Impacts of Drought: Water Resources in the Colorado River Basin

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1 Impacts of Drought: Water Resources in the Colorado River Basin Connie Woodhouse, NOAA Paleoclimatology Branch, National Climatic Data Center, and INSTAAR, University of Colorado, Boulder, CO with contributions from Robert S. Webb, NOAA/OAR Climate Diagnostics Center, Boulder, CO Jeff Lukas, INSTAAR, University of Colorado, Boulder, CO Funding for this work has been provided by NSF-ATM, NOAA/OGP and Denver Water

2 Overview Drought in a historical context; impact on political and economic culture of Colorado The recent drought in Colorado Droughts of the past reconstructed from tree rings Implications of paleo record for the Colorado River Compact

3 Droughts are a prominent part of the climate of Colorado and the southwestern U.S. People living in this region have developed a culture that acknowledges the challenges of maintaining water supplies during frequent and sometimes extended periods of drought. Four ancient reservoirs were recently discovered at Mesa Verde which were likely used for drinking water, dating to AD In addition, check dams were to support agriculture. Cliff Palace at Mesa Verde (photo: NPS) Centuries later, John Wesley Powell, in his survey of the Colorado Plateau, witnessed the communal water systems in Mormon and Native American communities. He later advocated local water districts and water laws that would help farmers survive periods of drought. Grand Canyon National Park Museum Collection

4 Timeline of Drought in Colorado and Important Policy and Legal Changes Drought Duration Region Impacted Statewide Eastern Colorado Southwestern Colorado Statewide Statewide Mountains & Western Slope Policy and Legal Changes 1872: Colorado Territorial Supreme Court announces basic water law principles of water scarcity and public access to water sources for beneficial use. 1897: Colorado General Assembly adopts first statutes allowing exchanges of water rights between agricultural, municipal, and other users. 1902: Congress adopts Reclamation Act providing federal aid for irrigation projects. 1937: General Assembly creates Water Conservancy and Conservation Districts, and the Colorado Water Conservation Board. Congress authorizes C-BT Project 1948 Upper Colorado River Basin Compact 1956: Congress adopts Colorado River Storage Project Act; authorizes Glen Canyon, Flaming Gorge, Aspinall, and Navajo dams 1965: Colorado Groundwater Management Act 1981: Water Availability Task Force established; Colorado Drought Response Plan developed From The Role of Climate on Water Institutions in the Western Americas By Justice Gregory J. Hobbs, Jr

5 The Recent Drought In Colorado, drought conditions, starting in late 1999, peaked in the summer of 2002, and extended through Snowpack in 2002 was extremely or exceptionally low throughout the state.

6 Water year values for streamflow were the lowest on record for many gages in the state. This, coupled with high level of water demand, led to unprecedented impacts on water supplies. Photo: Denver Water Blue River annual flow, acre feet year acre feet South Platte annual flow, year

7 Drought conditions in Colorado had far-reaching impacts Headlines in the Denver Post, Winter 2003

8 Is spite of the role drought has played in the past, this drought appeared catch water managers by surprise Why? Colorado experienced persistently above average moisture for most of the last two decades of the 20th century, so there had been no great stress on water supplies. Drought planned has been based on the drought of record, typically the 1950s drought, and this had been adequate to meet water growing needs. However, this wet period coincided with unprecedented growth and increased demands, so impacts were far greater than in the 1950s. Lake Powell Storage, March 1963-Dec

9 By the spring of 2002, Front Range water managers became acutely interested in several key questions: How rare an event was the drought of 2002 or the three year event, ? Is the 20th century record of flow an adequate frame of reference for planning, with regard to extreme years, multi-year droughts, and sequences of drought years? The gage record is too short to answer these questions, but records can be extended with paleoclimatic data from tree rings.

10 How is past streamflow reconstructed from tree rings? Moisture-sensitive tree species growing on open, well drained sites reflect moisture variability in their ring widths and are targeted for collection. Cores collected from about 20 trees are dated, measured, and averaged into site tree-ring chronologies.

11 Recent tree-ring collections from moisture-sensitive sites

12 Lees Ferry reconstruction and gage values A linear combination of 7 chronologies explains 80.6% of the variance in the Lees Ferry gage record. The 19.4% unexplained variance is most often in the extreme wet and dry values. Additional steps are taken to validate the model using independent data (data not used in the calibration).

13 Reconstruction of Lees Ferry Streamflow, Water Year Flow, Woodhouse et al. in review How representative is the 20 th century part of the record relative to the past 4 centuries? Gage record

14 Lees Ferry Reconstruction, Year Running Mean Assessing the drought in a multi-century context Data analysis courtesy of Dave Meko

15 Reconstruction of Lees Ferry Streamflow smoothed to show low-frequency variability and 5 wettest/driest 20-yr periods

16 Streamflow gages for which reconstructions have been generated, and water provider collaborators Denver Water Northern Colorado Water Conservancy District Bureau of Reclamation Hydrosphere Resource Consultants (Cities of Boulder, Aurora) City of Westminster/Coors Rio Grande Water Conservation District

17 How are streamflow reconstructions being used by water providers and other decision makers in drought management and planning? In water supply models to assess the ability of a system to meet demands under a broader range of drought conditions, and the impact of management decisions on water supplies during periods of drought Results from analysis of streamflow reconstructions now appear to be a consideration, along with information on snowpack, runoff, and estimated diversions, in setting allocations and enacting conservation measures. As part of the basis for planning and legal decisions regarding well permits, and compact negotiations?

18 1922 Colorado River Compact ARTICLE III III (a). There is hereby apportioned... in perpetuity to the Upper Basin and to the Lower Basin... the exclusive beneficial consumptive use of 7,500,000 af per annum... III (b). In addition to... paragraph (a) the Lower Basin is hereby given the right to increase its beneficial consumptive use...by one million af per annum. III (c). Provides that water for Mexico shall be supplied first from the waters over and above... paragraphs (a) and (b); and if such surplus shall prove insufficient... the burden... shall be equally borne by the Upper Basin and the Lower Basin and the Upper Division states shall deliver at Lee Ferry water to supply (its obligation)... in addition to that provided in paragraph (d). III (d). The states of the Upper Division will not cause the flow... at Lee Ferry to be depleted below an aggregate of 75,000,000 af for any ten consecutive years... From Eric Kuhn, Colorado River Water Conservation District

19 How often is annual flow at Lees Ferry 16.5 MAF or more? Only 173 out of 462 years (37% of years) How often has Lees Ferry flow been less than 7.5 MAF? 33 out of 462 years but only 4 times in the 20 th century (up to 1997) Lees Ferry Water Year Flows Year Acre Feet ,300, ,000, ,900, ,400, ,900, ,000, ,400, ,100, * 10,000,000 AVG ,082,305 Woodhouse et al. in review

20 If the 7.5 MAF is not available in Upper Basin storage (Lake Powell), a call on the river would be made. Users with rights junior to the compact would have to forgo their water to fulfill obligations to the lower basin. 25% of Colorado s share of Colorado River water is diverted out of basin, primarily to the Front Range. These diversions were developed after 1922, so are based on junior rights.

21 To date, there has never been a shortage in the Lower Basin, and there are currently no shortage guidelines. The Secretary of the Interior has requested the 7 basin states to come up with guidelines by Fall The tree-ring records can provide evidence for past droughts, and the range of natural variability that has occurred over past centuries. This information should be considered in drought planning, and there is some indication that it is now being considered. Planning Workshop to Develop Hydroclimatic Reconstructions for Decision Support in the Colorado River Basin Tucson, AZ, May 4, 2005