This paper is measuring stuff in Australia, but seems to mimic the dropoff in runoff we’re seeing on the Rio Grande and other western U.S. rivers compared to the precipitation deficits we’re experiencing:
Annual rainfall and runoff records from south-eastern Australia are used to examine whether interdecadal climate variability induces changes in hydrological behavior. We test statistically whether annual rainfall-runoff relationships are significantly different during extended dry periods, compared with the historical norm. The results demonstrate that protracted drought led to a significant shift in the rainfall-runoff relationship in ~44% of the catchment-dry periods studied. The shift led to less annual runoff for a given annual rainfall, compared with the historical relationship.
From “The influence of multiyear drought on the annual rainfall-runoff relationship: An Australian perspective”, Saft et al., Water Resources Research, 10.1002/2014WR015348