Introduction
Worldwide, 324 million hectares of land, which contribute 40% of the
total food production, are equipped for irrigation (FAO, 2016). At a
global scale, average annual water consumption by irrigation is 7,700
m3 per hectare. In Brazil, irrigation’s water uptake
is approximately 1000 m3/s, characterizing the major
consumption in the territory. That quantity is distributed through
irrigations systems covering 7 million ha with potential to expand by
43.3% (ANA, 2017). In the countryside, rivers and small dams are the
major sources of irrigation, despite the substantial volumetric impact
on water resources during the dry season. Considering that both urban
and rural water supplies are provided by superficial water, scarcity has
been a critical concern. For example, in the biennium 2014–2015, the
reservoirs of São Paulo metropolitan region were reduced to single-digit
percentages, leading to a profound water crisis (Millington, 2018).
Almost 20% of the Brazilian territory is covered by wetlands, and their
distribution has been dramatically reduced by land conversion, thus
affecting their natural water storage and ecological functions (Junk et
al., 2013; Rosolen, Oliveira & Bueno, 2015). Around the world,
uncontrolled exploitation is damaging ecosystem services that depend on
wetlands (Verones, Bartl, Pfister, Vilchez & Hellweg, 2012).
In the Cerrado region—the world’s most threatened savanna environment
and Brazil’s last remaining agricultural frontier areas under native
vegetation—the pronounced seasonality necessitates the use of
extensive irrigation for expanding agricultural land use (Rodrigues et
al., 2018). This biome is characterized by long dry periods and by the
presence of herbaceous plants and scattered woody plants and trees that
cover well-drained lateritic soils of low fertility. The Cerrado’s
wetlands represent islands of natural water storage. In the southeast
portion of the ecosystem, our study area in this research, the water
dynamic and volume are closely related to rainfall seasonality and the
fluctuation of shallow groundwater, comprising a freshwater system
integrated with rivers in the catchments. Surrounding the study area,
large farms with center-pivot irrigation systems have been using the
wetland’s water for grain cultivation. Although the allocation of water
is legally required by some farmers, the resource’s conservation is
jeopardized by a lack of knowledge about the balance between input and
output of water in wetlands, and about the hydrological dynamic between
aquifers and soils with hydromorphic properties. Despite closed wetlands
store water and promote groundwater infiltration, influencing the
hydrologic cycle at the watershed scale, few studies have relied on
field data to further our understanding of the impacts, either
individual or cumulative, of natural landscape characteristics and
wetland drainage at the local scale (Haque & Badiou, 2018).
Hydrodynamic models typically partition rainfall into: soil moisture
storage, groundwater recharge and surface runoff. This partitioning
largely depends not only on the hydrological conditions, but also on the
pedogeological conditions. Invasive data acquisition techniques such as
drilling or trench excavations are labor-intensive, costly and involve
extensive fieldwork sampling and laboratory analysis. They can be
efficiently complemented by non-invasive geophysical and remote sensing
methods which are time and cost effective, i.e. they succeed in covering
large areas of land cheaper and quicker than the soil sampling
techniques.
Unmanned Aerial Vehicles (UAVs), and in particular micro-UAVs (payload
less than 1.5–2 kg), represent the latest frontier in land and water
monitoring because of low-altitude flight and flexible payload design
(Anderson and Gaston, 2013). In recent years, miniaturized components
(GNSS receivers, inertial measurement units, autopilots) have advanced
(Watts et al., 2012), and UAVs have been used also for a wide range of
hydrological applications such as fluvial monitoring; river bathymetry
and photogrammetric DEM generation using very high resolution (VHR)
imagery (Lejot et al., 2007).
Innovative quantification techniques that use remote sensing, and
especially those using high spatio-temporal resolution imagery, have
been applied in non-invasive mapping of soil surface furthering our
understanding of the relationship between surface soil water and aquifer
in wetlands (Lin, 2012).
In this field, UAVs equipped with digital cameras had the capability to
acquire VHR and describe the distribution of soil moisture in wetlands.
Traditional techniques emphasized in pedological studies (e.g., grain
size analysis, mineralogy, chemical etc) carried out in a few meters
thick do not provide adequate information about soil-water relationships
as well the possibility to quantify the volume of water in soil. The use
of images generated by UAV allow obtain information not only from
surface features but also to provide excellent estimated values of,
e.g., the water volume that the wetland can storage.
Another technique is electrical resistivity tomography (ERT), which can
be successfully applied in waterlogged areas, since it is highly
sensitive to the presence of water and can provide valuable information
regarding the structure and volume of storage in rocks and soils
(Carrara, Pece & Robertini, 1994; Zhang et al, 2016; Luo et al., 2019).
These techniques have been widely applied in resource exploration and
hydrogeology surveys, having the advantage of being rapid and
cost-effective means of generating information (Kearey & Brooks, 1991;
Helaly, 2017).
Besides, geophysical ERT technique allow to assess soil architecture,
from surface to saprolite, highlighting zones with distinctive presence
of water in pore space. That information is a key to understand the soil
itself; soil-water transfers and to provide a model of water flow and
hydrological process.
Aerial imaging is a widely applied technique in the study of wetlands,
but digital aerial photogrammetry, i.e., the calculation trough UAV
images of the area and the potential volume of water that can be stocked
in the wetlands is a lack of knowledge, so that is one of our
propositions in this work. Still, the second aim of this work is to
propose an empirical model for estimating the spatial distribution of
water in a specific Cerrado wetland of Minas Gerais state, Brazil, using
UAV and ERT techniques to assess groundwater flow and aquifer recharge.
Combining both, UAV and ERT techniques, we employed noninvasively tools
to directly quantify soil architecture and hydrodynamic. The
methodological choice aimed to improve the needed linkage between
digital architecture soil mapping with hydropedology (Lin, 2012).