25th Annual Meeting and Symposium of the
Desert Tortoise Council, April 21-24, 2000
Abstracts

Past and Future Climate Variation in the Mojave Desert: Surface Processes and Land Management Issues

Richard Hereford
U.S. Geological Survey, Western Earth Surface Processes Team, Flagstaff, Arizona 86001

As part of the USGS Mojave Desert Ecosystem Science Program, we are studying historic-age climate variation and its affects on the desert landscape. Two components of landscape change amenable to study are sediment yield and the frequency of surface runoff. These should vary jointly with precipitation. Generally, sediment yield (work done cooperatively with R. H. Webb, USGS, WRD-NRP, Tucson) and runoff frequency increase during wet climate episodes and decrease during dry episodes. The problem is whether the magnitude of past climate variation was sufficient to effect hillside runoff and landscape change. Although preliminary and subject to revision, results suggest that runoff frequency and sediment yield have varied with climate. Runoff in over 50 percent of the cases was more frequent in roughly the first half of the 20th century when climate was relatively wet but it was less frequent during the latter half when climate was dryer or when precipitation was less intense. Information presently available suggests that runoff frequency did not increase substantially during the present wet climatic regime, probably because of a concurrent long-term decrease of rainfall intensity. Thus, the physical landscape-the substrate of the desert ecosystem-was affected by the precipitation patterns of past and present climatic regimes.

Climate variation caused by the Pacific decadal oscillation is a high-visibility topic of recent climatological research. This global climate phenomenon is caused by subtle but important changes of sea-surface temperature (SST) and atmospheric pressure in the eastern Pacific Ocean. These changes, which trigger a sharp transition from one climatic regime to another, alter the climate of North America for periods of 25-35 years. The weather, SST, and surface pressure patterns of the past 1-2 years suggest to climatologists that the transition to another regime is underway. If so, the present relatively wet climatic regime in the Mojave Desert should become dryer in the near future.

The climate in the near future may resemble that of the 1940s to mid-1970s, an era of generally cool SST and increased atmospheric pressure linked to drought in the Mojave Desert and Southwest in general. Precipitation from the 1940s to mid-1970s was generally below the long term normal, whereas it was mostly above normal from 1976-1998, when SST was warm and atmospheric pressure was low, conditions that enhance precipitation in the Southwest (see figure).

This anticipated change to drier and possibly warmer conditions will likely affect the biological and physical environments of the Mojave Desert. Persistent dry conditions stress the flora and fauna of the region, decrease surface runoff and replenishment of shallow aquifers, and increase recovery times from human disturbances. Restoration projects, investigations of landscape recovery, and studies of floral and faunal population dynamics undertaken in the previous 20-25 years were done when conditions were usually favorable. Researchers and land management agencies should consider the potential influence of this new, relatively dry climatic regime when planning restoration projects and in monitoring biological components of the ecosystem.

Additional Information

Climate History Web Site: http://mojave.usgs.gov/climate-history/

Selected References

Dettinger, M.D., D. R. Cayan, H. F. Diaz, and D. M. Meko. 1998. North-south precipitation patterns in western North America on interannual-to-decadal time scales. Journal of Climate 11:3095-3111.

Gatewood, J. S., Wilson, Alfonso, H. E. Thomas, and L. R. Kiser. 1963. General effects of drought on water resources of the Southwest, 1942-1956. U.S. Geological Survey Professional Paper 372-B:B1-B55.

Minobe, Shoshiro. 1997. A 50-70 oscillation over the north Pacific and North America. Geophysical Research Letters 24: 683-686.

Zhang, Yuan, J. M. Wallace and D. S. Battisti. 1997. ENSO-like interdecadal variability: 1900- 93. Journal of Climate 10: 1004-1020.