REVIEW ARTICLE Real-time river level estimation based on variations of radar reflectivity—a case study of the Quitandinha River watershed, Petrópolis, Rio de Janeiro (Brazil) Fabricio Polifke da Silva 1,2 & Otto Corrêa Rotunno Filho 1 & Maria Gertrudes Alvarez Justi da Silva 3 & Rafael João Sampaio 1 & Gisele Dornelles Pires 1,2 & Afonso Augusto Magalhães de Araújo 4 Received: 2 July 2020 /Accepted: 14 January 2021/ # The Author(s), under exclusive licence to Springer Nature Switzerland AG part of Springer Nature 2021 Abstract The real-time warning for flash floods represents one of the major challenges for monitoring centers and civil defenses due to the short time frame in which such phenomena occur. That situation often takes place particularly in mountainous regions where high slope rate favors an increase in the velocity of the water flow and it might generate conditions to impact the water level due to potential for provoking erosion, landslides, and debris flow. An initial one new and unique prototype of hydrometeorological relationship for real-time, site-tunable, water level for flash flood prediction is presented. Time variations of the reflectivity measured by horizontal polarization radar are evaluated and correlated to generate the local estimate of the elevation of the Quitandinha River water level for the next hour. The proposal of this relationship is not intended to act as a simplified hydraulic model but to characterize if a critical elevation of the water level could be obtained from the radar reflectivity data and consequently whether the flash flooding event should be expected. We verified that the proposed relationship that tended to underestimate the water level peaks, however, qualitatively was initially able to indicate if a flood event could occur or not in the next hour. The expectation related to this initial relationship is to improve the obtained results in order to be used as a tool for decision- making in relation to the issuance of warnings delivered by environmental monitoring centers. Keywords Reflectivity . River level . Weather radar reflectivity . Nowcasting 1 Introduction Flash floods are related to the most frequent and dangerous hydrometeorological extreme events which produce millions of socioeconomic losses in several countries worldwide Bulletin of Atmospheric Science and Technology https://doi.org/10.1007/s42865-021-00030-z * Fabricio Polifke da Silva fabricio@coc.ufrj.br Extended author information available on the last page of the article