Hydropeaking induces losses from a river reach: observations at multiple spatial scales

In humid regions, where gaining river conditions generally prevail, daily hydroelectric dam releases alter downstream surface water–groundwater interactions by reversing the head gradient between river and adjacent groundwater. Previously, it has been noted that artificial stage changes due to dam releases enhance hyporheic exchange. Here we investigate the regulated Deerfield River in northwestern Massachusetts at multiple scales to evaluate how changing downstream geologic conditions along the river mediate this artificial hyporheic pumping. Water budget analysis indicates that roughly 10% of bank‐stored water is permanently lost from the 19.5‐km river reach, likely as a result of transpiration by bank vegetation. An adjacent reference stream with similar dimensions and geomorphology, but without hydropeaking, shows predictable gaining conditions. Field observations from streambed piezometers and thermistors show that water losses are not uniform throughout the study reach. Riparian aquifer transmissivity in river sub‐reaches largely determines the magnitude of surface water–groundwater exchange as well as net water loss from the river. These newly documented losses from hydropeaking river systems should inform decisions by river managers and hydroelectric operators of additional tradeoffs of oscillatory dam‐release river management. Copyright © 2015 John Wiley & Sons, Ltd.     

Impacts of regional warming on long‐term hypolimnetic anoxia and dissolved oxygen concentration in a deep lake

Although few reports have described long‐term continuous anoxia in aquatic systems, Lake Ikeda in Japan experienced such conditions in the hypolimnion from 1990 to 2010. The present study aimed to assess temporal fluctuations in the lake's thermal stability from 1978 to 2011 to understand the influence of regional climate change on hypolimnetic anoxia in this lake. Because complete vertical mixing, which supplies dissolved oxygen (DO) to the hypolimnion, potentially occurs on February, we calculated the Schmidt stability index (S) in February and compared it with hypolimnetic DO dynamics. Vertical water temperature profiles were calculated using a one‐dimensional model, and calculated temperatures and meteorological data were used to analyse annual fluctuations in water temperatures, thermocline depth, meteorological variables and S. We estimated that mean annual air and volume‐weighted water temperatures increased by 0.028 and 0.033 °C year^?1, respectively, from 1978 to 2011. Between 1986 and 1990, S and water temperature increased abruptly, probably due to a large upwards trend in air temperature (+0.239 °C year^?1). We hypothesize that a mixing regime that lacked overturn took effect at this time and that this regime lasted until 2011, when S was particularly small. These results demonstrate that abrupt climate warming in the late 1980s likely triggered the termination of complete mixing and caused the 21‐year period of successive anoxia in Lake Ikeda. We conclude that the lake response to a rapid shift in regional climate conditions was a key factor in changing the hypolimnetic water environment and that thermal stability in winter is a critical environmental factor controlling the mixing regime and anoxic conditions in deep lakes. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrologic alteration along the Middle and Upper East River (Dongjiang) basin, South China: a visually enhanced mining on the results of RVA method

This paper presents a visually enhanced evaluation of the spatio-temporal patterns of the dam-induced hydrologic alteration in the middle and upper East River, south China over 1952–2002, using the range of variability approach (RVA) and visualization package XmdvTool. The impacts of climate variability on hydrological processes have been removed for wet and dry periods, respectively, so that we focus on the impacts of human activities (i.e., dam construction). The results indicate that: (1) along the East River, dams have greatly altered the natural flow regime, range condition and spatial variability; (2) six most remarkable indicators of hydrologic alteration induced by dam-construction are rise rate (1.16), 3-day maximum (0.91), low pulse duration (0.88), January (0.80), July (0.80) and February (0.79) mean flow of the East River during 1952–2002; and (3) spatiotemporal hydrologic alterations are different among three stations along Easter River. Under the influence of dam construction in the upstream, the degree of hydrologic changes from Lingxia, Heyuan to Longchuan station increases. This study reveals that visualization techniques for high-dimensional hydrological datasets together with RVA are beneficial for detecting spatio-temporal hydrologic changes.     

Hydropeaking indicators for characterization of the Upper-Rhone River in Switzerland

River channelization and the construction of high-head storage schemes have been the basis of agricultural and socio-economic development in many alpine regions. One example is the Upper-Rhone River in Switzerland. The Upper-Rhone’s morphology changed considerably between 1863 and 1960 as a result of two major channelizations and, from 1950 on, the construction of a large number of high-head storage hydropower schemes in the catchment. These modifications have brought large benefits to the local population, at the cost, however, of substantial disturbances in aquatic and terrestrial ecosystems in and along the river. A primary factor behind these disturbances is the alteration of the natural flow regime, namely hydropeaking due to the operation of the high-head storage hydropower plants. For sustainable river-restoration projects on regulated rivers, scientists and engineers now widely accept the necessity of integrated management of the river. Different aspects such as river morphology, sediment management, water quality, temperature, and the naturally variable flow regime should be considered simultaneously. Mitigation of non-natural, sub-daily flow fluctuations due to hydropeaking is a crucial step in restoring natural flow regimes, but is especially challenging due to the economic constraints such mitigation places upon hydropower plants. With the goal of addressing this challenge, this paper proposes three indicators to describe the flow regime of rivers in alpine catchments with and without high-head storage hydropower plants. The indicators quantify: (1) the seasonal distribution and transfer of water, (2) sub-daily flow fluctuations, and (3) the intensity and frequency of flow changes. Indicators are evaluated in a case study of the Upper-Rhone River for pre- and post-impact situations, and the benefit of a multipurpose project reducing hydropeaking on hydrologic conditions is quantified. Furthermore, the paper explores the possibility of using these indicators to link aquatic and terrestrial ecosystem well being to their hydrology.     

T‐NET,a dynamic model for simulating daily stream temperature at the regional scale based on a network topology

Currently, the distribution areas of aquatic species are studied by using air temperature as a proxy of water temperature, which is not available at a regional scale. To simulate water temperature at a regional scale, a physically based model using the equilibrium temperature concept and including upstream‐downstream propagation of the thermal signal is proposed. This model, called Temperature‐NETwork (T‐NET), is based on a hydrographical network topology and was tested at the Loire basin scale (10⁵ km²). The T‐NET model obtained a mean root mean square error of 1.6 °C at a daily time step on the basis of 128 water temperature stations (2008–2012). The model obtained excellent performance at stations located on small and medium rivers (distance from headwater <100 km) that are strongly influenced by headwater conditions (median root mean square error of 1.8 °C). The shading factor and the headwater temperature were the most important variables on the mean simulated temperature, while the river discharge influenced the daily temperature variation and diurnal amplitude. The T‐NET model simulates specific events, such as temperature of the Loire during the floods of June 1992 and the thermal regime response of streams during the heatwave of August 2003, much more efficiently than a simple point‐scale heat balance model. The T‐NET model is very consistent at a regional scale and could easily be transposed to changing forcing conditions and to other catchments. Copyright © 2016 John Wiley & Sons, Ltd.     

The Runoff Declining Process and Water Quality in Songhuajiang River Catchment,China under Global Climatic Change

The runoff in Songhuajiang River catchment has experienced a decreasing trend during the second half of the 20th century. Serially complete daily rainfall data of 42 rainfall stations from 1959 to 2002 and daily runoff data of five meteorological stations from 1953 to 2005 were obtained. The Mann–Kendall trend test and the sequential version of Mann–Kendall test were employed in this study to test the monthly and annual trends for both rainfall and runoff, to determine the start point of abrupt runoff declining, and to identify the main driving factors of runoff decline. The results showed an insignificant increasing trend in rainfall but a significant decreasing trend in runoff in the catchment. For the five meteorological stations, abrupt runoff decline occurred during 1957–1963 and the middle 1990s. Through Mann–Kendall comparisons for the area‐rainfall and runoff for the two decreasing periods, human activity, rather than climatic change, is identified as the main driving factor of runoff decline. Analysis of land use/cover shows that farmland is most related with runoff decline among all the land use/cover change in Nenjiang catchment. From 1986 to 1995, the area of farmland increased rapidly from 6.99 to 7.61 million hm². Hydraulic engineering has a significant influence on the runoff decline in the second Songhuajiang catchment. Many large‐scale reservoirs and hydropower stations have been built in the upstream of the Second Songhuajiang and lead to the runoff decline. Nenjiang and the Second Songhuajiang are the two sources of mainstream of Songhuajiang. Decreased runoff in these two sub‐catchments then results in runoff decrease in mainstream of Songhuajiang catchment. It is, therefore, concluded that high percent agricultural land and hydraulic engineering are the most probable driving factors of runoff decline in Songhuajiang River catchment, China.     

A spatial assessment of hydrologic alteration caused by dam construction in the middle and lower Yellow River,China

The ‘range of variability approach’ (RVA) and mapping technique are used to investigate the spatial variability of hydrologic alterations (HA) due to dam construction along the middle and lower Yellow River, China, over the past five decades. The impacts of climate variability on hydrological process have been removed during wet and dry periods and the focus is on the impacts of human activities, such as dam construction, on hydrological processes. Results indicate the following: (1) The impacts of the Sanmenxia reservoir on the hydrologic alteration are relatively slight with a mean HA value of 0·48, ranking in the last place among the four large reservoirs. (2) Xiaolangdi reservoir has significantly changed the natural flow regime downstream with mean HA value of 0·56, ranking it in first place among the large reservoirs. (3) The results of ranked median degrees of 33 hydrologic alteration indicators for 10 stations in the Yellow River show that the hydrologic alteration of Huayuankou ranks the highest among 10 stream gauges. (4) Impacts of reservoirs on hydrological processes downstream of the dams are closely associated with the regulating activities of the reservoirs. At the same time, alterations of streamflow regimes resulting from climatic changes (e.g. precipitation variability) make the situation more complicated and more hydrological observations will be necessary for further analysis. The results of the current study will be greatly beneficial to the regional water resources management and restoration of eco‐environmental systems in the middle and lower Yellow River characterized by intensified dam construction under a changing environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Assessment of a hydrologic model's reliability in simulating flow regime alterations in a changing climate

It is a common practice to employ hydrologic models for assessing present and future states of watersheds and assess the degree of alterations for a range of hydrologic indicators. Previous studies indicate that the hydrologic model may not be able to replicate some of the indicators of interest, which raises questions on the reliability of model simulated changes. Hence, we initiated a study to evaluate the replicability of the streamflow changes by employing the widely used variable infiltration capacity hydrologic model for sub‐basins and mainstem of the Fraser River Basin, Canada. Given that the hydrologic regime of the region is known to be influenced by teleconnections to the Pacific Decadal Oscillation (PDO) and El Niño–Southern Oscillation (ENSO), we used hydrologic responses to the PDO and ENSO states as analogues for evaluating the model's ability to simulate climate‐induced changes. The results revealed that the qualitative patterns of response, such as lower flows for the warm PDO state compared to the cool state, and progressively higher flows for the warm, neutral and cool ENSO states, were generally well reproduced for most hydrologic indicators. Additionally, while the directions of change between the different PDO and ENSO states were mostly well replicated, the magnitude of change for some of the indicators showed considerable differences. Hence, replicability of both magnitude and direction of change need to be carefully examined before using the simulated indicators for assessing future hydrologic changes, and a reliable replication increases the confidence of projected changes. Copyright © 2016 Her Majesty the Queen in Right of Canada. Hydrological Processes. © 2016 John Wiley & Sons, Ltd.     

Spatial assessment of hydrologic alteration across the Pearl River Delta,China, and possible underlying causes

The alterations of the water level across the Pearl River Delta (PRD) were investigated using a ‘range of variability approach’ (RVA) based on monthly water level datasets extracted from 17 gauging stations. A mapping method was used to illustrate the spatial patterns in the degrees of alteration of water levels. The results indicated that more stations showing moderate and high alterations in monthly mean maximum and minimum water levels when compared with monthly maximum and minimum water levels. River channels characterized by higher alterations of water levels were observed mainly in the regions north of 22° 30′N. Alterations of water levels across the PRD were a consequence of various influencing factors. However, changed hypsography due to extensive and intensive human activities, particularly the large‐scale dredging and excavation of the river sand, may be taken as one of the major causes for the substantial hydrologic alteration. This study indicated that the river channels characterized by altered water levels are mostly those characterized by highly and moderately intensive sand dredging. The changed ratio of the streamflow between Makou and Sanshui stations, the major upstream flow control stations, also influenced the water level alterations of the Pearl River delta. The results of this study will be of great significance in water resources management and better human mitigation of the natural hazards due to the altered water level under the changing environment. Copyright © 2009 John Wiley & Sons, Ltd.     

The effects of dam construction and precipitation variability on hydrologic alteration in the Lancang River Basin of southwest China

This paper presents an evaluation of the spatio-temporal patterns of hydrologic alteration induced by dam construction and precipitation variability in the Lancang River Basin of southwest China from 1957 to 2000. Analyses were conducted using the linear regression method, the Mann–Kendall test, and the Range of Variability Approach. The results indicate that there was considerable variation in the average monthly precipitation between the pre- and post-dam periods in the Lancang River Basin. Second, the magnitude of monthly runoff was strongly related to precipitation, which showed an up-down annual variation, and was significantly altered by dam construction and precipitation variability. In the modified series (hydrologic series with the precipitation impacts removed), runoff deviations between the pre- and post-dam periods became larger. Third, the extreme runoff cycles were influenced by dam construction and precipitation variability downstream from the dam, and the monthly maximum runoff increased from the pre-dam to post-dam period at all hydrologic stations. Fourth, the degree of hydrologic alteration (DHA) indicates that the precipitation variability not only affected the hydrologic regime of unregulated river reach but also modified the negative impacts of dam construction, which could provide a modest mitigation of the hydrologic alterations induced by dam construction, possibly decreasing the level of DHA. Last, the overall degree of hydrologic alteration in the observed series reached 25.2, 25.3, and 29.1 % for the upstream, midstream, and downstream areas, respectively. These results show that the hydrologic regimes of the Lancang River during the 1957–2000 period were affected by damming and precipitation variability, but the hydrologic alteration was relatively low in the upstream areas of the river without a dam.     

A comparison of stream water temperature regimes from open and afforested moorland,Yorkshire Dales,northern England

Despite the known importance of water temperature for river ecosystems, the thermal regime of streams and rivers can be heavily modified by afforestation. Although the nature of the heat budget affecting streams in forested catchments shows high variability in space and time, most of the studies of stream temperature response to afforestation have lacked replication among streams. This study examined the impacts of coniferous forest plantations on stream water temperature at six sites (three forested and three open moorland) in the Yorkshire Dales, northern England. Our aim was to test the hypothesis that afforestation would alter the thermal regime of streams, leading to reduced year‐round thermal variability, and cooler summer/warmer winter water temperatures, relative to streams flowing across open moorland. Data collected from April 2007 to March 2009 showed similar thermal dynamics among all six streams over the study period, although variability in forested streams was markedly lower as expected. Mean and maximum daily water temperatures were significantly higher in open moorland streams for much of the year but while some forested streams were warmer than individual moorland streams during winter months (November to February), there was considerable overlap in water temperature between moorland and forest streams. Most stream temperature records showed evidence of low/no winter flow and freezing. These results contrast with many previous studies that have reported warmer temperatures in forested versus open moorland streams during winter, a finding that most likely reflects site‐specific hydrological, geomorphological and climatological influences on water temperature in addition to afforestation. This study demonstrates the need for replication of hydrological monitoring when examining the effects of basin‐scale management practices and provides further evidence for changes in stream thermal regime following afforestation, a practice that is likely to increase in future due to growing demands for increased forest cover in the UK uplands. Copyright © 2010 John Wiley & Sons, Ltd.     

Model complexity and data requirements in snow hydrology: seeking a balance in practical applications

We investigate the problem of balancing model complexity and input data requirements in snow hydrology. For this purpose, we analyze the performance of two models of different complexity in estimating variables of interest in snow hydrology applications. These are snow depth, bulk snow density, snow water equivalent and snowmelt run‐off. We quantify the differences between data and model prediction using 18 years of measurements from an experimental site in the French Alps (Col de Porte, 1325 m AMSL). The models involved in this comparison are a one‐layer temperature‐index model (HyS) and a multilayer model (Crocus). Results show that the expected loss in performance in the one‐layer temperature‐index model with respect to the multilayer model is low when considering snow depth, snow water equivalent and bulk snow density. As for run‐off, the comparison returns less clear indications for identification of a balance. In particular, differences between the models' prediction and data with an hourly resolution are higher when considering the Crocus model than the HyS model. However, Crocus is better at reproducing sub‐daily cycles in this variable. In terms of daily run‐off, the multilayer physically based model seems to be a better choice, while results in terms of cumulative run‐off are comparable. The better reproduction of daily and sub‐daily variability of run‐off suggests that use of the multilayer model may be preferable for this purpose. Variation in performance is discussed as a function of both the calibration solution chosen and the time of year. Copyright © 2016 John Wiley & Sons, Ltd.     

Assessing the potential of reservoir outflow management to reduce sedimentation using continuous turbidity monitoring and reservoir modelling

In‐stream sensors are increasingly deployed as part of ambient water quality‐monitoring networks. Temporally dense data from these networks can be used to better understand the transport of constituents through streams, lakes or reservoirs. Data from existing, continuously recording in‐stream flow and water quality monitoring stations were coupled with the two‐dimensional hydrodynamic CE‐QUAL‐W2 model to assess the potential of altered reservoir outflow management to reduce sediment trapping in John Redmond Reservoir, located in east‐central Kansas. Monitoring stations upstream and downstream from the reservoir were used to estimate 5.6 million metric tons of sediment transported to John Redmond Reservoir from 2007 through 2010, 88% of which was trapped within the reservoir. The two‐dimensional model was used to estimate the residence time of 55 equal‐volume releases from the reservoir; sediment trapping for these releases varied from 48% to 97%. Smaller trapping efficiencies were observed when the reservoir was maintained near the normal operating capacity (relative to higher flood pool levels) and when average residence times were relatively short. An idealized, alternative outflow management scenario was constructed, which minimized reservoir elevations and the length of time water was in the reservoir, while continuing to meet downstream flood control end points identified in the reservoir water control manual. The alternative scenario is projected to reduce sediment trapping in the reservoir by approximately 3%, preventing approximately 45 000 metric tons of sediment from being deposited within the reservoir annually. This article presents an approach to quantify the potential of reservoir management using existing in‐stream data; actual management decisions need to consider the effects on other reservoir benefits, such as downstream flood control and aquatic life. Copyright © 2012 John Wiley & Sons, Ltd.     

A method to detect and characterize sub‐daily flow fluctuations

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.     

Assessment of annual high‐water events for the Mackenzie River basin,Canada

River ice break‐up is known to have important morphological, ecological and socio‐economic effects on cold‐regions river environments. One of the most persistent effects of the spring break‐up period is the occurrence of high‐water events. A return‐period assessment of maximum annual nominal water depths occurring during the spring break‐up and open‐water season at 28 Water Survey of Canada hydrometric sites over the 1913–2002 time period in the Mackenzie River basin is presented. For the return periods assessed, 13 (14) stations are dominated by peak events occurring during the spring break‐up (open‐water) season. One location is determined to have a mixed signal. A regime classification is proposed to separate ice‐ and open‐water dominated systems. As part of the regime classification procedure, specific characteristics of return‐period patterns including alignment, and difference between the 2 and 10‐year events are used to identify regime types. A dimensionless stage‐discharge plot allows for a contrast of the relative magnitudes of flows required to generate maximum nominal water‐depth events in the different regimes. At sites where discharge during the spring break‐up is approximately one‐quarter or greater than the magnitude of the peak annual discharge, nominal water depths can be expected to exceed those occurring during the peak annual discharge event. Several physical factors (location, basin area, stream order, gradient, river orientation, and climate) are considered to explain the differing regimes and discussed relative to the major sub‐regions of the MRB. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.     

Modelling production of migratory catfish larvae (Pimelodidae) on the basis of regional hydro‐ climatology features of the Madre de Dios Basin in southeastern Peru

A simple stochastic model is presented to describe the influence of the natural flow regime of the Madre de Dios River (southeastern Peru) on the presence and downstream transportation of catfish larvae (Siluriformes: Pimelodidae), an important migratory species in commercial fisheries in the Peruvian Amazon region. One year of daily river stage records were related to weekly larval catches to determine the association between floods and spawning events, and based on the hydro‐climatologic characteristics of Andean‐Amazon regions, available long‐term historical rainfall records were used to determine the inter‐annual variability of floods within the Madre de Dios Basin. Major larval drift occurred during the high water season, specifically in association with stages of over 5 m, which served as an indicator triggering spawning responses of these species, termed a ‘biologically significant event’ (BSE). Timing of these BSEs, estimated from the historical rainfall records, described a uniform distribution during the wet season, and their inter‐arrival times were exponentially distributed. These observations provided the basis of the stochastic model describing the likelihood of larvae releases from this headwater region to the lowland Amazon. Copyright © 2011 John Wiley & Sons, Ltd.     

Probabilistic assessment of one‐step‐ahead rainfall variation by Split Markov Process

In this paper, Split Markov Process (SMP) is developed to assess one‐step‐ahead variation of daily rainfall at a rain gauge station. SMP is an advancement of general Markov Process and specially developed for probabilistic assessment of change in daily rainfall magnitude. The approach is based on a first‐order Markov chain to simulate daily rainfall variation at a point through state/sub‐state transitional probability matrix (TPM). The state/sub‐state TPM is based on the historical transitions from a particular state to a particular sub‐state, which is the basic difference between SMP and general Markov Process. The cumulative state/sub‐state TPM is represented in a contour plot at different probability levels. The developed cumulative state/sub‐state TPM is used to assess the possible range of rainfall in next time step, in a probabilistic sense. Application of SMP is investigated for daily rainfall at four rain gauge stations – Khandwa, Jabalpur, Sambalpur, and Puri, located at various parts in India. There are 99 years of record available out of which approximately 80% of data are used for calibration, and 20% of data are used to assess the performance. Thus, 80 years of daily monsoon rainfall is used to develop the state/sub‐state TPM, and 19 years data are used to investigate its performance. Model performance is assessed in terms of hit rate (HR), false alarm rate (FAR), and percentage captured. It is found that percentage captured is maximum for Khandwa (70%) and minimum for Sambalpur (44%) whereas hit rate is maximum for Sambalpur and minimum for Khandwa (73%). FAR is around 30% or below for Jabalpur, Sambalpur, and Puri. FAR is maximum for Khandwa (37%). Overall, the assessed range, particularly the upper limit, provides a quantification possible extreme value in the next time step, which is a very useful information to tackle the extreme events, such as flooding, water logging and so on. Copyright © 2011 John Wiley & Sons, Ltd.     

Hydrological characterization of dammed rivers in the NW Mediterranean region

Water scarcity and climatic variability in the Mediterranean region have traditionally required the construction of dams to guarantee water supply for irrigation, industrial and urban uses and hydropower production. Reservoirs affect the hydrology of the river downstream, but the magnitude and persistence of these effects are still poorly unknown. Understanding the magnitude of these effects is the objective of this paper, in which we analyse the flow regimes of twelve rivers located in the NW Mediterranean region. Different temporal scales (daily, monthly and annual) are used for the analysis and also to estimate flow variables associated with flow magnitude, frequency, duration and variability. It is shown that dams alter the hydrological regime of most of the studied rivers, with special influence on monthly flows and flood magnitude and frequency. The most altered rivers (Muga and Siurana, NE Iberian Peninsula) experience a complete overturn in their flow regime with, for instance, flood reduction reaching up to 76% for the 2‐year flood event. Other rivers showed lower changes in hydrology (e.g. Orb and Têt). Annual runoff showed a pattern of decrease in all the studied rivers (regulated and non‐regulated) indicating that besides dams (i.e. reservoir evaporation), other factors likely affect water yield. A general recovery downstream from dams is also observed at all temporal scales, mainly because of the inflow from tributaries. Although dams have a clear impact on the hydrology of Mediterranean rivers, water withdrawals and diversions for irrigation and other consumptive uses also affected the hydrological patterns. Copyright © 2015 John Wiley & Sons, Ltd.     

The impact of climate change on extreme precipitation in Sicily,Italy

Increasing precipitation extremes are one of the possible consequences of a warmer climate. These may exceed the capacity of urban drainage systems, and thus impact the urban environment. Because short‐duration precipitation events are primarily responsible for flooding in urban systems, it is important to assess the response of extreme precipitation at hourly (or sub‐hourly) scales to a warming climate. This study aims to evaluate the projected changes in extreme rainfall events across the region of Sicily (Italy) and, for two urban areas, to assess possible changes in Depth‐Duration‐Frequency (DDF) curves. We used Regional Climate Model outputs from Coordinated Regional Climate Downscaling Experiment for Europe area ensemble simulations at a ~12 km spatial resolution, for the current period and 2 future horizons under the Representative Concentration Pathways 8.5 scenario. Extreme events at the daily scale were first investigated by comparing the quantiles estimated from rain gauge observations and Regional Climate Model outputs. Second, we implemented a temporal downscaling approach to estimate rainfall for sub‐daily durations from the modelled daily precipitation, and, lastly, we analysed future projections at daily and sub‐daily scales. A frequency distribution was fitted to annual maxima time series for the sub‐daily durations to derive the DDF curves for 2 future time horizons and the 2 urban areas. The overall results showed a raising of the growth curves for the future horizons, indicating an increase in the intensity of extreme precipitation, especially for the shortest durations. The DDF curves highlight a general increase of extreme quantiles for the 2 urban areas, thus underlining the risk of failure of the existing urban drainage systems under more severe events.     

Analysis and attribution of trends in water levels in the Vietnamese Mekong Delta

In the Vietnamese Mekong Delta (VMD), water levels at some stations have increased. However, the factors that cause this rise in the VMD have not been identified. We considered four factors that may have contributed to the water level rise: (1) increased runoff from upstream, (2) sea‐level rise, (3) land subsidence, and (4) decrease in flood mitigation function because of construction of high dykes. We analysed daily maximum and minimum water levels, and mean daily water levels from 24 monitoring stations from 1987 to 2006. Using daily and annual water level differences, we classified the delta into two groups: one is dominated by flows from upstream, while the other is tide dominated. We then tested the trends of annual maximum and minimum water levels using the Mann–Kendall test, and identified the slope of the trend using the method of Sen. The areas of dyke construction were estimated using the Enhanced Vegetation Index (EVI) and Land Surface Water Index (LSWI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Results show (1) river inflow has little impact on rising water levels in the VMD, (2) the influence of high dykes on water level rise could not be quantified in this study, and (3) both maximum and minimum water levels significantly increased in the tide‐dominated area. Trend of annual minimum water level can be considered as the sum sea‐level rise and land subsidence. Therefore, we attribute 6.05 mm year^?1 (80%) to land subsidence and 1.42 mm year^?1 (20%) to sea level rise, indicating that inundations have been severe in the VMD, caused primarily by land subsidence. Copyright © 2015 John Wiley & Sons, Ltd.