  The GRACE and GRACE-FO missions have provided an unprecedented quantification
of large-scale changes in the water cycle. However, it is still an open problem
of how these missions' data sets can be referenced to a ground truth.
Meanwhile, stationary optical clocks show fractional instabilities below
$10^{-18}$ when averaged over an hour, and continue to be improved in terms of
precision and accuracy, uptime, and transportability. The frequency of a clock
is affected by the gravitational redshift, and thus depends on the local
geopotential; a relative frequency change of $10^{-18}$ corresponds to a geoid
height change of about $1$ cm. Here we suggest that this effect could be
further exploited for sensing large-scale temporal geopotential changes via a
network of clocks distributed at the Earth's surface. In fact, several projects
have already proposed to create an ensemble of optical clocks connected across
Europe via optical fibre links. Our hypothesis is that a clock network with
collocated GNSS receivers spread over Europe - for which the physical
infrastructure is already partly in place - would enable us to determine
temporal variations of the Earth's gravity field at time scales of days and
beyond, and thus provide a new means for validating satellite missions such as
GRACE-FO or a future gravity mission. Here, we show through simulations how
ice, hydrology and atmosphere variations over Europe could be observed with
clock comparisons in a future network that follows current design concepts in
the metrology community. We assume different scenarios for clock and GNSS
uncertainties and find that even under conservative assumptions - a clock error
of $10^{-18}$ and vertical height control error of $1.4$ mm for daily
measurements - hydrological signals at the annual time scale and atmospheric
signals down to the weekly time scale could be observed.
