Development and Comparison of Long-Term Planning Hydrologies using Alternate Climate Information Sets
Long-term evaluations typically involve multi-decade simulations of operations forced by a hydrologic time series consistent with the instrumental record. The implied assumption is that hydrology of the instrumental record adequately portrays the hydrologic variability envelope that could occur during the evaluation's look-ahead horizon.
Reclamation has begun to test this paradigm and experiment with modified hydrologic assumptions that account for both instrument record and paleoclimate or projected climate information (UC, LC, MP, PN). However, joint accounting for paleoclimate and projected climate information has yet to be demonstrated within Reclamation.
(1) How do planning hydrologies vary when developed using alternative climate information sets (e.g., blends of instrument record, paleoclimate data, climate projections)?
(2) How can paleoclimate and projected climate data be jointly accounted for within long-term planning hydrologies?
Need and Benefit
Reclamation has considerable experience conducting long-term evaluations of project operations scenarios. Such evaluations often feature multi-decade look-ahead horizons. The objectives can vary, including assessment of proposed scenario benefits to Reclamation's mission of efficiently delivering water or generating power, and exploring how the proposed scenario could create operational conditions that impact various resource areas.
Potential hydrologic norms and variability are key assumptions framing these evaluations (among several others, including natural and managed water use, regulatory conditions, environmental factors affecting operations descretion, etc). Traditionally, the assumed hydrologic norms and variability are implied by simulating operations given a hydrologic sequence from the observed instrument record (~starting in sometime during the 20th Century). Implicit in this assumption is that the climate norms and variability coincident with the hydrologic instrument record (e.g., monthly mean temperatures and precipitation; monthly to annual extremes of both) will persist into the evaluation's look-ahead horizon.
Reclamation has recently questioned the significance of assuming persistent "instrument-record" climate in long-term evaluations. Alternate climate data sets have been proposed, including evidence on variability from the paleoclimate record, or projected climate changes reported in science synthesis reports (e.g., IPCC Fourth Assessment Report, February 2007, Working Group I). Related literature articles on regional climate and hydrologic impacts have also been receiving consideration (e.g., Lettenmaier & Christensen (2006) study on Colorado River basin runoff response to projected climate change; Maurer (2007) study on CA Sierra Nevada runoff response to climate change).
To date, Reclamation's efforts have been focused on taking advantage of either of these two types of information, but not both types at the same time. For example, the LC and UC regions have collaborated on exploring the use of literature methods that blend hydrologic instrument record with tree-ring based reconstructions of hydrologic variability predating the instrument record (Appendix N, Draft EIS Colorado River Interim Guidelines for Lower Basin Shortages and Coordinate Operations for Lake Powell and Lake Mead). In another example, the MP region has collaborated with Reclamation's Science & Technology program (#X6253, "Assessing the Risks of Shifting Climate Variability on Reclamation's Water and Power Operations," FY2006-2007) and the CA Department of Water Resources to demonstrate how projected climate information can be translated into natural runoff change from headwater basins, and subsequently into adjusted reservoir inflow assumptions framing long-term simulation of Central Valley Project and State Water Project operations. The latter methods involve a "period-norms" perspective where base to future period changes in mean monthly climate and runoff are identified, contrasting from a "transient" approach where evolving changes in climate and runoff are identified (e.g., Lettenmaier & Christensen (2006)).
Reclamation capabilities are currently underdeveloped in several areas:
-- being able to represent evolving change in projeced climate and runoff.
-- being able to jointly consider paleoclimate climate evidence and projected climate data.
-- being able to raionalize which climate information set is preferrable for a given evaluation(instrument record, instrument + paleo, instrument + projected, or instrument + paleo/projected blend), and what the ramifications would be given the choice of information set.
This proposed research addresses each of these areas. Although the case study basins are focused in the GP and UC regions, the methodologies evaluated in these basins would be broadly applicable throughout Reclamation.
Contact the Principal Investigator for information about partners.
Bureau of Reclamation Review
The following documents were reviewed by experts in fields relating to this project's study and findings. The results were determined to be achieved using valid means.
Long-Term Planning Hydrology based on Various Blends of Instrumental Records, Paleoclimate, and Projected Climate Information (final, PDF, 2.7MB)
By Levi Brekke
Publication completed on July 01, 2009