Enhancing drought resilience with conjunctive use and managed aquifer recharge in California and Arizona

The southwestern US has been subjected to many long-term droughts within the past century. California is currently in its fourth year of drought (2012–2015) with 100% of the state in drought and 58% in exceptional drought during its maximum extent on October 2014 (figures 3 and S3) [29]. The most extreme drought on record occurred in 1976–1977, followed by a six year, less intensive drought (1987–1992), and more recent droughts in 2007–2009 and 2012–2015 (figure S4) [29]. Many droughts end in floods, with atmospheric rivers ending 35%–40% of droughts in California [1]. The 1976–1977 drought in California was followed by a wet 1978 and the 1987–1992 drought was followed by a very wet 1993 in southern California. These meteorological droughts translate into hydrologic droughts as shown by markedly different annual runoff between dry and wet years (e.g. 1990, dry and 1993, wet) in southern California (figure S5). Long wet periods occurred mostly in the 1980s (1978–1986) and 1990s (1992–2000). There is no strong relationship between wet and dry periods and El Niño Southern Oscillation (ENSO) in California [29].

Arizona is in its fifth year of drought (2011–2015) but the drought is less severe than that in California with 98% of the state in drought in July 2014 at its maximal extent but 0% under exceptional drought (figure S3). Time periods of historical droughts in Arizona are generally similar to those in California, mid 1970s, late 1980s, late 1990s to early 2000s and 2005–2009 (figure S4) [30, 31]. The mid 1970s drought ended in floods in south central Arizona in 1978 [30]. Tropical storms also result in flooding, e.g. Tropical Storm Octave, September–October, 1983. There are strong teleconnections between droughts and wet periods in Arizona and ENSO: droughts associated with La Niña and wet periods mostly related to El Niño [31]. Drought in the mid-1970s is associated with a strong La Niña and wet years in 1983 and 1997–1998 are related to strong El Niños (table S1). These teleconnections may extend into southern California.

Long-term planning and investment was required to develop infrastructure to transport and store water from the source to the demand regions in the southern Central Valley and central Arizona. Construction of the Central Valley Project (CVP) project began in 1937 with the last of 22 reservoirs completed in the early 1970s (figure 1) The Delta-Mendota Canal (188 km) and Friant Kern Canal (F-K Canal, 245 km long (figure 1) are the primary CVP canals transporting international experiences of water transfers: relevance to India water from northern California to the southern Central Valley. The estimated construction costs of the CVP is ∼$3 billion [32]. Construction of the State Water Project (SWP) began in the late 1950s with the original canals and 20 reservoirs completed in the early 1970s. The main transport system is the California Aqueduct (715 km long) (figure 1).

The Central Arizona Project (CAP) includes a 542 km pipeline from Lake Havasu on the Colorado River to Phoenix and Tucson (figure 2). The CAP was constructed between 1973 and 1993 to increase use of Arizona's Colorado River annual allocation water (3.5 km³, 2.8 million acre feet, maf), with some of its allocation previously going to California (SI, section 2). The estimated construction cost of CAP is ∼$3.6 billion [33].

Water sources for MAR systems in the southern Central Valley include the Kern River, local rivers, and imported water through the CVP (e.g. Friant Kern Canal and Delta Mendota Canal) and SWP (California Aqueduct), and connections between the two (Cross Valley Canal) (figure 1). Water deliveries to the MAR systems in the Central Valley totaled ∼14 km³ (mid-1960s–2013 (figure 8(a)). Nine of the 10 spreading basins are in Kern County. The top four of the 10 systems represent ∼75% of total deliveries (figure S18). Deliveries to MAR systems in Kern County during peak wet years (e.g. 2005, 2011, ∼1.3 km³) represent ∼30% of total surface water deliveries and ∼45% of CVP and SWP deliveries to Kern County. Water deliveries were ≤0.1 km³ yr⁻¹ during droughts.

Water delivered for MAR in Arizona totaled 7.3 km³ (1994–2013), 75% from the Colorado River delivered through the CAP aqueduct (5.5 km³), 19% from reclaimed municipal waste water (MWW, 1.4 km³) and 6% from locally derived surface water (0.43 km³) (figure 6). CAP deliveries to MAR spreading basins represent ∼18% of total CAP deliveries to central Arizona, ranging from 0.05 km³ yr⁻¹ (1994) when the first spreading basin became operational to 0.3–0.6 km³ yr⁻¹ from 2003 to 2013. Reclaimed MWW is used for MAR because direct reuse of MWW is illegal in Arizona. MAR systems based solely on MWW represent 65% of MAR systems by number (50/77) but only 15% by permitted storage volumes in 2015 [47]. Waste water treatment plants are widely distributed in the AMAs (figure S19). Excess water from the Salt and Verde rivers as part of the Salt River Project, is input to the GRUSP MAR system in the Phoenix AMA (figure S25(a)). Water delivered to spreading basins totaled 4.1 km³ to the Phoenix AMA, 2.5 km³ to Tucson AMA, and 0.1 km³ to Prescott AMA (figures S20 and S21). Storm water provides an additional source of recharge through an estimated 51 507 registered dry wells, with ∼95% of the dry wells located in the Phoenix area (figure S22) [48].

Groundwater levels respond to natural recharge, recharge of excess applied irrigation, CU, and MAR, making it difficult to isolate impacts of MAR. In the Central Valley, water in MAR systems is generally stored over multiple wet years and extracted during droughts. For example, hydrographs adjacent to the Kern Water Bank, show large groundwater level rises during wet periods (≤40 m yr⁻¹) and marked declines during droughts (5–7 m yr⁻¹) (figures 8(b) and S24). To evaluate the impacts of MAR we compare these hydrographs with the GWS from the regional model of the Central Valley which shows large declines in GWS during droughts with much less recovery during wet periods, resulting in an overall net storage decrease of 1.4 km³ yr⁻¹ [36] (figure S9(a)). The impact of MAR is also seen in the contrast in well hydrographs in the MAR regions (figure 8(b)) and areas where recent subsidence has been recorded, such as the estimated 2 m of subsidence in the El Nido area (southern San Joaquin Basin, figure S2) between 2007 and 2014 [41].

Spreading basins in the southern Central Valley recharge the shallow unconfined aquifer; however, extraction wells are screened from the water table to deeper semiconfined to confined aquifers. Well nests screened at different depth intervals indicate that hydraulic gradients are downward in nearly all cases indicating the potential for downward movement from the shallow to deeper flow system (figure S24). The contribution from MAR to the deeper system will depend on lateral spread of increased storage through displacement in the shallow system relative to leakage rates through the confining layers. Lag times less than months with nearly equal magnitude change with depth suggest that the influence on shallow and deep systems is similar in many regions. The small head differences between the hydrographs in the Kern Water Bank suggest that the aquifers in this region are not strongly confined. Groundwater in the region of the MAR systems is a closed basin and does not discharge to surface water. Therefore, water inputs through MAR should not be lost through baseflow to streams.

In the Arizona AMAs, hydrographs from wells adjacent to spreading basins show rapid groundwater-level rises within a year after imported water was delivered to the basins although water levels in some basins were deeper than 100 m before spreading began (figure S25). The coarse sediments, particularly in the Tucson AMA, allow relatively rapid water transport to the aquifer. Groundwater level monitoring near the SAVSARP MAR system indicate that it took ∼6 weeks for water to percolate from the spreading basin to the water table at a depth of 122 m and laterally 150 m to the monitoring well [49].

CAP deliveries to the Tucson AMA are primarily for MAR (1.5 km³, 69% of total CAP deliveries to Tucson AMA). The composite water-level hydrograph based on water-level records at 995 wells for the post 1980 period shows water-level responses to wet and dry periods, peaking in 1993 and a decline through 2004 (0.5 m yr⁻¹), followed by a rise through 2012 (0.5 m yr⁻¹) (figure 7(a)). The regional groundwater model indicates that the increases in water levels in the early 1980s and 1990s are related to increased recharge in response to anomalously high precipitation, particularly in 1983 and 1993 [38] (figure S14(c)). The more recent increases in water levels correspond to simulated increases in recharge, consistent with MAR from CAP. The groundwater model provides a long-term context for the recent data and shows that GWS declined by ∼8.5 km³ from the 1940s to early 2000s because pumpage exceeded recharge, except during brief wet periods in 1983 and 1993. Stabilization of GWS in the mid-2000s corresponds to recharge exceeding pumpage and may be attributed in part to CAP MAR deliveries. The MAR spreading basins are mostly located in the Avra Valley subbasin and GWS reversed trends in 1980 in the subbasin and has increased since 2000 (figure S14(c)). Therefore, the impact of MAR can be seen in the well hydrographs and regional model.

The impact of the MAR systems in the AMAs is evident when comparing water level hydrographs in the AMAs with those in basins outside the CAP delivery zone and little or no access to local surface water. In contrast to the increases in groundwater levels in the AMAs that receive CAP water, groundwater levels in basins outside the CAP delivery zone show declines ranging from 0.5 to 1.3 m yr⁻¹ ranging from 1980 to 1997 onwards (figure 7(b)). These analyses underscore the importance of surface water replenishing GWS through CU and MAR when contrasted with nearby basins that do not have access to surface water. However, it is difficult to isolate the impacts of CU and MAR in systems subjected to wet and dry climate cycles with natural recharge also varying and groundwater pumpage changing in response to these climate cycles.

Large groundwater level declines related to irrigated agriculture (∼2.5 m yr⁻¹, figure S26) led to creation of the AEWSD and purchasing of water to reduce groundwater depletion in the extreme southeastern end of the Central Valley. AEWSD is typical of irrigation districts in the Central Valley and purchases water from the CVP. AEWSD is almost hydrologically isolated by the surrounding mountains on most sides, protecting stored groundwater [50]. The area served by AEWSD is ∼526 km², with 445 km² irrigated, 50% sourced from imported surface water and 50% from groundwater (figures S27 and S28). Low mean annual precipitation (155 mm, 6 inches), 90% in winter months (November–March), is out of phase with summer crop production (figure S29). In 1966, AEWSD began operation of its spreading basins, 7.1 km² in area, ∼1% of the total area. Between 1966 and 2012 AEWSD delivered 6.4 km³ directly to users for irrigation, and applied 2.6 km³ in three spreading basins. Estimated evaporation losses are low (∼1% of applied water). AEWSD is in an ideal location with access to CVP through the Friant Kern Canal, supplying 64% of total applied water, SWP through the Cross Valley Canal and Intertie Pipeline, accounting for 30% of the water, and the Kern River, representing 6% of the applied water (figure S30). Water application from the Kern River was restricted to very wet years, peaking in 1998. AEWSD operates ∼70 km of lined canals, 275 km of pipelines, ∼50 pumping stations (figure S31) [51] and is continually investing in infrastructure to enhance operations. AEWSD is constantly expanding its network of partners with ∼20 partners.

The water budget reflects the balance between deliveries and extractions (figure 9). A total of 2.6 km³ was delivered to the spreading basins, and an estimated 1.8 km³ was extracted from groundwater, resulting in a net delivery of 0.6 km³. Water deliveries peaked during wet years, ranging from 0.12 to 0.16 km³ whereas extractions peaked during drought years, ranging from 0.10 to 0.19 km³ yr⁻¹. AEWSD has also enhanced the canal system to allow reverse flow [51].

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Water percolation is slow, 30–140 mm d⁻¹ based on data for Oct 2011 (AEWSD, 2013). The average percolation rate based on annual deliveries and spreading basin areas is 27 mm d⁻¹, peaking in 1993 (77 mm d⁻¹) with little or no percolation during droughts. Water extraction is also slow and includes 76 extraction wells ranging in depth from 240 to 320 m (figure S27). Monitoring wells (81) show that the average groundwater depth ranged from 115 to 193 m beneath the different spreading basins in August 2013 [52]. A long-term hydrograph shows stabilization of groundwater levels in the late 1960s, increases in the early 1980s, declines during the drought from 1987 to 1992 and increases in the late 1990s (figure S26). More recent monitoring data show increases during wet periods and declines during droughts, similar to well hydrographs near the Kern Water Bank (figure 8(b)). There are limited data for the AEWSD system during the current drought; however, hydrographs near the Kern Water Bank show that declines during droughts exceed recoveries, resulting in net depletion during this drought. Depletion in the vicinity of these MAR systems is much less than the regional depletion simulated by the Central Valley Hydrologic Model for the Tulare Basin (1.4 km³ yr⁻¹, figure S9(a)). Analysis by AEWSD indicates that groundwater levels would be about 100 m deeper (200 m versus 100 m deep) if MAR had not been operating and assuming pumping would have occurred at the same level (figure 10).

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The economics of water storage is based on the ability to store water during wet years when water is available and prices are low and to extract water during dry years when the value of water is high. Deliveries of imported water are reduced during droughts because of reduced availability and endangered species issues (Delta Smelt), amplifying variability related to wet and dry periods. Because surface water is much less expensive than groundwater when available, AEWSD prices both sources similarly so that irrigators maintain both irrigation systems. The common water volume unit used in the US is the acre-foot, which is equivalent to 1230 m³ or 1.2 × 10⁶ liters (l). AEWSD has long-term (30–40 yr) contracts with the CVP to purchase water at ∼$10–$20 af⁻¹. The selling price of water by AEWSD to its members increased from $15 af⁻¹ (1.2 × 10⁶ l) in the mid-1960s to mid-1970s, to ∼$60 af⁻¹ in 1990, $80 af⁻¹ (2000) and $140 af⁻¹ in 2012 [52]. The ∼100 m shallower water table depth as a result of MAR reduces pumping costs by an estimated $50–$80 af⁻¹ [53] resulting in 50%–60% savings in power costs for water. Power costs represent about half of the total water cost. During the current drought AEWSD promoted an internal market among its members with purchase costs for water pools highest at the beginning of the summer season to incentivize water movement to higher value users, ranging from $400 af⁻¹ in May to $200 af⁻¹ in July [51]. These costs are much lower than the external market, which ranges from $1000 to $2000 af⁻¹ [54].

Farmers may pump groundwater into the canal for transfer to other users or to be purchased by AEWSD. The reliability of the AEWSD water supply has allowed farmers to switch from annual crops to more profitable perennial crops over time, mostly almonds and citrus, hardening their water demand, similar to trends throughout Kern County and in the Tulare Basin (figure S32).

Avra Valley (1400 km³) (figure 2(b)) evolved from a major agricultural area after 1940, a water supply that augmented local groundwater in Tucson in the 1990s, to a prominent reservoir for MAR after the mid-1990s. Additional details related to the Avra Valley system are provided in SI, section 5. Agriculture was supported by groundwater withdrawals that far exceeded annual recharge rates resulting in water-level declines of 1.3–1.4 m yr⁻¹ (figure S33). Groundwater levels declined markedly and then stabilized in the 1980s corresponding to a wet period. With the Arizona GW Law of 1980, Avra Valley and the adjacent Tucson Basin were grouped into the Tucson AMA. When the CAP aqueduct reached Tucson, the City of Tucson developed MAR systems, mostly in Avra Valley to store water. The City of Tucson operates two MAR facilities that infiltrate CAP water in central and southern Avra Valley Storage and Recovery Projects (CAVSARP, SAVSARP) (figure 2).

Simulated groundwater withdrawals in Avra Valley were as much as 0.18 km³ yr⁻¹ from ∼1940 to 1980, resulting in ∼3.5 km³ of storage depletion (figure S14(c)) [38]. Associated water-level declines were more than 30 m. Water levels and storage slowly recovered beginning in ∼1980 concurrent with a wet period that continued through the late 1990s (figures S14(c), S33 and S34). Storage recovery rates increased after about 2000 following the establishment of the two recharge facilities (CAVSARP and SAVSARP) and two smaller facilities in northern Avra Valley.

Artificial recharge of CAP water in Avra Valley began in 1996 at the CAVSARP facility (11 basins, 1.3 km² in area, 33 withdrawal wells) (figure 2). The SAVSARP facility began recharge operations in 2009 (9 basins, 0.9 km² in area, 11 withdrawal wells). Cumulative deliveries of CAP water for recharge in Avra Valley increased from ∼0.1 km³ before 2001 to ∼2.3 km³ by 2015 (∼65% to CAVSARP and SAVSARP) with an annual delivery rate of ∼0.2 km³ (figure S35), equivalent to the previous maximum annual groundwater withdrawal rate. Evaporation losses are low (≤1% of storage) and there is a 5% cut to the aquifer, resulting in 94% of deliveries being credited as long-term storage for future extraction. The City of Tucson plans to expand the CAVSARP facility storage capacity (Thompson, 2015).

There is adequate subsurface storage capacity based on naturally large depths to groundwater and past groundwater depletion in the southern Central Valley and in the Arizona AMAs to support future expansion of MAR. MAR is also being considered in the northern Central Valley, i.e. the Sacramento Valley. However, storage capacity is not as great (shallower water table depths, figure S6), soils are finer grained (figure S2), and surface water and groundwater are highly connected which could result in discharge of recharged groundwater as baseflow. Flood irrigation on fallow lands during winter periods is being evaluated to enhance recharge in this region and also in perennial crops during winter periods further south [24, 57, 58]. The similarity in recharge rates from irrigated agriculture and CU in Pinal AMA suggests that flood irrigation should be very effective in recharging aquifers. Inefficient flood irrigation based on surface water with efficient drip irrigation based on groundwater should increase groundwater recharge. The effectiveness and economic feasibility of different options will be critical in assessing various approaches.

Future planning will need to consider the general over-allocation of water resources in many of these regions and increasing allocations for the environment, such as the San Joaquin River Restoration program in the Central Valley.

In contrast to the California Central Valley where deliveries through the CVP and SWP are reduced during droughts (figure 5), CAP deliveries in Arizona have been maintained despite the long-term drought since the year 2000. However, future CAP deliveries are vulnerable because CAP has a junior water right entitlement among the lower basin states including Colorado River exports to California (SI, section 2) [59] and the CAP allocation would be reduced first [60]. If this happens, Arizona may revert to pumping groundwater to maintain water supplies. The existing models could be used to test different scenarios relative to their impacts on GWS. Other options are also being considered to cope with reduced water allocations during droughts [61].

Expansion of CU and MAR in California and Arizona needs additional water sources (figure 4). The California Dept. of Water Resources is mandated to determine how much additional water is available for groundwater replenishment under the California Sustainable Groundwater Management Act. Flood flows to the Pacific Ocean from California that exceed environmental flow requirements could provide additional water to support CU and MAR expansion. Preliminary estimates of such flood flows in excess of 170 m³ s⁻¹ (6 000 cfs) required for environmental flow quality standards from the California Delta based on data for December through March range from 0.6 to 2.0 km³ yr⁻¹ (figure 11, table S6). However, considering both the proposed Twin Tunnel project (part of the Bay Delta Conservation Plan) that would provide an additional 255 m³ s⁻¹ (9000 cfs) and the current limitations of the Delta pump capacity, these flows are restricted to ≤1.0 km³ yr⁻¹. Expansion of the San Luis Reservoir or building of a new surface reservoir could increase water availability for these transfers by up to 1.6 km³ yr⁻¹. In the Colorado River, surplus flows relative to the required allocation of 1.8 km³ yr⁻¹ to Mexico have also occurred in the wet periods of the early 1980s (up to 21 km³ yr⁻¹ in 1983) and also in 1993 (up to 6 km³ yr⁻¹) and 1997–1998 (up to 6 km³ yr⁻¹) (figure S36). The big question is how much infrastructure would be developed to support such episodic flows and what are the economics of such programs?

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While current practices in California and Arizona do not require operators to show hydrologic linkages between water inputs and outputs, future studies should move toward linking inputs and outputs in terms of upstream versus downstream and vertically relative to shallow and deep aquifers for infiltration and extraction. Large differences in head in wells screened at different depth intervals may indicate different degrees of confinement. For example, the City of Cottonwood has decided to use ASR rather than spreading basins for MAR because of confining layers between the surface and deeper extraction wells [62]. More detailed monitoring programs, including unsaturated zone monitoring, and modeling analyses could be used to assess the effectiveness of the CU and MAR programs.

The detailed performance data provided by CU and MAR systems in the southwestern US should provide data input for many other regions planning or advancing beyond 'spontaneous' CU [17] to more rigorous planned management. The complementary nature of surface water and GWS should be relevant to approaches to harvesting flood flows in different regions for drought management [63]. The relative merits of CU and MAR within the context of droughts and floods could be used in regions with proposed or built large scale inter-basin transfers [64], e.g. the South to North Water Transfer from the Yangtze River to the North China Plain [65, 66], and proposed east to west linkages in India [67].