water Year Types Changes if
Historical Thresholds Held Constant
Sacramento Valley
San Joaquin Valley
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Null and Viers. 2013. Water Resources Research
Water Year Classification Thresholds
Reduced to Maintain Historical Distribution
of Water Years
Sacramento Valley
San Joaquin Valley
Null and Viers. 2013. Water Resources Research
Water Allocations Vary by Year Type
Wet
Above Normal,
Below Normal
Dry,
Critical
(%)
(%)
(%)
Delta Farming
3
7
12
Water Supply
10
27
37
Environmental
Uses
87
69
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(Bay Delta Conservation Plan, 2010)
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Climate Change Affects Water Allocations
Environmental Uses
Water Exports
Delta Farming
Average Annual Water Deliveries (2051-2099)
Water Supply
(pumped exports)
Delta Farming
Environmental
(consumptive uses) (incl. undev. water)
(taf)
(taf)
(taf)
Hist. Thresholds
+70
+49
-3135
Hist. Distribution
-124
-31
-2861
4,484
1,386
17,20318
Average Annual
Delivery
taf= thousands of acre-feet
Example Bay-Delta water balance for
2051-2099
 Average annual
water allocation
for Delta outflow,
exports, and
consumptive use
(taf)
 Average annual
change (from
historical) in bold
values (taf)
 Black bars
quantify
uncertainty
between A2 & B1
 Arrow & bar
length represent
flow percentage
Upper Yuba River
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Yuba Watershed
2 cfs
 1200 mi2
 22 dams (> 1 taf)
 1.43 maf surface storage
 12 hydropower facilities
2-3 cfs
 424 MW online capacity
5 cfs
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Instream Flow Requirement (ft3/s)
Proposed instream flows by water year type
90
80
70
60
50
40
30
20
10
0
ECD
CD
D
BN
AN
W
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Modeling methods
 Monthly optimization model:
 A 12-month multi-reservoir linear programming model run each
month
 Hydropower flows, Instream flows, Spill
 Inflows from a hydrology model using downscaled climate
projections
 4 time periods (historical, near term, mid-term, far term)
 Eight downscaled projections:
WCRP CMIP5 Climate
Model (GCM) ID
End-of-century
precipitation
MIROC-ESM
Drier
MIROC5
CCSM4
CNRM-CM5
Slightly drier
Slightly wetter
Wetter
RCP
4.5
Projection Name
miroc-esm.1.rcp45
8.5
miroc-esm.1.rcp85
4.5
miroc5.1.rcp45
8.5
miroc5.1.rcp85
4.5
ccsm4.1.rcp45
8.5
ccsm4.1.rcp85
4.5
cnrm-cm5.1.rcp45
8.5
cnrm-cm5.1.rcp85
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Preliminary Results – Water allocation by use
Hist.
Nearterm
Midterm
Farterm
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Rheinheimer, Null, Viers. In Prep.
Water Year Type Alternatives
 Assumptions about hydrologic stationarity matter
in both modeling and real-world decision making.
 Climate change adaptation will likely require
alternative WYT-based allocation schemes.
HP
HP
W
N
D
HP
Env.
Env.
HP
W
N
D
Env.
W
N
D
Env.
WYT
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Rheinheimer, Null, Viers. In Prep.
Major Findings
 Current classifications less representative with
climate change
 SVI: 34-38% dry & critical years by 2051-2099
 SJI: 66-69% dry & critical years by 2051-2099
 Climate change adaptations affect allocations as
much as impacts
 Burden of climate change to different users
 Other adaptation options:
 Change seasonal weights in indices
 Change carryover storage weight
 Many regulatory processes assume climate
stationarity
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Questions?
Contact Info: Sarah Null (sarah.null@usu.edu)
Climate Data: Climate Research Division of Scripps
Institute for Oceanography (Dan Cayan & Mary Tyree)
Funding: California Energy Commission
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