Speaker Presentations | 2022 Proceedings
Exploring Extreme Conditions Impacting Water Supply from Colorado River Basin
April 19-20, 2022
Why This Theme?
The Colorado River basin provides water to nearly 40 million people and irrigates nearly 5.5 million acres of agricultural lands, both within and outside of the basin. Areas outside the basin receiving water include Denver, Salt Lake City, Albuquerque, Los Angeles, and San Diego for public (municipal) supply, and the Imperial Valley in California and the Mexicali Valley in Mexico for agricultural water supplies.
Droughts and floods are the historic extreme climatic conditions in the basin. Recent climate change impacts are altering these extreme conditions. Exploring the past, current, and potential future impacts from climate changes on droughts and floods will be covered by ten speakers over two half-days.
The Press and Pulse of Extreme Events in the Colorado River Basin
Extremes in temperature and precipitation are associated with damaging floods, prolonged drought, destructive wildfires, agricultural challenges, compromised human health, vulnerable infrastructure, and threatened ecosystems and species. Often, the steady and progressive trends (or presses) of rising global temperature are the central focus in how climate impacts are described. However, observations of extreme weather events (or pulses) increasingly show that the intensity, duration and/or frequency of acute events are also changing, resulting in greater impacts on communities and the environment. Describing how the influence of extreme events may shape water management in the Colorado River Basin in clear terms is critical to sound future planning and efforts to manage risk.
Three scenario planning workshops in 2019 and 2020 were held as part of a Colorado River Conversations series, identifying potential impacts from multiple intersecting extreme events. Water managers identified important climate-related events of concern in the Colorado River Basin that are escalated by climate presses and pulses and necessitate greater attention, description, and adaptive responses. To promote broadly accessible climate communications and support efforts to include climate-change-driven extremes in water management and planning, we explore the current state of knowledge at the confluence of long-term climate shifts and extreme weather events in the Colorado River Basin.
Water Balance of the Turn-of-the-Century Drought in the Southwestern United States
A water balance model was used to estimate time series of monthly water balance components (i.e., actual evapotranspiration, soil moisture, and runoff) for the southwestern United States (SWUS) for the period 1900 through 2018. Also, a subset of these data were used to evaluate the magnitude of the turn-of-the-century (TOC) drought (2000 through 2018).
Results indicate that the warm season (April through September) soil moisture and runoff during the TOC drought were among the lowest values of the 1900 through 2018 period. Additionally, increases in temperature were identified as a significant driver of low soil moisture and runoff conditions during the warm season. In contrast, during the cool seasons (October through March) and the water year (October 1 through September 30) during the TOC drought, soil moisture and runoff did not indicate extremely dry conditions even though temperatures were the highest of the 1900 through 2018 period.
Water Supply Forecasting Challenges in Colorado River Basin
[Abstract not available]
Examining Hydrological Modeling with Remote Sensing Products in the Colorado River Basin: Surface Temperature and Snowpack
The Colorado River, the largest river and the most important water resource in the Southwestern United States, has been experiencing severe water supply shortages in the 21st century. The Upper Basin's annual unimpaired streamflow at Lees Ferry declined >15% (3 km3) over the last 100 years. Previous studies based on large-scale modeling techniques report that at least 30% of the streamflow decline is associated with the warming temperature. More than half of the river's naturalized flow originates as mountain snowpack, which becomes substantially vulnerable due to increased temperature in the Upper Basin. Nonetheless, accurately capturing the energy exchanges near the surface and the distribution of mountain snowpack remain major challenges in the land surface modeling community.
This presentation will focus the following research question: How can we improve the spatial patterns and unconstrained parameters of hydrologic modeling with remote sensing products? Starting from a well-established modeling framework, we tested the possibility of incorporating satellite-based MODIS data products of land surface temperature and snow cover fraction to enhance confidence in hydrological simulations of the basin. This work helps to understand the hydrologic patterns of the Colorado River Basin and possible deficiencies in current land surface modeling techniques where further improvements are possible.
Characteristics and Origins of Extreme Snow Days and Weeks in the Yampa River Basin — The Role of Inland Penetrating Atmospheric Rivers
The Colorado River is the largest river in the U.S. Southwest and the most important surface water source to the agricultural irrigation and domestic water supply. Its natural streamflow is mainly from water released by snowmelt and is highly influenced by snowpack in the sub-basins of the Upper Colorado River Basin, including the Yampa River Basin.
This study investigates the characteristics and origins of extreme snow days and weeks in the Yampa River Basin with a focus on the role of inland penetrating atmospheric rivers (ARs). The inland penetrating ARs can bring significant water vapor inflow from coastal areas to the Yampa River Basin and produce extreme snow events in the inland area. During January 4-6, 2022, an extreme AR made landfall over the Washington and Oregon coast and its inland penetration produced 7.6 inches of snow water equivalent (SWE) in only three days at the SNOTEL site Tower (the wettest site in the Yampa River Basin), which is 16% of normal maximum annual SWE at that site. This kind of AR-related extreme snow event plays a critical role in the inter-annual variability of snowpack in the Yampa River Basin. We examine the linkage between inland penetrating ARs and extreme snow events in the Yampa River Basin and explore the relevant synoptic-scale weather conditions.
Panel Discussion: Speaker Panel Discussion #1
Using Tree Rings to Inform Water Resource Management in Colorado River Basin
Tree rings have long been used to develop proxy records of past climate and hydrology. In the Upper Colorado River basin, reconstructions of water year streamflow at Lees Ferry extend back over 12 centuries, lengthening the record of hydroclimatic variability far beyond what is provided by gage records. These records offer valuable context for assessing current and recent droughts. They also provide insights on resource allocation decisions made a century ago and still the basis for Colorado River management.
Reconstructions of the Colorado River and streamflow in neighboring watersheds document the range of hydroclimatic conditions that are possible under natural climate variability, including drought duration and intensity. Climate and hydrology are increasingly being influenced by the effects of anthropogenic climate change, so the record of the past is no longer an analogue for the future. However, these records remain valuable for water resource management as future conditions will be some combination of natural hydroclimatic and anthropogenic climate change.
This presentation will assess the current Colorado River drought and the worst droughts in the 20th century in the context of Colorado River streamflow reconstructions. The spatial extent of the current drought will also be compared to droughts of the past that impacted other major western US watersheds. The presentation will include a discussion of how these records have been used to inform water resource management, with prospects for their use in the facing of a warming climate.
How Warming Since the 1880s Has Impacted Colorado River Basin's Hydrology Over Time and During the 2020-2021 WY Drought
While the 2020-2021 Water Year (WY) drought was largely driven by a large negative anomaly in precipitation associated with natural variability, long-term anthropogenically forced warming exacerbated water resources during the event.
In this study, we use a hydrologic model of the western US and GCMs of the historical period to explore how warming since the pre-Industrial era has impacted water availability (runoff) and snowpack conditions from 1954-2021 across the Colorado River Basin. Our analysis focuses on climate change impacts by using a model calibrated to natural or unimpeded streamflow. We find that accelerated reductions in runoff and snowpack have occurred since the 1980s due to a non-linear increase in warming, with larger percent reductions in total runoff occurring during dry years when water resources matter most.
Specifically, we found that for every 1°C increase, the Colorado River Basin experiences a ~9.6% reduction in runoff, and during the most recent 2020-2021 WY drought experienced a ~16.8% reduction in runoff due to long-term anthropogenically forced warming. To diagnose this issue, we further explore the mechanisms that are driving reductions in Colorado River's runoff. We found that increases in temperature primarily drive increases in evapotranspiration (reductions in runoff) during the winter season; however, increases in temperature have had a small impact on evapotranspiration during the spring to summer since even prior to anthropogenic warming the Colorado River Basin was already in a state that evaporated nearly all precipitation. We also found that reductions in runoff are occurring at roughly twice the rate in areas of the Colorado River Basin with snowpack compared to areas without snowpack. This is due to a positive feedback mechanism where snowpack loss leads to a darker ground surface that absorbs more solar radiation (decreased albedo) and thus greater evapotranspiration and less runoff.
Colorado River Flow Dwindles as Warming-Driven Loss of Reflective Snow Energizes Evaporation
When Brad Udall and Jonathan Overpeck wrote their widely referenced 2017 paper laying out scenarios for future runoff in the Upper Colorado River Basin, they had to contend with the enormous range in uncertainty of the sensitivity of the river to warming. At that time, previous research had suggested that (setting aside any changes in precipitation) the river's flow would decrease by anywhere from a modest 2% per degree C of warming to almost 20% per degree C. At a symposium on the matter around the same time, I became intrigued by the lack of clarity on a number so crucial to the fate of the already-fully-allocated water availability of the US Southwest. One rarely comes across such a fascinating, clearly defined scientific question that has direct relevance to 40 million people and their trillion-dollar economy. I undertook
- to understand why the sensitivity estimates differed so widely,
- to identify key physical processes that determine the sensitivity, and
- to provide a better-constrained estimate of the sensitivity.
After a year of long days and nights of hypotheses, computations, and (mostly) dead-ends, followed by a months-long publishing delays, we published our findings:
- the high-end estimates arose from fallacious statistical reasoning, and the low-end estimates arose from neglect of a critical physical process;
- the sensitivity is controlled mainly by a radiation feedback, as stated in the title of this abstract; and
- the runoff into the Upper Colorado River Basin decreases by about 9% per degree Celsius of warming.
The Impact of Declining Western U.S. Snowpacks on Annual River Discharge in a Warming Climate
The role of declining mountain snowpack on the annual discharge of the Colorado River is analyzed in a recent Science paper by Milly and Dunne (2020). The primary mechanism they identify is increasing net radiation, especially in spring, resulting from decreased headwater snow cover extent, which in turn drives increased evapotranspiration. Decreasing snow extent leads to reduced albedo and hence increased net solar radiation. Their analysis was specific to the Colorado River basin (and specifically, the upper Colorado headwaters, where much of the Colorado\'s runoff originates). However, it remains uncertain the degree to which this snowmelt-radiation feedback controls/affects river discharge across the heterogeneous hydroclimatic regimes of the Western U.S.
We examine warming-induced streamflow changes and their drivers for the HUC-8 river basins of the WUS (west of the Continental Divide) for the period 1915 to 2018. We evaluate the role of the snowmelt-radiation feedback on runoff volumes using output from the Variable Infiltration Capacity (VIC) model implemented at the 1/16th degree spatial resolution. We classify the 616 WUS HUC-8 basins in the region by their elevation, mean winter and mean annual temperature, summer to winter precipitation ratio, primary vegetation type, and the fraction of forest cover. We define the snowmelt feedback coefficient as the correlation between springtime net radiation change and annual runoff change.
Our preliminary results demonstrate a significant negative snowmelt feedback impact on runoff, especially at watersheds with low runoff coefficients, which have greater evapotranspiration. Within the Upper Colorado River Basin HUC-8 basins, the correlation between snowmelt feedback impact on runoff and runoff coefficient is 0.59, at a significance level of 0.01.
Developing a New Long-Term Soil Moisture Observing Network in the Upper Yampa River Basin
With support from the Upper Yampa Water Conservancy District (UYWCD), the Center for Western Weather and Water Extremes (CW3E) has partnered with Colorado Mountain College (CMC) and Yampa Valley Sustainability Council (YVSC) to design a new soil moisture monitoring network in the Upper Yampa River Basin. The goal of this network is to provide data and scientific insight into the reduction of runoff by dry soils, provide a continuous record of changing landscape conditions with a changing climate, and to support operational model and forecast improvements. A preliminary basin analysis has been conducted to identify key observation gaps in the region and develop a proposed plan of potential sites for new hydrometeorological station installations.
This presentation will provide an overview of the basin analysis to date, which incorporated input from stakeholders and experience installing sensors in other western basins. The analysis includes the assessment of atmospheric drivers of soil moisture variability and their changes over time, assessment of land surface factors that modulate water movement throughout the basin, and identification of water collection zones.
We combined results from this analysis with information on existing station locations and input from the UYWCD and the Colorado Basin River Forecast Center (CBRFC) to identify a top priority for the first station, to be installed in summer 2022. The plan for this pilot station as well as future hydrometeorological stations in the Upper Yampa will be presented, with emphasis on their potential operational value for water management.