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Water is a precious commodity underpinning not only the U.S. economy, but also the quality of life here. The U.S. federal agencies managing the availability and distribution of this basic necessity face a variety of challenges every year in ensuring a high-quality, always-ready resource to meet public and private demands, and to manage highly changeable flood risks and drought responses around the nation. Recognizing that climate change is exacerbating these challenges, and to help managers better plan for and respond to climate change effects on water resources, the Bureau of Reclamation (Reclamation), U.S. Army Corps of Engineers (USACE), and National Center for Atmospheric Research (NCAR) are collaborating to improve our understanding of water resource-related climate effects. In addition, they are testing and improving some of the important management tools that the water resources community relies on to fulfill their federal missions to manage water resources in the best national interest.
“In 2010, Roy Rasmussen, who heads NCAR’s Hydrometeorological Applications Program, talked to scientists at Reclamation about the effects of climate change on winter precipitation, snowpack, and runoff processes in Colorado's headwater basins, which is what launched this collaboration,” says Martyn Clark, a scientist in the Hydrometeorological Applications Program who leads the water resource collaboration at NCAR.
The Headwaters Research Program, led by Rasmussen, used a high-resolution, fully coupled atmospheric-hydrologic model to generate a detailed perspective on how the atmosphere interacts with the region’s complex topography to control water availability under changing climate. Using projections of warmer, moister climate generated by global climate models, the research shows that Colorado’s Headwaters region will receive greater than expected amounts of precipitation in winter, Clark says. This finding contradicts results from some Colorado River Basin climate change impact studies using less sophisticated methods for generating future hydro-climate conditions, methods that U.S. agencies, decision makers, scientists, and water managers are beginning to explore in preparation for developing plans related to regional-scale climate variation and change and its effects on water resources.
“The outcome surprised us, making us stop and think for a minute as to whether the methods used to assess climate change impacts on hydrology need to be reconsidered,” Clark says.
Key scientists at Reclamation and USACE, the two chief water management agencies in the United States, became intrigued by Rasmussen’s findings. Eager to work with NCAR to develop a better understanding of the issues and the implications for their missions, Reclamation and USACE designed and funded a partnership together with NCAR to understand how a modeler’s methodological choices might affect the portrayal of climate change impacts on hydrology. Given the potential for any methodology to introduce biases into the model output, the group worked together to identify common or potential areas of weakness, focusing on “downscaling” issues and other relevant modeling and hydrologic questions.
Downscaling methods seek to extract local-scale information from global climate models and then use these data as input to regional-scale hydrologic models to examine effects of climate change on water resources. One commonly used approach, statistical downscaling, offers a number of benefits, including speed and ease of use; however, these statistical methods often cannot adequately capture the important subtle effects of climate change on terrestrial surface water hydrology.
“The statistical downscaling methods used by many water resource agencies can introduce artifacts in both precipitation occurrence and magnitude that affect projections of overall availability of water resources and flooding potential,” Ethan Gutmann, a scientist in NCAR’s Hydrometeorological Applications Program.
Dynamic downscaling methods, on the other hand, use physically based regional- or local-scale numerical models to simulate more accurately regional-scale weather and climate dynamics. These methods, though, can be too computationally intensive and expensive to run and analyze for the large numbers of simulations needed to help inform water resources decisions, Gutmann says.
So, despite the shortcomings, water managers asking questions about climate change effects have in the past often relied on statistical downscaling methods because of their speed and ease of implementation, and because they have appeared to estimate the meteorological means in temperature and precipitation, at monthly time-scales reasonably well. Values at finer time scales or for extremes in the hydrologic cycle like floods and droughts are not so well estimated.
Eager to characterize and understand the shortcomings and their possible effects on water management decisions, and having the hope of developing better methods, USACE and Reclamation have extended their collaboration with NCAR to a new climate change project. That project seeks to improve both climate downscaling methods and hydrologic modeling approaches by creating a hybrid methodology that relies on both statistical downscaling and dynamical modeling.
“We have been developing this innovative downscaling approach using an atmospheric model of intermediate complexity that provides a good representation of natural processes and feedbacks,” says Gutmann. “This method provides physically meaningful representations of local-scale changes, and it can be applied to hundreds of different GCMs and emissions scenarios.”
“Many projections of changes in water resources are based on a single hydrological model that may not account for the uncertainty of simulating hydrologic processes,” explains Clark. “Our modeling approaches seek to improve the fidelity of hydrologic simulations over the contiguous U.S., as well as better characterizing model uncertainty.”
The group is working on developing new methods for dealing with weaknesses in climate impact methods, especially, new downscaling methods and novel hydrologic modeling approaches, Clark explains. Substantially improving climate impact assessment methods are critical to ensuring that when water mangers make long-term water resource plans that these plans are developed using robust research assumptions and planning tools.
“Our collaboration with NCAR and Reclamation is a vital component of the very active program at USACE to characterize the specific impacts of climate variability and change on our water management missions and on the roughly half-trillion dollar investment in water infrastructure USACE has made just since the 1960s," says Jeff Arnold, senior scientist and co-director of the USACE Responses to Climate Change programs. “This collaborative work to understand shortcomings in existing methods and to develop new tools for rapid deployment is significantly helping us prepare for the diverse climate change threats to surface water supplies and to plan possible responses and future investments.”
"Agencies are looking for ways to mainstream actionable climate science into water management decisions, all while the climate science community continues to produce new sources of information that must be reconciled with planning activities," says Levi Brekke, Water and Climate Research Coordinator in Reclamation’s Science and Technology Program. "Reclamation and its partners are taking leading roles to develop an understanding on how these new sources of information and how they may be considered in water planning and management.”