  The Seyfert 1 galaxy Arp 151 was monitored as part of three reverberation
mapping campaigns spanning $2008-2015$. We present modeling of these
velocity-resolved reverberation mapping datasets using a geometric and
dynamical model for the broad line region (BLR). By modeling each of the three
datasets independently, we infer the evolution of the BLR structure in Arp 151
over a total of seven years and constrain the systematic uncertainties in
non-varying parameters such as the black hole mass. We find that the BLR
geometry of a thick disk viewed close to face-on is stable over this time,
although the size of the BLR grows by a factor of $\sim 2$. The dynamics of the
BLR are dominated by inflow and the inferred black hole mass is consistent for
the three datasets, despite the increase in BLR size. Combining the inference
for the three datasets yields a black hole mass and statistical uncertainty of
$\log_{10}($M$_{\rm BH}/\rm{M}_{\odot})=6.82^{+0.09}_{-0.09}$ with a standard
deviation in individual measurements of 0.13 dex.
