As another data point, the annual groundwater withdrawal from the Colorado river basin is estimated at 5.6 km^3/year over 2004-2013. (http://onlinelibrary.wile

As another data point, the annual groundwater withdrawal from the Colorado river basin is estimated at 5.6 km^3/year over 2004-2013. (http://onlinelibrary.wiley.com/doi/10.1002/2014GL061055/abstract http://onlinelibrary.wiley.com/doi/10.1002/2014GL061055/abst...) Since 1m^3 is 1000 kg, that's 5.6e12 kg/year in that one basin. The above groundwater figure (and other water-related quantities, like snowpack and reservoir storage) are pretty much at the edge of what can be detected by current gravity-based techniques using GRACE (https://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_Experiment#Discoveries_and_applications https://en.wikipedia.org/wiki/Gravity_Recovery_and_Climate_E...). Figure 2 in the linked paper (if you have access) shows a nice sinusoidal-like graph of mass in the Colorado basin over this time period. It's a monthly measurement, so daily mass flows like cars would not be present (even if the measurement were precise enough to get at them).