Speaker Presentations | 2018 Proceedings
Southwest US Droughts and Floods: Past Events and Future Changes
March 29, 2018
Robert Paine Scripps Seaside Forum
Scripps Institution of Oceanography, UCSD
Recent Developments in Understanding the Meteorological Origins of Extreme Precipitation in Southwestern US
Western US water resources are dependent on precipitation that exhibits extreme interannual variability. Along the West Coast, and in California in particular, only a few atmospheric river (AR) events per winter season define the water year. However, throughout the southwestern US, single extreme precipitation events have an even greater influence over annual precipitation. Adding to the complexity in understanding the role of extreme events in southwestern US flooding and drought development is the region's bimodal precipitation distribution; featuring significant contributions from both AR precipitation during winter and the North American monsoon in summer.
The North American monsoon provides vital rainfall in the semiarid southwestern United States and, occasionally, produces extreme rainfall and severe thunderstorm events. Ralph et al. (2014) used 30 years of rain gauge data to show that the top 10 precipitation days over south-central California to eastern New Mexico mostly occurred during the summer monsoon season. In addition to monsoon influence, ARs are responsible for up to 25% of winter precipitation. However, they also contribute disproportionately to extreme precipitation events with significant regional impacts.
Extreme precipitation events associated with ARs can often lead to flooding, as demonstrated by Neiman et al. (2013) for an AR event that impinged upon Arizona in January 2010, resulting in heavy orographic precipitation, increases in snowpack, and flooding in northeastern Arizona. Despite the large body of published work on both ARs and the North American monsoon, gaps remain in understanding, observations, weather predictions, and climate projections, particularly as they pertain to the transport of water vapor to the Southwestern US.
Understanding the current state and future fate of water resources in this region will require continued investigation of the origins and impacts of both ARs and monsoon precipitation, as well as their role in flooding and drought development.
Mechanisms and Impacts of Recent California Drought 2011-2016
Given the variability of its climate, California has made great strides to proactively manage drought related risks by utilizing information and resources to better prepare for, mitigate, and respond to the effects of drought. With a mediterranean-like climate characterized by warm to hot, dry summers and mild, moderately wet winters, drought is defined by a few large precipitation events each year such as from Atmospheric Rivers and the North American Monsoon.
This presentation will explore the mechanisms and impacts of the recent historic California drought. While drought impacted much of the state, the extent of drought impacts was regionally and sectorally specific. The extreme drought progression will be examined over the 5 years, including the 2016-2017 drought relief amidst challenges from extreme precipitation and flooding. Lastly we will reflect on how the CA-NV Drought Early Warning System (DEWS) can continue to move forward activities and networks to make climate and drought science readily available, easily understandable and usable for decision makers; and to improve the capacity of stakeholders to better monitor, forecast, plan for and cope with the impacts of drought.
Lake Sediments Reveal Southern California's Long History of Droughts, Pluvials, and Floods
Global warming is changing Earth's climate. While California will certainly warm, it remains uncertain whether precipitation amounts will increase or decrease. Even if the trend is towards more persistent and sustained drought, Southern California will continue to experience occasional pluvials (periods of above average wetness) as well as event scale weather, such as floods. Critical to planning for, and mitigating against, future risks from changes in precipitation requires a geological perspective, longer than our instrumental records.
Here, we present various lake sediment records from the pacific southwest United States used to reconstruct the history of droughts, pluvials, and floods. Importantly, these lake-based reconstructions extend our knowledge of hydroclimatic variability beyond the tree ring records, which, for Southern California rarely extend beyond 1000 years. Moreover, tree ring records cannot record flood events and do not cover the driest and low-lying areas of the state where some of our largest cities are found.
Our results indicate drought, pluvial, and flood conditions of longer duration (and magnitude?) than anything observed in the tree-ring records. Regional comparisons show a probable link between tropical Pacific Ocean forcing and drought/pluvials in the study region. Floods, on the other hand, seemingly occur during both wet and dry climate states, suggesting that flood producing precipitation events can occur regardless of the mean climate state. Learning from our state's history of droughts, pluvials, and floods will help to inform our state's water management program.
Atmospheric Rivers — Drought Busters, Flood Generators & the Very Stuff of Life in the Southwest
Atmospheric rivers (ARs) are long avenues of water-vapor transport that form about one mile up in the atmosphere. They are about 250-500 miles across but extend for thousands of miles. When one reaches the U.S. West Coast and hits mountain ranges, such as the Sierra Nevada, it is forced up, cools off and condenses to form precipitation. If the AR is particularly vigorous (conducting particularly large volumes of vapor) the resulting precipitation rates can be prodigious.
ARs enter the southwestern US almost entirely from over the North Pacific, dousing the Pacific Coast states directly, forcing their ways up and over-or though gaps in — the major ranges like the Sierra Nevada and Cascades, or by pathways that skirt those massifs to enter the intermountain West.
Along the Pacific Coast, landfalling ARs have caused 80% or more of all floods in the Pacific Northwest and northern California, with this role declining so that they cause approximately 30% of floods in Southern California, and even fewer in the interior Southwest. Along the West Coast, ARs have also played important roles in the persistence and terminations of historical droughts, with one or two major ARs in a month marking the end of 75% of droughts in Washington, declining to about 30% in Southern California. In the interior Southwest, although not often yielding precipitation extremes, ARs that make landfall in Southern California and Baja California have been shown to be significantly correlated with summertime vegetation greenness (and associated wildfire risks) across Arizona, New Mexico, and southern and northern parts of the Great Basin. The ARs have historically been the very stuff of both hydrologic hazards and life in the Southwest.
Better Precipitation Forecasting, Better Colorado River Basin Drought Contingency Planning
Management of the interstate and international Colorado River is governed by a complex institutional framework. In the U.S. Lower Basin, the Bureau of Reclamation manages operations of Lakes Mead and Powell pursuant to interim guidelines adopted in 2007, guidelines that will sunset after 2025. Development of the guidelines was prompted by the drought that began in 2000, and it was not then foreseen that prolonged drought would continue.
The period from 2000 through 2017 was the driest 18-year period in the historical record for natural flow into Powell, with storage in Colorado River system reservoirs declining from near-full to about half of capacity. The probability of a first-ever Lower Basin shortage has significantly increased; Colorado River water contractors have been taking voluntary measures to conserve storage in Mead to reduce shortage risk. Water agencies in both the Lower and Upper Basins have been negotiating drought contingency plans to better prepare for levels of shortage that were simply implausible in 2000.
About 85 percent of Basin runoff comes from 15 percent of the high-elevation upper watershed. Current skill in sub-seasonal to seasonal precipitation forecasting is inadequate to support water management decision-making. But what if skill in this important part of the watershed could be improved? And in time for the development of the follow-on to the existing interim guidelines for reservoir operations when they sunset? Compiling a climatology of water supply-producing storms, understanding their predictability and gaps in observations, and examining modeling improvements are potential pathways to better forecasting. If skillful, how could better forecasting support drought contingency planning?
Panel Discussion: Drought History, Expected Changes, and Adaptation Strategies
Assessing Arid Area Extreme Precipitation Using Doppler Radar and Rain Gages
Advances in Doppler Radar technology have led to considerable new equipment and software advances for this application. However, much work continues to be needed in the improvement of Doppler Radar predictions including the estimation of precipitation quantities. Other applications of Doppler Radar include assessment of storm aerial extent versus the magnitude of the precipitation intensity, among other topics of research. In this paper, we review the literature regarding reported issues concerning the use of Doppler Radar for hydrometeorological purposes, and in addition, assessment of the use of Doppler Radar collected over the last two decades in the arid southwest of the United States, in the County of San Bernardino, California, as well as other arid regions. These measures are then compared with Depth-Area Adjustment Factors ("DARF") curves developed and adopted by several flood control agencies located in the study area.
The More Extreme Nature of North American Monsoon Precipitation in the Southwestern United States
Most severe weather in the Southwestern United States occurs during the North American monsoon. This research examines how monsoon extreme weather events will change with respect to occurrence and intensity.
A new technique to severe weather event projection has been developed, using convective perimitting regional atmospheric modeling of days with highest instabilty and atmospheric moisture. The guiding principle is to use a weather forecast based approach to climate change project, with a modeling paradigm in which organized convective structures and their behavior are explicitly physically represented in the simulation design. Of particular interest is the simulation of severe weather events caused by mesoscale convective systems (MCSs), which account for a greater proportion of monsoon rainfall downwind of the Mogollon Rim in Arizona, in the central and southwestern portions of the state. The convective-permitting model simulations are performed for identified severe weather event days for both historical and future climate projections, similar to an operational weather forecast.
There have been significant long-term changes in atmospheric thermodynamic and dynamic conditions that have occurred over the past sixty years. Monsoon thunderstorms are tending to be more 'thermodynamically dominated' with less tendency to organize and propagate. Though there are tending to be a fewer number of strong, organized MCS-type convective events during the monsoon, when they do occur their associated precipitation is now tending to be more intense. The area of central and southwestern Arizona, corresponding to the area of the state most impacted by MCSs during the monsoon, appears to be a local hot spot where precipitation and downdraft winds are becoming more intense. These types of changes are very consistent with the historical observed precipitation data and model projections of historical and future climate, from dynamically downscaled CMIP3 and CMIP5 models.
Staying Ahead of Risk: Adapting to a Changing World
The Salt River Project (SRP) manages seven reservoirs on the Salt River, Verde River, and East Clear Creek in central Arizona, delivering approximately 987 million cubic meters (800,000 acre-feet) of water annually to the Phoenix Metropolitan Area. Tree-ring research suggests that extended wet periods and dry periods prior to the late 19th century were more severe than those in the recent observational record. Managing for historic droughts and floods is challenging in this highly variable and high evaporative demand climate, where annual runoff efficiencies range from 3 percent to 26 percent. Five-day events on the Verde basin can account for more than 40 percent of the water year discharge and spring snowmelt on the Salt basin can contribute from 1 percent to 35 percent of annual discharge. Additional challenges that will continue this century arise from population growth, wildfires, and reservoir sedimentation. Corresponding opportunities include reduced per capita demand, forest restoration, ground water recharge, cloud seeding, and securing additional water supplies. Fully capitalizing on these opportunities is paramount under a warming climate with an intensifying global hydrologic cycle, which portends to even greater magnitude floods and droughts in our future.
The Increasingly Robust Evidence for Increasing Precipitation Extremes in California
I will present evidence from our recent research efforts that precipitation extremes will increase significantly in California over the course of the 21st century.
We examined future climate projections done with models in the CMIP5 archive. We found that simulated increases in precipitation extremes in those parts of the world strongly affected by atmospheric rivers are tightly coupled with the well-documented and well-understood increase in global-mean precipitation.
We also analyzed a large ensemble of future simulations done with a single model to determine the extent to which California's significant internal precipitation variability could mask these anthropogenic increases in precipitation extremes. We found that the internal variability is generally not large enough to mask the anthropogenic signal.
All of this evidence points to formidable challenges to California's water resource infrastructure due to a significant increase in precipitation extremes.
The Santa Barbara County 1/9 Debris Flow of 2018: Extreme Runoff Response to Extreme Precipitation
Post-fire debris flows (PFDF) present a significant hazard to life and property in Southern California's Transverse Ranges. Many destructive events have occurred over the past few decades, and most recently, the tragic Montecito debris flow killed 21 people and destroyed over 100 homes. Observations demonstrate that short duration, high intensity rainfall is key in triggering post-fire debris flows, but what rainfall intensity and what debris flow magnitude constitutes an extreme event?
To initiate a PFDF, several factors are needed:
- wildfire creating sufficient soil burn severity and lack of vegetative cover;
- topography consisting of steeply sloping terrain generally greater than 42%;
- sediment, including dry-ravel and alluvium, stored in watercourses; and
- high intensity rainfall.
In the Santa Barbara area, our current understanding of precipitation intensity-durations that initiate PFDF suggests that a rate of 25.4 mm/hr for 15 minutes would provide an overall 50% chance of debris flow occurrence. According to the NOAA Atlas 14, this equates to less than a 1-year recurrence storm. During the 1/9 event, numerous precipitation gages within and around the Thomas Fire burn area reported rain rates that are triple the empirical PFDF thresholds. These observations generally range from a 2- to 50- year recurrence at the 15-minute duration based on Atlas 14 estimates.
Here we compare the 1/9 event rainfall observations and PFDF magnitude with historical observations from the region to evaluate whether, as a whole, this event was truly extreme.
Informational Needs for Long-Term Water Resource Management Planning
The world is getting warmer and with it come changes in meteorology and hydrology.
Historically, water management in California has operated on a seasonal cycle. Reservoir operations maintain a flood reserve space in winter months to manage winter extremes and refill with snowmelt for supply management in the dry summer months. In a warming world, managing water by event may become necessary requiring new information feeds for support.
Over the past decade the Department of Water Resources has partnered with local and federal agencies and the academic community in the development and deployment of advanced observing systems for atmospheric rivers and snowpack assessment, improved forecasting capabilities across timescales, and developed decision support tools to inform integrated water management. These efforts include the Hydrometeorology Testbed (HMT), the Airborne Snow Observatory (ASO), Forecast-Informed Reservoir Operations (FIRO), Reservoir Forecast-Coordinated Operations (F-CO), and the Atmospheric Research Program.
In this talk current capabilities are outlined, gaps are identified, and some ideas on new directions are offered.