Multi-Sensor Monitoring System of Water Levels in Wetlands

Summary | Methods | Results | Additional

Summary

Figure 1: Everglades wetland area. (a) Land cover adjusted from the Statewide Land Use Land Cover map from the Florida Department of Environmental Protection; (b) The Everglades Depth Estimation Network (EDEN) Digital Elevation Model (DEM); (c) An example water level map; and (d) An example water depth map for October 11, 2021 from the EDEN water surfaces products. [Source: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2023WR035422]

Wetlands store roughly 10% of global surface water in the terrestrial portion of the water cycle, cover roughly 9% of the Earth’s surface, and provide critical habitat for a wide variety of plant and animal species. Over the past century, many wetland areas have been lost, degraded, or stressed mainly due to anthropogenic activities, as water diversion, agricultural development, and urbanization, but also in response to natural processes, as sea level rise and climate change. Global and regional monitoring of wetland health and response to their natural and anthropogenic stressors are important and are best conducted using space-based remote sensing techniques, due to wetlands’ vast extent and often inaccessibility. The research project focuses on remote sensing monitoring of surface water availability in wetlands using a multi-sensor approach that integrates observation from Synthetic Aperture Radar (SAR), optical imagery, radar and laser altimetry, and upcoming missions such as Surface Water Ocean Topography (SWOT) and NISAR when data become available. The multi-sensor system generates detailed multi-temporal maps of wetland inundation extent, water levels, and water level changes. The multi-sensor monitoring system is being developed over the south Florida Everglades, which can be considered as a natural laboratory due to its variability and the availability of ground-based hydrological observations. After development, the system will be tested in two other wetland areas located in Louisiana, and Peace–Athabasca Delta (Alberta, Canada).

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Methods


The multi-sensor system integrates high spatial resolution relative observations of water level changes (SAR phase) with limited coverage ‘absolute’ measurements of water level (altimetry) to generate maps of water levels and water level changes. Optical imagery is used for assessing vegetation type and vegetation conditions to refine the observations.

SAR interferometry – Maps of Relative Water Level Changes (RWLC) in wetlands:

SAR Interferometry (InSAR) measures water-level changes in wetlands with high spatial resolution and centimetric accuracy. However, it only provides Relative Water Level Changes (RWLC), requiring in-situ data for calibration.


Figure 1: SAR interferometry approach. (1) InSAR geometry in wetlands for relative water change detection. (2) Sample interferogram from Sentinel-1 (2021/08/08 – 2021/08/20) showing fringe patterns associated with water level changes between the acquisition dates. The interferogram allows us to generate high spatial resolution maps of relative water level changes in vast wetland areas.



Satellite Altimetry – Along track Absolute Water Level (AWL) observations:

Altimetric observations provide along track absolute water level observations that can be used as a substitute for in-situ data in calibrating InSAR observation over ungauged wetlands.


Figure 2: Satellite Altimetry approach using ICESat-2 laser altimeter. (1) Schematic of the ICESat-2 observation geometry in wetlands for (1)(a) Dry condition and (1)(b) Inundation condition. The Schematic is adapted from the ATBD ICESat-2 ATL08 [Source (Neuenschwander et al., 2021) https://nsidc.org/sites/default/files/documents/technical-reference/icesat2_atl08_atbd_r005.pdf]. (2) Sample ICESat-2 ATL08 ground track for 2021-10-11. (2)(a) Derived water depth for the sample ground track. (2)(b), (c) and (d) subplots present the latitudinal water depth along the ICESat-2 ground tracks (P1, P2, and P3 of panel (a), respectively). The EDEN water depth gauge-based reference is also presented. [Source: (Palomino-Ángel et al., 2024 - manuscript) https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2023WR035422 and (Palomino-Ángel et al., 2024 - dataset) https://dataverse.fiu.edu/dataset.xhtml?persistentId=doi:10.34703/gzx1-9v95/4JETAA]



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Results


1. Spatial-temporal evaluation of ICESat-2 and GEDI missions for water level and depth retrievals over the South Florida Everglades:

Here we present results of our recent study using ICESat-2 and GEDI altimetry data for water level and depth retrievals over the entire south Florida Everglades wetlands (Palomino-Ángel et al., 2024 - manuscript https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2023WR035422). We developed a systematic accuracy assessment of ICESat-2 ATL08 and GEDI L2A products to monitor spatial-temporal water level and depth dynamics over the South Florida Everglades wetlands. The evaluation was performed on data acquired between 2020–2021, using gauge-based water level and depth estimates as references. The results showed an RMSE of 0.17 m (water level) and 0.15 m (water depth) for ICESat-2 and 0.75 m (water level) and 0.37 m (water depth) for GEDI. The analysis suggested that nighttime acquisitions were more accurate for both missions than daytime ones. The low-power beams achieved slightly higher accuracies than those of the high-power beams over the evaluated wetlands. Water level retrieval was more problematic in densely vegetated areas; however, we derived a correction model based on the leaf area index that improved the accuracy by up to 75% for water depth retrievals from GEDI. Furthermore, the analysis provides new insights to understand the potential of the altimeters in monitoring the spatial-temporal dynamics of water levels in the evaluated wetlands.


Figure 1. Spatial variation of the water depth along different transects. (a) Presents the transect location and direction. Each subplot presents the water depth for the (b) ICESat-2 Transect 1 (I-T1); (c) ICESat-2 Transect 2 (I-T2); (d) GEDI Transect 1 (G-T1); (e) GEDI Transect 1 corrected (G-T1-C); (f) GEDI Transect 2 (G-T2); and (g) GEDI Transect 2 corrected (G-T2-C). The solid black line represents the reference water depth obtained from EDEN. The blue dots represent the ICESat-2 and GEDI observations in strong (SB) and full (FB) beams. The red dots represent ICESat-2 and GEDI observations in weak (WB) and coverage (CB) beams. [Source: (Palomino-Ángel et al., 2024 - manuscript https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2023WR035422)]

2. Calibration of InSAR Relative Water Level Changes (RWLC) maps using Altimetry Absolute Water Level (AWL) observations:

We preliminarily tested the calibration methodology using four Long Temporal Baseline Interferograms: (1) Aug-08/2021 - Sept-01/2021 (24 days); (2) Feb-27/2020 - May-21/2020 (84 days); (3) May-28/2021 - Aug-20/2021 (84 days); (4) Feb-21/2021 - May-28/2021 (96 days). We first generated Relative Water Level Change (RWLC) maps using Sentinel-1 data and subsequently calibrated the RWLC maps to obtain Absolute Water Level Changes (AWLC) maps using ICESat-2 observations for calibration (InSAR Calibrated). We compared our results with the Everglades Depth Estimation Network (EDEN) water level surfaces as reference and calculated the difference.


Figure 2. Preliminary Absolute Water Level Change maps generated from Sentinel-1 and ICESat-2 observations for the Water Conservation Area 1 (WCA1) in the Everglades wetlands.

Publications


Palomino-Ángel, S., Wdowinski, S., & Li, S. (2024). Wetlands water level measurements from the new generation of satellite laser altimeters: Systematic spatial-temporal evaluation of ICESat-2 and GEDI missions over the South Florida Everglades.. Water Resources Research, 60, e2023WR035422. https://doi.org/10.1029/2023WR035422.

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