Yes! 11 trillion gallons of water is required regarding to the calculation from NASA satellite data (NASA news, 16 December 2014). If this number can not give you a specific idea, well, it means around 17 million full Olympic swimming pools. Such large amount required water seems hard to believe after severe floods and mudslides occurred in southern California (BBC news, 13 December 2014). So, what about droughts situation in California or U.S.? Does the drought get more severe? Just like the news what I found on the website that said the drought is the worst has seen in 1200 years in California (USA TODAY, 5 December 2014).
Figure1 the U.S. drought condition (Source: United States Drought Monitor)
After analysis among lots of indices such as SPI (standard precipitation index), NPP (net primary productivity) and NCE (net carbon exchange), Chen et al. 2012 indicates that there was no apparent change of droughts in southern U.S. from 1895 to 2007, although there was an increasing trend in drought intensity in many east regions. This change seems to bring a large loss to this area, as we can see from the resultant obviously drop in NNP which can even reach to 40% during extreme droughts (Chen et al. 2012). There is a wetting trend in most parts of U.S. according to the simulation of soil moisture and runoff except the southwest part in U.S. which has a few declining trend of soil moisture and runoff with increasing drought duration and severity (Andreadis and Lettenmaier 2006).
In regard to this opposite trend compared with the rest regions of the U.S., what might be the underlying reason? Kam et al. 2014 indicates that putting a positive phase of the AMO (Atlantic Multidecadal Oscillation) and a negative phase of the PDO (Pacific Decadal Oscillation) and ENSO (El Niño–Southern Oscillation) together, they would play a worse influence on droughts in the southern U.S. A continuous ridge of high atmospheric pressures generate offshore which would transfer storms track from California to Alaska during a majority of current winters may be the cause of droughts, because California gets most of precipitation in Decembers and Januaries (Dettinger and Cayan 2014). They also mentioned that droughts are going to be more severe and frequent due to climate change and there is a deep relationship between droughts and extreme storms in California. An increasing trend of temperatures results in the decreasing trend of mountain snowpack and earlier melting time of spring snow, which would play an important role in a declining trend of total precipitation (Seager and Vecchi 2010).
To sum up, California seems to be a special region in U.S. with different drought trends which may be not very significant but still increasing and especially in duration and severity of droughts. In addition, there are several causes behind this trend. The first reason is the influence from the positive phase of the AMO, negative phase of the PDO and ENSO; The second is regarded as anomalous atmospheric pressure which further cause the change of storms; The third is global warming which bring effect on amount of snowpack and melting time.
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