3.2.2 Verification against High Water Marks
Because a direct assessment for flood inundation is not feasible, some
watermarks or stains in the aftermath of a flood event can be used as
proxy data for model evaluation. The USGS team routinely publishes their
surveyed High Water Marks (HWMs) after some major flood events, which
can subsequently be used for model evaluations (Chen et al., 2021; Li et
al., 2021b; Sebastian et al., 2021; Wing et al., 2017). Figure 7 shows
the cell-wise maximum flood depth of the two schemes compared to the
HWMs. Both schemes present better performance upstream of Halls Bayou,
with a difference smaller than 0.5 meters. However, the model
over-predicts water depth in Greens Bayou up to 1.5 meters. This is
consistent with the over-prediction of in-channel water depth, as shown
in Figure 6. The distribution of the differences is shown in Figure 7c,
pointing to a generally better performance of the re-infiltration
scenario than without it, as the absolute mean depth difference of the
re-infiltration (0.51 m) is 17.2% smaller than that of the scenario
without re-infiltration (0.60 m). It is worth noting that HWMs
themselves come with uncertainties that are due to the data quality, and
errors could be up to 0.2 meters (Koeig et al., 2016). For instance,
tranquil water represents a smooth trend that has small uncertainties.
There are also spurious errors that are related to human mistakes or
values being rounded off. This is particularly true for the recorded
geographical coordinates which requires more floating points to pin down
the location exactly. In Figure 7d and e, the two HWMs are marked with
an absolute difference greater than 1 m but only several pixels away
(i.e., tens of meters) from their true values. Despite this, the
re-infiltration greatly alleviates the over-prediction of the previous
model.
[INSERT FIGURE 7 HERE]