Brief communication: Western Europe flood in 2021: mapping agriculture flood exposure from SAR

In this communication, we present the exposure of agriculture lands to the flooding caused by extreme precipitation in western Europe from 12th to 15th of July 2021. Overlaying the flood inundation maps derived from the near-real-time RAdar10 Produced Inundation Diary (RAPID) system on the CORINE land cover map we estimate a 2470 km2 area affected by the flooding, with 57% representing agricultural land. Among the inundated agricultural land, 36% of the area is pastures while 33% is arable land. Most agricultural flood exposure is found in south-eastern France (~1680 km2) along Rhône River and the coastal area of Marseille and Montpellier.

flooding, which will inform crop loss estimates, especially for countries where agriculture plays an important role in the national economy, e.g., France and Germany. Near-real-time (NRT) flood mapping capability from satellite observations is vital to facilitate rapid assessment of flood loss and damage [Shen et al., 2019].
In this brief communication, we use NRT inundation extents from the near-real-time RAdar-Produced Inundation Diary 35 (RAPID) system combined with CORINE land cover data to depict the flood-affected areas in western Europe, and particularly the agriculture land.

Methodology
We focus this communication on western Europe, which is mostly affected by the July 12-15 heavy precipitation event. The area extends from 1.5° E to 11. 6° E,and 42.9° N to 53.1° N,and encompasses the Netherlands,Belgium,Luxembourg,40 Switzerland and portions of Germany, France, and Italy. This region is dominated by marine climate with abundant moisture supplemented by Atlantic Ocean. The weather is therefore moist and mild in winter, and moist and cool in summer.
We extract half hourly precipitation data of the event from the Integrated Multi-satellitE Retrievals for Global Precipitation Mission (IMERG) Late Precipitation L3 V06 product with 0.1-degree spatial resolution [Huffman et al., 2019]. IMERG Late Run is computed about 14 hours after observation time, which integrates more data from sensors aboard on satellites to improve 45 the accuracy. We used IMERG data to calculate the maximum hourly precipitation rate and precipitation accumulation between 12 and 15 of July for each grid We generate inundation extents in NRT using the RAPID system and archive these maps on Amazon Web Services (AWS) [available at https://rapid-nrt-flood-maps.s3.amazonaws.com/index.html#Global_Flood_Event/Europe_Flood_2021/ ]. RAPID is a fully automated system delineating NRT inundation extents from high resolution (10 m) synthetic aperture radar 50 (SAR) imagery. To rule out false positives caused by glaciers and snow, we threshold the Height Above Nearest Drainage (HAND) data to mask out permafrost areas in Alps. The HAND used in this study is obtained from the Multi-Error-Removed Improved-Terrain (MERIT) Hydro Dataset [Yamazaki et al., 2019;Nobre et al., 2011]. Pixels over the Alps where HAND values are greater than 20 meters are removed from the inundated pixels. The threshold is determined by exploring the distribution of HAND for glaciers and perpetual snow recorded in CORINE land cover data and is large enough to avoid the 55 removal of any true positives.
We obtain the latest land cover map over western Europe from Coordination of information on the environment (CORINE) Land Cover (CLC) inventory data [available at https://land.copernicus.eu/pan-european/corine-land-cover/clc2018 ]. CLC uses a Minimum Mapping Unit (MMU) of 25 hectares (ha) and a minimum width of 100 meter for linear elements The standard CLC nomenclature includes 44 land cover classes, grouped in a three-level hierarchy. Five main categories used in this study 60 are "artificial surfaces", "agricultural areas", "forest and semi-natural areas", "wetlands" and "water bodies". The detailed description of CORINE program and its nomenclature can be found in https://www.eea.europa.eu/publications/COR0-part1 .

Results
The spatial pattern of the maximum hourly precipitation and accumulated precipitation from the July 12-15 heavy precipitation event are shown in Figure 1 (a) and (b). Heavy precipitation (peak rate > 20 mm/hr) is observed in western Germany, south-65 eastern France, western Switzerland, and western Italy. The most intense precipitation (peak rate > 50 mm/hr) is found in south France, as well as western Switzerland and Italy over the Alps. Heavier than 200 mm accumulated precipitation is found in eastern France, Luxembourg, southern Belgium, western Germany, Switzerland and Italy, which represent an equivalent of two-month precipitation accumulation in these areas. Furthermore, accumulated precipitation is shown to exceed 250 mm in some parts of the region (e.g., western Switzerland and Italy, south France). 70 Figure 2 shows the inundation extents over western Europe. The total inundated area determined from RAPID is around 2470 km 2 . We find extensive inundated areas in south-eastern France, especially the coastal area, including Marseille and Montpellier. The upstream region of Rhône River exhibits extensive flood inundation as well. The total inundated area over France is approximately 1680 km 2 . In Germany, the main inundated area is found in the west, along the Rhine River (about 162 km 2 ). In the northern Netherland, regions near Markermeer and Ijsselmeer, and regions around Hollands Diep are largely 75 affected by the flood, which represents a total area of 245 km 2 . In Belgium and Luxembourg, the inundated areas are 116 km 2 and 2 km 2 , mostly along Meuse River and Sauer River, respectively. In western Italy, an area of around 135 km 2 along the Po River is affected by flooding. The flash floods in Switzerland also cause a 131 km 2 inundation.
Figure 4 (a) shows inundated area of land use grouped by countries over western Europe. Specifically, in France, 1085 km 2 of agricultural land cover is affected by the flood. Among those inundated agricultural areas in France (Figure 4 (b)), 363 km 2 , 360 km 2 , 271 km 2 and 91 km 2 are pastures, arable land, heterogeneous agricultural areas, and permanent crops, respectively.
Especially, the non-irrigated arable land in France is severely affected, the area is up to 283 km 2 which is larger than the sum 90 of inundated non-irrigated arable land in other countries. Besides, the rice fields and vineyards in France are also hit by flood.
More than 70 km 2 of rice fields and vineyards, mainly in the coastal areas, are inundated. In Netherlands, 135 km 2 of agricultural land is inundated, mostly are pastures (74 km 2 ), followed by heterogeneous agricultural areas (36 km 2 ). The