This post continues a monthly series about our weather and California’s drought. The initial post, “A Year With No Winter“, appeared in February 2014.
The high pressure system has moved from California, so the jet stream is finally bringing a series of storms to California. Our November rainfall was 2/3 of normal, but rain is continuing into December. As shown in the above figure from the November forecast, the US Weather Service predicts that California’s “drought remains but improves” through February 2015, upgraded from the “Drought persists or intensifies” in October.
Our rain year starts in July, and we normally get 75% of our annual rainfall through February. With drought predicted to impact three-quarters of our annual rain, California’s drought could extend to a fourth year unless the prediction is wrong or our spring is unusually wet.
California continues to pump much more groundwater than is being recharged. To meet customer demand for water during the drought, we have overdrawn groundwater in our Santa Clara Valley to a level that caused the valley to sink up to 3′ by 1929. Since we border the San Francisco Bay, land subsidence increases our risk of flooding, just as sea level rise does. California passed a bond election to build new water storage, but this will probably impact water quality in the San Francisco delta.
Los Altos Weather
The high pressure system off the California has moved inland, opening the storm door. Despite a storm at the end of November that dropped half an inch of rain, our November rainfall was still 2/3 of normal. Storms and rain continue in December.
With the below-normal November rainfall, our cumulative rainfall fell further behind normal, as the drought deepened. Since the beginning of 2013, we have received only a third of our normal rainfall, and we’re an arid area so we don’t get much rain.
The trend of above-normal overnight temperatures continued. Combined with the low rainfall, the higher temperatures increased pressure on the drought-stricken plants and wildlife. For example, we haven’t had a killing frost yet, so some fruit trees and roses still have green leaves. The first date for frost in our county is November 15.
Groundwater and Subsidence
To ensure that water flows when we turn the faucet during the drought, our Santa Clara Valley is pumping much more groundwater than is being replaced. In the 20th century, the valley sank up to 13′ before we started recharging the groundwater. Our water district suspended groundwater recharge this year to meet user demand. Pumping lowered the groundwater level by about 35′ in 2014, nearly twice as much as the 20′ drop in 2013. How much more groundwater can we overdraw before land subsidence resumes?
As shown in the below Recharge graph from our local water district, the recharge of groundwater was significantly curtailed this year. The second chart shows that groundwater pumping continues at the same level as before, more than 10,000 acre-feet per month. From the third chart, the groundwater elevation at a San Jose well dropped about 20′ during 2013 and additional 35′ during 2014, after recharge was curtailed. The groundwater elevation, the level of the groundwater relative to sea level, at the beginning of November was about 25′ above sea level.
Until the 1960s, our valley pumped much more groundwater than was recharged. Less water in the aquifer reduced the water pressure holding up the valley floor, and our valley sunk up to 13′. To put this into perspective, past groundwater overdrafts caused our valley to sink two to three times more than the sea level rise projected through 2100 due to global warming.
The groundwater elevation and subsidence graph below shows the relationship between groundwater elevation and land subsidence. The blue bars show the groundwater elevation, which started at 100′ above sea level in 1915 and fell to 130′ below sea level in 1964, when water deliveries from the Sierras started. The red line shows the resulting land subsidence. Through 1964, the land subsided as the groundwater elevation declined. Then state water deliveries began: the groundwater elevation rose, and land subsidence was arrested.
The groundwater elevation for the well published by the water district is approximately 25′ above sea level. In 1929, when the groundwater elevation sank to 25′ above sea level, we had about 3′ of subsidence. But in 1988 the groundwater elevation sank to -5′, and no land subsidence was measured.
The water district hasn’t posted an update showing how much our valley might sink as the drought continues. The valley already sank up to 13′, and the sea level rise due to global warming is projected to be between 2.5 and 6.5 feet by 2100. Since our valley borders the San Francisco Bay, land subsidence would increase local flooding, just as sea level rise would. If the drought continues, ground subsidence could be just as significant for local sea level rise as global warming.
Future Water Storage Might Degrade Delta Water and Wildlife
California approved $2.7B for water storage projects, dams and reservoirs. A study from the University of California, Davis, says that California’s water storage can be increased only 15% because dams must be built where there is rainfall and California is an arid region. As one co-author said, “Reservoirs cannot supply water without a water supply to fill them first.” A second co-author said “Our current water supplies are over-allocated, and we need to invest in a much smarter strategy to upgrade our water system and meet multiple water needs with an eye to the future and changing climate conditions.”
Two candidates for new water storage are in the Sacramento-San Joaquin watershed, the large area in the USGS map. Capturing nearly half of California’s rainfall, that watershed drains into the delta east of San Francisco. Wildlife in the delta is already declining due to poor water quality. Storing more water that would flow into the delta will make life harder for the delta wildlife.