Speaker Presentations | 2019 Proceedings
Complexities of Flood and Water Resource Predictability in Southwest U.S.
March 27, 2019
Robert Paine Scripps Seaside Forum
Scripps Institution of Oceanography, UCSD
What Uncertainty in Seasonal Precipitation Forecasts Means for Managing Colorado River Water Supplies
The international Colorado River supports water uses in seven U.S. and one Mexican state, under a highly complex legal and institutional framework. Prolonged drought conditions since 2000 have reduced total system storage to about half of capacity, significantly increasing the risk of a first-ever Lower Basin shortage. Interim surplus and shortage and reservoir operations guidelines were adopted in 2007 to reduce shortage risk. However, continued drought has further elevated risk, leading to negotiations on drought contingency plans for responding to extraordinary reservoir storage levels. Present seasonal forecasting capacity lacks the ability to inform drought contingency planning, or the renegotiation of the existing guidelines which must begin in 2020. If skillful, seasonal forecasts would add value for shortage management. Early preliminary research suggests that there may be emerging seasonal predictive ability for western snowpack, can this ability be developed in time to be useful?
CW3E Research and Applications Development on Southwest U.S. Extreme Precipitation
The Center for Western Weather and Water Extremes (CW3E) seeks to provide 21st Century water cycle science, technology, and outreach. This presentation by CW3E Director, Marty Ralph, details the Center's ongoing hydrometeorology studies to advance the observation, physical understanding, and prediction of precipitation extremes in the Southwest U.S. The topics discussed include recent field campaigns to observe atmospheric rivers and their influence on hazardous precipitation in Southern California, as well as a large collaborative project to develop improved numerical weather prediction for the region. The presentation additionally addresses the meteorological complexity of the Southwest U.S. by exploring the wide range of extreme event types that impacted the region extending from Southern California to Colorado during the 2018-2019 water year. Efforts to improve forecasts of precipitation at all appropriate timescales and for all relevant processes have the potential to increase water supply reliability, mitigate flood risk, and improve ecosystem health in the Southwest.
Flood Hydroclimatology and Extreme Events in the Southwest: A Streamflow Perspective
When extreme flooding is analyzed from a "bottom up" perspective, beginning with the streamflow itself, a deeper understanding of the complexities involved in the flooding process emerges. Streamflow is — by nature — constrained by a drainage divide, distributed through a drainage network, and intrinsically connected to the landscape through which it flows, all of which can influence the development and impact of extreme flooding in response to different types of precipitation and atmospheric circulation features. This presentation will illustrate some of these complexities in floods of the Southwest,by revisiting the approach of "flood hydroclimatology" which identifies climate-based heterogeneities in peak-over-threshold streamflow events by examining them within the context of their history of variation and the spatial framework of the atmospheric circulation patterns and meteorological mechanisms involved.
In contrast to other parts of the Southwest, Arizona has a distinct bimodal precipitation seasonality which leads to a strikingly heterogeneous mixture of flood-causing atmospheric patterns. These include summer convective precipitation from individual storms or mesoscale systems, tropical cyclone remnant-enhanced precipitation, and synoptic-scale precipitation associated with atmospheric rivers, fronts, or cutoff lows. When this complex flood hydroclimatology is combined with drainage basin characteristics (such as size, shape, orientation, terrain complexity and other factors) it reveals more about the many ways in which a precipitation input can result in a flood — or might do so more frequently in the future. Throughout the Southwest, this approach of examining how flood hydroclimatology and basin characteristics interact can bring new insights into variations in the spatial patterns of some extreme floods and also aid in identifying areas especially susceptible to flooding from certain types of storms. Lastly, in certain watersheds, flood hydroclimatology can interact with some drainage basins in ways that provide insights into how flooding may — or may not — manifest itself in paleohydrologic reconstructions of streamflow based in tree rings.
Characteristics and Origins of Extreme Precipitation in the Lake Mead Watershed
The North American Monsoon (NAM) is the main driver of summertime climate and variability in the American southwest. While much work has been done on the NAM, most focuses on the region defined as Tier I in the seminal North American Monsoon Experiment (NAME), a region that stretches from central-western Mexico to southern Arizona and New Mexico. This work presents a climatological characterization of summertime precipitation, defined as July, August, and September (JAS), in the Lake Mead watershed, located in the NAME Tier II region, spanning from 1981-2016.
Spatial and temporal variability of JAS rainfall characteristics are examined using PRISM 4km daily precipitation data. The importance of the number of wet days (24hr rainfall > 1 mm) and extreme rainfall events (95th percentile of wet days) to the total JAS precipitation shows a strong west-to-east gradient of JAS precipitation with extreme events playing a larger role in the west and total number of wet days in the east. An investigation into the dynamical drivers of extreme rainfall events indicates that anticyclonic Rossby wave breaking in the midlatitude westerlies over the US west coast is the dominant flow regime, associated with 70% of precipitation events >10mm over the Lake Mead basin. This is in contrast to the NAME Tier I region, where extreme events are frequently associated with easterly upper-level disturbances such as inverted troughs.
Short-Duration, High-Intensity Precipitation in Southern California: Drivers, Predictability, and Changes in a Warming Climate
Short-duration (hourly to sub-hourly), high-intensity precipitation (e.g., >25 mmhr-1) can produce flash floods and trigger debris flows in areas recently burned by wildfires in Southern California. A growing body of research suggests intensification of short-duration rainfall in a warming climate. Global Climate Model (GCM) output at a daily resolution demonstrates precipitation intensification. However it is likely that sub-daily rainfalls are intensifying at a higher rate than seen at the daily timescale due to processes not well-resolved by GCMs. In southern California, model projections suggest weak and uncertain changes in multi-decadal mean precipitation as compared to the historical period, but point towards increasing numbers of dry days. This balance may be achieved through fewer, more intense storm events, which are likely associated with higher intensity sub-daily precipitation.
Here I will describe some of the common drivers of short-duration, high intensity precipitation in southern California and discuss challenges associated with forecasting these events. I will present analysis of model output from a recent pseudo-global warming modeling study that demonstrates a shift in the distribution of hourly precipitation intensities towards more frequent extreme events. Such changes may render existing precipitation frequency estimates and design criteria less effective. I will conclude with a discussion of the current needs, research activities, and future directions related to sub-daily precipitation extremes.
Drought Monitoring in Arizona's Changing Hydroclimate
Tracking and characterizing drought conditions across the southwest U.S. is a challenging task. The Southwest's seasonal-transitional climate coupled with steep topo-climatic gradients and high levels of inter-annual variability in precipitation conspire to create a complex pattern of potential drought impacts across the region. Temperatures have also been steadily increasing, but their role in driving changes in drought frequency and intensity also appear to be complex.
This presentation will highlight some of the main drivers of regional climate variability, their connection to multiple types of drought (short-term seasonal to longer-term multi-decade events), and some preliminary results from an effort to try and help optimize the use of drought indices by land managers in Arizona. This effort is using a soil moisture model to create an objective dataset against which numerous timescales of the Standardized Precipitation Index and Standardized Precipitation Evapotranspiration Index were compared. Shallow (10cm) modeled soil moisture values correlated well with shorter SPI and SPEI timescales (˜2 months), but strength of correlation varied through the year based on seasonality in precipitation. Correlations were weaker at deeper depths (˜30cm), but still corresponded reasonably well with SPI and SPEI windows in the 6 to 9 month timescale.
These results are being used to help land managers interested in tracking potential soil moisture conditions at different depths by providing guidance on optimal timescales to use in readily available indices like the SPI and SPEI. Work is being done to extend the analysis to additional soil types, soil profile complexity and with different driving climate datasets based on recent feedback from land managers.
Panel Discussion: Speaker Panel Discussion #1
Laboratory for Tree-Ring Research
Desert Research Institute
Western Regional Climate Center
Floodplain Management Challenges Across Scales and Future Climate Scenarios
Communities face a unique set of challenges with regard to management of storm drainage infrastructure, floodplain management and disaster warning/response. Technological advancements have significantly improved our ability to measure and document outcomes and simulate complex behaviors with mathematical modeling tools for either predicted, real time or post-event scenarios. While this leads to a better informed planning, design and public engagement processes, it also results in some new questions. Communities need to know more about their vulnerabilities both now and in the future as they attempt to address resiliency issues that relate to climate uncertainty.
This presentation will describe some of the approaches that are being used in our industry to simulate event behaviors and estimating risk. Climate uncertainty complicates those processes since the future precipitation patterns (frequency, intensity, duration, and type) will result in changes in average and extreme events that need to be considered since we are designing and planning infrastructure that has expected design life of over 50 years.
Addressing Impacts of Precipitation Forecast Uncertainty on Stormwater Capture and Infiltration, Orange County, California
Stormwater represents a significant source of water used by the Orange County Water District (OCWD) to recharge the Orange County groundwater basin. Over the last 20 years, OCWD has captured and recharged an average of 50,000 acre-feet per year (afy) of stormwater with a maximum of 85,000 af in 2005, which was a record wet year in Orange County. On average, storm water provides approximately 15 percent of the total recharge to the basin every year. This recharge comes from a combination of investments made in OCWD's recharge system to capture storm water flowing down the Santa Ana River and the capture of stormwater at Prado Dam, which is located upstream of OCWD's recharge system. OCWD has and continues to work closely with the US Army Corps of Engineers (USACE) to temporarily store storm flows behind the dam and manage outflows from Prado Dam so they can be captured by OCWD.
This partnership with the USACE has increased the amount of storm flow capture and recharge by OCWD. However, there is the potential for further increasing the amount of storm water that can be captured with improved weather and runoff forecasting. This is particularly important as future climate change modeling suggests that although total future precipitation in the watershed is not likely to change significantly, the precipitation events that do occur will be less frequent and more intense. To this end, OCWD is working with the Center for Western Weather and Water Extremes (CW3E) to explore the application of Forecast Informed Reservoir Operations (FIRO) at Prado Dam, following the model applied to Lake Mendocino by Sonoma Water and CW3E. The goal is to develop tools that will allow the USACE to operate Prado Dam for its primary purpose, flood risk management, while also allowing for increased storm water capture, which ultimately leads to increased recharge to the groundwater basin.
Operating for Hydrologic Change — Adapting to Changing Distributions
Operation of water management systems involves decisions at time scales of hours to years, with varying available hydro-climatological information, and differing levels of consequences of actions. Operators consider historical and forecast information in near-term operational, but the integration of longer-term predictive or change information is generally only considered through distinct technical approaches. A large cause for the discontinuity of approaches is based on the skill of predictive information over time scales of concern. Skill in absolute prediction, however, is not the only requisite metric for usefulness for many decisions. Since many decisions are based on historical observed distributions in the absence of predictions, information that would suggest changes in the distributions can be important for improving decisions even at longer lead times. This presentation will highlight some evolving water operation and management applications in the western U.S. that explicitly incorporate future changes in hydro-climatological distributions.
Climate Preparedness and Resiliency at U.S. Army Corps of Engineers
As a Federal agency, USACE has statutory responsibilities to provide our authorized services to the Nation under changing conditions, now and in the future. The US Army Corps of Engineers (USACE) has a long history in dealing with climate change impacts. Our involvement began with drilling ice cores in Greenland and Antarctica in the 1950s, to drought in the 1970s, sea level change in the 1980s, economic impacts of climate change in the 1990s, and addressing climate issues in water resources in the 2000s. We are now planning and implementing climate preparedness and resilience measures in accordance with our overarching climate policy from 2011 (updated 2014) and with our Climate Adaptation Plans submitted to the Council on Environmental Quality and Office of Management and Budget.
Our approach to climate preparedness and resilience is designed to reduce USACE water resources vulnerabilities and increase our resilience to observed and reasonably foreseeable climate disruptions to new and existing projects. An important requirement of our adaptation policy is that we use the best available and actionable information and embrace uncertainty as we make progress. We fully embrace and support the need for science agencies, academia, and others to produce new science, whether this is realized in the form of incremental improvements or significant advances. At the same time, science is necessary but not sufficient for operating agencies. Rather, we aggregate and integrate science, and then translate it for use in decision-making targeted to our agency missions and responsibilities. Our decision-makers especially need to consider surprises or cascading events that could result in large consequences. This talk will provide insight into USACE climate preparedness and resilience activities with a focus on the nexus between flood and drought in the Southwest US, and how variability and uncertainty impact our water resources planning and management here.
Reclamation Research for Water Management Under Uncertain Hydrologic Conditions
Reclamation's Research and Development (R&D) Office funds research to address needs across a range of topics associated with Reclamation's water management mission: Water Infrastructure, Power and Energy, Environmental Issues for Water Delivery and Management, Water Operations and Planning, and Developing Water Supplies. Water managers contend with uncertainty across a continuum of timescales and objectives. Activities funded by the R&D Office that aim to improve water management in the face of uncertainty include improving sub-seasonal forecasts in the Western U.S. and exploring new methods to enhance decision making processes.
The Sub-Seasonal Climate Forecast Rodeo is a recently completed year-long, real-time prize competition (i.e. crowdsourcing). Participants were asked to develop and implement sub-seasonal forecasting methods that could outperform current operational forecasts at 3-4 week and 5-6 week lead times. Improved information at this timescale (i.e., reduced uncertainty) will help water managers to better prepare for shifts in hydrologic regimes such as the onset of drought or occurrence of wet weather extremes. Three teams shared $525,000 in prizes based on their performance in the competition.
The second effort complements the "Rodeo" in paradigm for contending with uncertainty. Whereas the Rodeo sought to reduce uncertainty through more skillful forecasts, this project is developing resources to facilitate use of an emergent class of techniques collectively referred to as "Decision Making under Deep Uncertainty (DMDU)". These methods aim to improve the outcomes of decisions made in the face of deep uncertainty, which refers to uncertainty that is not well known or easily characterized with traditional methods. These examples of novel Reclamation activities contend with uncertainty in water management by improving the information upon which decisions are based and by advancing the decision making process itself.