{ "id": "R40979", "type": "CRS Report", "typeId": "REPORTS", "number": "R40979", "active": true, "source": "EveryCRSReport.com, University of North Texas Libraries Government Documents Department", "versions": [ { "source": "EveryCRSReport.com", "id": 444330, "date": "2015-08-14", "retrieved": "2016-04-06T18:36:39.134717", "title": "California Drought: Hydrological and Regulatory Water Supply Issues", "summary": "Congressional Research Service\n7-5700\nwww.crs.gov\nR40979\nSummary\nCalifornia is in its fourth year of drought. As of late July 2015, 47% of California was experiencing exceptional drought\u2014the most severe U.S. federal drought classification\u2014and approximately 71% of the state was experiencing extreme drought. The 2014 water year (October 2013 through September 2014) was the third driest on record. Water users that receive water supplies from the state of California and federal water projects are experiencing unprecedented water supply shortages due to the drought. Severe water supply shortages also hampered the state during a recent three-year drought (2008-2010). Paleontological and tree-ring records indicate that California has experienced many multiyear droughts over several millennia; however, some experts estimate that the current drought may be the most severe in the past 1,200 years. \nThis report focuses on hydrological and regulatory compliance issues that affect operation and management of two large water supply projects that serve farms and communities throughout California: the federal Central Valley Project (CVP), owned and operated by the Bureau of Reclamation (Reclamation) in the Department of the Interior, and the State Water Project (SWP), owned and operated by the California Department of Water Resources (DWR). Reductions in water supplied by these projects in drought years result in economic disruption across the state such as concentrated crop and financial losses in agricultural areas throughout the Central Valley, including portions of the San Joaquin Valley. At the same time, several fish species\u2014one of which may be close to extinction\u2014whose habitat lies at the heart of California\u2019s water supply system and throughout its northern rivers are in decline. Declining fish species, exacerbated by drought, also may have economic implications, resulting in job and income losses in coastal areas. In addition, the drought and low water supplies affect recreation, power production, other industries, and small and large communities.\nWith below-average snowpack and severe water shortages again in 2015, a short-term issue for Congress is how to respond to demands for increased water deliveries given other policy concerns, such as avoiding harm to threatened and endangered species. Other issues include how to address water supply availability broadly and how to finance water supply or other projects. These are challenges given fiscal constraints and policies discouraging site-specific project authorizations and appropriations. A long-term issue for Congress is how to address federal water delivery reliability without destabilizing the aquatic ecosystems and related resources (e.g., clean water, commercial and recreational fisheries, etc.) upon which many communities depend. \nSeveral bills were introduced in the 113th Congress to address short- and long-term water shortage issues and CVP and SWP water management (e.g., H.R. 3964, H.R. 5781, and S. 2198). These bills each passed their respective chambers but were not enacted. Legislation with similar aims\u2014to maximize CVP and SWP water supplies\u2014has been introduced in the 114th Congress (H.R. 2898 and S. 1894). H.R. 2898 passed the House on July 16, 2015, and S. 1894 was introduced on July 30, 2015. Other bills introduced in the 114th Congress that address California drought and water management include several authorizing conservation and water H.R. 291 recycling initiatives, among other activities (e.g., and S. 176; H.R. 2983 and S. 1837; and H.R. 3045). \nQuestions and debate are likely to continue as Congress considers legislation addressing CVP and SWP management and other activities to lessen the impacts of drought. This report provides background and analysis on factors affecting CVP and SWP water management. For information on legislation, see CRS In Focus IF10019, H.R. 5781: Legislation Proposed to Maximize Water Supplies to Address Drought in California; CRS Report R43820, Analysis of H.R. 5781, California Emergency Drought Relief Act of 2014; and CRS Insight IN10308, Drought Legislation: H.R. 2898.\nContents\nIntroduction\t1\nWhat Is Drought?\t6\nCalifornia Drought and Hydrological Conditions\t8\nRunoff and Storage\t10\nTiming\t13\nNature of the California Drought: Possible Causes and Consequences\t14\nAtmospheric Rivers\t15\nProspects for a Continuing Hydrologic Drought\t16\nGroundwater and Land Subsidence\t16\nCalifornia\u2019s Drought and CVP Operations\t22\nRegulatory and Statutory Restrictions Affecting CVP and SWP Water Deliveries\t23\nLimitations on Delta Pumping\t24\nEffects of Regulatory and Statutory Restrictions\t28\nRecent Delta Export Reductions: Selected Examples\t28\nHow Do Recent Exports Compare to Previous Droughts?\t33\nCalifornia Water Rights: Acquisitions and Allocations\t34\nWater Rights and Allocations for Water Delivered via the CVP\t35\nIssues for Congress\t39\nOutlook\t41\n\nFigures\nFigure 1. Sacramento-San Joaquin Delta\t3\nFigure 2. Major Rivers and Water Infrastructure Facilities in California\t4\nFigure 3. U.S. Drought Conditions\t8\nFigure 4. Comparison of California Drought Conditions for April 2014 and April 2015\t10\nFigure 5. Major California State and Federal Reservoirs\t12\nFigure 6. Comparison of Runoff Amounts for California Between a Wet Year, Dry Year, 30-Year Average, 2014, and 2015 Through June\t13\nFigure 7. Land Subsidence in the San Joaquin Valley Southwest of Mendota Between 1925 and 1977\t17\nFigure 8. Percentage of Wells with Groundwater Levels at or Below Historical Lows\t19\nFigure 9. Estimated Potential for Future Land Subsidence in California\t20\nFigure 10. CVP and SWP Delta Water Exports, 1976-2014\t32\nFigure 11. CVP and SWP Service Areas\t36\n\nTables\nTable 1. Average and Observed Statewide Precipitation, by Month\t14\nTable 2. Central Valley Project (CVP) and State Water Project (SWP) Delta Exports, 1976-2014\t31\nTable 3. CVP Contractors and 2015 Water Allocations\t37\nTable 4. CVP Contractors and 2009 Water Allocations\t38\n\nContacts\nAuthor Contact Information\t42\nAcknowledgments\t42\n\nIntroduction\nCalifornia is experiencing its fourth consecutive year of drought. As of late July 2015, 47% of California was experiencing exceptional drought and more than 94% of the state was facing drought conditions that ranged from severe to exceptional. According to the National Oceanic and Atmospheric Administration\u2019s (NOAA\u2019s) Climate Prediction Center (CPC), the outlook for California through the end of October calls for little if any chance for improvement in drought conditions. Unless there is an anomalous weather event, California likely will experience persistent drought conditions for the rest of the 2015 water year (October 2014-September 2015).\nThe Congressional Research Service (CRS) has analyzed a variety of data and information on current California hydrological conditions, regulatory factors affecting management of California\u2019s developed water supplies, and restrictions due to baseline water rights allocations and delivery priorities of the Central Valley Project (CVP), a large water supply project that serves farms and communities throughout California. This report provides a summary of California\u2019s 2012-2015 drought with comparisons, where applicable, to previous droughts; a summary of the key regulatory requirements that at certain times limit water deliveries (or exports) from the San Joaquin and Sacramento Rivers\u2019 Delta; and a brief discussion of California water rights and how they relate to different types of federal contracts and their associated water allocations. \nThe report specifically addresses issues related to management of the U.S. Bureau of Reclamation\u2019s (Reclamation\u2019s) CVP and to reductions in water deliveries to CVP water users\u2014specifically, to irrigation districts, water districts, and others that have long-term contracts for delivery of CVP water (i.e., CVP contractors). The CVP delivers water to contractors throughout the state, largely serving agricultural water contractors as well as some municipal and industrial (M&I) contractors. A somewhat parallel state system, the State Water Project (SWP) serves primarily M&I water users and some agricultural users. The CVP and SWP are operated in conjunction under a Coordinated Operations Agreement (COA) pursuant to P.L. 99-546. \nMajor CVP and SWP pumps that supply water for central and southern California are located at the southern portion of the Sacramento and San Joaquin Rivers\u2019 Delta confluence with San Francisco Bay (Bay-Delta, or Delta, see Figure 1). An estimated 25 million people get some, if not all, of their drinking and agricultural water supplies from the Bay-Delta\u2014often referred to as the hub of California\u2019s water supply system. Figure 2 shows an overview of CVP and SWP facilities statewide. \nMany factors affect CVP water deliveries. These include hydrologic factors, state and federal laws and regulations, court orders implementing those laws and regulations, CVP contract and allocations policies, and the state\u2019s long-established state water rights system. The current hydrological drought in California (2012-present) has lasted longer than the previous California drought (2007-2009) and probably is more severe. For example, the 2014 water year (October 2013 through September 2014) was the third driest in California since record-keeping began in 1895, and 2013 was the driest calendar year on record for the state. The human factors\u2014laws and regulations, water rights, and CVP allocations policies\u2014can magnify the impacts of drought on water deliveries to some contractors. The system of state water rights and water code, together with CVP contract allocations and priorities, largely dictates who gets how much water and when, particularly in times of drought or other changes in the hydrologic cycle. As with the previous California drought, some stakeholders are questioning the extent to which compliance with environmental laws such as the federal Endangered Species Act (ESA; P.L. 93-205; 16 U.S.C. \u00a7\u00a71531 et seq.) and with state water quality laws and regulations are worsening drought impacts and resulting in significantly reduced project water deliveries\u2014particularly CVP and SWP exports of water from the Delta.\nFigure 1. Sacramento-San Joaquin Delta\n(with CVP and SWP pumping plants)\n\nSource: DeltaRevision, Delta Maps, at http://www.deltarevision.com/maps/Delta_land_use_ownership/legal_delta.jpg, 1990. \nNotes: State and federal facilities that pump and export water out of the Delta for users south of the Delta are located on the southwestern edge of the legally defined Delta area. CVP = Central Valley Project; SWP = State Water Project.\nFigure 2. Major Rivers and Water Infrastructure Facilities in California\n(state, federal, and local water project facilities)\n\nSource: California State Department of Water Resources, California Water Plan Update 2013, Investing in Innovation & Infrastructure, Vol. I, Chapter 3, Figure 2-3, issued Oct. 30, 2014.\nWater shortages due to hydrologic variability, regulatory export, and other management restrictions have resulted in unequal impacts on CVP water contractors because of differences in priority of water rights and other legal agreements underlying different water contracts and federal and state allocation policies. Combined Delta exports have increased on average since the 1980s and early 1990s, even with implementation of several regulatory restrictions; nonetheless, CVP water allocations for some contractors have been reduced significantly.\nControversy over CVP water supply deliveries persists in part because even in years with high levels of precipitation and runoff, such as 2011, water deliveries still are often reduced below their contract amount for some contractors\u2014typically south-of-Delta (SOD) CVP water service contractors. Due to low precipitation levels during the 2011-2012 winter months (2012 water year), and again throughout the 2012 and 2013 calendar years, water storage levels leading into the 2013-2014 winter were very low. The result was a low amount of water in reserve going into the spring of 2014\u2014an unusually warm year with below-average precipitation. CVP and SWP contract deliveries were allocated at historic lows in 2014, and even senior water rights users were affected. Some municipalities nearly exhausted water supplies, and some unincorporated areas experienced wells going dry. Historically low levels of winter precipitation and snowpack for the 2015 water year have resulted in reduced water allocations again, including zero allocations for many agricultural water users throughout the CVP. Some communities reliant on groundwater also have experienced dry wells.\nThe California State Water Resources Control Board (SWRCB) summarized 2014 drought impacts as follows:\n$2.2 billion in total economic costs due to drought;\n$1.5 billion direct loss to agriculture (3% of the state\u2019s total agricultural production value);\n17,100 jobs lost (3.8% of farm employment);\n428,000 acres idled (5%) in the Central Valley, Central Coast, and Southern California;\n$447 million in added groundwater pumping costs;\n95% winter-run Chinook egg mortality due to high temperatures;\nhigh spring-run Chinook mortality due to high temperatures;\nlowest survey level of Delta smelt on record;\nsecond-lowest level of longfin smelt on record;\npopulation indexes of the striped bass, American shad, and threadfin shad near record lows; and\npotential (unaccounted) impacts to commercial and recreational fishing, recreation, and other nonagricultural water-dependent industries.\nThe 113th Congress responded to the 2014 drought by reauthorizing several drought programs, including the Reclamation States Emergency Drought Relief Act (RSEDR), the National Integrated Drought Information System (NIDIS), and agricultural assistance programs (2014 farm bill; Agricultural Act of 2014 [P.L. 113-79]). Congress also included provisions to facilitate water banking, water transfers, and new storage projects in the FY2014 Consolidated Appropriations Act (P.L. 113-76). In addition, the 113th Congress debated California-specific legislation, including S. 2016, S. 2198 (which passed the Senate in May 2014), H.R. 3964 (which passed the House in February 2014), and H.R. 5781, a compromise bill that passed the House in December 2014; however, none were enacted. \nSeveral bills have been introduced in the 114th Congress. For example, H.R. 2898 passed the House on July 16, 2015. The bill is similar in several aspects to previously passed House bills (H.R. 3964 and 5781 from the 113th Congress). Several titles of H.R. 2898 focus on maximizing CVP and SWP water deliveries, while other titles address Bureau of Reclamation project authorization and financing throughout the West. With regard to California-specific provisions, a key challenge for legislators is whether to increase water supplies for CVP and SWP water users, particularly those in the San Joaquin Valley and Southern California areas (SOD), and how this could be accomplished without further threatening or endangering the survival of several fish species and degrading water quality for in-Delta water users. Other bills introduced in the 114th Congress would address drought management in California more broadly by focusing on increasing the provision of water supplies through conservation and recycling, among other activities (e.g., H.R. 291 and S. 176; H.R. 2983 and S. 1837; and H.R. 3045). The state also has been active in addressing the drought, including funding specific water projects and conservation activities and calling for mandatory statewide reductions in water use. \nWhat Is Drought?\nDroughts have affected the United States, particularly the American West, for centuries. Drought is defined in a number of ways; the simplest may be as a deficiency of precipitation over an extended period of time, usually a season or more, resulting in a water shortage for some activity, group, or environmental sector. The deficiency usually is evaluated relative to some long-term average condition or balance between precipitation, evaporation, and transpiration by plants. Drought, which has a beginning and an end, is distinguished from aridity, which describes to low-rainfall regions and is a relatively permanent feature of climate (e.g., deserts are regions of relatively permanent aridity). \nAt the national level, drought is monitored and reported in an index known as the U.S. Drought Monitor, which synthesizes various drought indexes and impacts and represents a consensus among academic and federal scientists of ongoing drought conditions. The U.S. Drought Monitor uses five key indicators as well as expert opinion and other indexes, such as those that account for conditions in the West where snowpack is relatively important and those that are used mainly during the growing season. Drought indexes typically are used to assess and classify the intensity and type of drought. The classification of drought intensity, such as that shown in Figure 3, may depend on a single indicator or on several indicators, often combined with expert opinion from the academic, public, and private sectors. \nThe U.S. Drought Monitor intensity scheme\u2014D0 to D4\u2014is used to depict broad-scale conditions but not necessarily drought circumstances at the local scale. For example, the large regions depicted as red and brown, most notably California and parts of Nevada, in Figure 3 faced extreme to exceptional drought conditions for the week of July 28, 2015, but they may contain local areas and individual communities that experienced less (or more) severe drought. The U.S. Drought Monitor in Figure 3 does not reflect availability of water supplied by federal or state reservoirs or projected water deliveries. It strictly represents the hydrological status of California from factors other than deliveries of water mandated or restricted by regulation (e.g., precipitation, snowpack, streamflow, soil conditions, as described above). In addition to the color-coded D0-D4 designations, U.S. Drought Monitor maps often include an S and L designation to provide additional information about the nature of drought. The S designation indicates a combination of drought indexes that reflect impacts that respond to precipitation over several days up to a few months (short-term effects). These effects would include impact to agriculture, topsoil moisture, unregulated streamflows, and aspects of wildfire danger. The L designation approximates responses to precipitation over several months up to a few years (long-term effects). These effects would include reservoir levels, groundwater, and lake levels. As Figure 3 shows, some regions of the United States, such as California, include both an S and an L designation, indicating that in late July 2015 those regions experienced both short- and long-term impacts.\nFigure 3. U.S. Drought Conditions\n/\nSource: United States Drought Monitor, at http://droughtmonitor.unl.edu/.\nCalifornia Drought and Hydrological Conditions\nAt the beginning of the 2015 water year\u2014as of October 1, 2014\u2014nearly 60% of California was experiencing exceptional drought, the most severe U.S. federal drought classification. The dry conditions resulted, in part, because the 2014 water year was the third driest on record in terms of precipitation (2013 was the driest calendar year on record, whereas the 1924 water year was the driest on record). California typically receives more than 80% of its annual precipitation in the November-April winter precipitation season, and the November 2013-April 2014 precipitation for the state as a whole was the sixth driest since records began in 1895. \nThe state is experiencing its fourth consecutive dry year. As of July 28, 2015, 47% of California still was experiencing exceptional drought and more than 94% of the state was facing drought conditions that ranged from severe to exceptional. \nWater users receiving supplies from California state and federal water projects are experiencing unprecedented water supply shortages due to the drought. Severe water supply shortages also hampered the state during the 2007-2009 drought. Other severe droughts in the last 100 years include a six-year drought (1987\u20131992); a two-year drought (1976\u20131977), and an extended dry period during the 1920s and 1930s which included the single driest water year on record\u20141924. (The driest winter on record was 1976-1977.) Studies of relict tree stumps, tree rings, and other evidence indicate that parts of California have experienced numerous multiyear droughts, some of which lasted for decades or even centuries, during the past 2,000 years. Whether the current drought is the worst in California\u2019s history is currently being debated. The role of human-induced climate change also is being debated, in particular whether the record high temperatures in 2014 combined with low precipitation are a fingerprint of human influence on climate (see text box below).\nFigure 4 provides a comparison of drought conditions in California on April 29, 2014, and April 28, 2015. Figure 4 shows that the area of exceptional drought expanded considerably over the course of the year. According to the U.S. Drought Monitor, exceptional drought nearly doubled in a year, from 25% of California at the end of April 2014 to nearly 47% in late April 2015.\nFigure 4. Comparison of California Drought Conditions for April 2014 and April 2015\n\nSource: U.S. Drought Monitor, http://droughtmonitor.unl.edu/MapsAndData/MapArchive.aspx, modified by CRS.\nNotes: Colors matching drought intensity are identical to those shown in Figure 3.\nCalifornia has experienced years of consecutive drought in the past. Observations of below-average runoff, reservoir levels, and groundwater levels for 2007-2009 were broadly comparable to those observed during previous episodes of drought in California (e.g., 1977-1978 and 1987-1992). Drought conditions for 2015 appear more severe and widespread in scope.\nRunoff and Storage\nThe 2014 water year was one of the driest on record, which has led to below-average storage levels and runoff for the year. According to the California Department of Water Resources (DWR):\nWater Year 2014\u2014overlapping with California\u2019s driest calendar year\u2014ended on September 30 as the state\u2019s third driest in 119 years of record, based on statewide precipitation. \nAs the Water Year ended on September 30, the state\u2019s major reservoirs collectively held only 60 percent of average storage for the date, or about 41 percent of capacity. Cumulative reservoir storage in 1977, California\u2019s driest calendar year on record, was approximately five million acre-feet less than this year, but the state had 16 million fewer people then.\nRunoff in California has declined since 2012 and is continuing its downward trend in 2015. According to the U.S. Geological Survey, runoff in California has shown a downward trend since 2006, despite relatively wet years in 2010 and 2011. Below-average runoff indicates an underlying deficit in precipitation, which would support a common definition of drought: less rain or snow than a region would receive compared with some long-term average (consistent with the description of hydrological drought; see \u201cWhat Is Drought?\u201d above). California has experienced three dry years in a row, and so far 2015 is a fourth dry year, compared with the long-term average.\nFigure 5 shows reservoir storage in early August 2015 for several of the major CVP and SWP water storage reservoirs. All the major reservoirs shown in Figure 5 are well below their historical average capacity. \nSnowpack is an essential component of water storage in California. Runoff from the snowpack typically contributes to a large portion of storage in many of the reservoirs shown in Figure 5. For a year with average or above-average snowpack levels in early May, runoff from spring snowmelt would be expected to contribute additional water storage in reservoirs through May and June before dropping off in the summer months. Figure 6 shows this pattern for a wet year (1983), a dry year (1977), the 30-year average, and for 2014 and 2015 (through June 2015). However, the very low snowpack levels in spring 2015 resulted in low amounts of runoff, compared with average values, in early summer. As the figure shows, June runoff in 2015 was about 0.15 inches and trending downward, whereas the 30-year average runoff for June is 0.54 inches. That value likely reflects the unusually low snowpack levels in the Sierras in 2015 (approximately 5% of average for April). Absent the unlikely event of significant amounts of summer precipitation, the trend of below-average runoff shown in Figure 6 probably will continue.\nFigure 5. Major California State and Federal Reservoirs\n\nSource: California Department of Water Resources, California Data Exchange Center, Current Conditions for Major Reservoirs, http://cdec.water.ca.gov/cgi-progs/products/rescond.pdf.\nFigure 6. Comparison of Runoff Amounts for California Between a Wet Year, Dry Year, 30-Year Average, 2014, and 2015 Through June\n\nSource: U.S. Geological Survey (USGS), California Water Science Center, Annual Runoff Estimate for California, at http://ca.water.usgs.gov/data/drought/runoff.html, viewed August 4, 2015.\nNotes: The original source is an interactive feature, enabling the user to scroll over the lines on the figure and extract runoff amounts in inches for the midpoint of each month. All 2015 data are provisional and subject to change.\nTiming\nPersistently dry or drought conditions in most of California since 2012 do not necessarily mean that all locations throughout California have experienced similar drought conditions at all times. Drought conditions change over time and vary by location. For example, January is normally the wettest month for California, averaging approximately 4.1 inches of precipitation in the state. (See Table 1.) In January 2015, however, California received only 0.47 inches, or 12% of average precipitation for the month. By contrast, in December\u2014also one of the wettest months for California\u2014the state received 7.3 inches of precipitation, or 202% of the monthly average of about 3.6 inches. Despite the relatively wet months of November and December 2014, very dry conditions in January and below-average precipitation through April resulted in the cumulative average of annual precipitation at 76% through the first seven months of the water year. During the months of October through March, California typically receives more than 80% of its annual precipitation for the 12-month water year. \nTable 1 also shows that July 2015 was an unusually wet month in California, receiving roughly 10 times the average amount of precipitation that normally falls during the month. However, the above-average rainfall likely will not affect drought conditions significantly. Even though California received more than 1.6 inches of precipitation averaged over the state, well above the normal 0.16 inches, the total amount that fell was less than what the state usually receives each month from November through March, and the total was equal to about the average precipitation California receives in April at the end of the wet season (Table 1). Despite the relatively very wet July, the first 10 months of the 2015 water year cumulatively were still below average (about 83% of normal), reflecting the dry winter months in early 2015. \nTiming of precipitation, and the consequent amount of runoff in the spring that flows into the state\u2019s reservoirs, is a critical factor leading to water delivery decisions in the spring. Where and how precipitation occurs (e.g., snow versus rain) also are critical to water delivery decisions for the SWP and CVP. Both projects rely on precipitation data, including data indicating the water content of snowpack and projected runoff, to decide how much water to allocate to water users early in the water year (February-May).\nTable 1. Average and Observed Statewide Precipitation, by Month\n(percentage of average by month and cumulatively for water year 2015, through July 31, 2015)\nMonth\nAverage Precipitation Statewide (inches)\nWater Year 2015 Observed Precipitation (inches)\n% of Average (by month)\n% of Average (cumulative)\n\nOctober\n1.16\n0.71\n61%\n61%\n\nNovember\n2.40\n2.0\n82%\n75%\n\nDecember\n3.60\n7.3\n202%\n139%\n\nJanuary\n4.08\n0.47\n12%\n93%\n\nFebruary\n3.58\n2.7\n75%\n88%\n\nMarch\n3.03\n0.55\n18%\n77%\n\nApril\n1.59\n1.1\n69%\n76%\n\nMay\n0.86\n0.77\n88%\n76%\n\nJune\n0.34\n0.21\n62%\n76%\n\nJuly\n0.16\n1.63\n1051%\n83%\n\nTotal \n20.82\n17.36\n83%\n83%\n\nSource: California Climate Tracker, at http://www.wrcc.dri.edu/monitor/cal-mon/frames_data.html. CRS calculated the average monthly values from using California Climate Tracker data back to 1895. \nNote: Totals may not sum due to rounding.\nNature of the California Drought: Possible Causes and Consequences\nThe immediate cause of the current California drought appears to be the region of persistent atmospheric high pressure over the northeast Pacific Ocean offshore Oregon and Washington. The resilient ridge, as it is sometimes referred to, has changed the atmospheric circulation patterns so that the wintertime streams of intense precipitation are blocked from reaching the northern California coastline. These weather features are referred to as atmospheric rivers (ARs) and are extremely important contributors to California\u2019s precipitation during the winter months. Some shifts in the high-pressure ridge in December 2014 led to a few AR storms in northern California, as discussed above, but since that time the resilient ridge has reestablished itself.\nAtmospheric Rivers\nARs play an extremely important role in making or \u201cbusting\u201d California droughts. The absence of ARs in January 2014 led to precipitation levels of only 0.43 inches statewide, or 11% of average for the month. January 2015 precipitation was 0.47 inches statewide, nearly mirroring the previous year. Again, the absence of ARs was in part responsible for the lack of significant precipitation during the crucial month of January. Further, some have speculated that up to half of California droughts are busted by ARs that provide a significant proportion of annual precipitation over a relatively short period. However, from May through September, California typically receives less than an inch of precipitation statewide per month, so the occurrence of a drought-busting AR in the 2015 water year is increasingly unlikely.\nDid Human-Induced Climate Change Cause the 2012-2015 California Drought?\nTwo studies published in late 2014 provide examples of the ongoing scientific discussion about whether emissions of heat-trapping greenhouse gases by human activities have influenced the current California drought. One study claimed that the current drought is the most severe in the past 1,200 years of California\u2019s history. The study stated that diminished snowpack, streamflows, and reservoir levels resulted in a convergence of reduced surface water supply with increased demand, a combination that appears unique in the state\u2019s history. The study stated that 2014 was the worst single drought year in at least the last 1,200 years in California but that it was not the driest year. What made the 2012-2014 drought\u2014and 2014 in particular\u2014stand out was the combination of lack of precipitation and record high temperatures, resulting in extreme dryness according to a soil-moisture metric known as the Palmer Drought Severity Index. Attributing a human influence is more tenuous, according to the study, as attribution of a human influence on California rainfall and Pacific storm tracks is equivocal. The study, however, stated that \u201cprojections for a continued trend toward higher mean and extreme temperatures are robust.\u201d Further, the study linked the future warming to human activities, claiming that \u201cfuture hot\u2019 droughts driven by increasing temperatures due to anthropogenic emissions of greenhouse gases ... are assured.\u201d \nAnother study also published in 2014 found that the 2012-2014 dry conditions were not without precedent in California\u2019s history. Even with the current drought, the study did not find a clear trend toward wetter or drier conditions over the past 120 years in California. It noted that the impacts of lack of precipitation were exacerbated by warm temperatures and that November 2013 through April 2014 was the warmest winter half-year on record. 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