From Inkstain (John Fleck):
Article VII of the Rio Grande Compact is one of the keys to allocating the river’s supply among Colorado, New Mexico, and Texas:
Neither Colorado nor New Mexico shall increase the amount of water in storage in reservoirs constructed after 1929 whenever there is less than 400,000 acre feet of usable water in project storage….
Operationally, this is critical. It means that in drought conditions, irrigators cannot store spring runoff in essentially all upstream reservoirs for summer use. There’s been some flexibility written into the law in practice, but it only operates at the margin. It basically means that during droughts, one of the water managers’ most important tools (storage) is constrained.
More Rio Grande River Basin coverage here.