Pima County and the City of Tucson have a cooperative project to study the regional water supply and demand. “The ultimate goal of this effort is to assure a sustainable community water source given continuing pressure on water supply caused by population growth.”
Water is vital to life, so there is concern about the current drought in the Western U.S. and its impact on our water supply. In Arizona, our supply from the Lower Colorado River system stands at just 56% capacity as of Jan. 19, 2010, according to the Bureau of Reclamation. The Salt River system, supplying Phoenix, stands at 79% capacity, and the Verde River system is at 34%.
Some claim that the current drought is the result of human-induced global warming; others blame the ozone hole. However, droughts are naturally occurring and cyclic.
According to NOAA, “Droughts occur throughout North America, and in any given year, at least one region is experiencing drought conditions.” “Droughts similar to the 1950s, in terms of duration and spatial extent, occurred once or twice a century for the past three centuries (for example, during the 1860s, 1820s, 1730s). However, there has not been another drought as extensive and prolonged as the 1930s drought in the past 300 years. Longer records show strong evidence for a drought that appears to have been more severe in some areas of central North America than anything we have experienced in the 20th century, including the 1930s drought.”
In the Pacific northwest, Knapp et al, found that widespread and extreme droughts were concentrated in the 16th and early 17th centuries when the planet was considerably colder than the 20th century.
In a separate study of mean water-year flow on the Columbia River, Gedalof et al. found that “persistent low flows during the 1840s were probably the most severe of the past 250 years,” and that “the drought of the 1930s is probably the second most severe.” They say also that ” recent droughts were not exceptional in the context of the last 250 years and were of shorter duration than many past events.”
In Montana and Idaho, Gray et al. (2004) found that “both single-year and decadal-scale dry events were more severe before 1900,” and that “dry spells in the late thirteenth and sixteenth centuries surpass both the magnitude and duration of any droughts in the Bighorn Basin after 1900.”
Researchers working in the Pyramid Lake area of Nevada found that for the past 2,740 years “intervals between droughts ranged from 80 to 230 years; while drought durations ranged from 20 to 100 years.” Another study in the same area found that the longest of these droughts occurred between 2,500 and 2,000 years ago and between 1,500 and 1,250, 800 and 725, and 600 and 450 years ago, with none recorded in more recent warmer times.
In the Rocky Mountains, Gray et al. (2003) found a pattern of droughts that they say “may ensue from coupling of the cold phase Pacific Decadal Oscillation with the warm phase Atlantic Multidecadal Oscillation.”
Research on the Upper Colorado River Basin shows “a near-centennial return period of extreme drought events in this region.” The major drought of 2000-2004 was not as severe as 1844-1848, and was similar to droughts in the early 1500s and early 1600s. They conclude, “these analyses demonstrate that severe, sustained droughts are a defining feature of Upper Colorado River hydroclimate.” And the results show that more severe droughts are associated with colder cycles.
Work in Arizona and New Mexico shows that “sustained dry periods comparable to the 1950s drought occurred in “the late 1000s, the mid 1100s, 1570-97, 1664-70, the 1740s, the 1770s, and the late 1800s.”
Drought cycles are most closely correlated with various solar cycles of 1,533 years (the Bond cycle), 444 years, 170 years, 146 years, and 88 years (the Gleissberg cycle). Asmerom,et al. report that periods of increased solar radiation correlate with periods of decreased rainfall in the southwestern United States (via changes in the North American monsoon). These solar cycles control the Pacific Decadal Oscillation and the El Nino system which control weather and climate in the southwest. We are just entering solar cycle 24 and it seems very sluggish. That could mean that we will be spared from an intensifying drought.
For specifics on Tucson’s water supply see:
For a primer on drought see:
To understand the proxies used in paleoclimate research see:
Papers reviewed by http://www.co2science.org/subject/d/summaries/droughtusawest.php
Asmerom, Y., Polyak, V., Burns, S. and Rassmussen, J. 2007. Solar forcing of Holocene climate: New insights from a speleothem record, southwestern United States. Geology 35: 1-4.
Benson, L., Kashgarian, M., Rye, R., Lund, S., Paillet, F., Smoot, J., Kester, C., Mensing, S., Meko, D. and Lindstrom, S. 2002. Holocene multidecadal and multicentennial droughts affecting Northern California and Nevada. Quaternary Science Reviews 21: 659-682.
Gedalof, Z., Peterson, D.L. and Mantua, N.J. 2004. Columbia River flow and drought since 1750. Journal of the American Water Resources Association 40: 1579-1592.
Gray, S.T., Betancourt, J.L., Fastie, C.L. and Jackson, S.T. 2003. Patterns and sources of multidecadal oscillations in drought-sensitive tree-ring records from the central and southern Rocky Mountains. Geophysical Research Letters 30: 10.1029/2002GL016154.
Gray, S.T., Fastie, C.L., Jackson, S.T. and Betancourt, J.L. 2004. Tree-ring-based reconstruction of precipitation in the Bighorn Basin, Wyoming, since 1260 A.D. Journal of Climate 17: 3855-3865.
Hidalgo, H.G., Piechota, T.C. and Dracup, J.A. 2000. Alternative principal components regression procedures for dendrohydrologic reconstructions. Water Resources Research 36: 3241-3249.
Knapp, P.A., Grissino-Mayer, H.D. and Soule, P.T. 2002. Climatic regionalization and the spatio-temporal occurrence of extreme single-year drought events (1500-1998) in the interior Pacific Northwest, USA. Quaternary Research 58: 226-233.
Mensing, S.A., Benson, L.V., Kashgarian, M. and Lund, S. 2004. A Holocene pollen record of persistent droughts from Pyramid Lake, Nevada, USA. Quaternary Research 62: 29-38.
Ni, F., Cavazos, T., Hughes, M.K., Comrie, A.C. and Funkhouser, G. 2002. Cool-season precipitation in the southwestern USA since AD 1000: Comparison of linear and nonlinear techniques for reconstruction. International Journal of Climatology 22: 1645-1662.
Woodhouse, C.A., Gray, S.T. and Meko, D.M. 2006. Updated streamflow reconstructions for the Upper Colorado River Basin. Water Resources Research 42: 10.1029/2005WR004455.