3.1.2 Spatiotemporal relationships
To explore the spatiotemporal differences of scenarios with and without
re-infiltration, we set three parameters to their default values, namely
an initially dry soil condition and normal soil infiltration rates
(i.e., a-priori setting). Figure 4 shows the difference in flood extent
for the two schemes. Despite a considerable amount of grid cells showing
positive agreements (i.e., both detect floods; TP=16.0%), there are
still 1.7% of the grid cells issuing false positives, amounting to
15.6k grid cells (~1.56 km2) in this
model configuration. Specifically, those false positives accumulate
around upstream floodplains while the downstream such as areas near the
basin outlet does not present discernable differences, as the flood
depth there due to accumulation is not sensitive to inundation
thresholds for flooded cells. Moreover, Figure 5 portrays the spatial
distribution of the differences with respect to maximum depth, initial
inundation timings, and total inundation duration. Figure 5a depicts the
major differences that are situated in floodplains and river channels
where surface water is accumulated via routing, and the maximum depth
difference is up to 3 meters in the river channel, especially downstream
of Halls Bayou. However, the initial inundation timings and durations
are scattered sparsely over the study area, with a majority of the grid
cells showing earlier and longer inundations for the case without
re-infiltration scenario. Therefore, it is likely that the flood is
over-predicted by models without the re-infiltration scenario.
[INSERT FIGURE 4 HERE]
Meanwhile, we notice that there are some samples in the opposite
distribution, indicating delayed and/or shorter inundation time.
Arguably, this could be some local effects when the soil reaches earlier
saturation in the re-infiltration scenario, thereby leading to earlier
flooding. A supporting material is found in Figure 5d, in which the
basin-average soil moistures of two schemes are compared. Notably,
evapotranspiration is not considered in this ideal test, so the soil
moisture does not deplete with time. Within the storm lifetime, soil
moisture surges from completely dry to 85% saturation for the scenario
without re-infiltration and to 95% saturation for scenario with
re-infiltration. Early saturation reduces infiltration rates later on
and thus has pronounced effects on local flooding when surface water is
not routed timely. Figure 5e presents the evolution of surface water
volume by integrating surface water depth along with grid cells.
Although both scenarios similarly reach the maximum surface water volume
concurrently, their recession limbs show considerable differences. The
re-infiltration scenario apparently has a steep exponential decay, as
both still water and running water infiltrates into the soil; for the
scenario without re-infiltration, in contrast, there is a mild decay and
even levels out at the end of the simulation. The difference between the
two increases with time, as shown in the shaded area, up to\(0.4\times 10^{8}\) m3 volume difference, which
equates to almost half of the total surface water volume.
[INSERT FIGURE 5 HERE]
In summary, re-infiltration scheme indeed influences flood magnitude and
timings via surface water-soil interaction, and it possibly reduces
flood magnitude and delays (shortens) flood timing (duration). Flood
magnitude differences are pronounced downstream or in depressions, while
flood timings are scattered. Such results are markedly tied to soil
condition (wet or dry) and soil characteristics (infiltration capacity).