There’s never been a “call” on the Colorado River – a need to curtail use in order to meet Colorado Compact delivery obligations. But folks in Western Colorado are preparing for the possibility. Via John Orr’s Coyote Gulch (a Colorado (state) water blog), a release from the Colorado River District explains how it works:

[U]sers with pre-1922 rights would be compensated for entering into an agreement to offer their senior water rights that are exempt from compact administration to junior users who would otherwise be called out by compact delivery requirements. Junior water users, in turn, could “subscribe” to the bank as a sort of insurance policy in the event of a compact call on their water rights. The bank would serve as the administrator and clearing house for those with senior, pre-1992 water rights and those with junior rights needing an alternative source of water. Temporary use of senior rights by juniors would only be permitted if a compact call were in effect or imminent.