THE SOURCE HYDROLOGY OF SEVERE SUSTAINED DROUGHT IN THE SOUTHWESTERN UNITED-STATES

This paper considers the risk of drought and develops drought scenario s for use in the study of severe sustained drought in the southwestern United States. The focus is on the Colorado River basin and regions t o which Colorado River water is exported, especially southern Californ ia, 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 reconstru ctions of streamflow based on tree ring widths. Drought scenarios in t he Colorado River are defined on the basis of annual flow at Lees Ferr y. Possible spatial manifestations of the Colorado River drought scena rios for input into a Colorado River system simulation model are devel oped by disaggregating the Lees Ferry flow to monthly flows at 29 sour ce locations required by the model. The risk, in terms of return perio d, of the drought scenarios developed, is assessed using stochastic mo dels applied to both the Colorado River basin and the combined flow in four major California rivers. The risk of severe sustained drought oc curring concurrently in the Colorado River basin and California is als o assessed.