This document summarizes a presentation on conjunctive use of surface and groundwater with examples from California. Conjunctive use aims to manage surface and groundwater resources together for better water supply, quality, economic and environmental outcomes. Potential benefits include increased water availability and reliability. The presentation reviews concepts, strategies and components of conjunctive use, and provides examples from various locations in California, including Yolo County, Sacramento metro area, Kern County Water Bank, and Santa Clara Valley. It also discusses California's new Sustainable Groundwater Management Act and challenges in achieving sustainable groundwater management.

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
presentation J. Lund
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Conjunctive use in groundwater
management- concepts and
California's experiences
Jay R. Lund
Civil and Environmental Engineering
University of California, Davis
13 Mars 2020, Montpellier
CaliforniaWaterBlog.com
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Summary: Conjunctive use in groundwater management-
concepts and California's experiences
The talk reviews concepts and methods in the
conjunctive use of surface and ground
waters. Many of these concepts are illustrated
by the context and examples from California,
which has a diverse range of conditions where
conjunctive use has been applied. The
discussion then shifts to the future of
groundwater management (and overall water
management) in California, with
implementation of a new state law seeking to
eliminate groundwater overdraft by 2040.

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
presentation J. Lund
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Thanks
Everything I know about groundwater I
learned from students:
Manuel Pulido-Velazquez, Julien Harou, Gui
Marques, Mimi Jenkins, Andy Draper, Ken
Kirby, Stacy Tanaka, Christina Connell, Tingju
Zhu, Gracie Yao, and others.
and some reading of the literature.
… I still don’t know much.
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Overview
1. Concepts for Conjunctive Use of
Surface and Groundwater
2. California Examples
3. Making Groundwater Sustainable in
California
4. Modeling uncertainties and
groundwater policy

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
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Groundwater in Complex
Water Management
1. Drought storage (largest by far!)
2. Seasonal storage
3. Water collection (infiltration)
4. Water reuse (infiltration of applied water)
5. Water treatment/contamination
6. Aquifer mining (overdraft, salt disposal)
7. Conjunctive use (possible mix of above)
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What is Conjunctive Use of Surface
and Groundwater?
Management of surface and ground-
water together for better performance.
Water supply, water quality, economic,
environmental, institutional

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
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Potential Benefits of Conjunctive Use of
Ground and Surface Waters
Water Supply
1. More water availability and reliability (“buffer”)
2. More effective storage capacity, operational flexibility
3. Less spills and evaporative losses
4. Less imported supplies in drought & emergencies; less groundwater depletion
Water Quality
1. Match sources with demands by quality
2. Better manage water quality
3. Water reuse through soil-aquifer treatment
4. Blending for waters of different qualities
Economic Performance
1. Economic benefits from deliveries and water quality
2. Manage peak demands, reduce conveyance size
3. Phase and reduce capital costs
4. Avoid canal lining; seepage becomes recharge
Environmental Performance
1. Less surface infrastructure damages
2. Less groundwater overdraft and damages
3. Less drainage and salinity problems Can better accommodate environmental demands
Institutional
1. More local control and autonomy, local flexibility in regional systems
2. Better coordinate inter-regional, regional, and local management
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Strategies for Conjunctive Use
Strategy Problem Period Strategy
Drought
cycling
Droughts, inter-annual
imbalances in water
availability and
demands
Annual to
decadal
Store and use surface water in
wetter years, use more
groundwater in drier years
Seasonal
cycling
Seasonal imbalances in
water availability and
demands
Seasonal,
within year
Greater wet season use and
recharge of surface water, and dry
season use of groundwater
Initial intensive
exploitation
Initial stage of region
development and
conjunctive use
Initial
regional
development
Initial pumping supports early
economic development, deferring
infrastructure investments.
Supplemental
recharge
Increase groundwater
availability
Continuous Increase recharge to increase
groundwater supplies overall
Saline intrusion,
contaminants
dispersion in aquifer
Continuous Reallocation of pumping and
surface water, recharge
management, fresh water injection.
Dilution Extending surface
supplies with saline
groundwater
Inter-annual Dilute saline groundwater
applications with available surface
water in drier years
Treatment Contaminated surface
water
Continuous Treatment of surface water in
unsaturated and saturated zones
of aquifer
Mixed Combination of above Mixed Integrated mixture of above
strategies

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
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Conjunctive Use Components
Water Source Options
Surplus surface water, Stormwater and floodwater, Reclaimed
wastewater, Reclaimed groundwater, Desalinated seawater, Conserved
urban or agricultural water use, Purchased water (lower economic value)
Managed Recharge Options
Deep percolation from surface use (irrigation, spreading, wetlands),
Deep percolation from conveyance or storage losses, Urban infiltration,
Recharge basins, Induced flood seepage, Injection wells, Stream-aquifer
interaction, In-lieu recharge or water exchanges
Groundwater Withdrawal Options
Well pumping, Aquifer seepage to stream or wetlands, Aquifer seepage
to ocean (to repel salinity)
Coordination – Regulation Options
Surface water prices, Groundwater pricing or taxes, including “net
metering” , Water right allocations for surface or groundwater use,
Agreements among water users and water managers, Water markets,
Single agency management of basin, Water accounting systems
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Passive vs Active Conjunctive Use
• Conjunctive use requires coordination among many
people – water users, managers, regulators, etc.
• Managing people is hard.
• Active management of all parts is often not possible.
• Passive management approach sets some
conditions, incentives, and infrastructure and then
lets sytem manage itself.

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Passive vs Active Conjunctive Use
Passive Management
Facilities • Design for incidental recharge
• Well capacity expansion
Water users • Relative prices and availability of surface and
groundwater
• Groundwater or surface water substitution
• Exchange agreements, contracts, water markets
Active Management
Facilities • Surface facility operations
• Artificial recharge
• Treatment of water to improve conjunctive use
• Pumping
• Coordinated operation of facilities
Water users • Exchange agreements & contracts, water
markets
Coordinated operations • Alternating use and alternating operations
• Managed stream/aquifer interaction
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Functional Institutional Needs
• Legal authorities
• Rights for surface water and recharged groundwater
• Enforceable water accounting or allocations
• Governance and communication within a larger
management framework
• Formal or informal agreements among water
managers and users
• Enforcement with credible incentives and threats for
behaviors supporting conjunctive use
• Financially and politically stable allocations of
benefits and costs among parties.

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Methods of Technical Integration
A wide range of options:
• None
• Institutional leader judgement, with observations
• Rough mass balances
• Detailed mass balances
• Rough basin groundwater flow models
• Detailed basin groundwater flow models
• Dynamic real-time groundwater flow modeling
• Dynamic real-time surface and groundwater flow
modeling
How to support institutionally?
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Some California Examples
General California Water Management
Conjunctive use examples:
1. Yolo County Irrigation District
2. Sacramento Metro Area
3. Kern County Water Bank
4. Tulare Basin
5. Santa Clara Valley Water Agency

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2014
1977
2015
2016
2013
Sacramento Valley Precipitation
Water and People in California
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Most annual rainfall variability in US
SOURCE: Dettinger, et al. 2011. “Atmospheric Rivers, Floods and the Water Resources of California.” Water 3(2), 445-478. .
NOTES: Dots represent the coefficient of variation of total annual precipitation at weather stations for 1951-2008. Larger values
have greater year-to-year variability.
Annual coefficient of variation
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 2000 4000 6000 8000 10000
Cumulative Irrigated Crop Area (1,000 Acres)
Cumulative Jobs and Revenues
Cumulative Jobs
Cumulative
Revenues
Vegetables,
Horticulture
&
Non-Tree
Fruits
Vines
Subtropical
Fruits
Almonds
and
Pistachios
Deciduous
(Orchards)
Processing
Tomato
Other Field, Grain, and Feed
Fresh
Tomato
+
Cucurbits
Onion
+
Garlic
Josue Medellin

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Modeled Groundwater Operations
480
490
500
510
520
530
540
550
Oct-21
Oct-24
Oct-27
Oct-30
Oct-33
Oct-36
Oct-39
Oct-42
Oct-45
Oct-48
Oct-51
Oct-54
Oct-57
Oct-60
Oct-63
Oct-66
Oct-69
Oct-72
Oct-75
Oct-78
Oct-81
Oct-84
Oct-87
Oct-90
Groundwater
Storage
(maf/mon)
Base2020 SWM2020 SWM2100
PCM2100 HCM2100
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0%
10%
20%
30%
40%
50%
60%
0% 20% 40% 60% 80% 100%
Annual Exceedence Probability
Total
Annual
Supply
-
%Goundwater
Base2020 SWM2020 SWM2100 PCM2100 HCM2100
Conjunctive Use – model results

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California’s Water System
• Diverse water supplies
– Mostly in north
– Mostly in wet season
• Diverse water demands
– Mostly central and south
– Mostly in dry season
• Extensive use of aqueducts,
reservoirs, and groundwater
• Delta “hub” for conveyance
• Most storage is groundwater
The Delta
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Coming changes…
1. Continued demand growth?
2. Demand hardening (urban growth, tree crops)
3. Sea level rise and Delta
4. Climate warming
5. Flood frequency worsens
6. Water markets
7. Greater system integration, new law
Groundwater becomes more important.

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
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Some California Examples
1. Yolo County Irrigation District
2. Sacramento Metro Area
3. Kern County Water Bank
4. Tulare Basin
5. Santa Clara Valley Water Agency
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Yolo County Irrigation District
1. - 62,000 hectares irrigated farming
2. Most years surface water available
3. Dry years have only groundwater
4. Unlined canals to increase recharge
5. Surface water prices set less than
pumping cost (passive CU)
6. Well balanced aquifers

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Conjunctive Use of Surface and Groundwaters with California Examples, 2020
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Sacramento Metro Area
Most local urban agencies coordinate
their surface and groundwater use
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Kern Water Bank
1. Wet-year recharge + wetland; Dry-year
pumping
2. Facilities owned by local irrigation agencies
3. Water owned by contracting agencies and
users statewide - lose 6% of water

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Tulare Basin Overdraft
• 2-3 bcm/year overdraft, ~15% of net use)
• How much net use reduction is needed locally?
Faunt et al., 2009
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Tulare Basin –
Groundwater Depletion
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SOURCE: What If California’s Drought Continues? (PPIC, 2015), Figure 3.Data through 2009 from DWR; author estimates after 2009. Projections since 2009
may underestimate depletions since the onset of the latest drought (2012+)..

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Santa Clara Valley Water Agency
1. San Jose area; large aquifer, water imports
2. Eliminated overdraft, improves reliability
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Making Groundwater
Sustainable in California
1. New State Law: Sustainable
Groundwater Management Act (SGMA)
2. Water Balances
3. Institutions
4. Hard problems: water, salinity, nitrate

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California Groundwater Law
Anita Milman
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Uncertainty in Overdraft
Hydrologic variance and modeling variance
Faunt et al., 2009
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Escriva-Bou et al, in press

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Model Uncertainty in Overdraft
Differences between older C2VSIM and CVHM models
Faunt et al., 2009
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Conclusions
1. Groundwater is mighty important
2. Conjunctive use has many forms
and can help in many ways
3. California has many examples
4. Groundwater often behaves very
different from surface water
5. Sustainability is hard and has
uncertainties

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Some further reading
Escriva-Bou, et al. “Planning for Groundwater Sustainability Accounting
for Uncertainty and Costs: an Application to California's Central Valley,”
Journal of Environmental Management, in press.
Llamas MR, Martínez-Santos P (2005) Intensive groundwater use: silent
revolution and potential source of social conflict. ASCE J Water Resour
Plan Manag 131(5):337–341
Pulido – Velázquez, M., et al. (in prep.), “Conjunctive Management of
Ground and Surface Waters: Concepts, Experiences, and Prospects”
Pulido – Velázquez, M., M.W. Jenkins, and J.R. Lund (2004), “Economic
Values for Conjunctive Use and Water Banking in Southern California,”
Water Resources Research, Vol. 40, No. 3, March.
Vaux HJ (1986) Water scarcity and gains from trade in Kern county,
California. In: Frederick K (ed) Scarce water and institutional change.
Resources for the Future, Washington, DC
Zhu, T., et al., “Hydroeconomic Optimization of Integrated Water
Management and Transfers under Stochastic Surface Water Supply,”
Water Resources Research, Vol. 51, Issue 5, pp. 3568–3587, May 2015.
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Questions?