Retro- and forecasting inundation extents for trend analysis in the Cambodian Mekong Delta - a new method combining Sentinel-1 and 2 and local water level measurements
Authors: Orieschnig Christina Anna , Belaud Gilles , Venot Jean Philipp , Massuel Sylvain 
Published: September 2022

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&lt;div&gt;&lt;span&gt;&lt;span&gt;Annual monsoon inundations are integral to the hydrological, ecological, and economic processes of the Mekong Delta&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;&lt;span&gt;However, &lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;the dynamics of seasonal flooding shifted over the past decades due to upstream influences like hydropower infrastructure, land use change, and modified precipitaiton patterns&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;&lt;span&gt;Characterizing these changes has been a major challenge in these ungauged areas with scarce environmental data, imposing constraints on modelling approaches&lt;/span&gt;&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/div&gt;
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&lt;div&gt;&lt;span&gt;&lt;span&gt;Here, hydrological remote sensing through Sentinel-1 and -2 provides valuable insights into inundation processes due to its enhanced revisit frequencies and SAR capabilities. &lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;However&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;, the &lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;relatively&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; short duration of the Sentinel mission makes it impossible to analyze long-term trends&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;&lt;span&gt;To address this, we developed a &lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;methodology&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; harnessing Sentinel-derived inundation maps and local water level measurements available for the past 30 years&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;&lt;span&gt;By linking inundation maps to water levels through a correlation model taking into account flood propagation delays we constructed a water level-flood link &lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; to retro- and forecast inundation extents&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. Its performance &lt;/span&gt;&lt;span&gt;&lt;span&gt;was assessed&lt;/span&gt;&lt;/span&gt;&lt;span&gt; using historical Landsat imagery and the TanDEM elevation model. &lt;/span&gt;&lt;span&gt;&lt;span&gt;Subsequently,&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; it &lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;was used&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; to analyse inundation dynamics since 1991 and to forecast future developments based on streamflow projections&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;/div&gt;
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&lt;div&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/div&gt;
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&lt;div&gt;&lt;span&gt;&lt;span&gt;Preliminary results yield an accuracy of up to 93% when compared to historical Landsat inundation maps&lt;/span&gt;&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;&lt;span&gt;The analysis indicates a marked decrease in inundation incidence in the first half of the flood season (-24%) and a decrease in&lt;/span&gt;&lt;/span&gt;&lt;span&gt; inundation durations by 19 days.&amp;#160;&lt;/span&gt;&lt;/div&gt;
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