Climate change in the Colorado River Basin: Water management implications Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington Ninth SAHRA Annual Meeting Tucson September 23, 2009

Barnett and Pierce (WRR 2008, “When will Lake Mead go dry?”)  Lakes Mead and Powell treated as a single storage unit  Initial condition: live storage as of June 2007 (25.7 MAF).  Storage in each reservoir assumed approximately equal  Baseline mean net inflow 15 MAF

Deterministic analysis: Constant inflow (initially 15 MAF/yr) with linear decrease to new assumed mean (10-30% reduction) Stochastic analysis: Assumes various models of time- varying inflows, with same long-term trends as in deterministic analysis

From Barnett and Pierce, WRR 2008

From Barnett and Pierce, PNAS 2009

Cumulative Probability CDFs of R4 (cumulative shortfalls over 30 years) for scenarios with Cv = 0.5, G = 0.75, C = 2.0, D = 0.8 (all units are number of mean annual reservoir inflows). Reservoir initial contents were lesser of first year’s inflow or capacity. 1: h = 0.5, ρ = h = 0.6, ρ = h = 0.7, ρ = h = 0.8, ρ = 0.4

The main issue: D/μ approaching or exceeding 1? “We emphasize that while D = 0.8 represents a severe demand level … it will likely become more common in the future” (Burges and Lettenmaier, 1982)

Storage Reservoirs Run of River Reservoirs CRRM Basin storage aggregated into 4 storage reservoirs –Lake Powell and Lake Mead have 85% of basin storage Reservoir evaporation = f(reservoir surface area, mean monthly temperature) Hydropower = f(release, reservoir elevation) Monthly timestep Historic Streamflows to Validate Projected Inflows to assess future performance of system Christensen et al (Climataic Change, 2004)

Total Basin Storage

Conclusions The main issue is that the system is approaching the intersection where D/μ > 1. When that occurs, “failures” are inevitable. D is increasing, and μ may be decreasing, accelerating the rate at which the intersection is being approached. Differences in various studies have to do with accounting for D (e.g., contingent reductions based on Lake Mead level) and μ (e.g., accounting for reservoir evaporation, bank storage, flow below Lees Ferry. It seems likely that the intersection will be reached within the next ~25 years – pessimistic assumptions say it may have already happened, optimistic assumptions push the date out a few decades All of the analyses are based on physical considerations only, and assume in particular that D is a hard number. In fact, if a “ soft landing ” is to be achieved, water allocation mechanisms (e.g. the entire structure of compacts, treaties, and legal decisions) will have to be revisited.