Abstract. Worldwide, flow regimes are being modified by various anthropogenic impacts and climate change induces an additional risk. Rising temperatures, declining snow cover and changing precipitation patterns will interact differently at different locations. Consequently, in distinct climate zones, unequal consequences can be expected in matters of water stress, flood risk, water quality, and food security. In particular, river ecosystems and their vital ecosystem services will be compromised as their species richness and composition have evolved over long time under natural flow conditions. This study aims at evaluating the exclusive impacts of climate change on river flow regimes in Europe. Various flow characteristics are taken into consideration and diverse dynamics are identified for each distinct climate zone in Europe. In order to simulate present-day natural flow regimes and future flow regimes under climate change, the global hydrology model WaterGAP3 is applied. All calculations for current and future conditions (2050s) are carried out on a 5 × 5 European grid. To address uncertainty, bias-corrected climate forcing data of three different global climate models are used to drive WaterGAP3. Finally, the hydrological alterations of different flow characteristics are quantified by the Indicators of Hydrological Alteration approach. Results of our analysis indicate that on the European scale, climate change can be expected to modify flow regimes remarkably. This is especially the case in the Mediterranean (due to drier conditions with reduced precipitation across the year) and in the boreal climate zone (due to reduced snowmelt, increased precipitation, and strong temperature rises). In the temperate climate zone, impacts increase from oceanic to continental. Regarding single flow characteristics, strongest impacts on timing were found for the boreal climate zone. This applies for both high and low flows. Flow magnitudes, in turn, will be predominantly altered in the Mediterranean but also in the Northern climates. At the end of this study, typical future flow regimes under climate change are illustrated for each climate zone.

Abstract:Changes in water temperature can have important consequences for aquatic ecosystems, with some species being sensitive even to small shifts in temperature during some or all of their life cycle. While many studies report increasing regional and global air temperatures, evidence of changes in river water temperature has, thus far, been site specific and often from sites heavily influenced by human activities that themselves could lead to warming. Here we present a tiered assessment of changing river water temperature covering England and Wales with data from 2773 locations. We use novel statistical approaches to detect trends in irregularly sampled spot measurements taken between 1990 and 2006. During this 17-year period, on average, mean water temperature increased by 0.03°C per year (±0.002°C), and positive changes in water temperature were observed at 2385 (86%) sites. Examination of catchments where there has been limited human influence on hydrological response shows that changes in river flow have had little influence on these water temperature trends. In the absence of other systematic influences on water temperature, it is inferred that anthropogenically driven climate change is driving some of this trend in water temperature.

Projection of future changes in river flow regimes and their impact on river ecosystem health is a major research challenge. This paper assesses the implications of projected future shifts in river flows on in-stream and riparian ecosystems at the pan-European scale by developing a new methodology to quantify ecological risk due to flow alteration. The river network was modelled as 33,668 cells (5' longitude x 5' latitude). For each cell, modelled monthly flows were generated for an ensemble of 10 scenarios for the 2050s, and for the study baseline (naturalised flows for . These future scenarios consist of combinations of two climate scenarios and four socio-economic water-use scenarios (with a main driver of economy, policy, security, or sustainability). Environmental flow implications are assessed using the new Ecological Risk due to Flow Alteration (ERFA) methodology, based on a set of Monthly Flow Regime Indicators (MFRIs). Differences in MFRIs between scenarios and baseline are calculated to derive ERFA classes (no, low, medium, high risk), which are based on the number of indicators significantly different from the baseline. ERFA classes are presented as colour-coded pan-European maps. Results are consistent between scenarios and show European river ecosystems are under significant threat with about two-third at medium or high risk of change. Four main zones were identified (from highest to lowest risk severity): (i) Mediterranean rim, southwest part of Eastern Europe, and Western Asia; (ii) Northern Europe, northeast part of Eastern Europe; (iii) Western and Eastern Europe; (iv) inland North Africa. Patterns of flow alteration risk are driven by climate-induced change, with socioeconomics a secondary factor. These flow alterations could be manifested as changes to species and communities and loss of current ecosystem functions and services.

Excessive sediment pressure on aquatic habitats is of global concern. A unique dataset, comprising instantaneous measurements of deposited fine sediment in 230 agricultural streams across England and Wales, was analysed in relation to 20 potential explanatory catchment and channel variables. The most effective explanatory variable for the amount of deposited sediment was found to be stream power, calculated for bankfull flow and used to index the capacity of the stream to transport sediment. Both stream power and velocity category were highly significant (p ≪ 0.001), explaining some 57% variation in total fine sediment mass. Modelled sediment pressure, predominantly from agriculture, was marginally significant (p<0.05) and explained a further 1% variation. The relationship was slightly stronger for erosional zones, providing 62% explanation overall. In the case of the deposited surface drape, stream power was again found to be the most effective explanatory variable (p<0.001) but velocity category, baseflow index and modelled sediment pressure were all significant (p<0.01); each provided an additional 2% explanation to an overall 50%. It is suggested that, in general, the study sites were transport-limited and the majority of stream beds were saturated by fine sediment. For sites below saturation, the upper envelope of measured fine sediment mass increased with modelled sediment pressure. The practical implications of these findings are that (i) targets for fine sediment loads need to take into account the ability of streams to transport/retain fine sediment, and (ii) where agricultural mitigation measures are implemented to reduce delivery of sediment, river management to mobilise/remove fines may also be needed in order to effect an improvement in ecological status in cases where streams are already saturated with fines and unlikely to self-cleanse.