San Francisco Estuary

California's primary hydrologic system, the San Francisco estuary and its upstream watershed, is vulnerable to the regional hydrologic consequences of projected global climate change. An understanding of the potential impacts is necessary to prepare mitigation strategies. Projected temperature anomalies from a global climate model are used to drive a combined model of watershed hydrology and estuarine dynamics. By 2090, a projected temperature increase of 2.1 degrees C results in a loss of about half of the average April snowpack storage, with greatest losses in the northern headwaters. Consequently, spring runoff is reduced by 5.6 cubic km, with associated increases in winter flood peaks. The smaller spring flows yield spring/summer salinity increases of 5 psu, with larger increases in wet years. Mitigating the impacts of climate change is likely to more difficult in wet years.

The figure to the left shows the modeling hierarchy used to assess potential impacts of climate change on the estuary and watershed. Climate change simulations from the Parallel Climate Model (PCM) are downscaled to drive the Bay-Delta Watershed Model (BDWM), yielding estimates of hydrologic impacts throughout the watershed (region highlighted). Simulated total watershed outflows are then used to drive the Uncles-Peterson (UP) model, generating estimates of estuarine impacts.

-

Simulated snowpack under warmed conditions depicts a severe loss of snow as indicated by changes in the snow water equivalent (SWE). By 2030, under the "business-as-usual" scenario, temerature is projected to rise about 0.6 degC, resulting in a minor decrease in April snowpack at lower elevations. However, by 2060 a temperature rise of 1.6 degC results in a loss of one-third of the total snowpack. This loss is focused in mid to lower elevations since the snowpack there is more sensitive to temperature changes than at higher, colder elevations. Regionally, this means that the northern Sierra and Cascades experience the greatest loss.

Simulated April snow water equivalent (SWE), expressed as a percentage of average 1995-2005 SWE, for 2050-2069 is shown in the figure to the right.

Knowles and Cayan, Potential effects of global warming on the Sacramento/San Joaquin watershed and the San Francisco estuary, to appear in Geophysical Research Letters. Click here for the full pdf file of this article.