Background
The authors write that "several studies drawing upon general circulation models have investigated the potential impacts of future climate change on precipitation and runoff to stream flow in the southwest United States," and they report that these studies suggest there will be reduced runoff in response to predicted "increasing temperatures and less precipitation." However, they say that "projected precipitation trends show substantial regional variations, seasonal cycles are poorly represented, and changes are more complex and less certain than those for temperature alone," citing the Intergovernmental Panel on Climate Change (IPCC, 2007), Milly et al. (2005) and Dominguez et al. (2009).

What was done
Concerned about these complexities, Murphy and Ellis examined the evidence for the prediction of reduced runoff for eight sub-basins of the larger Colorado River Basin (CRB), all of which, as they describe it, have "unregulated runoff to stream flow gages, providing a 22% spatial sampling of the CRB." This was done by using the stations' long-term records of surface temperature and precipitation, along with corresponding stream gage records, via time series analysis methods and testing criteria that have been established relative to statistical definitions of stationarity.

What was learned
The two researchers report that (1) "statistically significant temperature increases in all sub-basins were found," but that (2) "tests of precipitation and runoff did not reveal persistent reductions, indicating that they remain stationary processes," while further noting that (3) "transitions through periods of drought and excess have been characterized, with precipitation and stream flows found to be currently close to their long-term average," and finding that (4) "the evidence also indicates that resolving precipitation and runoff trends amidst natural modes of variability will be challenging and unlikely within the next several decades."

What it means
In concluding their study, Murphy and Ellis write that their assessment "has found non-stationary temperature but stationary precipitation and runoff in watersheds of the Colorado River Basin," which further suggests, as they continue, that "the global pattern of observed stream flow trends asserted by Milly et al. (2008) is not evident in the CRB," which they characterize as "one of the most important water sources in western North America."

References
Dominguez, F., Canon, J. and Valdes, J. 2009. IPCC-AR4 climate simulations for the Southwestern US: the importance of future ENSO projections. Climatic Change: 10.1007/s10584-009-9672-5.

IPCC. 2007. Synthesis report. In: Core Writing Team, Pachauri, R.K., Reisinger, A. (Eds.). Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Geneva, Switzerland.

Milly, P.C.D., Betancourt, J., Falkenmark, M., Hirsch, R.M., Kundzewicz, Z.W., Lettenmaier, D.P. and Stouffer, R.J. 2008. Stationarity is dead: whither water management? Science 319: 573-574.

Milly, P.C.D., Dunne, K.A. and Vecchia, A.V. 2005. Global pattern of trends in streamflow and water availability in a changing climate. Nature 438: 347-350.