30 October 2009

Surviving Mega-Droughts - on Groundwater?

At the recent California Biennial Groundwater Conference, I was assigned to chair a special session on "Thriving (or Surviving) in Times of Drought". Scott Stine, California State University East Bay, opened the session with his fascinating story and review of California's medieval mega-droughts. He is known perhaps mostly for his discovery and age-dating of old tree stands hidden below the surface of Mono Lake until recent water diversions to LA substantially lowered the lake level. At the meeting, Scott unfolded a story of varied evidence from many more places - from Point Reyes at the Pacific Coast to Walker Lake and Pyramid Lake in the Great Basin - that consistently tell of large mega-droughts, one between approximately 900-1100 AD and another from approximately 1200-1350 AD.

In fact, it turns out that the first half of the 20th century  - the hydrologic time period used by engineers as a basis to design our large Western U.S. water projects - was one of the wettest periods not just in the Colorado Basin, but also in California's climate history of the last two millenia. Tree stumps from periods of when lake levels in California were climatically low - sometimes well below current day discharge elevations - are found not just at Mono Lake, but also in Donner Lake, Fallen Leaf Lake, and Tenaya Lake, to name just a few. For entire forests of large trees to grow well below today's lake level of, for example, Tenaya Lake, weather in California must have been much drier than today - not just for three years, but for decades and centuries.

Scott took us on an amazing detective's journey of evidentiary materials for these mega-droughts, followed at times by mega-wet periods: Sierra's alpine meadow wetlands and geologic records of plant stands, ancient soils, lake shore levels, flood sediments, juxtaposed against time markers, such as known volcanic eruptions leaving behind widespread layers of ashes or tuff - scattered across California and neighboring states. The droughts were long and deep - Mono Lake was perhaps 50-60 ft (15 - 20m) below its early 20th century lake level, well below even its recent, man-induced low-stand.

It is a stark reminder of California climate's variability, even before global warming. Importantly, groundwater appeared to have played a critical role for some plants and animal species to survive these mega-droughts:  using pupfish as an example, Scott pointed out that species may have survived over generations in and near springs of Eastern California, fed by groundwater. Thus, these species celebrated a major comeback during wetter climate conditions, despite the fact that their habitat had almost completely been destroyed by drought. It is a concrete, if mostly metaphoric success story from which we may wisely take our cues as we manage an always uncertain water future in California: groundwater is an essential part of our water landscape.

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