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Animas-La Plata Compact Arkansas River Compact Colorado River Compact Costilla Creek Compact (Rev. 1963) La Plata River Compact Laramie River Equitable Apportionment Decree North Platte River Equitable Apportionment Decree South Platte River Compact Republican River Compact Rio Grande Compact Upper Colorado River Compact

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NEW MEXICO then the Upper Basin share may be no more than 6 MAF once downstream obligations are satisfied. Accepting this figure, Colorado’s 51.75 percent entitlement of the Upper Basin share translates to 3.1 MAF annually, rather than 3.9 MAF. The difference between these two figures, 800,000 acre feet, is significant. It roughly equals total current municipal and industrial demand in the South Platte basin. This problematic feature of the Colorado River Compact has the potential to get considerably worse due to climate change. Colorado already uses about 2.3 MAF a year of its entitlement, which theoretically make roughly 0.8 MAF, or 800,000 acre feet per year, legally available for new development, ignoring other constraints and considerations, or slightly more than the total projected state increase in new demands by 2030. But depending on which of several, highly debatable hydrologic and legal assumptions are used, just a 10 percent decline in average stream flow might be sufficient not only to completely erode this growth cushion, but also to threaten the reliability of existing water uses. Not surprisingly, several efforts to better estimate Colorado River flows and available supplies are ongoing, but are unlikely to provide certain answers. Possible declines in average Colorado River flows are not the only hydrologic challenge confronting Colorado water managers. Climate variability, i.e., drought, is a long-standing concern, especially given climate studies projecting increased droughts in the future, and the current drought

crisis that has gripped the region. River flows have been approximately 62 percent of the 30-year average in 2000, 59 percent in 2001, 25 percent in 2002, 51 percent in 2003, 49 percent in 2004, 105 percent in 2005, 7 percent in 2006, and 68 percent in 2007; 2008 is expected to be an average or aboveaverage year. So far, the major storage reservoirs in the Upper and Lower Basins have provided the needed cushion to prevent widespread shortages, and have provided the stimulus for the states to enact new basin-wide reservoir management rules that should reduce the possibility of curtailments being needed in Colorado to meet interstate obligations. The twin challenges of declining average flows and the enhanced probability of larger and more frequent droughts are being addressed by a variety of adaptation mechanisms. Of these two concerns, drought is the more familiar challenge, as is the solution: storage. However, while storage reservoirs can provide the buffer needed to smooth out flow variability across seasons and years, they are unlikely to boost system yields if longterm average streamflows decline, especially on a river like the Colorado that already can store roughly four full years of flow. This aspect of the climate change challenge calls for a much more diversified suite of adaptation strategies based on a combination of augmenting supplies and reducing demands, perhaps through mechanisms as diverse as transbasin imports; desalinization; weather modification, such as cloud seeding; efficiency im-

provements; pricing mechanisms; and shifting water among uses and users. Many examples of these mechanisms can now be found in the Colorado basin, at both the interstate scale and within particular states, including Colorado. Current drought conditions and the growing understanding of climate change dramatically accelerated these processes. The Special Issue of Water Rights Timing Another issue with an interstate component is the shift in timing of water flows in response to climate change. Colorado is a signatory to five interstate compacts—on the Arkansas, La Plata, Rio Grande, and South Platte rivers, and Costilla Creek—that feature apportionment formulas using specific calendar dates. These dates are usually in the spring and fall, and are used to distinguish between key water management seasons (such as the irrigation and storage seasons) and between periods of restricted versus unrestricted use. With every passing year, the formulas may become more out of step with the natural hydrograph and with patterns of water demand. A similar situation exists with many important intra-state water rights, including water rights purchased by cities from farmers. While this aspect of the climate change/water nexus has not been highly problematic yet, it is a reminder that climate change affects water systems in multiple ways, and solutions must address both the institutional and physical infrastructure of water management. q

Citizen’s Guide to Colorado Climate Change

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Citizen's Guide to Colorado Climate Change  

This guide presents a range of contemporary climate change information presented by Colorado experts.