# On the visual detection of non-natural records in streamflow time series: challenges and impacts

Laurent Strohmenger, Eric Sauquet, Claire Bernard, Jérémie Bonneau, Flora Branger, Amélie Bresson, Pierre Brigode, Rémy Buzier, Olivier Delaigue, Alexandre Devers, et al.

<figure><img src="image_1.png" /></figure>

## To cite this version:

Laurent Strohmenger, Eric Sauquet, Claire Bernard, Jérémie Bonneau, Flora Branger, et al. On the visual detection of non-natural records in streamflow time series: challenges and impacts. Hydrology and Earth System Sciences, 2023, 27 (18), pp.3375-3391. 10.5194/hess-27-3375-2023. hal-04214908

HAL Id: hal-04214908

https://hal.inrae.fr/hal-04214908

Submitted on 22 Sep 2023

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>
# On the visual detection of non-natural records in streamflow time series: challenges and impacts

Laurent Strohmenger¹, Eric Sauquet², Claire Bernard³, Jérémie Bonneau², Flora Branger², Amélie Bresson⁴,
Pierre Brigode¹,⁵, Rémy Buzier⁶, Olivier Delaigue¹, Alexandre Devers², Guillaume Evin⁷, Maïté Fournier⁸,
Shu-Chen Hsu¹, Sandra Lanini⁹,¹⁰, Alban de Lavenne¹, Thibault Lemaitre-Basset¹,¹¹, Claire Magand¹²,
Guilherme Mendoza Guimarães¹, Max Mentha¹³, Simon Munier¹⁴, Charles Perrin¹, Tristan Podechard¹⁵,
Léo Rouchy², Malak Sadki¹⁴, Myriam Soutif-Bellenger¹,¹⁶, François Tilmant¹, Yves Tramblay¹⁷, Anne-Lise Véron¹,
Jean-Philippe Vidal², and Guillaume Thirel¹

¹INRAE, HYCAR Research Unit, Université Paris-Saclay, Antony, France

²INRAE, UR RiverLy, Villeurbanne, France

³Chambre d'agriculture du Vaucluse, Avignon, France

⁴EPIDOR, Castelnaud-la-Chapelle, France

⁵Observatoire de la Côte d'Azur, CNRS, OCA, IRD, Géoazur, Université Côte d'Azur, Sophia-Antipolis, France

⁶URA IRSTEA, University of Limoges, Limoges, France

⁷INRAE, CNRS, IRD, Grenoble INP, IGE, Univ. Grenoble Alpes, Grenoble, France

⁸ACTeon – Environment, Research & Consultancy, Grenoble, France

⁹BRGM, unité EAU-RMD, Montpellier, France

¹⁰G-EAU, UMR 183, INRAE, CIRAD, IRD, AgroParisTech, Supagro, BRGM, Montpellier, France

¹¹UMR 7619 METIS, Sorbonne Université, CNRS, EPHE, Paris, France

¹²Office français de la biodiversité (OFB), Vincennes, France

¹³Safege-Suez Consulting, Paris, France

¹⁴CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse, France

¹⁵CEREG, Montpellier, France

¹⁶AgroParisTech, 75005, Paris, France

¹⁷HSM, University of Montpellier, CNRS, IRD, IMT, Montpellier, France

**Correspondence:** Laurent Strohmenger (laurent.strohmenger@inrae.fr) and Guillaume Thirel (guillaume.thirel@inrae.fr)

Received: 22 February 2023 – Discussion started: 12 April 2023

Revised: 3 July 2023 – Accepted: 23 August 2023 – Published: 22 September 2023

**Abstract.** Large datasets of long-term streamflow measure-
ments are widely used to infer and model hydrological pro-
cesses. However, streamflow measurements may suffer from
what users can consider anomalies, i.e. non-natural records
that may be erroneous streamflow values or anthropogenic
influences that can lead to misinterpretation of actual hydro-
logical processes. Since identifying anomalies is time con-
suming for humans, no study has investigated their propor-
tion, temporal distribution, and influence on hydrological in-
dicators over large datasets. This study summarizes the re-
sults of a large visual inspection campaign of 674 streamflow
time series in France made by 43 evaluators, who were asked

to identify anomalies falling under five categories, namely,
linear interpolation, drops, noise, point anomalies, and other.
We examined the evaluators’ individual behaviour in terms of
severity and agreement with other evaluators, as well as the
temporal distributions of the anomalies and their influence on
commonly used hydrological indicators. We found that inter-
evaluator agreement was surprisingly low, with an average
of 12% of overlapping periods reported as anomalies. These
anomalies were mostly identified as linear interpolation and
noise, and they were more frequently reported during the
low-flow periods in summer. The impact of cleaning data
from the identified anomaly values was higher on low-flow
indicators than on high-flow indicators, with change rates lower than 5% most of the time. We conclude that the identification of anomalies in streamflow time series is highly dependent on the aims and skills of each evaluator, which raises questions about the best practices to adopt for data cleaning.

# 1 Introduction

Water is essential for the wellbeing of our societies, as it supports recreational activities, biodiversity, agriculture, industrial development, and fresh water supply. Yet water, and also lack of water, can become a threat during extreme events such as floods (Merz et al., 2021) and droughts (Blauhut et al., 2022). This highlights the importance of management strategies based on scientific understanding of hydrological processes in order to mitigate their impacts. The starting point of the learning framework proposed by Dunn et al. (2008) is the acquisition of field data (e.g. river streamflow) to hypothesize and conceptualize the functioning of a catchment before making predictions.

Acquiring long-term data is a crucial step in studying the water cycle and its interactions with natural (i.e. without human influences) and anthropogenic drivers in the atmosphere, biosphere, and lithosphere (Gaillardet et al., 2018). A well-distributed monitoring network is required to cover the spatial heterogeneity of these interactions within large territories and to improve the robustness of statistical analyses and models by increasing the number of available observations from a wide range of probability distributions (Andréassian et al., 2006; Gupta et al., 2014; Lloyd et al., 2014).

High-quality streamflow measurements are needed for detecting non-stationarity in river flow regimes due to global change. Long-term streamflow monitoring has enabled data analyses that revealed increasing trends in the frequency of severe floods (Hisdal et al., 2001; Kundzewicz et al., 2013; Blöschl et al., 2019; Gudmundsson et al., 2021; Hannaford et al., 2021), drought intensification (Vicente-Serrano et al., 2014, 2019; Blauhut et al., 2022), and changes in intermittent river flow regimes (Sauquet et al., 2021). In addition, Meerveld et al. (2020) highlighted inconsistencies between the proportion of potential temporary streams and the occurrence of zero flows reported over 730 gauging stations. Thus, checking flow records before applying statistical tests is a crucial but delicate task for which there are no common technical guidelines to date, especially for large datasets.

Beyond data analysis, streamflow datasets have been used to set up, calibrate, and evaluate hydrological models through a variety of studies. For example, Chauveau et al. (2013) described the Explore2070 project that used more than 1000 gauging stations to simulate changes in surface water in France by 2065. Forzieri et al. (2014) used a large set of 446 gauging stations to evaluate their model before addressing the future of streamflow drought characteristics across Europe.

de Lavenne et al. (2019) used streamflow time series of 1305 French gauged catchments to evaluate a constrained calibration method of semi-distributed hydrological models.

However, measurements may suffer from flaws that lead to streamflow values that do not reflect the reality (Beven and Westerberg, 2011; McMillan et al., 2012; Wilby et al., 2017). First, instruments are subject to surrounding factors, such as extreme temperatures or humidity that may alter their functionality. In addition, errors can occur when there is infrequent instrument maintenance because of access issues, for example, which can lead to bias in the measurement. Moreover, streamflow is estimated from the conversion of the river water level with respect to a rating curve (Herschy, 2008), which is only valid for given conditions. Thus, measurement errors can occur when the river water level is outside the measurement range of the instrument or the rating curve. Besides, the riverbed may change during extreme events, which can compromise the validity of the rating curve. Finally, missing data are often filled using interpolation methods during post-processing. Streamflow time series may also include anthropogenic influences (known or unknown to data users), such as agricultural practices with artificial drainage or irrigation, river management with compensation flows, industry with water intakes/releases, and reservoir management for hydroelectric power plants (Wilby et al., 2017).

All these flaws, hereafter called “anomalies” (Leigh et al., 2019), are considered disinformative data (Beven and Westerberg, 2011) that may not reflect the natural streamflow dynamics, eventually leading to misinterpretation of hydrological processes. Wright et al. (2015) found that the leverage effect of individual influential data points could substantially influence streamflow predictions depending on the catchment and the model structure. Lamontagne et al. (2013) argue that outliers (i.e. unusually small values compared to the rest of the data) might compromise the accuracy and validity of flood quantile estimation. Therefore, it is crucial to carefully examine the data before trying to make inferences about hydrological processes.

Promising techniques exist for detecting anomalies in water quality and urban water networks (Leigh et al., 2019). These methods often use multivariate analyses that include multiple covariates (when available) or a combination of methods to better detect short- and long-term anomalies in water quality time series (Muxika et al., 2007; Leigh et al., 2019; Rodriguez-Perez et al., 2020) and water level time series (van de Wiel et al., 2020). However, visual inspection of data is still highly recommended (Barthel et al., 2022) because automatic methods can suffer from misclassification of data points that may require user intervention afterward (Leigh et al., 2019; van de Wiel et al., 2020). In addition, choosing the most appropriate methods for anomaly detection depends on the goal and expertise of end users (Leigh et al., 2019). Sebok et al. (2022) used experts’ knowledge to qualitatively evaluate the ability of models to predict future hydrological and climate conditions and concluded that
expert elicitation can help to weight and thus decrease the influence of improbable hydrological models. Crochemore et al. (2015) compared numerical criteria of model performance with experts’ visual evaluations of hydrographs and concluded that none of the numerical criteria can fully replace expert judgement when rating hydrographs. Yet, identifying anomalies for a large dataset is time consuming, and, as far as we know, no study has focused on the identification of such data in a large dataset of streamflow time series. Therefore, questions remain on their proportion, temporal distribution, and influence on classic hydrological indicators.

This study aims at exploring the results of a large campaign of visual inspection of hundreds of daily streamflow time series in order to detect anomalies: (1) we evaluated the subjectivity of the individual evaluators in terms of the quantity of data reported as anomalies and the agreement with other evaluators; (2) we analysed the frequency and the temporal changes for each type of anomaly; and (3) we evaluated the influence of the anomalies on the calculation of classic high- and low-flow hydrological indicators. This study provides useful insights into disinformative hydrological data that might help researchers assess the quality of their datasets, in addition to drawing the attention of data producers on how streamflow monitoring could be improved in the future.

## 2 Methods

### 2.1 Data

An initial number of 674 gauged stations were selected in the French HydroPortail database (https://hydro.eaufrance.fr/, last access: 1 September 2021). The ultimate goal of this catchment set is to serve as a reference streamflow network for the Explore2 project (https://professionnels.ofb.fr/fr/node/1244, last access: 1 June 2023) that aims to assess the impact of future climate change on water resources in France during the 21st century. Consequently, it is necessary to identify a large set of streamflow stations that provide uninfluenced daily data both for qualifying the natural hydrology and for assessing the performance of daily rainfall-runoff models that are used to simulate future streamflows from climate projections. The criteria of selection of these stations, based on information given in different national databases, were as follows: (1) low and no anthropogenic influence displayed in the database; (2) good data quality for all flow regimes displayed in the database (Leleu et al., 2014); (3) available length of the time series greater than 26 years at a daily time step between 1976 and 2019, to mitigate decadal variability; and (4) a drained area larger than 64 km², which corresponds to the area of a pixel from the French SAFRAN reanalysis (Vidal et al., 2010b).

The catchment set covers metropolitan France well with a variety of hydrological, geological, topographical, and cli-

<figure><img src="image_3.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Locations and (b) number of available observed streamflow data (per day) for the 674 gauged stations in France.</figcaption></figure>

matic contexts, except for the western part because of the high agricultural influence on streamflow (Fig. 1a). The catchment area of the 674 selected gauged stations ranges from 64 to 111 570 km² (mean and median of 1278 and 263 km²). The available length of streamflow time series ranges from 26 to 44 years (mean of 39.5 years). The number of available streamflow observations for each day ranges from 503 to 667 stations between 1976 and 2019 (mean of 605 stations, Fig. 1b).

### 2.2 Visual inspection of streamflow time series

The Explore2 project focuses on natural streamflow to evaluate the hydrological models. However, the streamflow time series could be affected by errors or influences, even though they have been marked as having low anthropogenic influence and good quality. Thus, the first objective of the visual inspection of the dataset was to identify stations that were largely influenced by anthropogenic activity (i.e. hydropower production, low-water level support, and reservoir management). The visual inspection involved the participation of 43 evaluators, mostly academic (80%) and operational (20%) hydrologists of varying levels of experience (24%, 38%, and 38% were considered novice, advanced, and senior, respectively). Each evaluator analysed different number of time series (later on specified in Fig. 3), based on
(1) their availability and willingness to analyse a given number of time series; (2) their preference for some hydrographic zones, linked to their expertise, when pertinent; and (3) randomness to attribute the remaining time series. The first step led to the exclusion of 63 stations from the analysis, based on the general feedback given by the evaluators. For these excluded stations, the evaluators mentioned time series that contained too many anomalies to be reported individually, inconsistency of data over several years compared to the rest of the time series, the absence of any clean summer period, or too large a proportion of missing data filled in by linear interpolation. In addition, data producers helped to identify the potential presence of anthropogenic influences for some of these time series.

The second objective was to visually identify anomalies, i.e. periods when the dynamics of streamflows does not seem natural (e.g. due to dam operation, water withdrawal, instrument failures, unit conversion, or post-processing errors), for the remaining 611 stations. All subsequent analyses were conducted with these 611 stations and the remaining 42 evaluators (since one of the evaluators only analysed stations among those that were excluded).

An evaluation protocol was established in order to allow evaluators to report anomalies. All evaluators participated in online meeting sessions, the main objectives of which were to remind them of the goal of the work and of the evaluation protocol for the time series, to answer any questions about the work, and to enable discussions around any difficulties encountered. Each evaluator had access to a batch of streamflow time series (in HTML format; see an example in Fig. A1) and to a spreadsheet to report the period and type of anomaly as well as to provide any additional comments that they deemed necessary.

The HTML file was composed of three dynamic panels displaying the time series of (1) catchment-aggregated solid and liquid precipitation, and air temperature based on the SAFRAN analysis; (2) streamflow time series in a linear scale; and (3) streamflow time series in a logarithmic scale to highlight potential anomalies for low flows. In order to help identify anomalies, simulated streamflow time series were also displayed, although we emphasized that simulated streamflow could also be flawed and should not be considered an absolute reference. We used the GR5J lumped rainfall-runoff model (Pushpalatha et al., 2011) together with the CemaNeige snow accumulation and melt model (Valéry et al., 2014) calibrated on the original time series of streamflows in the airGR package (Coron et al., 2017, 2020) to produce these simulations.

One spreadsheet was provided to each evaluator for every analysis session (usually consisting of a set of 5–10 time series to analyse). This spreadsheet comprised one tab per station that each evaluator had to fill in. The required fields for each anomaly identified were its start date, its end date, and the type of anomaly. In addition, the evaluator could provide a general comment on the station. We proposed five types

<table><caption>**Table 1.** Examples of union and intersection of anomalies at a single time step.</caption><thead><tr><th>Evaluator 1</th><th>Evaluator 2</th><th>Union</th><th>Intersection</th><th>Case</th></tr></thead><tbody><tr><td>Anomaly A</td><td>Anomaly A</td><td>Anomaly A</td><td>Anomaly A</td><td>(1)</td></tr><tr><td>Anomaly B</td><td>Anomaly A</td><td>Disagreement</td><td>Disagreement</td><td>(2)</td></tr><tr><td>Anomaly A</td><td>—</td><td>Anomaly A</td><td>—</td><td>(3)</td></tr><tr><td>—</td><td>Anomaly A</td><td>Anomaly A</td><td>—</td><td>(4)</td></tr><tr><td>—</td><td>—</td><td>—</td><td>—</td><td>(5)</td></tr></tbody></table>

of anomaly, namely, linear interpolation, drops, noise, point anomalies, and other (Fig. 2). Linear interpolation consists of periods showing a straight line often due to a filling in of a period with missing data. Noise consists of a periodic pattern in the streamflow time series that may be related to hydro-electricity production or an unknown perturbation in the measurements. Drops consist of a sudden decrease in the measured streamflow that may be due to water management or technical failures of the instrument. Point anomalies are short-term variations of the streamflow that may be related to the maintenance of the instrument or, for example, to the presence of debris in the river. Evaluators could assign the rating of “other” when an anomaly did not fit any of the four previous types or was a combination of several of them.

We estimated the time needed to evaluate one station to be approximately 10–15 min per evaluator, although we have not set a time limit. Since this exercise is subjective, each time series was analysed by two different evaluators. This allowed a comparison to be made of the subjectivity in anomaly detection between each evaluator, as well as a better coverage of anomalies in the time series.

## 2.3 Data analysis

### 2.3.1 Combining the feedback

We tested two methods for combining the analyses from the pairs of evaluators for each station: the union and the intersection of the anomalies (Table 1). The union of the anomalies consists of considering a streamflow value to be an anomaly in the time series if at least one of the evaluators identified it as an anomaly. The type of union of anomaly attributed to a value is either the anomaly type identified by both evaluators if they agree (or if the second type of anomaly is “other”, case 1), the anomaly type identified by one evaluator if the other one identified no anomaly (Table 1, cases 3 and 4), or “disagreement” if both evaluators identified different types of anomaly (Table 1, case 2). A value is considered valid if none of the two evaluators identified an anomaly (Table 1, case 5). The pros of using the union of the anomalies is a better confidence in the number of anomalies detected in the time series, at the expense of an increased number of potentially false-positive anomalies detected.
<figure><img src="image_5.png" /><figcaption>Figure 2. Actual examples of the four types of anomalies that could be detected in the dataset.</figcaption></figure>

The intersection of the anomalies consists of considering a streamflow value to be an anomaly in the time series only if both evaluators identified an anomaly, even if the types of anomaly were different (Table 1, cases 1 and 2). A value is considered valid if at least one of the evaluators did not identify an anomaly (Table 1, cases 3–5). The pros of using the intersection of the anomalies is having better confidence in the anomaly of the values identified, at the expense of missing potential anomaly values in the time series (false negative). These two methods for combining feedback will be compared later, and the union method will be ultimately kept for further analyses (see Sect. 3.1).

## 2.3.2 Analysis by evaluator

We analysed the individual behaviour of each evaluator in terms of sensitivity and agreement in the identification of anomalies with other evaluators. First, we computed the percentage of time identified as an anomaly by each evaluator and by station (Eq. 1) in order to assess the variability among evaluators.

$$P_a = \frac{D_a}{D_Q}, \qquad (1)$$

where $P_a$ is the percentage of anomaly, $D_a$ and $D_Q$ are the duration (in days) of anomalies and available discharge time series. Second, we estimated the individual agreement of each evaluator with their associated pair when an anomaly was detected. We computed this inter-evaluator agreement for each station as the ratio between the sum of the intersec-

tion of anomalies and the union of anomalies (Eq. 2).

$$A_e = \frac{D_a^i}{D_a^u}, \qquad (2)$$

where $A_e$ is the inter-evaluator agreement, and $D_a^i$ and $D_a^u$ are the duration of the anomaly considering the intersection and the union of evaluators feedbacks, respectively. The inter-evaluator agreement ranges from 0% for no agreement at all to 100% for a total agreement between the two evaluators. We attributed an inter-evaluator agreement of 100% if none of the pairs of evaluators found any anomaly in a time series.

## 2.3.3 Analysis by type of error

We compared the inter-evaluator agreement for each type of anomaly by looking at the associated distribution of all anomaly types (i.e. how many times linear interpolation is associated with linear interpolation, noise, drops, point anomalies, other, and none). The proportional distribution of each type of anomaly was computed as the ratio between the length of the union of a specific type of anomaly and the length of union of all types of anomaly. We also computed the monthly and yearly temporal distributions for each type of union of anomalies.

## 2.3.4 Analysis by station

We assessed the impact of the detected anomalies by computing some hydrological indicators on initial time series and cleaned time series (i.e. time series for which the anomalies
were set as missing data). The change rates have been estimated following Eq. (3) for each time series.

$$C_r = \frac{I_c - I_o}{I_o}, \qquad (3)$$

where $C_r$ is the change rate of hydrological indicators using initial ($I_o$) and cleaned ($I_c$) discharge time series. We opted for nine hydrological indicators that reflect different parts of the hydrological regime of each river. The seasonal behaviour was computed as the monthly mean interannual streamflow. Low-flow periods were evaluated with the first and fifth quantiles of the total flow duration curve, the annual minimum monthly flow with a return period of 5 years (noted QMNA5), and the annual minimum of a 30 d moving average of flow with a 5-year return period (noted VCN30$_5$). High flows were evaluated with the 95th and 99th quantiles of the total flow duration curve and the annual maximum daily flow with a 10-year return period (noted QJXA10).

# 3 Results

## 3.1 Individual behaviour of the evaluators

Each evaluator analysed from 5 to 111 time series, with a mean of 29 stations per evaluator. The percentage of time identified as an anomaly ranges from 0% to 45% depending on the station and the evaluator, with a median of 0.7% (Fig. 3a), showing a high variability among evaluators. The median time reported as anomalies ranges between 0.04% and 2.92% according to the different evaluators (Fig. 3a). These median values seem to stabilize around 0.77% ± 0.26% (mean ± standard deviation) for the evaluators who analysed more than 40 stations in comparison with the other evaluators (0.86% ± 0.72%). The percentages of agreement between evaluators were unexpectedly low, with medians ranging from 0% to 43% with a mean of 12% for all the evaluators (Fig. 3b). Regarding the proportion of error identified, the inter-evaluator agreement seems more stable for evaluators who analysed more than 40 stations (12% ± 4%) than for the other evaluators (12% ± 10%).

The low percentage of inter-evaluator agreement was observed for every type of anomaly. Indeed, when an evaluator identified an anomaly, there was no anomaly identified by the second evaluator 59%–74% of the time (“none” type, Fig. 4). The higher agreement rates (i.e. both evaluators identified the same type of anomaly) were for the linear interpolation, noise, and drops with 20, 19, and 11%, respectively. The type of anomaly that was the most often associated with the “other” type was noise (16%), while the agreement rate for “other” was only 5%. Even worse, the agreement rate of the point anomalies was the lowest of all types with only 2% (Fig. 4).

Ideally, evaluators would identify roughly the same doubtful periods, and the intersection of anomalies should be recommended to clean up the time series, but the surprisingly

low inter-evaluator agreement that we observed would lead to marginal cleaning of the dataset. For this reason, we will focus on the union of the anomalies in the next sections.

## 3.2 Distribution of the anomalies

The most represented anomalies were the linear interpolations (35% of the anomalies, Fig. 5a), followed by noise (23%), other (20.5%), disagreement (10.2%), drops (8.2%), and point anomalies (3.1%). The seasonal distribution of the anomalies shows that they occur more frequently during summer than winter months, especially for linear interpolations and noise (Fig. 5b). The long-term evolution of the annual frequency of anomalies seems to decrease from 1976 to 2019 (Fig. 5c), mostly due to a decreasing number of days identified as linear interpolations and noise. A few years showed a higher number of anomalies, such as 1976, 1978, 1985, 1989, and 2003 (relative to surrounding years).

## 3.3 Changes in the length of the time series

The initial length of available data from 1976 to 2019 ranges between 26 and 44 years with a mean and median of 39.6 and 42 years, respectively (Fig. 6a). The average percentage of time series identified as an anomaly ranges between 0% (for 14 stations) and 46%, with a mean and median of 2.4% and 1.3%, respectively (Fig. 6b). Cleaning the time series from these anomalies resulted in a decrease of the mean and median length of the time series by 1 and 1.45 years, respectively. The length of the clean time series ranges between 23.5 and 44 years, with a mean and median of 38.6 and 40.6 years, respectively (Fig. 6a).

## 3.4 Changes in hydrological indicators

We assessed the impact of removing anomalies from the dataset on hydrological indicators by calculating change rates in hydrological indicators before and after cleaning the time series. Removing the anomalies had little or no impact on the high-flow indicator values, while those of low-flow indicators could slightly increase.

The higher impacts were observed for Q1, Q5, VCN30$_5$, and QMNA5, with most (first to third quartiles) of the change rates being between 0% and 6.4%, 0% and 3.9%, 0% and 2.1%, and 0% and 3.0%, respectively (Fig. 7a), while a substantial proportion of the stations showed higher change rates for those indicators. Removing the anomalies from the time series had a lower impact on Q50 with most of the changes rates being between 0% and 2.7% and a negligible impact on Q95, Q99, and QJXA10 with most of the change rates lower than 1%. The change rates of monthly mean streamflow were also negligible overall (Fig. 7b). Most of the stations showed change rates in the monthly mean streamflow from 0% to 1% in winter months and from 0% to 2.6% during summer months; i.e. the monthly mean streamflow in-
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Individual statistics on (a) the percentage of time reported as an anomaly and (b) the inter-evaluator percentage of agreement for time steps with an anomaly detection. Statistics are displayed by box plots, where the lower and upper hinges correspond to the 25th and 75th quantiles, the vertical bar is the median, the upper and lower whiskers extend up to 1.5 times the interquartile distance from the 25th and 75th quantiles, and the dots are the outliers beyond the end of the whiskers. Each row relates to one evaluator and the number of time series they analysed. The vertical blue line is the median value considering all evaluators.</figcaption></figure>

creased by less than 2.6% after we removed the anomalies
from the time series.

# 4 Discussion

## 4.1 Subjectivity of the evaluators

One of the main results of our study is the high subjectiv-
ity in detecting anomalies, as reflected by the variability in
the percentage of time identified as anomalies, as well as
in the low inter-agreement on the timing and on the type of
anomaly identified among evaluators. We assessed the rela-
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Percentage of agreement between evaluators for each type of anomaly. Each bar displays the distribution of the type of anomaly identified by one evaluator when the other evaluator identified the type of anomaly indicated in the bar label. Self-associations (i.e. the same anomaly identified by the two evaluators) are indicated by black-bordered bars (n = 78 025, 60 108, 21 639, 10 695, and 59 305 d identified by one evaluator as linear interpolation, noise, drops, point anomalies, and other, respectively).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Distribution of the types of anomalies (a) in percentage of total time identified as anomalies, (b) with respect to month, and (c) with respect to year for the 611 stations. Each colour relates to a type of error such as displayed in (a).</figcaption></figure>

tionship between these subjective statistics with the level of experience of the evaluators. The percentage of anomaly reported seems to decrease as the experience of the evaluator increases; however, the variability between individuals is too high to conclude a strong relationship.

The high variability in the percentage of time identified as anomalies (Fig. 3a) might reflect the quality of the time series that the evaluators had to analyse, which may be heterogeneous among the 611 stations. Another explanation could be that this variability reflects differences in the level of expectations of the evaluators, therefore suggesting that the individ-
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Distribution of (a) length of initial observed streamflow time series and cleaned observed streamflow time series, and (b) percentage of time identified as anomalies using the union of anomalies by station. Statistics are displayed by box plots, where the lower and upper hinges correspond to the 25th and 75th quantiles, the vertical bar is the median, the upper and lower whiskers extend up to 1.5 times the interquartile distance from the 25th and 75th quantiles, and the dots are the outliers beyond the end of the whiskers. The dashed blue line relates to the mean of each population.</figcaption></figure>

ual subjectivity is high during visual inspection of stream-
flow time series. This variability seems to stabilize for the
evaluators who analysed more than 40 stations, which raises
two questions: is it related to the variability of the quality
of the station time series that is better estimated over 40 sta-
tions analysed? Did the evaluators gain experience in visual
inspection during the operation, so that their expertise con-
verged toward a value similar to that of other evaluators?

The low overall values of inter-evaluator agreement (Fig. 3b) reflect the high subjectivity in evaluators that makes the identification of anomalies highly challenging. This is true for the identification of the timing of anomalies, although including a margin of 3 or 7 d on the start and end date slightly increased the inter-evaluator agreement to 15 % and 17 %, respectively. The subjectivity of evaluators also reflects in the attribution of a type of anomaly as shown by the high proportions of “other” and disagreement types of anomaly identified (Fig. 5a) and by the low percentage of self-association of type of anomaly (Fig. 4). It seems that each evaluator has their own intuition of what is a proper time series of streamflow and what type of anomaly is identified. This subjectivity might reflect the variety of expertise or the level of expectations of hydrologists. One might be more focused on flood events, thus looking more at high-flow periods. Another might be more interested in groundwater flows and the seasonality of the streamflow dynamics. And yet another might be more interested in droughts, thus looking for the low-flow dynamics. The subjectivity of hydrologists has also been observed during visual evaluations

of model performance (Alexandrov et al., 2011; Crochemore
et al., 2015; Melsen, 2022) and of groundwater hydrographs
(Barthel et al., 2022).

Some anomalies appeared easier to identify than others,
such as linear interpolation and noise (Fig. 5a). One reason
that may explain why linear interpolation and noise were the
most represented anomalies is that they can last for weeks
or months, while point anomalies and drops may last a few
days (mean length of 22 and 73 d for linear interpolation and
noise, respectively), which should also make them easier to
detect, even though the inter-evaluator agreement for linear
interpolation was only 20 % (Fig. 4).

## 4.2 Temporal distribution and impacts of the anomalies

The annual frequency of anomalies showed a long-term de-
crease (Fig. 5c) that is not related to the number of avail-
able data (see Fig. A2 for the proportion of anomalies by
year). Thus, it may reflect an overall improvement of the
streamflow monitoring techniques or more efficient data
post-processing thanks to software improvements. Another
option is that the data supplier did not have the means to thor-
oughly verify old data that might be inherited from other ser-
vices or from old records. However, the long-term decrease
in anomalies might also be explained by the greater focus
of evaluators on streamflow analysis in the beginning of the
time series, which is supported by the decreasing number
of disagreements after 2000 (Fig. 5c), depicting a behaviour
where evaluators only picked the most obvious anomalies.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Change rates of hydrological indicators after cleaning the anomalies from the streamflow time series relative to indicators calculated with the initial streamflow time series. Panel (a) is the distribution of the change rates for quantiles (1, 5, 50, 95, and 99th), low-flow indicators (QMNA5 and VCN30$_5$), and a high-flow indicator (QJXA10), and panel (b) is the distribution of the change rates of monthly mean streamflows. Statistics are displayed by box plots, where the lower and upper hinges correspond to the 25th and 75th quantiles, the vertical bar is the median, the upper and lower whiskers extend up to 1.5 times the interquartile distance from the 25th and 75th quantiles, and the dots are the outliers beyond the end of the whiskers.</figcaption></figure>

The few years showing a higher number of anomalies (1976, 1978, 1985, 1989, and 2003; Fig. 5c) reflect some climatic events in France, such as the heatwaves and droughts in 1976 and 2003, which may have affected the proper functioning of the instruments. Indeed, Vidal et al. (2010a) identified all these years as major drought events related to exceptional precipitation or soil moisture deficits.

In addition, the temporal distribution of anomaly showed a seasonal pattern (Fig. 5b) that can be explained by many factors. The summer months are characterized by low flows, which are harder to monitor than the medium-to-high flows because of the resolution of the flow measurement instruments. The summer period is also traditionally a holiday period in France, meaning that instruments can be fixed less promptly. In addition, low-flow periods are proportionally more impacted by anthropogenic influences such as water withdrawal, energy production, or water releases. Another reason might be the use of the logarithmic scale to plot the

streamflow panel of the HTML files provided to the evaluators. This scale might have disproportionately magnified some anomalies that were minor in reality.

Our results show that the mean proportion of time series identified as anomalies remains relatively low (Fig. 6), even though we excluded the union of anomalies reported by evaluators. Excluding these periods from the initial dataset had little influence on the length of available data for most of the stations. It should be noted that these data were previously labelled as good-quality data in the metadata provided by the producer. This means that the time series had been considered as valid before they were made available in the database, which may explain the low percentage of data identified as anomalies. However, since the anomalies were more frequently reported during the summer months (Fig. 5b), their removal from the time series might induce a seasonal bias that may impact the calculation of hydrological indicators.
Indeed, the change rates were higher for hydrological in-
dicators from July to November and for Q1, Q5, VCN30$_5$, and QMNA5 (Fig. 7), which could have implications from an operational point of view. We assessed the impact of re-moving random time steps from streamflow time series in the same proportion as reported by evaluators (results not shown). We observed that the change rates for these indi-cators were larger when removing anomalies using evalua-tors' feedback than when removing random time steps. In addition, the rare stations with a negative change rate sug-gest that lower flows were more often identified as anomalies (Fig. 7). These results suggest that wet periods were better monitored overall than dry periods or that anomalies were easier to detect during low-flow periods because of the log-arithmetic scale on the streamflow time series we provided to the evaluators. Some stations displayed as outliers (dots in Fig. 7) showed changes that are nonetheless larger than de-scribed above. These higher change rates might be related to specific cases in our sample of stations or to the proportion of data removed from the time series. Indeed, a low initial value of streamflow or few remaining data after removing the anomalies from the time series can have a large impact on hy-drological indicators, especially for lower quantiles such as Q1 and Q5 and for calendar-constrained indicators such as QMNA5.

5 Lessons and perspectives

This section provides feedback on the design of the visual in-
spection campaign and some considerations about how such
a campaign could be used to improve the use of streamflow
datasets. These comments are intended for data producers,
data users, and anyone interested in repeating a similar vi-
sual inspection campaign of streamflow time series.

**5.1 Considerations for data producers**

The number of available streamflow data in France is huge
and of good quality most of the time, as shown by the low
proportion of anomalies found in the time series (Fig. 6). The
data provided over large spatial and temporal scales at high
resolution are essential to build and test hypotheses on hy-
drological processes or to estimate catchment indicators. We
noticed an improvement in river monitoring in terms of quan-
tity as the available data increased from 1976 to 1990 but also
potentially in terms of quality as anomaly rates decreased
from 1976 on. Nevertheless, we observed that available data
have been decreasing slightly since 2005 and sharply since
2015 (Fig. 1b). The latter decrease might be due to a de-
lay between streamflow monitoring and publishing in the
database; nonetheless, we are concerned about the slight de-
crease, which seems to be related to a gradual closure of
gauged stations. Crochemore et al. (2020) also observed a
decrease in the availability of data worldwide for political,

economic, or privacy reasons. We would like to emphasize
that long-term data are essential for hydrologists, especially
for those studying the effect of climate or land use change
on the water cycle. In addition, we noticed that medium- to
high-flow events appear to be well monitored, perhaps due
to the aggregation of measured data to daily discharge; how-
ever, the higher rates of anomaly during the drought periods
and drought years such as 1976 and 2003 (Fig. 5) showed
that an improvement of the techniques of low-flow monitor-
ing is still possible (Horner et al., 2022). Such an improve-
ment would be very valuable to the hydrological community
(Meerveld et al., 2020).

**5.2 Considerations for data users**

Data analysts and modellers are aware that perfect time series
do not exist for many reasons (Wilby et al., 2017); thus pre-
processing is mandatory before any use of the data. One of
the main findings of this study is that data cleaning is highly
subjective, as shown by the large proportion of heterogene-
ity in the temporality and types of anomalies that were re-
ported by the evaluators (Fig. 3 and 4). This high subjectivity
raises questions on the best practices for data cleaning, such
as how many evaluators should be involved in the detection
of anomalies and how their feedback should be combined.
For example, we could picture a process that combines the
anomalies coming from three or more evaluators and con-
sider the periods for which at least two evaluators agree to be
actual anomalies.

Our results show that the union of anomalies of two eval-
uators had a marginal effect on high-flow hydrological indi-
cators but could impact the low-flow indicators such as Q1
and QMNA5, which could have implications for water man-
agement regulations. Low impacts were observed for most
of the stations of the large dataset we used (611 stations),
but we emphasize that the impact of data cleaning on indi-
vidual stations might still be high for particular cases, and
the awareness of data users should increase as the size of the
dataset decreases. We did not find any clear relationship be-
tween change rates in low-flow hydrological indicators and
the proportion of data removed, while the absolute change
rates in Q50, Q95, and Q99 seem to increase with the pro-
portion of data removed from the time series (see Fig. A3).

Hydrological models may also be influenced by anoma-
lies, and further studies should investigate how parameters
and simulated streamflow change when removing anomalies
from the training period data. van den Tillaart et al. (2013)
and Brigode et al. (2015) showed that systematic errors, out-
dated rating curves, or a single extreme event (e.g. extreme
flood) could affect model performance and parameter esti-
mation. Thébault et al. (2023) reported that artificially cor-
rupting time series has little effect on model calibration over
a large dataset. Nevertheless, Perrin et al. (2007) and Ayzel
and Heistermann (2021) showed that modellers can remove a
large proportion of data from the calibration set, such as po-
tential anomalies, without compromising the estimates of the model parameters, as long as the calibration period covers dry and wet periods.

## 5.3 Feedback on the visual inspection campaign

This section aims at providing suggestions to those interested in reproducing such a campaign of visual inspection of streamflow time series. The objective of our campaign was to analyse a large dataset of streamflow time series in order to remove anomalies that could impact the evaluation of hydrological models. The dataset comes from a wide range of catchment conditions with different responses to rainfall events, hence the need to simultaneously compare temperature, precipitation, and streamflow time series. The first suggestion is that the number of anomaly types provided should be as low as possible to avoid confusion. Indeed, our results suggest potential confusion between the drops and point anomaly types (Fig. 4). If the types of anomaly are not well defined, each evaluator can picture different kinds of anomalies for a given period or may consider that the proposed types share similarities, which makes it difficult to choose one type or another. A phase of inter-calibration of evaluators, and even better with the data producer when possible, is highly recommended as it could reduce the subjectivity of such an exercise. This calibration phase could be completed by assessing the ability of the evaluator to detect fictitious anomalies in streamflow time series. Furthermore, we suggest adding a confidence rate in addition to the periods and type of anomaly, which would encourage evaluators to identify more doubtful periods, therefore increasing the intersection of identified anomalies. Including the confidence rate in a study such as ours might also make it possible to deepen the investigation on the subjectivity of each evaluator.

One important feature of visual inspection is the figure layout of the time series provided to the evaluators, as also noted by Barthel et al. (2022). Indeed, using a logarithmic scale for the streamflow axis might have facilitated the identification of anomalies in low-flow conditions; thus it may have artificially increased the anomaly frequency during low-flow periods (Fig. 5). In addition, evaluators seemed to lose focus when analysing long time series (44 years). Consequently, one should consider splitting streamflow time series to investigate and perhaps avoid this potential effect of weariness or consider simplifying the anomaly-reporting procedure with an interface that allows one to select the period and type with a mouse click.

An automatic detection of anomalies could avoid these issues of subjectivity and weariness. As a first step, an automatic detection could identify suspicious parts of streamflow time series that would be the subject of a visual inspection afterwards, instead of inspecting the whole time series. Using the bias between model simulations and measured time series could be a starting point for identifying these potential anomalies. Unfortunately, to our knowledge, these tech-

niques still require improvements. Such an algorithm should be flexible regarding the types of anomaly to identify and might be trained for each study to avoid the risk of removing data of interest (e.g. using a visual inspection such as the one reported in this study). Ideally, hydrologists should share a common library of anomaly types such as those suggested by Wilby et al. (2017). Promising perspectives in anomaly detection could be using covariates, such as biogeochemistry, conductivity, or water temperature time series, in addition of streamflow, since they may reflect the hydrological conditions and flow paths, or using paired catchment and comparing their double-mass curves of discharge time series.

## 6 Conclusions

This study explored the results of a large visual inspection campaign of streamflow time series in France that aimed at detecting non-natural records. The objectives of this study were to evaluate the subjectivity of evaluators in detecting anomalies, to analyse the frequency and temporal distributions of anomalies, and to assess their impact on commonly used hydrological indicators.

Each of the 611 time series was visually inspected by 2 out of 42 evaluators in order to report doubtful periods such as linear interpolation, noise, drops, point anomalies, or other. The feedback was combined using the union and the intersection of the periods of anomaly reported.

The main result of this study is the high subjectivity of the evaluators' behaviour in detecting anomalies. Indeed, the surprisingly low inter-evaluator agreement rates (mean value of 12%) reflect the variety of feedback by the hydrologists. Evaluators more frequently reported periods of anomaly during summer, which could be related to low flows that are more difficult to monitor than high flows but also to relatively higher anthropogenic influences during this period. Consequently, we observed higher impacts of the anomalies when analysing low-flow indicators calculated on the initial time series and on time series where the anomalies were removed, although the change rates remained low overall, below 5% for most of the time series in our large dataset.

This study also provided recommendations for future campaigns of visual inspection of time series. We strongly suggest setting up a phase of inter-calibration of evaluators in order to assess their subjectivity, as well as adding a confidence rate to the reported anomalies in order to identify more doubtful periods. Ideally, the development of automatic detection of anomalies, or at least doubtful periods, could greatly improve data cleaning stage.
Appendix A

<figure><img src="image_11.png" /><figcaption>Figure A1. Example of an HTML file displaying streamflow time series that was sent to evaluators.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure A2.** Proportion of anomalies (100 times the number of anomalies divided by the number of available $Q_{\text{obs}}$) with respect to year for the 611 stations.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure A3.** Absolute change rates of hydrological indicator vs. the proportion of anomalies for the 611 stations.</figcaption></figure>
*Data availability.* Observed streamflow data are available from the French HydroPortail database (http://www.hydro.eaufrance.fr/, last access: 1 September 2021; Leleu et al., 2014) and via the hub'eau API (https://hubeau.eaufrance.fr/page/api-hydrometrie, Mauclerc and Vilmus, 2018). The information on the time steps reported as anomalies and the type of anomalies can be found at https://doi.org/10.57745/SO2WOV (Strohmenger and Thirel, 2023).

*Author contributions.* LS, GT, and CP conceived the evaluation framework; GT prepared the HTML files (Delaigue et al., 2020); LS and GT organized the online meetings; LS analysed the feedback and drafted the article; and all authors analysed the time series and reviewed the article.

*Competing interests.* The contact author has declared that none of the authors has any competing interests.

*Disclaimer.* Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

*Acknowledgements.* The Explore2 project funded Laurent Strohmenger's postdoctoral position. We thank the additional evaluators, Patrick Arnaud, Audrey Bayle, François Bourgin, Yvan Caballero, François Colleoni, Joël Gailhard, Thibault Hallouin, Enola Henrotin, Baptiste Lévêque, Arlette Robert-Vassy, Paul Royer-Gaspard, Michael Savary, and Gaëlle Tallec, who participated in the visual analysis campaign. The French government (SCHAPI) and the HydroPortail database are acknowledged for providing hydrological data.

*Financial support.* This research has been supported by the French Ministry for Ecology (contract no. 2103295967).

*Review statement.* This paper was edited by Jan Seibert and reviewed by Martin Gauch, Frederik Kratzert, and Alexander Gelfan.

## References

Alexandrov, G., Ames, D., Bellocchi, G., Bruen, M., Crout, N., Erechtchoukova, M., Hildebrandt, A., Hoffman, F., Jackisch, C., and Khaiter, P.: Technical assessment and evaluation of environmental models and software, Environ. Model. Softw., 26, 328–336, https://doi.org/10.1016/j.envsoft.2010.08.004, 2011.

Andréassian, V., Hall, A., Chahinian, N., and Schaake, J.: Introduction and synthesis: Why should hydrologists work on a large number of basin data sets?, Large sample basin experiments for hydrological parametrization: results of the models parameter experiment – MOPEX, IAHS Red Books Series no 307,

IAHS Press, Wallingford, https://hal.inrae.fr/hal-02588687 (last access: 1 June 2023), 2006.

Ayzel, G. and Heistermann, M.: The effect of calibration data length on the performance of a conceptual hydrological model versus LSTM and GRU: A case study for six basins from the CAMELS dataset, Comput. Geosci., 149, 104708, https://doi.org/10.1016/j.cageo.2021.104708, 2021.

Barthel, R., Haaf, E., Nygren, M., and Giese, M.: Systematic visual analysis of groundwater hydrographs: potential benefits and challenges, Hydrogeol. J., 30, 359–378, https://doi.org/10.1007/s10040-021-02433-w, 2022.

Beven, K. and Westerberg, I.: On red herrings and real herrings: disinformation and information in hydrological inference, Hydro. Process., 25, 1676–1680, https://doi.org/10.1002/hyp.7963, 2011.

Blauhut, V., Stoelzle, M., Ahopelto, L., Brunner, M. I., Teutschbein, C., Wendt, D. E., Akstinas, V., Bakke, S. J., Barker, L. J., Bartošová, L., Briede, A., Cammalleri, C., Kalin, K. C., De Stefano, L., Fendeková, M., Finger, D. C., Huysmans, M., Ivanov, M., Jaagus, J., Jakubínský, J., Krakovska, S., Laaha, G., Lakatos, M., Manevski, K., Neumann Andersen, M., Nikolova, N., Osuch, M., van Oel, P., Radeva, K., Romanowicz, R. J., Toth, E., Trnka, M., Urošev, M., Urquijo Reguera, J., Sauquet, E., Stevkov, A., Tallaksen, L. M., Trofimova, I., Van Loon, A. F., van Vliet, M. T. H., Vidal, J.-P., Wanders, N., Werner, M., Willems, P., and Živković, N.: Lessons from the 2018–2019 European droughts: a collective need for unifying drought risk management, Nat. Hazards Earth Syst. Sci., 22, 2201–2217, https://doi.org/10.5194/nhess-22-2201-2022, 2022.

Blöschl, G., Hall, J., Viglione, A., Perdigão, R. A. P., Parajka, J., Merz, B., Lun, D., Arheimer, B., Aronica, G. T., Bilibashi, A., Boháč, M., Bonacci, O., Borga, M., Čanjevac, I., Castellarin, A., Chirico, G. B., Claps, P., Frolova, N., Ganora, D., Gorbachova, L., Gül, A., Hannaford, J., Harrigan, S., Kireeva, M., Kiss, A., Kjeldsen, T. R., Kohnová, S., Koskela, J. J., Ledvinka, O., Macdonald, N., Mavrova-Guirguinova, M., Mediero, L., Merz, R., Molnar, P., Montanari, A., Murphy, C., Osuch, M., Ovcharuk, V., Radevski, I., Salinas, J. L., Sauquet, E., Šraj, M., Szolgay, J., Volpi, E., Wilson, D., Zaimi, K., and Živković, N.: Changing climate both increases and decreases European river floods, Nature, 573, 108–111, https://doi.org/10.1038/s41586-019-1495-6, 2019.

Brigode, P., Paquet, E., Bernardara, P., Gailhard, J., Garavaglia, F., Ribstein, P., Bourgin, F., Perrin, C., and Andréassian, V.: Dependence of model-based extreme flood estimation on the calibration period: case study of the Kamp River (Austria), Hydrolog. Sci. J., 60, 1424–1437, https://doi.org/10.1080/02626667.2015.1006632, 2015.

Chauveau, M., Chazot, S., David, J., Norotte, T., Perrin, C., Bourgin, P.-Y., Sauquet, E., Vidal, J.-P., Rouchy, N., and Martin, E.: What will be the impacts of climate change on surface hydrology in France by 2070?, Houille Blanche, 44, 5–15, https://doi.org/10.1051/LHB/2013027, 2013.

Coron, L., Thirel, G., Delaigue, O., Perrin, C., and Andréassian, V.: The suite of lumped GR hydrological models in an R package, Environ. Model. Softw., 94, 166–171, https://doi.org/10.1016/j.envsoft.2017.05.002, 2017.

Coron, L., Delaigue, O., Thirel, G., Dorchies, D., Perrin, C., and Michel, C.: airGR: Suite of GR Hydrological Models
for Precipitation-Runoff Modelling, R package version 1.7.0,
https://doi.org/10.15454/EX11NA, 2020.

Crochemore, L., Perrin, C., Andréassian, V., Ehret, U., Seibert, S. P., Grimaldi, S., Gupta, H., and Paturel, J.-E.: Comparing expert judgement and numerical criteria for hydrograph evaluation. Hydrolog. Sci. J., 60, 402–423, https://doi.org/10.1080/02626667.2014.903331, 2015.

Crochemore, L., Isberg, K., Pimentel, R., Pineda, L., Hasan, A., and Arheimer, B.: Lessons learnt from checking the quality of openly accessible river flow data worldwide. Hydrolog. Sci. J., 65, 699–711, https://doi.org/10.1080/02626667.2019.1659509, 2020.

Delaigue, O., Génot, B., Lebecherel, L., Brigode, P., and Bourgin, P.-Y.: Database of watershed-scale hydroclimatic observations in France, https://webgr.inrae.fr/base-de-donnees (last access: 1 June 2023), Université Paris-Saclay, INRAE, HYCAR Research Unit, Hydrology group, Antony, 2020.

de Lavenne, A., Andréassian, V., Thirel, G., Ramos, M.-H., and Perrin, C.: A regularization approach to improve the sequential calibration of a semidistributed hydrological model, Water Resour. Res., 55, 8821–8839, https://doi.org/10.1029/2018WR024266, 2019.

Dunn, S. M., Freer, J., Weiler, M., Kirkby, M., Seibert, J., Quinn, P., Lischeid, G., Tetzlaff, D., and Soulsby, C.: Conceptualization in catchment modelling: simply learning?, Hydrol. Process., 22, 2389–2393, https://doi.org/10.1002/Hyp.7070, 2008.

Forzieri, G., Feyen, L., Rojas, R., Flörke, M., Wimmer, F., and Bianchi, A.: Ensemble projections of future streamflow droughts in Europe, Hydrol. Earth Syst. Sci., 18, 85–108, https://doi.org/10.5194/hess-18-85-2014, 2014.

Gaillardet, J., Braud, I., Gandois, L., Probst, A., Probst, J.-L., Sanchez-Pérez, J. M., and Simeoni-Sauvage, S.: OZCAR: The French network of critical zone observatories, Vadose Zone J., 17, 1–24, https://doi.org/10.2136/vzj2018.04.0067, 2018.

Gudmundsson, L., Boulange, J., Do, H. X., Gosling, S. N., Grillakis, M. G., Koutroulis, A. G., Leonard, M., Liu, J., Müller Schmied, H., and Papadimitriou, L.: Globally observed trends in mean and extreme river flow attributed to climate change, Science, 371, 1159–1162, https://doi.org/10.1126/science.aba3996, 2021.

Gupta, H. V., Perrin, C., Blöschl, G., Montanari, A., Kumar, R., Clark, M., and Andréassian, V.: Large-sample hydrology: a need to balance depth with breadth, Hydrol. Earth Syst. Sci., 18, 463–477, https://doi.org/10.5194/hess-18-463-2014, 2014.

Hannaford, J., Mastrantonas, N., Vesuviano, G., and Turner, S.: An updated national-scale assessment of trends in UK peak river flow data: how robust are observed increases in flooding?, Hydrol. Res., 52, 699–718, https://doi.org/10.2166/nh.2021.156, 2021.

Herschy, R. W.: Streamflow measurement, CRC Press, Taylor & Francis, London, UK, https://doi.org/10.1201/9781482265880, 2008.

Hisdal, H., Stahl, K., Tallaksen, L. M., and Demuth, S.: Have streamflow droughts in Europe become more severe or frequent?, Int. J. Climatol., 21, 317–333, https://doi.org/10.1002/joc.619, 2001.

Horner, I., Le Coz, J., Renard, B., Branger, F., and Lagouy, M.: Streamflow uncertainty due to the limited sensitivity of controls at hydrometric stations, Hydrol. Process., 36, e14497, https://doi.org/10.1002/hyp.14497, 2022.

Kundzewicz, Z. W., Pińskwar, I., and Brakenridge, G. R.: Large floods in Europe, 1985–2009, Hydrolog. Sci. J., 58, 1–7, https://doi.org/10.1080/02626667.2012.745082, 2013.

Lamontagne, J. R., Stedinger, J. R., Cohn, T. A., and Barth, N. A.: Robust National Flood Frequency Guidelines: What Is an Outlier?, American Society of Civil Engineers, Reston, Va, https://doi.org/10.1061/9780784412947.242, pp. 2454–2466, 2013.

Leigh, C., Alsibai, O., Hyndman, R. J., Kandanaarachchi, S., King, O. C., McGree, J. M., Neelamraju, C., Strauss, J., Dilini Tala-gala, P., Turner, R. D. R., Mengersen, K., and Peterson, E. E.: A framework for automated anomaly detection in high frequency water-quality data from in situ sensors, Sci. Total Environ., 664, 885–898, https://doi.org/10.1016/j.scitotenv.2019.02.085, 2019.

Leleu, I., Tonnelier, I., Puechberty, R., Gouin, P., Viquendi, I., Cos-bos, L., Foray, A., Baillon, M., and Ndima, P.-O.: La refonte du système d'information national pour la gestion et la mise à disposition des données hydrométriques, Houille Blanche, 100, 25–32, https://doi.org/10.1051/lhb/2014004, 2014.

Lloyd, C. E., Freer, J. E., Collins, A., Johnes, P., and Jones, J.: Methods for detecting change in hydrochemical time series in response to targeted pollutant mitigation in river catchments, J. Hydrol., 514, 297–312, https://doi.org/10.1016/j.jhydrol.2014.04.036, 2014.

Mauclerc, A. and Vilmus, T.: Hub'Eau-Les données sur l'eau à portée de clic, 106ème Comité Technique de l'OGC-Open Day [data set], https://hubeau.eaufrance.fr/page/api-hydrometrie (last access: 1 June 2023), 2018.

McMillan, H., Krueger, T., and Freer, J.: Benchmarking observational uncertainties for hydrology: rainfall, river discharge and water quality, Hydrol. Process., 26, 4078–4111, https://doi.org/10.1002/hyp.9384, 2012.

Meerveld, H. I., Sauquet, E., Gallart, F., Sefton, C., Seibert, J., and Bishop, K.: Aqua temporaria incognita, Hydrol. Process., 34, 5704–5711, https://doi.org/10.1002/hyp.13979, 2020.

Melsen, L.: It Takes a Village to Run a Model–The Social Practices of Hydrological Modeling, Water Resour. Res., 58, e2021WR030600, https://doi.org/10.1029/2021WR030600, 2022.

Merz, B., Blöschl, G., Vorogushyn, S., Dottori, F., Aerts, J. C., Bates, P., Bertola, M., Kemter, M., Kreibich, H., and Lall, U.: Causes, impacts and patterns of disastrous river floods, Nature Reviews Earth & Environment, 2, 592–609, https://doi.org/10.1038/s43017-021-00195-3, 2021.

Muxika, I., Borja, A., and Bald, J.: Using historical data, expert judgement and multivariate analysis in assessing reference conditions and benthic ecological status, according to the European Water Framework Directive, Mar. Pollut. Bull., 55, 16–29, https://doi.org/10.1016/j.marpolbul.2006.05.025, 2007.

Perrin, C., Oudin, L., Andreassian, V., Rojas-Serna, C., Michel, C., and Mathevet, T.: Impact of limited streamflow data on the efficiency and the parameters of rainfall-runoff models, Hydrolog. Sci. J., 52, 131–151, https://doi.org/10.1623/hysj.52.1.131, 2007.

Pushpalatha, R., Perrin, C., Le Moine, N., Mathevet, T., and Andréassian, V.: A downward structural sensitivity analysis of hydrological models to improve low-flow simulation, J. Hydrol., 411, 66–76, https://doi.org/10.1016/j.jhydrol.2011.09.034, 2011.

Rodriguez-Perez, J., Leigh, C., Liquet, B., Kermorvant, C., Pekterson, E., Sous, D., and Mengersen, K.: Detecting techni-
cal anomalies in high-frequency water-quality data using artifi-
cial neural networks, Environ. Sci. Technol., 54, 13719–13730,
https://doi.org/10.1021/acs.est.0c04069, 2020.

Sauquet, E., Shanafield, M., Hammond, J. C., Sefton, C., Leigh, C., and Datry, T.: Classification and trends in intermittent river flow regimes in Australia, northwestern Europe and USA: A global perspective, J. Hydrol., 597, 126170, https://doi.org/10.1016/j.jhydrol.2021.126170, 2021.

Sebok, E., Henriksen, H. J., Pastén-Zapata, E., Berg, P., Thirel, G., Lemoine, A., Lira-Loarca, A., Photiadou, C., Pimentel, R., Royer-Gaspard, P., Kjellström, E., Christensen, J. H., Vidal, J. P., Lucas-Picher, P., Donat, M. G., Besio, G., Polo, M. J., Stisen, S., Caballero, Y., Pechlivanidis, I. G., Troldborg, L., and Refsgaard, J. C.: Use of expert elicitation to assign weights to climate and hydrological models in climate impact studies, Hydrol. Earth Syst. Sci., 26, 5605–5625, https://doi.org/10.5194/hess-26-5605-2022, 2022.

Strohmenger, L. and Thirel, G.: Result of a visual detection of non-natural records in streamflow time series for the Explore2 project, V2, Recherche Data Gouv [data set], https://doi.org/10.57745/SO2WOV, 2023.

Thébault, C., Perrin, C., Andréassian, V., Thirel, G., Legrand, S., and Delaigue, O.: Impact of suspicious streamflow data on the efficiency and parameter estimates of rainfall–runoff models, Hydrol. Sci. J., 68, 1627–1647, https://doi.org/10.1080/02626667.2023.2234893, 2023.

Valéry, A., Andréassian, V., and Perrin, C.: 'As simple as possible but not simpler': What is useful in a temperature-based snow-accounting routine? Part 1-Comparison of six snow accounting routines on 380 catchments, J. Hydrol., 517, 1166–1175, https://doi.org/10.1016/j.jhydrol.2014.04.059, 2014.

van den Tillaart, S. P., Booij, M. J., and Krol, M. S.: Impact of uncertainties in discharge determination on the parameter estimation and performance of a hydrological model, Hydrol. Res., 44, 454–466, https://doi.org/10.2166/nh.2012.147, 2013.

van de Wiel, L., van Es, D. M., and Feelders, A. J.: Real-Time Outlier Detection in Time Series Data of Water Sensors, in: Advanced Analytics and Learning on Temporal Data, edited by: Lemaire, V., Malinowski, S., Bagnall, A., Guyet, T., Tavenard, R., and Ifrim, G., AALTD 2020, Lecture Notes in Computer Science, Springer, Cham, 12588, https://doi.org/10.1007/978-3-030-65742-0_11, 2020.

Vicente-Serrano, S. M., Lopez-Moreno, J.-I., Beguería, S., Lorenzo-Lacruz, J., Sanchez-Lorenzo, A., García-Ruiz, J. M., Azorin-Molina, C., Morán-Tejeda, E., Revuelto, J., and Trigo, R.: Evidence of increasing drought severity caused by temperature rise in southern Europe, Environ. Res. Lett., 9, 044001, https://doi.org/10.1088/1748-9326/9/4/044001, 2014.

Vicente-Serrano, S. M., Peña-Gallardo, M., Hannaford, J., Murphy, C., Lorenzo-Lacruz, J., Dominguez-Castro, F., López-Moreno, J. I., Beguería, S., Noguera, I., Harrigan, S., and Vidal, J.-P.: Climate, Irrigation, and Land Cover Change Explain Streamflow Trends in Countries Bordering the Northeast Atlantic, Geophys. Res. Lett., 46, 10821–10833, https://doi.org/10.1029/2019GL084084, 2019.

Vidal, J.-P., Martin, E., Franchistéguy, L., Habets, F., Soubeyroux, J.-M., Blanchard, M., and Baillon, M.: Multilevel and multiscale drought reanalysis over France with the Safran-Isba-Modcou hydrometeorological suite, Hydrol. Earth Syst. Sci., 14, 459–478, https://doi.org/10.5194/hess-14-459-2010, 2010a.

Vidal, J.-P., Martin, E., Franchistéguy, L., Baillon, M., and Soubeyroux, J.-M.: A 50-year high-resolution atmospheric reanalysis over France with the Safran system, Int. J. Climatol., 30, 1627–1644, https://doi.org/10.1002/joc.2003, 2010b.

Wilby, R. L., Clifford, N. J., De Luca, P., Harrigan, S., Hillier, J. K., Hodgkins, R., Johnson, M. F., Matthews, T. K., Murphy, C., and Noone, S. J.: The 'dirty dozen' of freshwater science: detecting then reconciling hydrological data biases and errors, WIREs Water, 4, e1209, https://doi.org/10.1002/wat2.1209, 2017.

Wright, D. P., Thyer, M., and Westra, S.: Influential point detection diagnostics in the context of hydrological model calibration, J. Hydrol., 527, 1161–1172, https://doi.org/10.1016/j.jhydrol.2015.05.047, 2015.