  Solar flares form and release energy across a large number of magnetic loops.
The global parameters of flares, such as the total energy released, duration,
physical size, etc., are routinely measured, and the hydrodynamics of a coronal
loop subjected to intense heating have been extensively studied. It is not
clear, however, how many loops comprise a flare, nor how the total energy is
partitioned between them. In this work, we employ a hydrodynamic model to
better understand the energy partition by synthesizing Si IV and Fe XXI line
emission and comparing to observations of these lines with IRIS. We find that
the observed temporal evolution of the Doppler shifts holds important
information on the heating duration. To demonstrate this we first examine a
single loop model, and find that the properties of chromospheric evaporation
seen in Fe XXI can be reproduced by loops heated for long durations, while
persistent red-shifts seen in Si IV cannot be reproduced by any single loop
model. We then examine a multi-threaded model, assuming both a fixed heating
duration on all loops, and a distribution of heating durations. For a fixed
heating duration, we find that durations of 100 -- 200 s do a fair job of
reproducing both the red- and blue-shifts, while a distribution of durations,
with a median of about 50 -- 100 s, does a better job. Finally, we compare our
simulations directly to observations of an M-class flare seen by IRIS, and find
good agreement between the modeled and observed values given these constraints.
