Lake Argyle Water Extent

Lake Water Extents derived from a Sentinel-2 time series

ICube SERTIT

16 January 2023

Context

In the context of global change, water bodies are considered as climate sentinels, being sensitive to climatological and meteorological conditions. In fact, lakes were defined as an Essential Climate Variable (ECV) by GCOS (Global Climate Observing System) . Lake Water Level and Extent (LWL & LWE) are part of the 6 ECVs identified through the ESA’s Climate Change Initiative (CCI) to better understand the physical responses of lakes to climate change.

Introduction

Lake Argyle, Australia

Within the CCI Lakes project, ICube-SERTIT is in charge of LWE production, exploiting ExtractEO , an in-house automatic water detection workflow developed and based on the open-source EOReader library (also developed in-house). This is applied to a Sentinel-2 time series over Lake Argyle, from January 2016 to August 2022. This lake is a Ramsar site located in the northeast of Western Australia, within the Timor Sea Drainage Division. It is also the Western Australia's largest lake by area and Australia's second-largest artificial lake by volume.

*Lake Argyle, the second-largest artificial lake by volume in Australia*

The ExtractEO pipeline downloaded 800 Sentinel-2, distributed over 2 tiles, and extracted water extents based on a Multi-Layer Perceptron (MLP) algorithm combined with the use of the Global Surface Water dataset for sampling.

As ExtractEO mapped, for each date, the Lake Water Extent (LWE) at 10m ground resolution with the cloud percentage over the lake, it is possible to automatically reject cloudy dates from the LWE database. Then, the results are fed into the Lake Water Extent curve.

LWE curve

LWE curve derived by ICube-SERTIT from Sentinel-2 data acquired between January 2016 and August 2022:

800 Sentinel-2 images processed (400 dates)
229 lake masks produced from non-cloudy dates
Lake surface areas calculated within the AOI
Maximum lake area (15.04.2017): 1138 km²
Minimum lake area (10.12.2020): 431 km²

Regarding the lake's fluctuation, a very important dynamic can be clearly noticed. In fact, Lake Argyle is a non-regulated structure, located in the dry tropics, with an area that fluctuates seasonally. Eighty percent of rainfall occurs in the wet season (December to February) and on average, evaporation exceeds rainfall 11 months of the year. Source

Illustration of the Lake Argyle surface variation. Sentinel-2 images acquired the 10/11/2020 (left) and the 05/05/2017 (right) in false colour SWIR, NIR and RED band composites

Hence, correlations should be observed between surface variations, rainfall, drought and other climatic variables.

Between December 2016 and May 2017, a strong increase in LWE is observed, from less than 700 km² to greater than 1100 km², its second-highest level since 2010 ( Hydroweb platform ). This is most likely due to record high rainfall that occurred in the region between December 2016 and Mars 2017.

This is followed by a decrease of its surface until the end 2020, losing 2/3 of its surface. A part of this decrease is explained by drought conditions occurring in the region, especially in 2019.

However, within this trend, a slight LWE increase can be noticed at the beginning of each year. Thereafter, the LWE will almost double in size again between the end of 2020 and April 2021, most likely due to well above-average rainfall.

Since April 2021, the dynamic seems to be similar to that of 2017.

For more detailed rainfall maps .

Occurrence

Occurrence products are also derived from LWE over the 5.5 year observation period. Water occurrence layers are generated from the set of 229 LWE masks.

Water Occurrence (January 2016 - August 2022)

This occurrence was also compared to the Global Surface Water (GSW) database, developed by the European Commission's Joint Research Centre within the Copernicus programme. This GSW database is a reference in the domain as it accurately quantifies the changes in global surface water over the last 37 years at a resolution of 30 metres. With a shorter time period, the occurrence calculations, identify the same patterns as the GSW, illustrating the dynamics of this lake.

Comparison between the Sentinel-2 occurrence (10m resolution, on the right) with the GSW occurrence (30m resolution, on the left)

Hypsometric curve

Comparing the LWE extracted from Sentinel-2 data and the water level (LWL) time series, calculated using satellite altimetry and available on the Hydroweb platform , a polynomial relationship can then be established for this lake. Knowing this function LWE = f(LWL), LWL can be related from altimetry to LWE using hypsometry. This is also a good way to estimate the quality of the LWE extracted through a correlation assessment. As an RMSE of 8.13km² was obtained and regarding the distribution of the scatter plot along the hypsometric curve, it can be assumed that the ExtractEO derived LWEs are accurate over this lake.

Subsequently, due to its size and dynamics, it would be interesting to analyze the performance obtained in terms of surface area, height and volume variations derived from the newly launched Franco-American SWOT mission.

Contact

ICube-SERTIT helps space serve the Earth producing geo-information from space imagery. As a regional, national and European operator, it responds to society’s needs offering solutions in risk and disaster management, land planning, natural resource management, the environment and sustainable development.

Thomas Ledauphin: tledauphin[@]unistra.fr
Hervé Yésou: herve.yesou[@]unistra.fr
Jérôme Maxant: maxant[@]unistra.fr

LEGOS (Laboratory of Space Geophysical and Oceanographic Studies) is a joint research unit (UMR5566) under the auspices of CNES, CNRS and IRD. Its research and teaching scientific fields include oceanography and the water cycle in the broadest sense.

Jean-François Crétaux: jean-francois.cretaux[@]legos.obs-mip.fr