Modeling Extreme Precipitation Events and Impacts in a Changing Climate to Support Western Water Management
This proposal is intended to build on previous work related to extreme event estimation and implications,
including work being performed at Reclamation and NCAR, and work being performed under Reclamation's
Science & Technology (S&T) Program. Previous S&T and other Reclamation-funded projects have focused on
extreme precipitation and flood risks under changing climate, often for the purpose of determining failure levels
for infrastructure such as water-storage and flood-control dams. From Extreme Value Theory (EVT), the
Generalized Extreme Value (GEV) distribution (Coles 2001) has been used, including with climate covariates, to
develop a spatial-temporal model of precipitation extremes in the Western US (Bracken et al. in prep), to
determine flood frequency (e.g., Condon et al. 2015), and water quality impacts (Towler et al. 2010). The goals of this research are to:
(i) build on previous work and develop additional EVT tools to examine the frequency and magnitude of extreme
threshold exceedances, and
(ii) generalize the results to increase the usability of these tools by Reclamation Managers, Constituents, and
Partners to support long-term water resources management.
The following are the Research Questions associated with this proposal.
1. What is the frequency and magnitude of extreme precipitation events that lead to decision-relevant impacts
(e.g., flooding, exceedance of reservoir storage capacity) under historical climate conditions?
2. How can we best use this characterization of past conditions, along with existing climate projections, to
understand how the frequency and magnitude of extreme precipitation events and impacts are likely to change in
3. How can this information be used effectively by Reclamation and its constituents for long-term water resources
planning? That is, can managers move from managing for escalating risks to planning for ways to more effectively
use the changing water supply?
Need and Benefit
The Bureau of Reclamation, through the Research Office, Technical Services Center, and Regions, has become a leader in the development of projections of the hydrologic impacts of climate change. The "Downscaled CMIP3 and CMIP5 Climate and Hydrology Projections" website that Reclamation hosts has become a "go-to" source for hydrologic projections for the continental United States. However, these projections emphasize changes to water supply and demand. The technique used to bias correct the projections at a local level restricts the variability in temperature and precipitation – and subsequently the occurrence of extreme events – to conditions that have occurred in the past. Therefore, there is concern that the technique masks changes in extreme events that may be embedded in the future GCM simulations. The GEV tools proposed here can be tested with climate covariates from observations and the Reclamation website to better understand changes in extremes.
The proposed research addresses a need identified in Reclamation's forthcoming "2016 SECURE Report to Congress – Congressional Report". Under "Priority Technical and Research Needs", the report calls to: "Improve the Understanding of Impacts Associated with Extreme and Unusual Events." It specifically states: "Studies should be encouraged to more thoroughly investigate the full spectrum of impacts and uncertainties associated with extreme events, to help inform appropriate infrastructure investments." It also specifically points out that in some basins, such as the Rio Grande and Pecos Basin in New Mexico, there may be opportunities to potentially exploit changes in extreme events. This makes case study selection of these New Mexico basins a logical place to develop and demonstrate this research. For example, in the Rio Grande, most reservoirs are designed to capture snowmelt runoff, but increasing temperatures and decreasing snowpack threaten to reduce upstream storage potential. As such, Reclamation is wondering if they will need more downstream storage or detention capability to manage water from extreme precipitation events, such as during the summer monsoon. Previously developed tools such as the GEV are limited in that they only look at a single maximum event per year, whereas the POT approach proposed here is more suitable to this question because it estimates the frequency of extreme events, as well as the magnitude of each of those events.
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