A method to detect and characterize sub-daily flow fluctuations Franz Greimel, 1 * Bernhard Zeiringer, 1 Norbert Höller, 2 Bettina Grün, 3 Reinhold Godina 4 and Stefan Schmutz 1 1 Institute of Hydrobiology and Aquatic Ecosystem Management, BOKU University, Vienna, Austria 2 Center for IT Services, BOKU University, Vienna, Austria 3 Department of Applied Statistics, Johannes Kepler University, Linz, Austria 4 Austrian Hydrographic Service, Vienna, Austria Abstract: Hydro peaking causes an important environmental impact on running water ecosystems. Many affected rivers have a poor ecological status. In rivers affected by hydro peaking, the flow conditions are highly complex and difficult to grasp. To develop a general framework for detecting and characterizing sub-daily flow fluctuations, we analysed more than 500 Austrian hydrographs, covering the whole range from unimpacted to heavily impacted rivers. Different fluctuation types could be identified according to the potential source: e.g. sub-daily flow fluctuations caused by hydro peaking, rainfall or snow and glacier melt. Additionally, the term ‘hydro fibrillation’ was established, to indicate frequently occurring artificial fluctuations with comparably low intensities. An automatic procedure was used to detect frequency and intensity of each flow fluctuation. Using variables based on duration curves of flow fluctuation rates (ramping rates), amplitudes, flow ratios, durations and daily numbers of fluctuations, a predictive model (linear discriminant analysis) was fitted to classify hydrographs into predominant fluctuation regimes. This is the basis for a detailed investigation of present sub-daily flow regimes and to analyse the differences between the regimes. Based on the results, we finally propose a general framework that enables a standardized assessment of flow fluctuations regarding event intensities and/or event timing. The proposed framework offers a standardized selection of particular flow fluctuations referring to increase and decrease events separately. The selection of specific flow fluctuations can be defined with respect to several research questions (e.g. ecologically relevant fluctuations), which offers a wide range of applications. Copyright © 2015 John Wiley & Sons, Ltd. KEY WORDS sub-daily fluctuation regimes; hydro peaking; hydro fibrillation; fluctuation intensity; fluctuation frequency; sub- daily fluctuation types; time series; fluctuation characteristics; ecologically relevant sub-daily fluctuations; duration curves Received 25 February 2015; Accepted 12 December 2015 INTRODUCTION Flow is a major driver for physical habitat in streams and a major determinant of biotic composition. Aquatic organisms have evolved life history strategies primarily in direct response to natural flow regimes (Poff et al., 1997; Bunn and Arthington, 2002). In consequence, flow fluctuations (FFs) play an important role for aquatic organisms. River discharge typically varies significantly during the annual cycle, depending on climate and catchment conditions. Beside these seasonal changes in flow, intra-seasonal flow variations can be observed. Intra-seasonal flow variations can be separated into sur- daily and sub-daily variations associated with run-off generating mechanisms: Sur-daily variations occur during a period of a couple of days. These changes are primarily related to the infiltration capacity of catchments and soil drainage. Natural sub-daily FFs with durations from minutes to hours depend on the volume of precipitation (or snow and glacier melt) contributing to direct run-off. The focus of this work is to analyse natural sub-daily FFs and human impacts on the natural regimes. Seasonal and sur-daily FFs as well as regulations on the mean flow and coherent ecological effects are not considered. Aside from natural phenomena, sub-daily FFs can be induced by human activities: Hydro peaking – discontin- uous release of turbined water because of peaks of energy demand – causes artificial FFs downstream of reservoirs. Also, run-of-the-river (ROR) power plants and other human activities may create artificial hydrographs because of turbine regulation, gate manipulations and pumping stations. Anthropogenic sub-daily FFs with high intensities (e.g. hydro peaking) result in unnatural changes in hydraulic parameters such as water level, flow velocity and bed shear stress, which may affect river *Correspondence to: Franz Greimel, Institute of Hydrobiology and Aquatic Ecosystem Management, BOKU University, Vienna, Austria. E-mail: franz.greimel@boku.ac.at HYDROLOGICAL PROCESSES Hydrol. Process. (2016) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10773 Copyright © 2015 John Wiley & Sons, Ltd.