The West Without Water by B. Lynn Ingram and Frances Malamud-Roam discusses recent weather research and what it tells us about California weather — we should expect longer droughts and larger floods than we have seen in the last 100 years. Ingram is a Professor of Earth and Planetary Science and Geography at the University of California (UC) at Berkeley, and the The West Without Water was published the the University of California Press in 2013.
The worst drought in written history occurred in the winter of 1976-1977, when state precipitation was less than half of normal. River flows declined to a quarter of the normal, increasing the salinity of the San Francisco Bay and delta and severely disrupting migration and spawning of migratory fish such as salmon and trout. In the Bay Area, we had mandatory water rationing where we couldn’t wash cars or water lawns. Fortunately, the drought lasted only a year.
In November and December 1861, early snow blanketed the Sierras with 10-15′ feet of snow. Starting at Christmas, a series of warm storms dropped 60-102 inches of rain, melting the snow. The entire Central Valley, 250-300 miles long and 20-60 miles wide, was flooded. The state capital, Sacramento, remained underwater for six months, so the state government was temporarily moved to San Francisco.
What causes this variation in the weather? Storms develop from heat and moisture from the ocean. California is on the western edge of the Pacific Ocean, the world’s largest ocean, which covers a fourth of the world. Storms develop over the Pacific and flow into the west coast of North America. The strength of the storms and where they hit is governed by several factors, such as El Nino and La Nina. The authors describe and explain the factors, but they’re outside the scope of this post. I especially appreciated the clear description of the mechanism for upwelling and our dry summers:
“just off the coasts of North and South America, the surface waters blown to the west by the trade winds are replaced by colder water from beneath the surface in a process known as “upwelling.” These cold, upwelled waters cool the overlying atmosphere, forming high pressure, resulting in very dry conditions in the adjacent land areas, including Ecuador, Peru, and Chile in the south and the American Southwest to the north”.
Water vapor from the Pacific is carried through corridors of concentrated moisture, dubbed “atmospheric rivers”. These atmospheric river storms contribute about 30-50% of California’s precipitation, so where the atmospheric river flows determines California’s rainfall.
Written records go back 100-150 years. The next section of the book discusses techniques to look farther back and and the findings from those investigations.
Researchers examine “mud from the bottom of lakes and ponds, microscopic organisms living in the oceans, bubbles frozen in glaciers, pencil-thin wood cores drilled from trees, and salts precipitating in dried-up lake bottoms”. The authors cite research, techniques, conclusions, and publications for each kind of evidence.
Based on tree rings, bay sediments, seabed sediments, and coral, the West had two prolonged “megadroughts” separated by a wet interval around AD 1100-1200. This would explain why the ancestral Pueblo cultures of Chaco Canyon and Mesa Verde abruptly collapsed. Going back farther in time, scientists examined large tree stumps submerged over twelve feet below the surface of Lake Tahoe. Some trees were 3 feet in diameter, with one over 150 years old. This indicates a prolonged drought lasting more than 150 years. The trees are more than 4800 years old.
As for floods, “floods at least the size of the 1861-62 event struck Northern California on the average every 100 to 120 years”, based on floodplain sediments from the Sacramento River. In the past 800 years, four floods dwarfed the 1861-62 flood.
Megadroughts and flooding the Central Valley for months is scary stuff, and then there’s recent climate change! Are these megadroughts and floods credible? The West Without Water is written by a UC Berkeley professor, cites journals, and is published by the University of California Press. It’s not clear that current water and flood planning and preparation account for the droughts and floods discussed in this new book.