The source hydrology of severe sustained drought in the southwestern United States

This paper considers the risk of drought and develops drought scenarios for use in the study of severe sustained drought in the southwestern United States. The focus is on the Colorado River basin and regions to which Colorado River water is exported, especially southern California, which depends on water from the Colorado River as well as the four major rivers in northern California. Drought scenarios are developed using estimates of unimpaired historic streamflow as well as reconstructions of streamflow based on tree ring widths. Drought scenarios in the Colorado River are defined on the basis of annual flow at Lees Ferry. Possible spatial manifestations of the Colorado River drought scenarios for input into a Colorado River system simulation model are developed by disaggregating the Lees Ferry flow to monthly flows at 29 source locations required by the model. The risk, in terms of return period, of the drought scenarios developed, is assessed using stochastic models applied to both the Colorado River basin and the combined flow in four major California rivers. The risk of severe sustained drought occurring concurrently in the Colorado River basin and California is also assessed.