[benj111]

To be pedantic:

You say "It all melts", and then "I trust that the experts know the difference between permanent snow....".

I hypothesised satellite photos, I'm not sure if depth of snow is relevant for more than just volume, and whether you could get good estimates of depth (and density???) from a satellite.

[gwbas1c]

A few years ago I read an article on Hacker news about the drought. It discussed the experts looking at satellite photos and then climbing to mountain tops to perform more detailed measurements.

Don't forget that modern satellites are topographical with a high degree of accuracy. We can use satellites to learn snowpack depth.

This is how we know that the aquifers are depleted because the satellites measure elevation above the aquifer.