and poor suitability as a GDP proxy in rural areas (Gibson et al., 2021).

To address challenges associated with using VIIRS nightlights data as a proxy for economic activity (Skoufias et al., 2021), we apply filters to remove pixels with extreme values (i.e., we restrict the sample to values comprised between the 1<sup>st</sup> and 99<sup>th</sup> percentiles) and account for the number of observations available per pixel.<sup>1</sup> We calculate the average nightlight radiance monthly spanning from 1 to 6 months before and after the occurrence of the 2019 floods in Mozambique and the 2022 floods in Nigeria. Two variables are computed: "avg_rad," representing the nightlight radiance at the flooded pixel level, and "avg_radBuff05" which averages for each flooded pixel the nightlight radiance of the pixel itself and adjacent pixels within a 0.5-kilometer buffer.<sup>2</sup> The latter variable is preferred to ensure maximum observation availability and to capture the impact on indirectly affected grid cells.<sup>3</sup> We also extracted the associated variables "cf_cvg" and "avg_cvgBuff05", which indicate the number of cloud-free observations in the month used to calculate average nightlight radiance.<sup>4</sup>

# _Flood data_

Flood events are determined based on the methodology outlined by DeVries et al. (2020). We use S1 Ground Range Detected scenes from the Synthetic Aperture Radar sensors onboard the Sentinel-1 satellite, part of the European Space Agency's Copernicus program (ESA, 2023). These scenes provide data on Z-scores derived from SAR backscatter time series of single band co-polarization vertical transmit vertical receive (VV) and dual cross-polarization vertical transmit horizontal receive (VH). Since October 2014, this data has been available every 6 days at a 10-meter resolution.

floods from permanent or seasonally occurring surface water, we utilize the historical Landsatderived monthly water probabilities dataset produced by the European Commission’s Joint Research Centre (Pekel et al., 2016). Flood confidence is categorized as high if both VV and VH Z-scores fall below the identified thresholds, and as medium if only one of these polarizations is below the thresholds. We classify floods in areas not designated as permanent open water (with a probability of water greater than 95%) or with a historical inundation probability less than or equal to 25%. For each case study, we preselect a historical reference period based on existing knowledge of past flooding events in the respective area.

##

Event Data Project (ACLED) database (ACLED, 2023), covering the period from January 2012 to December 2023 for Nigeria and from January 2016 to December 2023 in Mozambique. For the purpose of this study, conflict, as defined by the WBG (2024) is, “a state of acute insecurity resulting from the use of lethal force by a group— encompassing state forces, organized non-state entities, or other irregular bodies— driven by a political purpose or motivation. Such force may manifest

> 1 Pixels with no cloud-free observations are excluded.

> 2 A pixel is around 100m2, we tested without buffer, 500m and 1km and chose a 500m buffer to introduce more variation of nightlight intensity within flooded pixels.

> 3 A time series depicting both variables is provided in Figure 16 in the appendices for Nigeria.

> 4 The corresponding time series for is presented in Figure 17 for Nigeria.

4