International Symposium on Groundwater Sustainability (ISGWAS)
Economic and Water Management Effects of a NO Overdraft Policy:
California’s Tulare Basin
Harou, Julien J. and Lund, Jay R.
Department of Civil and Environmental Engineering
University of California – Davis
Davis, California 95616, USA.
[email protected]
[email protected]
http://cee.engr.ucdavis.edu/faculty/lund/
ABSTRACT
While unsustainable, groundwater overdraft is a common and often useful feature of groundwater
management systems. This paper focuses on the end of groundwater overdraft. We examine options for
ending overdraft and deferring economic groundwater exhaustion, and present economic-engineering
optimization model results indicating the economic impact and water management actions that could
economically accompany the end of groundwater overdraft in the Tulare Basin of California, USA.
INTRODUCTION
Groundwater is in overdraft in many regions of the world. Groundwater overdraft has attracted much
attention both for its potentially harmful environmental and economic consequences as well as for its
potentially beneficial social and economic effects (Llamas and Martínez-Santos 2005). Here, we are
concerned with the end of groundwater overdraft.
All overdraft must come to an end, eventually. In a purely technical sense, overdraft is ultimately
unsustainable. Economically, sustained overdraft often raises pumping costs to an uneconomical level,
reducing economical pumping rates before supplies are exhausted physically. Overdraft also can end if
lower groundwater tables eventually induce additional stream infiltration enough to balance groundwater
withdrawals (Bredehoeft, et al. 1982; Bredehoeft 1997). And where escalating pumping costs do not
reduce pumping, (such as for shallow aquifers with highly valued water uses) and decreasing water tables
do not create a compensating increase in aquifer recharge, sustained overdraft will eventually physically
exhaust groundwater storage.
Groundwater overdraft occurs when more groundwater leaves regional aquifers than replenishes them,
over a period long enough to overlook the effects of seasonal and drought effects. Typically, overdraft
occurs on a multi-decade, historical time-scale, as opposed to a seasonal, annual, or drought time scales.
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
Groundwater overdraft also generally is considered to be a regional phenomenon, averaged over a
spatially sizable area, not to be confused with local cones of depression.
In this paper we explore the economics of groundwater overdraft, discuss some historical patterns of
groundwater overdraft, present options and strategies for ending overdraft, examine and analyze overdraft
and the potential for ending overdraft in the Tulare Basin of California, and finally provide some overall
conclusions.
ECONOMICS AND HISTORICAL PATTERNS OF OVERDRAFT
Most groundwater pumping, and therefore most overdraft, is for economic purposes. Individual farmers or
water users use groundwater because this source is less expensive and more available (economically,
legally, or physically) than other sources for their intended water applications, and is less expensive than
the economic value they expect to produce from the water use. As overdraft continues, groundwater
pumping increases until the cost of pumping begins to exceed the value of economic production from that
groundwater pumping, by which point other sources are becoming more economical or water use
decreases, until the marginal value of production from water application equals the marginal pumping
cost.
Groundwater is a local resource, and even ancient civilizations found ways to tap and enhance this natural
cistern using wells, qanats, and artificial infiltration (Pulido-Bosch and Ben Sbish, 1995; Evenari, et al.
1982). With the rise of diesel and electrical power and centrifugal pumps, and mechanical well-drilling
technology in the 20th century, groundwater pumping became more affordable and efficient in large
quantities. Consequently, groundwater use greatly increased and overdraft became more widespread.
The accumulation of overdraft has led to problems in many regions. These problems include land
subsidence, intrusion of poor quality waters, and threats to long-term sustainability of the overlying
economic activities and society. Land subsidence can lead to increased flooding and drainage problems (an
ironic consequence of a water supply issue) and disruption of infrastructure such as sewers and aqueducts.
Overdraft, by lowering the groundwater table can induce intrusion of seawater or other lower quality
waters (ancient saline groundwater, contaminant plumes, or contaminants held in the soil matrix) into
aquifers. Long-term overdraft also can uneconomically increase pumping costs and raise the risk of future
water shortages.
In response to such problems and the specter of such problems, governments and groundwater users have
often sought economic or legal means to control overdraft. Lawsuits are common in such situations as
pumpers sue each other and the government for redress. Groundwater legislation or regulation are often
proposed and sometimes occur. In southern California and elsewhere, multiple lawsuits often have been
combined and become the motivation for settlements among pumpers to allocate pumping allocations,
institute detailed monitoring and pumping allocation enforcement, and organize aquifer users to better
employ imported surface and groundwater resources conjunctively (Blomquist 1992). Arizona developed
special groundwater management areas for troubled aquifers (Blomquist et al. 2004).
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Harou, Julien J. and Lund, Jay R.
OPTIONS FOR ENDING OVERDRAFT
Table 1 summarizes how overdraft can come to an end. Most of these means of ending overdraft imply a
variety of management options and opportunities.
Table 1. Options for Ending Overdraft
Groundwater Strategy
Exhaust groundwater
Variations
Unmanaged exhaustion
Managed exhaustion
Declining water tables increase infiltration
Retaining local surface waters
Importing surface water
Active management
Passive management
Natural surface augmentation
Surface water substitution
Conjunctive use
Exhaustion of groundwater
Exhaustion of groundwater is the end of overdraft which seems to come to mind most frequently for of
the public and media. In cases where aquifers cannot be physically or economically recharged, isolated
confined aquifers of ancient groundwater or aquifers in arid areas remote from surface waters,
groundwater is an exhaustible resource, and effectively non-renewable. Much like a mineral ore deposit,
groundwater is a mined resource. And economically, it makes sense to mine this resource until the marginal
cost of extraction exceeds the marginal value of the resource for production. The marginal cost of
extraction of course increases with pumping head, as the groundwater table declines, until the marginal
extraction cost becomes infinite at the point where the resource is exhausted.
Where aquifer pumping is unmanaged by a large number of pumpers, each pumper sees that no pumper
has an individual incentive to curtail pumping, each pumper has incentive to cheat in any voluntary
curtailment arrangement, and the resource is coming to an end. This leaves each pumper’s only individually
rational recourse as pumping water to the maximum economic extent. Unmanaged exhaustion is likely to
occur in this situation, with declining groundwater tables until the point where the resource is physically
exhausted or, for deep aquifers, to the point where marginal pumping costs exceed all marginal water use
values.
Where pumping can be managed (by fees, taxes, or transferable rights) this same condition arrives more
slowly, with some low-valued pumping uses ending earlier than with the unmanaged case, allowing some
higher-values uses of the finite groundwater to be supported longer (Hotelling, 1931). In the case of
transferable property rights to groundwater, growers of high-valued orchard crops would purchase some
or all or the rights of alfalfa producers so as to extend the production of their orchards. Groundwater
pumping fees would similarly reduce groundwater use by making pumping uneconomical for some water
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
users. These two situations, of unmanaged versus economically managed groundwater exhaustion, are
depicted in Figure 1.
Figure 1. Unmanaged vs. Optimally Managed Pumping (Q) and Resulting Storage (S)
Natural surface water augmentation
Natural surface water augmentation occurs where declining groundwater tables should increase recharge
from (and decrease discharge to) overlying surface waters (streams, lakes, and wetlands) and adjacent
groundwater (Bredehoeft, et al. 1982). Such declining water tables simultaneously raise pumping costs
(and thus lower pumping quantities), with the possibility of eventually bringing pumping and recharge into
a new equilibrium condition. Such behavior appears in Figure 3, with the end of overdraft at a lower water
table elevation. Of course such induced net recharge augmentation might come at some cost to
downstream, instream, or up-gradient groundwater water users, especially in drier years.
Figure 3. Groundwater pumping and storage when recharge increases and pumping decreases with lower water tables
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Harou, Julien J. and Lund, Jay R.
Surface water substitution
Surface water substitution is a common historical response to overdraft. Steadily declining groundwater
levels raise fears of excessive pumping costs or exhaustion of groundwater supplies, motivating local water
users and their governments to seek additional substitute surface water supplies. Two surface sources are
often available, retained local surface waters and imported surface waters. Both approaches require the
construction and maintenance of a new local surface water distribution system, along with institutions and
finance for this new surface water system. Local surface waters may be local streams or captured local
precipitation. Imported surface waters require additional capital expenditures, as well as a willingness and
ability to appropriate water for importation, requiring additional capital, legal, and political resources. With
pure surface water substitution, the new surface water sources replace groundwater use entirely. This can
occur where the new surface water sources, one built, provide water at a cost lower than that of pumping
groundwater, leading to abandonment or severe curtailment in the use of groundwater. This situation arises
from the greater economies of scale often seen in surface water, once developed, compared with
economies of scale and sometimes greater operating costs for groundwater development.
Conjunctive use
Conjunctive use is an increasingly common approach to ending groundwater overdraft. Table 2 summarizes
various types of conjunctive use from the perspective of different time-scales of operation. Llamas and
Martínez-Santos (2005) note that groundwater overdraft frequently occurs in a historical context as the
first step in conjunctive use. Initially high groundwater tables are taken advantage of as an inexpensive
source of water allowing a region to intensify agricultural development and accumulate capital to support
more sustainable water development. In addition to this macroeconomic advantage, early overdraft, by
emptying part of the aquifer, makes space available for later storage of surface water, a necessary
component of a functioning conjunctive use system. A period of historical overdraft essentially becomes a
profitable and effective means of underground reservoir construction. If the aquifer is always full,
conjunctive use is very limited.
Table 2. Temporal Strategies for Conjunctive Use
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
Drought cycling of groundwater storage and use relies more on surface waters during wetter years and
shifts water deliveries more to groundwater during drought years. This approach takes advantage of
temporal changes in water availability (and cost) and is commonly desirable in regions with significant
inter-annual variability of surface water where surface storage of wet-year surpluses is not economically
available, would suffer excessive evaporative losses, or would cause unacceptable environmental
disruption. While such a system might seem to require institutional control of groundwater to limit
groundwater use in wetter years, this is often not the case. Where the surface water supply system has the
financial ability to offer surface water at a price slightly lower than the cost of pumping groundwater, most
users will chose surface water when it is available, and resort to groundwater only in drier years when
surface supplies are unavailable (Vaux 1986; Jenkins 1991).
Seasonal cycling of groundwater storage and use employs the same logic as drought cycling of
groundwater use, only on a shorter seasonal time-scale. Seasonal cycling requires seasonal imbalances in
water availability or demands. Seasonal recharge can be artificial in special facilities or induced by recharge
from streams during the drier season.
Continuous conjunctive use is often employed in coastal areas, using treated wastewater to prevent loss
of groundwater to flows to the sea or seawater intrusion into aquifers. The approach can similarly be used
to restrict the migration of contaminants into portions of an aquifer system used for water supply.
Many actual water resource and groundwater problems lend themselves to mixed conjunctive use
strategies, with a more complex mix of drought and seasonal cycling and continuous activities.
In developing a groundwater or conjunctive use policy, it is important to keep in mind that groundwater is
not employed alone. As summarized in Table 3, a vast array of water management options exist and are
utilized by the many water users and managers operating within any basin. Thus, we do not want to merely
manage groundwater, but to manage groundwater and overdraft within the context of the economic,
social, and environmental goals of the region and the many other water management activities available
and occurring simultaneously, and variably over very long and short periods of time.
Table 3. Water Management Options to Accompany End of Overdraft
Demand Management and Allocation
General
Pricing
Subsidies, Taxes
Regulations (allocation, water quality, contract authority, rationing, etc.)
Water transfers, options, markets, exchanges (within and/or between regions/sectors)
Insurance (drought insurance)
Demand Sector Options
Urban water use efficiency
Urban water scarcity (reduce demand through pricing or rationing)
Agricultural water use efficiency
Agricultural water scarcity
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Harou, Julien J. and Lund, Jay R.
Ecosystem restoration/improvements (dedicated flow and non-flow options)
Ecosystem managed water use efficiency
Environmental water scarcity
Recreation water use efficiency
Recreation improvements
Recreation scarcity
Supply Management
Operations Options (Water Quantity and/or Quality)
Conjunctive use of surface and ground water
Surface water storage facilities (new or expanded)
Cooperative operation of surface facilities, operational changes
Conveyance facilities (new or expanded)
Conveyance and distribution facility operations
Supply Expansion Options (Water Quantity and/or Quality)
Supply expansions through operations options (e.g. reduced losses and spills
Agricultural drainage management
Urban water reuse (recycling)
Water treatment and desalination
Urban runoff/stormwater collection and reuse (in some areas)
INSTITUTIONAL ASPECTS OF ENDING OVERDRAFT
The ending of overdraft might have economic and environmental motivations, but actual implementation
requires institutions to coordinate and enforce the actions of the many individuals and stakeholders
involved. The actual pattern of conjunctive use, surface water substitution, or managed exhaustion often
is shaped as much or more by the prevailing local water law and political context as by the physical and
economic context (Blomquist et al., 2004; Blomquist 1992). Institutional options for groundwater
management institutions commonly discussed and applied in the American West include:
•
•
•
•
•
•
•
•
Adjudicated water rights with an aquifer water master (Blomquist 1992)
Aquifer pumping taxes
Surface water prices set below the cost of surface pumping (Vaux 1986)
Aquifer pumping conducted by a unified irrigation district
Contracts and agreements between pumpers and surface water suppliers
Joint ventures of overlying land owners or irrigation districts
Special aquifer management districts
Laws or court rulings supporting rights to recharged water (Kletzing, 1988)
The prevailing local legal and political situation will affect the particular institutional approaches available
and selected for implementing overdraft management. An institutional means of implementing overdraft
management is a necessary condition for managed approaches to ending overdraft. Such institutions must
be politically, legally, and financially viable, as well as technically capable.
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
On some occasions, ending overdraft requires no implementing institution. These would include
unmanaged exhaustion and when a surface water substitute is being supplied at a cost lower than the
cost of groundwater pumping. Natural augmentation also might not require a special institution, although
it is likely to require a legal and political context where the additional induces groundwater recharge from
surface streams cannot also induce legal actions from downstream surface water users.
Figure 4. Tulare is a closed hydrological basin in the southern part of California’s Central Valley
(modified from DWR, 2003).
TULARE BASIN, CALIFORNIA
California’s Tulare region is one of the most productive agricultural regions of California, its most waterconsuming region, and a region with only modest natural inflows (see Table 4). The region depends heavily
on groundwater use and groundwater overdraft. Groundwater use amounts to 41% of total water use
(DWR, 2003). Estimates of groundwater overdraft for this region vary from 400 million cubic meters (mcm)
to over 1,000 mcm/year (CDWR 1998). This part of the paper explores the economic engineering aspects
of ending overdraft in the Tulare Basin. We begin with a brief hydrologic description.
California’s Tulare region is a 44,000 km2 closed hydrological basin (Figure 4). The southern part of the
Central Valley extends into the Tulare basin. The valley is underlain by a series of interconnected mostly
unconfined aquifers of area 21,500 km2 and depth of 1.3 km. These aquifers have been historically
overexploited which has resulted in low piezometric levels and high land-subsidence in some parts of the
basin (USGS, 1995).
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Harou, Julien J. and Lund, Jay R.
The Tulare Basin is one of the state’s greatest water-importing regions. The region has major water imports
from the San Joaquin River (through Friant-Kern Canal) and the California Aqueduct, which brings water
from northern California. Both imported water sources are used in conjunctive use operations by entities
such as the Kern Water Bank and various irrigation districts (Marques et al., in press). Despite the
importation of large quantities of water in recent decades, overdraft continues in this basin.
Table 4: Approximate annual flow volumes of the Tulare basin. (California Water Plan Update, 1995, p.8-45;
DWR, 2003 and CVGSM, 1997)
Flow Category
Mountain surface inflow
Groundwater recharge
Surface water imports
Tulare regional consumption
Estimated groundwater overdraft
Approx. Average Annual Water Volumes (mcm)
3,500
4,100
10,000
16,000
400-1,000
ECONOMIC-ENGINEERING ANALYSIS OF ENDING OVERDRAFT
Tulare Basin overdraft and groundwater use is analyzed using the CALVIN economic-engineering
optimization model for California’s water supply system (Draper, et al. 2003). The CALVIN model optimizes
surface and groundwater operations as well as water use and water allocations to maximize statewide net
economic benefits. Water demands are represented as economic penalty functions, representing each
water user’s economic willingness-to-pay for water deliveries. Operating costs for pumping and treatment
also are represented. While the model necessarily has many limitations, it does provide interesting and
useful regional scale insights for water management.
For this study, the California-wide model has been restricted to the Tulare Basin region, with boundary
flows coming in from diversions from the San Joaquin River and the California Aqueduct (bringing water
from the Sacramento-San Joaquin River Delta) and water being passed through and subsequently exported
via the California Aqueduct to the Southern California metropolitan area to the south. A schematic network
representation of the basin appears in Figure 5. The Tulare basin is divided into eight agricultural and urban
water demand regions. Each agricultural region overlies one of eight semi-interconnected groundwater
basins. The eight sub-regions of the Tulare are visible in the network of Figure 5 and the map of Figure 6.
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
Figure 5. Schematic of Tulare Basin CALVIN model
Figure 6. CALVIN groundwater sub-basins overlaying Tulare Basin public water agencies (modified from SAIC, 2003)
62
Harou, Julien J. and Lund, Jay R.
As a large-scale regional economic engineering model, CALVIN does not provide a detailed representation
of groundwater processes. Groundwater representation in CALVIN and its limitations are discussed by
Davis and Jenkins (2001) and Pulido-Velasquez et al. (2004). CALVIN optimizes water operations and
allocations over the 72-year hydrologic record (representing hydrologic variability) for a particular level of
infrastructure and land use development (projected to the year 2020 in this case) with a generalized
network flow optimization formulation. The model does not dynamically model groundwater hydraulics
within and between the groundwater sub-basins. Instead, CALVIN uses fixed series of flows between subbasins derived from a regional groundwater model called CVGSM (USBR, 1997). Groundwater recharge
time series also are taken from the CVGSM model. Pumping lifts are constant for each sub-basin and based
on average recent water levels. Overdraft quantities for each sub-basin are based on average volumes
estimated during the 1990s (USBR, 1997). These levels were extrapolated out for 72 years (USBR, 1997)
to derive final groundwater basin storage volumes (Table 5).
Table 5: CVPEIS NAA initial and ending storage levels.
Modeling Scenarios
The model is run under four conditions, all with projected year 2020 water demands and using 72 years
of historical monthly time series of inflows to represent hydrologic variability:
a) 375 mcm/year of average overdraft, with current capacities for conjunctive use (recharge, pumping,
and conveyance) within the basin
b) 375 mcm/year of average overdraft, with lesser capacity for conjunctive use common about a
decade ago
c) no overdraft, with current capacities for conjunctive use within the basin
d) no overdraft, with lesser capacity for conjunctive use common about a decade ago
No overdraft scenarios constraint ending groundwater basin storages to equal initial storages. This
combination of model runs allows us to examine the effects of overdraft and ending overdraft on water
shortages (scarcity), economic water scarcity costs, water users’ willingness-to-pay to import additional
water, and the value of conjunctive use facilities (recharge, pumping, and conveyance) within the region.
As an optimization model, CALVIN optimally re-operates and re-allocates water for the entire basin to
maximize net regional economic well-being. This includes changing surface and groundwater operations
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
and reallocating or marketing water, so water goes to its highest economic uses, consistent with
environmental flow restrictions imposed on the optimization. Thus, the model employs all available water
management options (most of Table 3) to economically adapt to the presence or absence of groundwater
overdraft.
Results
CALVIN provides time series of optimal operations at each location in the network. The economic penalty
function used in the optimization implies a water supply target for each demand, and economic costs of
not providing a full allocation of water. The delivery target is the water allocation which would result in no
economic harm to water users in the region, i.e., the point where the marginal value of water is zero.
Because water is limited and less than the region’s full demands, any physically feasible water allocation
will result in water scarcity to some water users. The cost of this scarcity is evaluated using the economic
penalty functions.
For the four alternative management cases, total groundwater storage in the Tulare Basin varies with time
as shown in Figure 7. In addition to the long-term trends of overdraft (or non-overdraft), there are also
significant patterns of groundwater recharge and withdrawal over seasonal and drought time-scales. The
time period of drought drawdown and refill is commonly about a decade or longer (Lettenmaier and
Burges 1982; Pulido, et al. 2004). Having additional conjunctive use facilities allows faster refilling of
aquifers during wetter periods, and the beginning of drawdown periods with greater amounts of water in
groundwater storage (Figure 7).
Figure 7. Groundwater storage trends with estimated overdraft averaging 375 mcm/year and no overdraft, with and
without additional conjunctive use facilities
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Harou, Julien J. and Lund, Jay R.
Overall results from the four alternatives are summarized in Table 6. The cost of scarcity from ending an
overdraft of 375 mcm/year in the Tulare Basin would be roughly $46 million/year. The resulting average
cost of ending overdraft is $124/thousand cubic meters (tcm) ($153/acre-ft). This average cost of ending
overdraft exceeds the marginal value of additional surface water imports to the basin, due to conveyance
and storage constraints on imported surface water at various locations and times within the basin and the
ability to reuse return flows within the basin. A 1 tcm reduction in supply (from overdraft) decreases water
available for agricultural application by more than 1 tcm due to reuse of return flows within the basin.
If conjunctive use capacity had not been recently expanded, costs of implementing a no overdraft policy
under optimal operations would be greater, reaching $59 million/year. Expanded conjunctive use capability
reduces the cost of ending overdraft in the Tulare Basin by 20%, from $59 to $46 million/year. The greater
flexibility provided by the conjunctive use facilities also lowers the average economic value of imported
water (from $137 to $117/tcm), and so would reduce the economic motivation of the region to seek
additional imported supplies.
Water scarcity and scarcity cost are available at each location of the network for each monthly time period
and year modeled. A regional time series of scarcity and scarcity cost under the four alternatives appears
in Figure 8. Results for each sub-region appear in Table 7. Because urban and industrial water users have
higher marginal willingness to pay for water, these sectors are not directly affected by water scarcity
imposed by the no overdraft policy (although it will increase their costs of acquiring water). Agricultural
water-right holders sell or lease their water to farmers or cities with higher willingness-to-pay. Thus, the
agricultural sector sees scarcities almost double that of both no-overdraft cases.
Table 6. Effects of ending overdraft and conjunctive use expansion on water scarcity, scarcity cost, and the average
marginal value of imported water
Overdraft
No overdraft
Average Annual Water
Scarcity (mcm)
CU
Less CU
493
778
942
1240
Annual Scarcity Cost
($ million)
CU
Less CU
25
59
72
118
65
Marginal Value of Imports
($/tcm)
CU
Less CU
54
101
117
135
Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
Table 7. Marginal willingness to pay for water and average annual deliveries, scarcities, scarcity costs for eight
agricultural and three urban service areas
Delivery
Area
Agri. area 14
Agri. area 15
Agri. area 16
Agri. area 17
Agri. area 18
Agri. area 19
Agri. area 20
Agri. area 21
Fresno
Bakersfield
Santa Barbara
Totals
Target
Delivery
mcm
1,845
2,456
611
1,029
2,663
1,179
834
1,433
469
321
171
13,012
No Overdraft; With CU
With Overdraft; With CU
WTP
Scarcity Delivery Scarcity WTP
Scarcity Delivery Scarcity
$/mcm Cost $M/yr mcm
mcm $/mcm Cost $M/yr mcm
mcm
51
11.7
1,695
149
24
8.1
1,745
99
40
7.8
2,311
145
21
1.8
2,400
56
53
1.9
582
29
13
0.1
605
6
60
3.1
979
50
15
0.4
1,012
17
79
40.1
2,228
436
29
10.2
2,460
203
38
3.3
1,118
62
27
3.0
1,123
56
49
2.9
785
49
33
2.6
790
45
36
1.5
1,403
29
26
1.3
1,406
26
0
0
469
0
0
0
469
0
0
0
321
0
0
0
321
0
0
0
171
0
0
0
171
0
72.3
12,063
949
27.5
12,503
508
Figure 8. Agricultural water shortages and scarcity costs for different management scenarios
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Harou, Julien J. and Lund, Jay R.
In addition to internal changes in water allocations and operations, the model suggests the marginal
willingness to pay of basin water users for additional water imports. For the two major import sources (the
San Joaquin River and the State Water Project), the marginal economic values of additional water imports
appear in Table 6 and Figure 9. These economic values for additional water imports show significant
seasonal variation, following seasonal patterns of local water demands and availability. The California
Aqueduct shows significant increases during major droughts, when local water availability is further
constrained. The availability of additional conjunctive use capacity (storage, recharge, and withdrawal
capabilities) tempers the value of increased water imports, by raising the overall regional use efficiency of
existing water supplies.
Figure 9. Marginal willingness to pay for additional water imports from two major sources, Friant-Kern Canal and
California Aqueduct
Discussion
An important lesson from this study is the demonstrated beneficial impact of conjunctive use management
on implementing a sustainable yield policy in a region that intensively uses groundwater for irrigation. The
study underlines the importance of considering groundwater overdraft, and policies aimed at reversing it,
within the context of diverse systemwide water management activities and objectives. Overdraft is not
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Economic and Water Management Effects of a NO Overdraft Policy: California’s Tulare Basin
easily reversed by considering groundwater management options alone, and for many semi-arid basins,
this would also be unrealistic.
Because the historical record for the Tulare Basin begins and ends in drought periods, the optimal trajectory
of groundwater storage with no overdraft stays mostly well above current storage levels. This provides a
key insight into potential benefits of intensive groundwater use – initial drawdown of aquifers creates
storage capacity that allows later conjunctive use. Initially overdrafting aquifers to make space for later
flexible operations may be optimal. While implementing a no overdraft policy for the Tulare basin,
willingness to pay for additional water deliveries stays well below the cost of seawater desalination. Links
at Santa Barbara allowing desalination were not activated.
When interpreting model results, one should be aware of a few key assumptions and limitations. Here, both
groundwater levels and flows between subbasins are fixed based on historical data. Groundwater pumping
costs are static and do not vary with storage. Thus the model does not provide insight into one of the major
benefits of ending overdraft – stabilizing pumping costs. This issue may have less effect than expected
since agricultural revenues are often more than an order of magnitude greater that pumping costs. Another
aspect of the model application to keep in mind is that all model runs portray perfectly optimized
operations with no institutional constraints. They represent an economic-engineering ideal, which might
not be institutionally feasible.
CONCLUSIONS
While groundwater overdraft can be good or bad, depending on one’s perspective and time in history, all
overdraft must inevitably end. However, when and how overdraft is ended has substantial economic,
environmental, and social importance. As pointed out here and by Llamas and Martínez-Santos (2005),
groundwater overdraft can have an important social and economic function for the over-lying society and
an important engineering function in providing a reservoir for conjunctive use operations if they come to
replace overdraft operations.
Fortunately, a variety of approaches are typically available to end overdraft and conjunctive use allows a
region to continue benefiting from the services of groundwater storage. The selection of policies to end
overdraft should carefully consider the optimality of the timing and rate of ending overdraft, to avoid
unnecessary economic disruption, and the mix of complementary water resource management policies
available to help improve or mitigate water management in a region currently experiencing groundwater
overdraft. In the case of the Tulare Basin, California, ending overdraft will have considerable, but not
regionally catastrophic, economic consequences. These would be somewhat mitigated by additional
conjunctive use capability and surface water imports.
ACKNOWLEDGEMENTS
We thank Guilherme Marques, Manuel Pulido-Velázquez, Marion Jenkins, and Stacy Tanaka for useful
discussions on conjunctive use and help with modeling conjunctive use for the Tulare Basin.
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Harou, Julien J. and Lund, Jay R.
REFERENCES
Blomquist, W. (1992), Dividing the Waters: Governing Groundwater in Southern California, ICS Press, San Francisco,
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