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
h/t Kevin Anchukaitis at Woods Hole
Curious what they hydrologists say causes it. More aquifer recharge in the surface/ground connection? Just drier soil absorbing more? Drought period warmer temp? All of the above?
Mike – As I read it, section 4.1 of the paper essentially says “dunno”.
Thanks John. I was thinking of the 50s when Reynolds came on and tried to figure out why the river didn’t have enough for the Compact and learned about groundwater, specifically how all the pumping for the post war pop boom was not only draining the aquifer but pulling water out of the river. Wondering if there was some evidence from Australia to add to the understanding of how all this works. Couldda read the thing of course but easier just to ask (: