Increased Interannual Precipitation Extremes over California under Climate Change

American Meteorological Society (AMS) | August 10th, 2015

Changes to mean and extreme wet season precipitation over California on interannual time scales are analyzed using twenty-first-century precipitation data from 34

Changes to mean and extreme wet season precipitation over California on interannual time scales are analyzed using twenty-first-century precipitation data from 34 global climate models. Models disagree on the sign of projected changes in mean precipitation, although in most models the change is very small compared to historical and simulated levels of interannual variability. For the 2020/21–2059/60 period, there is no projected increase in the frequency of extremely dry wet seasons in the ensemble mean. Wet extremes are found to increase to around 2 times the historical frequency, which is statistically significant at the 95% level. Stronger signals emerge in the 2060/61–2099/2100 period. Across all models, extremely dry wet seasons are roughly 1.5 to 2 times more common, and wet extremes generally triple in their historical frequency (statistically significant). Large increases in precipitation variability in most models account for the modest increases to dry extremes. Increases in the frequency of wet extremes can be ascribed to equal contributions from increased variability and increases to the mean. These increases in the frequency of interannual precipitation extremes will create severe water management problems in a region where coping with large interannual variability in precipitation is already a challenge. Evidence from models and observations is examined to understand the causes of the low precipitation associated with the 2013/14 drought in California. These lines of evidence all strongly indicate that the low 2013/14 wet season precipitation total can be very likely attributed to natural variability, in spite of the projected future changes in extremes.

Increasing precipitation volatility in twenty-first-century California

Nature Portfolio (Springer Nature) | April 23rd, 2018

Mediterranean climate regimes are particularly susceptible to rapid shifts between drought and flood—of which, California’s rapid transition from record multi-yea

Mediterranean climate regimes are particularly susceptible to rapid shifts between drought and flood—of which, California’s rapid transition from record multi-year dryness between 2012 and 2016 to extreme wetness during the 2016–2017 winter provides a dramatic example. Projected future changes in such dry-to-wet events, however, remain inadequately quantified, which we investigate here using the Community Earth System Model Large Ensemble of climate model simulations. Anthropogenic forcing is found to yield large twenty-first-century increases in the frequency of wet extremes, including a more than threefold increase in sub-seasonal events comparable to California’s ‘Great Flood of 1862’. Smaller but statistically robust increases in dry extremes are also apparent. As a consequence, a 25% to 100% increase in extreme dry-to-wet precipitation events is projected, despite only modest changes in mean precipitation. Such hydrological cycle intensification would seriously challenge California’s existing water storage, conveyance and flood control infrastructure.

Inferring efficient operating rules in multireservoir water resource systems: A review

Wires Water (Wiley) | November 20th, 2019

Coordinated and efficient operation of water resource systems becomes essential to deal with growing demands and uncertain resources in water‐stressed regions. System a

Coordinated and efficient operation of water resource systems becomes essential to deal with growing demands and uncertain resources in water‐stressed regions. System analysis models and tools help address the complexities of multireservoir systems when defining operating rules. This paper reviews the state of the art in developing operating rules for multireservoir water resource systems, focusing on efficient system operation. This review focuses on how optimal operating rules can be derived and represented. Advantages and drawbacks of each approach are discussed. Major approaches to derive optimal operating rules include direct optimization of reservoir operation, embedding conditional operating rules in simulation‐optimization frameworks, and inferring rules from optimization results. Suggestions on which approach to use depend on context. Parametrization–simulation–optimization or rule inference using heuristics are promising approaches. Increased forecasting capabilities will further benefit the use of model predictive control algorithms to improve system operation.

Influence of temperature and precipitation variability on near-term snow trends

Climatic Change (Springer) | October 17th, 2014

Snow is a vital resource for a host of natural and human systems. Global warming is projected to drive widespread decreases in snow accumulation by the end of the century

Snow is a vital resource for a host of natural and human systems. Global warming is projected to drive widespread decreases in snow accumulation by the end of the century, potentially affecting water, food, and energy supplies, seasonal heat extremes, and wildfire risk. However, over the next few decades, when the planning and implementation of current adaptation responses are most relevant, the snow response is more uncertain, largely because of uncertainty in regional and local precipitation trends. We use a large (40-member) single-model ensemble climate model experiment to examine the influence of precipitation variability on the direction and magnitude of near-term Northern Hemisphere snow trends. We find that near-term uncertainty in the sign of regional precipitation change does not cascade into uncertainty in the sign of regional snow accumulation change. Rather, temperature increases drive statistically robust consistency in the sign of future near-term snow accumulation trends, with all regions exhibiting reductions in the fraction of precipitation falling as snow, along with mean decreases in late-season snow accumulation. However, internal variability does create uncertainty in the magnitude of hemispheric and regional snow changes, including uncertainty as large as 33 % of the baseline mean. In addition, within the 40-member ensemble, many mid-latitude grid points exhibit at least one realization with a statistically significant positive trend in net snow accumulation, and at least one realization with a statistically significant negative trend. These results suggest that the direction of near-term snow accumulation change is robust at the regional scale, but that internal variability can influence the magnitude and direction of snow accumulation changes at the local scale, even in areas that exhibit a high signal-to-noise ratio.

Influence of Urban Areas on Surface Water Loss in the Contiguous United States

American Geophysical Union (AGU) | February 7th, 2022

Insights into estuary habitat loss in the western United States using a new method for mapping maximum extent of tidal wetlands

PLOS | August 14th, 2019

Effective conservation and restoration of estuarine wetlands require accurate maps of their historical and current extent, as well as estimated losses of these va

Effective conservation and restoration of estuarine wetlands require accurate maps of their historical and current extent, as well as estimated losses of these valued habitats. Existing coast-wide tidal wetland mapping does not explicitly map historical tidal wetlands that are now disconnected from the tides, which represent restoration opportunities; nor does it use water level models or high-resolution elevation data (e.g. lidar) to accurately identify current tidal wetlands. To better inform estuarine conservation and restoration, we generated new maps of current and historical tidal wetlands for the entire contiguous U.S. West Coast (Washington, Oregon, and California). The new maps are based on an Elevation-Based Estuary Extent Model (EBEEM) that combines lidar digital elevation models (DEMs) and water level models to establish the maximum historical extent of tidal wetlands, representing a major step forward in mapping accuracy for restoration planning and analysis of wetland loss. Building from this new base, we also developed an indirect method for mapping tidal wetland losses, and created maps of these losses for 55 estuaries on the West Coast (representing about 97% of historical West Coast vegetated tidal wetland area). Based on these new maps, we estimated that total historical estuary area for the West Coast is approximately 735,000 hectares (including vegetated and nonvegetated areas), and that about 85% of vegetated tidal wetlands have been lost from West Coast estuaries. Losses were highest for major river deltas. The new maps will help interested groups improve action plans for estuarine wetland habitat restoration and conservation, and will also provide a better baseline for understanding and predicting future changes with projected sea level rise.

Integrated Environmental Modeling of Estuarine Systems: Lessons for the Sacramento-San Joaquin Delta

Delta Stewardship Council (Delta Council) | April 18th, 2017

Complexity in estuarine systems calls for integrated and community-based approaches for using and developing models and data. Environmental and hydrodynamic models have h

Complexity in estuarine systems calls for integrated and community-based approaches for using and developing models and data. Environmental and hydrodynamic models have helped organize and extend knowledge and predictions for physical, biological, and chemical aspects. However, specialization has often steered science and management to fragmentation among models, data, and management of estuarine systems. Integration of models and data in platforms that increase collaboration, interdisciplinary work, organization and transparency have been successful in other systems. Mathematical modeling developed in the past few decades to capture current understanding and inform immediate decisions and long-term strategies remains the most viable science-based approach for supporting management of these complex systems. Changes and uncertainties require that such technical activities occur in a long-term adaptive management framework. However, strategies and responsibilities for selecting, developing, and employing models and data, selecting modeling questions, and making the best use of multiple modeling efforts among diverse institutions remain unresolved. A new approach is needed to build and evolve scientific and technical understanding and analysis capability for policy and management for large estuarine systems across institutions, interests, and different areas of expertise. A modeling Collaboratory is proposed that has capacity to build, refine, maintain and upgrade models, compare and contrast different scientific approaches, quantify uncertainty in predictions, synthesize current data, and accelerate the discovery of knowledge to inform policy and management for these challenging problems. A Collaboratory will focus on specific problems with a defined time-line, with the intent of providing broadly credible information for the many parties involved in decision-making and management. For the Sacramento-San Joaquin Delta, a partnership of the federal government, the State of California, and contractors, perhaps hosted by an academic institution, would establish this center with access to computational and scientific resources from agencies, consultants, NGOs, and academics.  

Integrated Hydrologic Model of Pajaro Valley, Santa Cruz and Monterey Counties, California

U.S. Geological Survey (USGS) | September 5th, 2014

Increasing population, agricultural development (including shifts to more water-intensive crops), and climate variability are placing increasingly larger demands on avail

Increasing population, agricultural development (including shifts to more water-intensive crops), and climate variability are placing increasingly larger demands on available groundwater resources in the Pajaro Valley, one of the most productive agricultural regions in the world. This study provided a refined conceptual model, geohydrologic framework, and integrated hydrologic model of the Pajaro Valley. The goal of this study was to produce a model capable of being accurate at scales relevant to water management decisions that are being considered in the revision and updates to the Basin Management Plan (BMP). The Pajaro Valley Hydrologic Model (PVHM) was designed to reproduce the most important natural and human components of the hydrologic system and related climatic factors, permitting an accurate assessment of groundwater conditions and processes that can inform the new BMP and help to improve planning for long-term sustainability of water resources. Model development included a revision of the conceptual model of the flow system, reevaluation of the previous model transformed into MODFLOW, implementation of the new geohydrologic model and conceptual model, and calibration of the transient hydrologic model. The conceptual model identified inflows and outflows that include the movement and use of water from natural and human components. The groundwater flow system is characterized as a layered geologic sedimentary system through which downward flow is driven by the combined effects of the application of irrigation water at the land surface and the pumping of groundwater from deeper in the system. Overall, groundwater meets most of the agricultural demand in the initial part of the growing season, augmented by precipitation during wet winter and spring seasons. In addition, the amount of groundwater used for irrigation varies from year to year in response to climate variation and can increase dramatically in dry years. Data on agricultural pumpage is a major component of simulated outflow that was previously unavailable; therefore, a coupled farm-process model was used to estimate historical pumpage for Pajaro Valley by subregion (water-balance subregions) as well as the delivery of surface water to and from the Harkins Slough Aquifer-Storage-and-Recovery System (HS-ASR) and related Coastal Distribution System (CDS) since 2002. The new, integrated hydrologic model includes new water-balance subregions; delineation of natural, municipal, and agricultural land use; streamflow networks; regions of tile drains; and, the groundwater flow system. The redefinition of the geohydrologic framework and incorporation into the simulation revealed the importance of the confining units at the base of the alluvial deposits and between the upper and lower Aromas Sand for regional groundwater flow.

Integrated Surface and Groundwater Modeling of Martis Valley, California, for Assessment of Potential Climate Change Impacts on Basin-Scale Water Resources

U.S. Bureau of Reclamation (USBR) | April 15th, 2015

This study develops a modeling framework to better understand surface and groundwater interactions in the Martis Valley Basin. Prior to the current study there were no ph

This study develops a modeling framework to better understand surface and groundwater interactions in the Martis Valley Basin. Prior to the current study there were no physical or statistical models of the Martis Valley Basin that could be used for long term water resource planning in the Basin. Currently, groundwater is the primary source for municipal and industrial water supply in Martis Valley, and surface water from the surrounding watersheds are partly stored in Donner, Prosser reservoirs and allowed to runoff into the Truckee River. This study presents the development, calibration, and verification of the physically based surface and groundwater GSFLOW model for Martis Valley. The model was calibrated for the historical 1981-2011 observed period. Various observations were used to calibrate the model during different periods, which helped verify the model’s ability to capture the dynamics of the hydrologic system. The model was calibrated to multiple fluxes and states in the hydrological system including streamflow, snow water equivalent, groundwater heads, and wetland and spring locations. In addition to comparing fluxes to multiple observations, consistency at internal gauges was also verified apart from simulating fluxes at the outlet. For future projections of hydrology, the GSFLOW model was forced with five future climate scenarios provided by CH2M HILL. Future simulations of snow water equivalent indicate reductions for all scenarios due to increasing temperatures. Groundwater recharge and its contribution to streamflow also decrease in the future for the warmer drier and hotter drier scenarios. The reduction in snowpack and earlier melt complicates the seasonally dependent nature of water resources and when combined, these results will likely add further stress to long term water availability in the Martis Valley Basin and the downstream consumers of the waters generated in the Basin.

Intensification of hydrological drought in California by human water management

American Geophysical Union (AGU) | January 25th, 2017