Monitorização de ecossistemas dependentes de águas subterrâneas usando imagens de radar de abertura sintética (SAR) na Austrália

Institutional setting

Water management is a fundamental issue in Australia. While legislative action was taken since the late XIX century to make streams protected by water authorities, only in the 1980s the water management actions began to consider other than those water bodies and started to take into consideration water allocation procedures coupled with environmental (and social) objectives, laying the basis for a water economy. In the 1990s, the incremental cost of water, increasing demand and arise of environmental problems push the public institutions to adopt alternative methods of management and moderate conflicts between stakeholders (Tisdell et al., 2002).

Doody et al. (2017) noted that since water reform began in 1994 the environment has been recognised as a legitimate user of water. Prioritization was given in environmental water uses and, in the case of GDEs, water allocations were to be clearly defined to maintain riparian, terrestrial, wetland, marine and subterranean ecosystems that depend on groundwater. The identification and mapping of GDEs has been pointed as a critical investment to achieve the water reform objectives.

The acquisition of water information is primal to a sustainable water planning, water trading and particularly environmental management (Doolan et al., 2016).

Geographical setting

Australia shows a high variability climate with clear evidence of recurrent droughts followed by extreme flooding events. Highly irregular rainfall and high evapotranspiration create an increased challenge in water resources management, particularly for surface water. The Murray-Darling Basin authority accounted groundwater for around 30% of the water used in the continent and being in many areas the only reliable water source (Dabovic et al., 2019).

The construction of the GDE Atlas concluded that 34% of Australia’s landscape potentially contains GDEs of which 5% are classified with a high GDE potential (Doody et al., 2017). The use of SAR for GDE detection was tested in two contrasting study sites in Australia. one on Victoria and South Australia and one on the Northern Territory (Fig. 2). Site 1 (Mount Gambier area) corresponds to a karstic aquifer system and Site 2 (Wildman-Kakadu area) corresponds to a monsoonal coastal floodplain to inland semi-arid grassland (Castellazzi et al., 2019)

Historical overview

Historical overview
The first space-borne SAR system has been launched in 1978 and has been used in a wide range of applications (Moreira, 2007). Globally acquiring SAR satellites opened the door to potential applications in GDE monitoring. Automated and global SAR acquisition began in 2014 with the launch of Sentinel-1A satellite, followed by its synchronous twin satellite Sentinel-1B in 2016 (Castellazzy et al., 2019).

Concerning GDEs, in 1994 the Council of Australian Governments (COAG) endorsed reforms to move towards a sustainable water industry that included allocations for the environment and greater environmental accountability of water resource developments. In 1996 the National Principles for the Provision of Water for Ecosystems was signed in 1996 to provide a basis for considering Ecological Water Requirements (EWR) as part of water allocation decisions by water resource managers. EWR should be based on the best available scientific information. For GDEs groundwater is be considered in terms of flow, level, pressure and quality required by an ecosystem, and are to be studied through strategic scientific research. The EWR contribute with socioeconomic evaluation and water consumption demands, to the development of Ecological Water Provisions (EWPs) within water management plans. EWPs are a management tool used to achieve ecological objectives often expressed in terms of a target to maintain, restore or rehabilitate (Richarson, et al., 2011).

Evidence of benefits from implementation

The identification of GDEs often requires site-specific information on various indicators such as plant information on water use and groundwater depth which at a regional scale is impractical and cost-prohibitive (Kuginis et al., 2016). Field-based GDEs inventories are not convenient for state-wide, regional, national, or global maps, are labour intensive, and represent one point in time. The use of remote sensing greatly improves data acquisition and is a cost-effective technique (Pérez Hoyos et al., 2016).

Referring specifically to SAR, if compared with the imagery from multispectral platforms, available since the early 1970s, SAR offers the advantage of global day-and-night sensing capability and insensitivity to cloud cover, making SAR data particularly suitable for monitoring changes in Earth’s surface (Castellazzy et al., 2019). The technique presented – SARGDE index – offers improved yearly monitoring, with high resolution, and even under clouded areas. It is also relatively user-independent, as it requires only routine SAR processing.

Such methods coupled with already existing tools (GDE Atlas) ensure that consistent data is available to provide the basis for better-informed decisions.

Replication potential in SUDOE region

The identification of GDE is particularly relevant in the southern European regions, prone to scarcity phenomena and in which these ecosystems can take a significant role in mitigating extreme events. Terrestrial GDE protection is enforced by the European Union Water Framework Directive (WFD) and member-states are required to prevent damages (Rohde et al., 2017). Monitoring networks of surface and groundwater bodies play an important role and ecosystem identification may significantly benefit from the spaceborne acquired information.

Concerning SARGDE possible implementations outside the territories in which it was tested the authors note that some additional research is needed to adapt it to non-Australian vegetation. The index lacked testing for the ability to temporally monitor GDEs impacted by groundwater pumping and to be compared with in situ monitoring networks acquired data (Castellazzi, et al, 2019).

European projects such as SAR2CUBE are looking into defining prototypes to integrate SAR data into everyday processing chains and reduce the entry-level barrier of the InSAR-derived products by providing analysis-ready data (ARD) specifically defined to achieve efficiency and flexibility. This may significantly increase the user base and within the range of SAR applications in European territories.

Future outlook

Moreira (2007) reported that, at the time of publication, increased demand for this technology drove several nations to project and launch more than 20 spaceborne SAR systems. Other authors also noted (Lu et al., 2007) that InSAR promising applications can drive to scientific breakthroughs. These advances are based on longer wavelength SAR images, fully polarized SAR sensors for better characterization of vegetation and ground features, or the lessening of InSAR atmospheric delays which increase the technology’s accuracy. It is also noted that advances in data-mining, with multi-temporal and multi-dimensional techniques, can allow, for example, mapping of time-variant ground surface deformations (natural or caused by human actions) by improving deformation measurements.

Innovation in monitoring procedures, which scientists refer to as a necessary step forward (Carvalho et al., 2019), can imply the integration of technologies such as InSAR in real-time monitoring with greater spatial cover with decreased costs. As an example, the use of satellite data for surveillance of water bodies status within the framework of the EU WFD has encouraged several projects based on European Space Agency’s Copernicus programme, looking into aiding almost real-time machine-learning supported decision making in water resources management.

Advances in the technology may help in its application. The development of a miniaturized version of SAR satellites, by both private and public initiatives, are expected to decrease the cost of the technology and open the way to SAR satellite constellations that can increase global coverage.

Key points of the innovative method

• SAR imagery can provide a lower cost and be less time consuming if compared to field-based GDEs inventories.
• The developed index generated good results when compared with existing GDE mapping with ~90% of similarities.
• The technique is of rather simple use but require the use of commercial software for image manipulation.
• As it is band-related, testing is still necessary if the index is to be applied in other regions, with a different type of vegetation.
• Further advancements are expected with the development of more cost-effective SAR systems with increased resolution and acquiring capabilities.