A trend of decreasing snowmelt runoff in northern California

During the last couple of decades, the fraction of annual natural runoff during the April through July period in California's Sacramento Basin has decreased. The decline seems to have started around 1960. Estimates of natural or unimpaired runoff are developed by adjusting measured flow for upstream water development. There is no evidence of changes in the way the estimates are derived. Although the trend is compatible with global warming, it is doubted that this is the reason. Some subtle shifts in patterns of precipitation appear to be the primary cause, along with some prominent midwinter rain floods which have increased the winter fraction of runoff. The trend may be part of some broad scale regional shifts in ocean or weather patterns.