Michael Campana offers some cautions about over-interpreting what the Castle et al. paper on the loss of Colorado River Basin groundwater is telling us. It’s a question of stocks versus flows. The GRACE measurements that have gotten such extensive attention can tells us the latter, but not the former:

Why is such a number – the groundwater stock – important? In my class I likened it to managing a checking account without knowing the balance (S; the stock) but just the deposits (I; inflows) and withdrawals (O; outflows). As long as I = O, you’re fine, regardless of the S (we’re assuming no fees or problems with crediting deposits and floating checks). You may feel uncomfortable but you’ll be okay. However, as soon as O > I in a given time period you had better start worrying simply because you don’t know how long you can keep running a deficit before hitting bottom. That’s the problem we have with groundwater in many places.

In the CRB, as in many other groundwater basins, we are likely in the O > I phase, but we don’t know how much water is in storage. But we need to know the storage to properly manage the groundwater.

There is much more in the post worth reading if you are interested in what we know and don’t know about Colorado River Basin water.