Sustainable water deliveries from the Colorado River in a changing climate

Abstract

The Colorado River supplies water to 27 million users in 7 states and 2 countries and irrigates over 3 million acres of farmland. Global climate models almost unanimously project that human-induced climate change will reduce runoff in this region by 10-30%. This work explores whether currently scheduled future water deliveries from the Colorado River system are sustainable under different climate-change scenarios. If climate change reduces runoff by 10%, scheduled deliveries will be missed approximately 58% of the time by 2050. If runoff reduces 20%, they will be missed approximately 88% of the time. The mean shortfall when full deliveries cannot be met increases from approximately 0.5-0.7 billion cubic meters per year (bcm/yr) in 2025 to approximately 1.2-1.9 bcm/yr by 2050 out of a request of approximately 17.3 bcm/yr. Such values are small enough to be manageable. The chance of a year with deliveries <14.5 bcm/yr increases to 21% by midcentury if runoff reduces 20%, but such low deliveries could be largely avoided by reducing scheduled deliveries. These results are computed by using estimates of Colorado River flow from the 20th century, which was unusually wet; if the river reverts to its long-term mean, shortfalls increase another 1-1.5 bcm/yr. With either climate-change or long-term mean flows, currently scheduled future water deliveries from the Colorado River are not sustainable. However, the ability of the system to mitigate droughts can be maintained if the various users of the river find a way to reduce average deliveries.

Fig. 1.

Model simulations of total active storage in Lakes Mead and Powell for the CRBM model used here (black circles) and the full USBR Colorado River model (red crosses). The 2 inflow sequences and USBR model results (A, after figure N-8; B, after figure N-10) are taken from ref. , appendix N, figures 7–10 with elevations converted to total active storage.

Fig. 2.

Probability of experiencing delivery shortages (blue, %), and the mean delivery shortage (red, bcm/yr), for the cases with no climate change (A) and a reduction in Colorado River runoff of 10% (B) and 20% (C). Also shown for comparison is the largest lower-basin delivery cut included in the USBR's preferred alternative of reservoir operations, 0.74 bcm/yr, or 0.6 maf/yr (dashed line).

Fig. 3.

Probability of delivering <14.5 bcm (11.75 maf) of water in the indicated year (A) and probability of the reservoirs being at least 80% full (B). Lines show cases with no anthropogenic climate change and with reductions in runoff of 10% and 20% driven by anthropogenic climate change.

Fig. 4.

Mean water deliveries from the Colorado River system under various climate scenarios. Line D shows the mean water delivered across all model realizations and can be thought of as the upper limit of sustainable water deliveries. Line B10% shows mean water delivered during years that fall within the bottom 10% of deliveries. For comparison, line R shows requests for scheduled deliveries. (A–C) Computations with 20th-century values of Colorado River flow. (D–F) Computations using an assumed Colorado River flow of 17.38 bcm/yr (14.08 maf/yr) at Lees Ferry, AZ, the mean of estimates from 10 different tree-ring reconstructions. Wherever the D line drops below the R line (shaded regions), requests for water are exceeding sustainable deliveries.

Fig. 5.

Historical and Paleoclimate estimates of Colorado River flow. (A) Vertical line shows observed (naturalized) mean flow in the Colorado River at Lees Ferry, AZ from 1906–2005 compared with a histogram of the mean flow in all sliding 100-yr segments from a tree-ring-based reconstruction of Colorado River flow (21). (B) Time series of naturalized mean annual flow in the Colorado River at Lees Ferry, AZ, along with the 5-year running mean (heavy line) and mean over the period 1906–2005 (18.6 bcm/yr or 15.07 maf/yr) (dashed line).