Please cite this article as: Bejarano, M.D., Sordo-Ward, A., Alonso, C., Nilsson, C., Characterizing effects of hydropower plants on sub-daily flow regimes, Journal of Hydrology (2017), doi: http://dx.
Abstract 9A characterization of short-term changes in river flow is essential for understanding the 10 ecological effects of hydropower plants, which operate by turning the turbines on or off to 11 generate electricity following variations in the market demand (i.e., hydropeaking). The goal 12 of our study was to develop an approach for characterizing the effects of hydropower plant 13 operations on within-day flow regimes across multiple dams and rivers. For this aim we first 14 defined ecologically meaningful metrics that provide a full representation of the flow regime 15 at short time scales from free-flowing rivers and rivers exposed to hydropeaking. We then 16 defined metrics that enable quantification of the deviation of the altered short-term flow 17 regime variables from those of the unaltered state. The approach was successfully tested in 18 two rivers in northern Sweden, one free-flowing and another regulated by cascades of 19 hydropower plants, which were additionally classified based on their impact on short-term 20 flows in sites of similar management. The largest differences between study sites 21 corresponded to metrics describing sub-daily flow magnitudes such as amplitude (i.e., 22 difference between the highest and the lowest hourly flows) and rates (i.e., rise and fall rates 23 33 34 Key words: hydrological alterations; hydrological characterization; hydropeaking; impact 35 assessment; short-term; sub-daily flows 36 37 1. Introduction 38 Critical components of the flow regime such as magnitude, frequency, duration, timing 39 and rate of change control ecological processes in river ecosystems (Poff et al., 1997), and 40 modification of flow regimes constrains the distribution of species, their adaptive capacity, 41 survival, dispersal and reproduction (Lytle and Poff, 2004). Each of these five flow 42 components describes the variability over a wide range of spatial and temporal scales (Ward, 43 1989). Flow variability may be considered at long time scales, which are commonly 44 controlled by inter-and intra-annual variations in climate. Year-to-year variation in flows 45 associated to the Interdecadal Pacific Oscillation index and shifts in the El Niño Southern 46 Oscillation phenomenon (Biggs et al., 2005), and month-to-month variation in flows 47 3 associated to seasons (Bejarano et al., 2010) are examples of large time-scale flow variability. 48 Additionally, topography and geology are usually superimposed on climate and shape intra-49 annual flow variation in, for example, snowmelt-fed or groundwater-fed rivers (Bejarano et 50 al., 2010). Furthermore, flow variability may also be considered at shorter time scales, from 51 months to hours (or smaller). Day-to-day and within-day water gains or losses are ultimately 52 caused by varying rates of precipitation, evapotranspiration, infiltration, ...
