3.1 Potential of UAV imagery to study soil moisture and stock of
water in wetland
The selection of the modeling approach depends, besides the modelling
objectives, on both scientific and technical aspects as well as on the
resources available, which include the scale of the simulation domain,
topography, hydrological and topographical data available and the
complexity of the hydraulic regime (Wester et al., 2018). For these
reasons, to estimate water storage the studied wetland was subdivided
into three compartments, in order to facilitate the hydrodynamic
modeling. The compartments were defined through rigorous analysis of the
images (RGB and DMS) generated by UAV, which showed distinct
pixel-information about soil surface inside that vary according
topography. Thus, we considered the topography and terrain morphology:
the slope, from the border to the center of the topographic depression
that characterizes the wetland site, and the density of the
microtopography (mound fields), which is directly related to the level
of hydromorphism in the wetland (Rosolen et al., 2019).
Between the border and the center of the depression, compartment 1
corresponds to its boundary with the well-drained plateau; compartment 2
is the intermediate segment between compartments 1 (boundary) and 3
(center); and compartment 3 corresponds to the central part of the
topographic depression (Figure 2 C).
The data acquired by the UAV images allowed us to estimate the amount of
water that each compartment can support and, therefore, the total water
volume that can be stored on the surface of the studied wetland. Based
on the data of the altitude, geographical location and the slope in
relation to adjacent points of each pixel of the images we calculated
the volume by delineating a 3D polygon in each compartment. Because the
mounds (murundus ) do not fill up with water, their volumes were
subtracted from the total volume of the compartments.
The result of semi-automatic extraction through slope and visual
interpretation of the orthomosaic obtained 1751 mounds features in the
research area. The biggest mound have 3643.40 m² and the smallest mapped
shape was 1.01 m². Regarding the perimeter recorded in the extraction,
the smallest recorded form was 4.60 m and the largest with 476.63 m.
Figure 6 shows the histograms with murundus distribution by area and
perimeter in the research area. It is observed that the murundus of the
research area are mostly distributed with perimeters ranging from 24.48
m to 58.49 m and the area are distributed mostly between 67.41 m² and
167.37 m².
When each compartment is flooded, it displays a distinct water residence
time and dynamic associated with its topographic position. Water
flooding over the land surface reaches compartment 1, during the
rainfalls. In compartment 3 the water level remains high for a prolonged
period due the lateral water influx from compartment 2 and 1. Clearly,
there are variation in soil moisture pattern linked to topography, and
that variable has a major impact on the hydrology in landscape (Moore et
al., 1991). The largest amount of microtopography with little trees and
shrubs on their top occurs in Compartment 1 in agreement with lower soil
moisture. Wetland area also exhibit grassy cover in Compartment 3 in
agreement with the elevated soil moisture. These differences of soil
moisture observed in field are in accordance to features observed in the
aerial imaging.
Rainfall seasonality determines the wetland’s water supply. In the
field, we observed that the wetland area is a natural seasonal water
reservoir, filling up in rainy months and drying up during dry months
(Figure 7 A, B). We performed the photogrammetry at dry months in order
to estimate the water stock during the rainy period.
Table 2 shows each compartment’s total storage capacity, in cubic meters
and liters (Figure 2 D), its area in square meters, and the minimum and
maximum altitudes that were used in the calculations. The altitude
values were calculated for each pixel of the images of each
compartment.
In compartment 3, we conducted the electrical tomography tests to
provide an understanding of subsurface hydrodynamics and identify the
architecture of soil and rock, which acts as an infiltration zone.