Abstract. In many cases flood frequency analysis needs to be carried out on mean daily flow (MDF) series without any available information on the instantaneous peak flow (IPF). We analyze the error of using MDFs instead of IPFs for flood quantile estimation on a German dataset and assess spatial patterns and factors that influence the deviation of MDF floods from their IPF counterparts. The main dependence could be found for catchment area but also gauge elevation appeared to have some influence. Based on the findings we propose simple linear models to correct both MDF flood peaks of individual flood events and overall MDF flood statistics. Key predictor in the models is the event-based ratio of flood peak and flood volume obtained directly from the daily flow records. This correction approach requires a minimum of data input, is easily applied, valid for the entire study area and successfully estimates IPF peaks and flood statistics. The models perform particularly well in smaller catchments, where other IPF estimation methods fall short. Still, the limit of the approach is reached for catchment sizes below 100 km2, where the hydrograph information from the daily series is no longer capable of approximating instantaneous flood dynamics. 1 Flood frequency analysis using mean daily flows vs. instantaneous peak flows Anne Bartens1, Uwe Haberlandt1 1Institute of Hydrology and Water Resources Management, Leibniz University of Hannover, Germany Correspondence to: Anne Bartens (fangmann@iww.uni-hannover.de) 5 Abstract. In many cases flood frequency analysis needs to be carried out on mean daily flow (MDF) series without any available information on the instantaneous peak flow (IPF). We analyze the error of using MDFs instead of IPFs for flood quantile estimation on a German dataset and assess spatial However, embracing the true dimension of a peak requires continuous measurement of the flow on a high temporal resolution. Such data is rarely available and oftentimes FFA needs to be carried out on average daily flow records instead. The daily averaging naturally flattens the flood peak and the true maximum becomes unknowable. The degree of this smoothing, i.e. the difference between the true instantaneous peak flow (IPF) and the maximum mean daily flow (MDF) depends on the response 25 time of a system, which is controlled by a multitude of factors. (2008), 30 Muñoz et al. (2012) and Ding et al. (2015). Other IPF estimation methods aim at using the bare minimum of available data, i.e. solely the available daily flow record. In these cases, usually the shape of hydrographs are used to estimate the instantaneous peaks of events (e.g. Langbein, 1944; Ellis and Gray, 1966). Several approaches make use of the maximum daily flow and the discharge of the previous and successive day. Chen et al. (2017) compare t wo of these methods, namely those of Sangal (1983) and Fill and Steiner (2003). (1) This error is computed at each station for any desired quantity stat, like the mean annual maximum flow (MHQ), L-moments, distribution parameters and flood quantiles. 65 In order to improve the IPF estimation by MD F, several correction methods are applied, which make use of the peak -volume ratio. This ratio is computed for events in the average daily time series after baseflow separation using the direct peak flo w Qdir and the direct flood volume Voldir p/V [ 1 𝑑] = Qdir [m3d−1] Voldir [m³] . (2) The first IPF estimation method aims at correcting individual events. For calibration, all events are identified that contain a 70 monthly maximum instantaneous peak. 2 demonstrates by means of the mean annual maximum flow that the larger the area, the smaller the deviation between MDF and IPF. Also, errors appear to be especially large in https://doi.org/10.5194/hess-2021-466 Preprint. Discussion started: 17 September 2021 c⃝ Author(s) 2021. CC BY 4.0 License. 8 higher altitudes. Generally, the error seems to increase in north -south direction, which could be a secondary effect of both increasing altitude and decreasing catchment size. 185 Figure 2: Spatial distribution of the MDF error in the MHQ. When assessing the differences between average daily and instantaneous peaks, it is also meaningful to take a closer look at different types of floods. This indicates 205 that the average daily flow smooths significant peaks to a point where they are no longer relevant for the overall flood behavior. Fig. 4 a shows the percentage of annual maxima at each gauge that are attributed to the wrong season. Negative values are falsely attributed to summer, positive values to winter. It is obvious that with decreasing catchment size an increasing number of annual maxima are falsely identified in the winter half year, while the actual instantaneous maxima occur in summer. Long, voluminous winter floods on the other hand show a much smaller IPF-MDF ratio and are easier to model. This study has also shown that hydrograph characteristics, like the peak -volume ratio of flood events can be u sed to estimate instantaneous peak flows when only average daily series are available. The p/V ratio may be used to predict both IPFs of individual events and instantaneous flood statistics, including mean annual and seasonal maximum flows and flood quanti les. 465 Due to improper flood event separation, the event -correction method produced some outliers in our case but may work significantly better when flood events can be defined more carefully. and Gray, M.: Interrelationships between the peak instantaneous and average daily discharges of small prairie streams, Can Agr Eng–39, 1966. Fill, H. D. and Steiner, A. A.: Estimating instantaneous peak flow from mean daily flow data, J Hydrol Eng, 8, 365 -369, doi: 10.1061/(ASCE)1084-0699(2003)8:6(365), 2003. Fischer, S.: A seasonal mixed-POT model to estimate high flood quantiles from different event types and seasons, J Appl Stat, 525 45, 2831-2847, doi:10.1080/02664763.2018.1441385, 2018.