California precipitation varies more dramatically from year to year than elsewhere in the conterminous United States. This paper analyzes the extent to which contributions of the wettest days to overall precipitation dictate the state’s precipitation seasonality and frequent multiyear periods of drought (as precipitation deficit) and plenty is analyzed, historically and in projections of future climates. The wettest 5% of wet days in California contribute about a third of precipitation but about two-thirds of the variance of water-year precipitation. Year-to-year fluctuations in precipitation strongly reflect year-to-year fluctuations of contributions from the largest storms, with the large-storm contributions explaining about twice as much precipitation fluctuation as do contributions from all remaining storms combined. This extreme dominance of large storms is largely unique to California within the United States. In climate-change projections, eight of ten climate models considered here yield increases in precipitation from the largest storms, and when the increases are large, total precipitation follows suit. All of the models project declines in contributions from the smaller storms and models projecting total-precipitation declines reflect this decline. Projected changes in variance of water-year precipitation reflect changes in variance of large-storm contributions. The disproportionately large overall contributions from California’s largest storms, and their outsized year-to-year variability, ensure that the state’s largest storms dictate the state’s regimes of wet and dry spells, historically and in climate-change projections.

The uncertainties associated with climate-change projections for California are unlikely to disappear any time soon, and yet important long-term decisions will be needed to accommodate those potential changes. Projection uncertainties have typically been addressed by analysis of a few scenarios, chosen based on availability or to capture the extreme cases among available projections. However, by focusing on more common projections rather than the most extreme projections (using a new resampling method), new insights into current projections emerge: (1) uncertainties associated with future greenhouse-gas emissions are comparable with the differences among climate models, so that neither source of uncertainties should be neglected or underrepresented; (2) twenty-first century temperature projections spread more, overall, than do precipitation scenarios; (3) projections of extremely wet futures for California are true outliers among current projections; and (4) current projections that are warmest tend, overall, to yield a moderately drier California, while the cooler projections yield a somewhat wetter future. The resampling approach applied in this paper also provides a natural opportunity to objectively incorporate measures of model skill and the likelihoods of various emission scenarios into future assessments.

Current projections of climate change present a number of challenges to scientists and decision-makers. The projections predict a twenty-first-century climate in which many climate variables are likely to trend across broad geographical areas and at rates that are rapid by historical standards. The projections of change are likely to remain uncertain for many years to come, and complete surprises are possible. Responses to these changes will have to span large areas and many variables, and impacts will interact in complex ways. In the face of these challenges, we offer recommendations as to strategic approaches that the CALFED Science Program—which serves here as an important and illustrative example from among the many current scientific resource- and ecosystem-management programs—and the scientific and public-policy communities in central California, in general, may need to pursue. Recommended strategies include emphasis on long-term eco- and resource-system adaptability—rather than historical verisimilitude—in its restoration targets; major commitments to long-term monitoring of restoration and impacts; even more integration across scientific disciplines, observations, models, and across the study area; increased use of manipulative experiments; and recognition that climate-change issues must be addressed in all efforts undertaken by the program.

The extent to which winter precipitation is orographically enhanced within the Sierra Nevada of California varies from storm to storm, and season to season, from occasions when precipitation rates at low and high altitudes are almost the same to instances when precipitation rates at middle elevations ( considered here) can be as much as 30 times more than at the base of the range. Analyses of large-scale conditions associated with orographic precipitation variations during storms and seasons from 1954 to 1999 show that strongly orographic storms most commonly have winds that transport water vapor across the range from a more nearly westerly direction than during less orographic storms and than during the largest overall storms, and generally the strongly orographic storms are less convectively stable. Strongly orographic conditions often follow heavy precipitation events because both of these wind conditions are present in midlatitude cyclones that form the cores of many Sierra Nevada storms. Storms during La Nina winters tend to yield larger orographic ratios (ORs) than do those during El Ninos. A simple experiment with a model of streamflows from a river basin draining the central Sierra Nevada indicates that, for a fixed overall basin-precipitation amount, a decrease in OR contributes to larger winter flood peaks and smaller springtime flows, and thus to an overall hastening of the runoff season.

The State of Bay–Delta Science 2008 highlighted seven emerging perspectives on science and management of the Delta. These perspectives had important effects on policy and legislation concerning management of the Delta ecosystem and water exports. From the collection of papers that make up the State of Bay–Delta Science 2016, we derive another seven perspectives that augment those published in 2008. The new perspectives address nutrient and contaminant concentrations in Delta waters, the failure of the Delta food web to support native species, the role of multiple stressors in driving species toward extinction, and the emerging importance of extreme events in driving change in the ecosystem and the water supply.

The scientific advances that underpin these new perspectives were made possible by new measurement and analytic tools. We briefly discuss some of these, including miniaturized acoustic fish tags, sensors for monitoring of water quality, analytic techniques for disaggregating complex contaminant mixtures, remote sensing to assess levee vulnerability, and multidimensional hydrodynamic modeling.

Despite these new tools and scientific insights, species conservation objectives for the Delta are not being met. We believe that this lack of progress stems in part from the fact that science and policy do not incorporate sufficiently long-term perspectives. Looking forward half a century was central to the Delta Visioning process, but science and policy have not embraced this conceptual breadth. We are also concerned that protection and enhancement of the unique cultural, recreational, natural resource, and agricultural values of the Delta as an evolving place, as required by the Delta Reform Act, has received no critical study and analysis. Adopting wider and longer science and policy perspectives immediately encourages recognition of the need for evaluation, analysis, and public discourse on novel conservation approaches. These longer and wider perspectives also encourage more attention to the opportunities provided by heavily invaded ecosystems. It is past time to turn scientific and policy attention to these issues.

The State of Bay–Delta Science 2016 (SBDS) is a collection of papers that summarizes the scientific understanding of the Sacramento–San Joaquin Delta, emphasizing progress made during the past decade.It builds on the first SBDS edition (Healey et al. 2008). Paper topics for this edition address the most relevant scientific issues in the Delta identified by senior scientists and managers. The topical papers cover issues ranging from contaminants in the Delta to levee stability, and from Delta food webs to recent discoveries about salmon migration. These papers are written for a scientific audience. Two additional papers, one describing the challenges of managing water and ecosystems in the Delta and another that discusses policy implications of the recent scientific findings, are written for a general audience. The papers will be published in at least two issues of San Francisco Estuary and Watershed Science and will be available as a set electronically.

During the past year, an investigation has been initiated regarding how future climate changes may impact the hydrology of the Tijuana River Watershed – a binational watershed. The study has used gridded observed daily precipitation and temperatures and downscaled daily precipitation and temperature projections from three global climate models (GCM) to drive the VIC macroscale hydrologic model. Sensitivity analysis using VIC suggests about 2% reduction of runoff for each 1% reduction in precipitation. A 1oC increase in average temperature produces about 3% reduction of runoff. All three GCM simulations yield annual warming, with end-of-century temperature increases from approximately +1oC under a lower emission scenario in the less responsive PCM1 to +3oC in a higher emission scenario with the more responsive GFDL model. Climate projections suggest greater warming in the spring and summer months ranging between 2oC to 3oC under the higher emission scenario. Two of the three GCM simulations yield more frequent summer drying as gauged by VIC simulated soil moisture in the twenty-first century under the higher emission scenario. Summer soil moisture declines most, and most rapidly, in the later part of the twenty-first century. This initial evaluation provides perhaps the first direct estimates of the climate change impacts on the Tijuana River Watershed. However, transforming these results into a more useable projections and impacts will be a task for the future and will require collaboration and interaction between local stakeholders and the researchers.