  High-precision photometry from space-based missions such as K2 and TESS
enables detailed studies of young star variability. However, because
space-based observing campaigns are often short (e.g., 80 days for K2),
complementary long-baseline photometric surveys are critical for obtaining a
complete understanding of young star variability, which can change on
timescales of minutes to years. We therefore present and analyze light curves
of members of the Upper Scorpius association made over 5.5 years by the
ground-based Kilodegree Extremely Little Telescope (KELT), which complement the
high-precision observations of this region taken by K2 during its Campaigns~2
and 15. We show that KELT data accurately identify the periodic signals found
with high-precision K2 photometry, demonstrating the power of ground-based
surveys in deriving stellar rotation periods of young stars. We also use KELT
data to identify sources exhibiting variability that is likely related to
circumstellar material and/or stellar activity cycles; these signatures are
often unseen in the short-term K2 data, illustrating the importance of
long-term monitoring surveys for studying the full range of young star
variability. We provide the KELT light curves as electronic tables in an
ongoing effort to establish legacy time-series datasets for young stellar
clusters.
