  We analyze and model an M8.0 flare on 2005 May 13 observed by TRACE and
RHESSI to determine the energy release rate from magnetic reconnection that
forms and heats numerous flare loops. The flare exhibits two ribbons in UV 1600
{\AA} emission. Analysis shows that the UV light curve at each flaring pixel
rises impulsively within a few minutes, and decays slowly with a timescale >10
min. Since the lower atmosphere (transition region and chromosphere) responds
to energy deposit nearly instantaneously, the rapid UV brightening is thought
to reflect the energy release process in the newly formed flare loop rooted at
the footpoint. We utilize spatially resolved (down to 1 arcsec) UV light curves
and thick-target hard X-ray emission to construct heating functions of a few
thousand flare loops anchored at UV foot points, and compute plasma evolution
in these loops using the EBTEL model. The modeled coronal temperatures and
densities of these flare loops are used to calculate synthetic soft X-ray
spectra and light curves, which compare favorably with those observed by RHESSI
and GOES/XRS. The time-dependent transition region DEM for each loop during its
decay phase is also computed with a simplified model and used to calculate the
optically-thin C IV line emission, which dominates the UV 1600 {\AA} bandpass
during the flare. The computed C IV line emission decays at the same rate as
observed. This study presents a method to constrain heating of
reconnection-formed flare loops using all available observables independently,
and provides insight into the physics of energy release and plasma heating
during the flare. With this method, the lower limit of the total energy used to
heat the flare loops in this event is estimated to be 1.22e31 ergs, of which
only 1.9e30 ergs is carried by beam-driven upflows during the impulsive phase,
suggesting that the coronal plasmas are predominantly heated in situ.
