11 Apr 2022
11 Apr 2022
Status: this preprint is currently under review for the journal NHESS.

How uncertain are precipitation and peakflow estimates for the July 2021 flooding event?

Mohamed Saadi1,2, Carina Furusho-Percot1,2, Alexandre Belleflamme1,2, Ju-Yu Chen3, Silke Trömel3,4, and Stefan Kollet1,2 Mohamed Saadi et al.
  • 1Institute for Bio- and Geosciences (IBG-3, Agrosphere), Forschungszentrum Jülich, Jülich 52425, Germany
  • 2Centre for High-Performance Scientific Computing in Terrestrial Systems, Geoverbund ABC/J, Jülich 52428, Germany
  • 3Institute for Geosciences, Department of Meteorology, Universität Bonn, Bonn 53121, Germany
  • 4Laboratory for Clouds and Precipitation Exploration, Geoverbund ABC/J, Bonn 53121, Germany

Abstract. The disastrous July 2021 flooding events made us question the ability of current hydrometeorological tools in providing timely and reliable flood forecasts. This is an urgent concern since extreme events are increasing due to global warming. For the July 2021 events, we simulated the hourly streamflows of seven catchments in western Germany, by combining five, partly polarimetric, radar-based quantitative precipitation estimates (QPE) with two hydrological models: a conceptual lumped model (GR4H) and a physically-based, 3D distributed model (ParFlowCLM). GR4H parameters were calibrated with emphasis on high flows using historical discharge observations, whereas ParFlowCLM parameters were estimated based on landscape and soil properties. The key results are as follows: (1) All radar-based QPE products underestimated the total precipitation depth relatively to rain gauges due to intense collision-coalescence processes near the surface, i.e. below the height levels monitored by the radars. (2) The use of polarimetric radar variables led to clear improvements compared to reflectivity-based QPE products. (3) The probability of exceeding the highest measured peakflow before July 2021 was highly impacted by the QPE product, and depended on the catchment for both models. (4) The estimation of model parameters had a larger impact than the choice of QPE product, but simulated peakflows of ParFlowCLM agreed with those of GR4H for five of the seven catchments. This study highlights the need for the correction of vertical profiles of reflectivity and other polarimetric variables near the surface to improve radar-based QPE for extreme floods. It also underlines the large uncertainty in peakflow estimates due to model parameter estimation.

Mohamed Saadi et al.

Status: open (extended)

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  • RC1: 'referee comment on nhess-2022-111', Anonymous Referee #1, 03 May 2022 reply

Mohamed Saadi et al.

Mohamed Saadi et al.


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Short summary
In the 14 July 2021, heavy rainfall fell over western Germany, causing considerable damage and human fatalities. We analyzed how accurate were our estimates of rainfall and peakflow for these flooding events. We found that the rainfall estimates from radar were improved by including polarimetric variables. Our estimates of peakflow were highly uncertain due to uncertainties in model parameters and rainfall measurements.