Trends In Low Flows Research Articles

Trends in low flows of German rivers since 1950: Comparability of different low‐flow indicators and their spatial patterns

AbstractClimate change, land‐use shifts, reservoir storage, and water withdrawals impact low flows in rivers, creating challenges for ecological integrity and human uses. A systematic investigation of river discharges was carried out for 79 stream gauges in Germany. Available time series between 1950 and 2013 were analysed for trends in annual minimum low flows, discharge deficits, and low‐flow durations. The application of different low‐flow indicators led to similar spatial patterns, although each metric is used for different purposes in water management applications.Statistical tests identified significant discharge trends at more than half of the stations investigated. Low‐flow trends since 1950 tended to be catchment specific, suggesting that climate change has not been the dominant driver. Most of the gauges investigated showed statistically significant increases in low flows. This can be mainly attributed to reservoir management. For rivers showing snow‐ and icemelt‐dominated flow regimes, such trends are probably overlain by climate‐driven changes (increasing amounts of rainfall, earlier snowmelt in spring). In contrast, stations showing statistically significant decreases in low flows were correlated with areas of decreasing mining activity. Hydrologic impacts of climate change are widespread and significant, but the results here suggest that human river management remains the dominant hydrologic driver on many rivers.

Low Streamflow Trends in the United States

This research considered low streamflow features which are an important indicator of droughts and suggests significant information for water-resources decision making. Consecutive low flow conditions in a river can create chronic water scarcity. Water scarcity is one of the main drivers of a social and economic conflicts. This research has observed not only locations where suitable adaptive management measures are needed but also locations where low flow regime changes due to natural / climate effects or anthropogenic impacts. Trends in low flows in the United States were evaluated using Mann-Kendall and the Sen-Slope trend tests. 5% significance level is used and an evidence of mostly upward trends in low flows in the United States was found. Trends in referenced (not exposed to anthropogenic impacts) and non-referenced stations (exposed to anthropogenic impacts) were also investigated separately, and then compared. Results illustrate that the percentage of significant low flow trends in non-referenced stations are higher than referenced stations. Furthermore, wavelet, PCA and clustering analysis were also performed. Wavelet analysis was used to determine the amplitude and timing of such low streamflow discontinuities for specific wavebands. Clustering analysis helped to determine a grouping low streamflow patterns with similar annual flow behaviors.

A three-pillar approach to assessing climate impacts on low flows

Abstract. The objective of this paper is to present a framework for assessing climate impacts on future low flows that combines different sources of information, termed pillars. To illustrate the framework three pillars are chosen: (a) extrapolation of observed low-flow trends into the future, (b) rainfall–runoff projections based on climate scenarios and (c) extrapolation of changing stochastic rainfall characteristics into the future combined with rainfall–runoff modelling. Alternative pillars could be included in the overall framework. The three pillars are combined by expert judgement based on a synoptic view of data, model outputs and process reasoning. The consistency/inconsistency between the pillars is considered an indicator of the certainty/uncertainty of the projections. The viability of the framework is illustrated for four example catchments from Austria that represent typical climate conditions in central Europe. In the Alpine region where winter low flows dominate, trend projections and climate scenarios yield consistently increasing low flows, although of different magnitudes. In the region north of the Alps, consistently small changes are projected by all methods. In the regions in the south and south-east, more pronounced and mostly decreasing trends are projected but there is disagreement in the magnitudes of the projected changes. The process reasons for the consistencies/inconsistencies are discussed. For an Alpine region such as Austria the key to understanding low flows is whether they are controlled by freezing and snowmelt processes, or by the summer moisture deficit associated with evaporation. It is argued that the three-pillar approach offers a systematic framework of combining different sources of information aimed at more robust projections than that obtained from each pillar alone.

Innovative Trend Methodology Applications to Precipitation Records in Turkey

There are trend identification methodologies in the literature but they have three crucial points that should be cared with great attention in practical applications. These points are that the available time series should have independent serial correlation structure, Gaussian (normal) distribution and monotonic trend fitting to whole time series through the least square technique. The existing methods consider the whole duration of the time series and try to identify a monotonic trend in increasing or decreasing forms. This paper presents partial trend methodology as an innovative and simple trend identification method, which yields low, medium and high cluster trends separately. It provides the opportunity to segregate between the low, high and medium flow trends and their relative intensities, durations as well as magnitudes. The application of the partial trend methodology is presented for 7 precipitation stations from 7 different sub-climatic regions of Turkey leading to spatial and temporal trend interpretations.

Trends in low flows in Spain in the period 1949–2009

ABSTRACTThis study analyses trends in low flows in Spain in the period 1949–2009, based on daily flow data collected at 60 gauging stations located in near-natural catchments. Two low-flow indicators were considered: (i) the seven-day annual minimum streamflow and (ii) the 10th percentile of the yearly flow duration curve. Catchments were clustered into three regions in terms of monthly mean flows. The Mann-Kendall test was used considering four periods between 1949 and 2009. A multi-temporal trend analysis was also applied to the longest series to identify wet and dry periods that could influence the results. Lastly, a field significance test provided a regional assessment of the at-site detected trends at each region. The results for each indicator reveal a clearly decreasing trend in low flows throughout the northern half of Spain that was found to be field-significant over the (Atlantic and Mediterranean) regions.Editor Z.W. Kundzewicz; Associate editor not assigned

Changes in the low flow regime over the eastern United States (1962–2011): variability, trends, and attributions

We examine trends and variability in low flows over the eastern U.S. (S. Carolina to Maine) and their attribution in a changing climate. We select 149 out of 4878 USGS stations over the eastern U.S., taking into account data availability and minimal direct management. Annual 7-day low flows (Q7) are computed from the series of daily streamflow records for 1962–2011 and compared to an antecedent precipitation (AP) index calculated over the corresponding basin for each station. In general, a north–south (increasing-decreasing) dipole pattern in low flow trends is associated with trends in AP. The exception is in the southern part of the study area including Virginia and the Carolinas, where moderate increasing trends in AP may have been offset by water withdrawals and increasing potential evapotranspiration (PET) as driven by increasing temperature and vapor pressure deficit. A principal component analysis (PCA) of Q7 and AP indicates that the North Atlantic Oscillation (NAO) and Pacific North America (PNA) pattern show statistically significant correlations for Q7 at 1 and 2 month lead time, respectively, via large-scale pressure patterns. Our findings suggest that the inter-annual variability of low flows has increased due to significant anti-correlation between the NAO and PNA during recent decades, and the future risk of low flow extremes may be further enhanced with temperature driven increases in PET and persistence of the multi-decadal relationship between NAO and PNA.

Patterns of hydrological drought indicators in major U.S. River basins

Climate non-stationarity, changes in land use and water management practices affect regional hydrological extremes. This research considers extreme low streamflow (“low flow” henceforth) and flow deficit characteristics, which are important indicators of water scarcity within the broader context of droughts. The results indicate that the number of stations with positive significant trends in low flows is generally higher than the number of stations with negative significant trends; however, the fraction of total number of stations with negative trends has been increasing in the last four decades while the fraction of total number of stations with positive trends have been decreasing, making a gradual reduction in the wetting tendency in low flows in the conterminous U.S. Regional differences in low flow trends are notable, which echo the local climatic changes and soil moisture trends documented in the recent National Climate Assessment (NCA), as well as the changes in cropping and irrigation practices, and creation of forest plantations. A reversal of trend is seen for some northern regions since 1980s. Patterns in return periods and corresponding return values of low flows are also examined, which suggests changing risk conditions that are important for water-resources decision-making. Persistent low flow conditions in a river can lead to chronic water scarcity—a main driver of societal and cross-boundary conflicts. Thus, this research identifies “hotspot” locations where suitable adaptive management measures are most needed.

Evaluation of global impact models' ability to reproduce runoff characteristics over the central United States

AbstractThe central United States experiences a wide array of hydrological extremes, with the 1993, 2008, 2013, and 2014 flooding events and the 1988 and 2012 droughts representing some of the most recent extremes, and is an area where water availability is critical for agricultural production. This study aims to evaluate the ability of a set of global impact models (GIMs) from the Water Model Intercomparison Project to reproduce the regional hydrology of the central United States for the period 1963–2001. Hydrological indices describing annual daily maximum, medium and minimum flow, and their timing are extracted from both modeled daily runoff data by nine GIMs and from observed daily streamflow measured at 252 river gauges. We compare trend patterns for these indices, and their ability to capture runoff volume differences for the 1988 drought and 1993 flood. In addition, we use a subset of 128 gauges and corresponding grid cells to perform a detailed evaluation of the models on a gauge‐to‐grid cell basis. Results indicate that these GIMs capture the overall trends in high, medium, and low flows well. However, the models differ from observations with respect to the timing of high and medium flows. More specifically, GIMs that only include water balance tend to be closer to the observations than GIMs that also include the energy balance. In general, as it would be expected, the performance of the GIMs is the best when describing medium flows, as opposed to the two ends of the runoff spectrum. With regards to low flows, some of the GIMs have considerably large pools of zeros or low values in their time series, undermining their ability in capturing low flow characteristics and weakening the ensemble's output. Overall, this study provides a valuable examination of the capability of GIMs to reproduce observed regional hydrology over a range of quantities for the central United States.

Climate change impact assessment on water inflow to a coastal lagoon: the Ria de Aveiro watershed, Portugal

A climate change impact study was performed for the watershed of the Ria de Aveiro Lagoon in Portugal. The total freshwater inflow to the lagoon was analysed considering long-term average annual and seasonal changes in discharge, as well as extremes in terms of high/low-flow percentiles. The eco-hydrological model SWIM was calibrated at three gauges and driven by a set of 15 climate scenarios from the ENSEMBLES project for one reference and three future 30-year periods. The results indicate a moderate decrease of total inflow by 5–7% in the first two scenario periods, and up to 15% in the third period. High flows do not change significantly, whereas low flows show the strongest decreasing trend among the analysed variables (10–30%). Though the trends in average discharge and low flow are very clear, the uncertainty of projections driven by 15 scenarios is high, and it is increasing with time.Editor D. Koutsoyiannis; Guest editor M. White

An analysis of trends in baseflow recession and low-flows in rain-dominated coastal streams of the pacific coast

The work presented here centers on the identification and calculation of indices representative of annual low-flow conditions and baseflow recession form, and the evaluation of trends in these indices with time for rain-dominated streams of the Pacific coastal mountain ranges. Two different baseflow recession analysis techniques are employed, which when combined are capable of modeling the varied dry-season flow conditions that exist over the broad range of catchments included in the study area. Results indicate that over the past 40–80 years widespread trends of increasing rates of baseflow recession and decreasing annual low-flow conditions exist throughout the region. Of streamgages analyzed, 44% were identified as having a statistically significant trend in either low-flow conditions or recession form with time. While spring flow conditions show little change over the study period, trends of decreasing late-summer flow conditions and increasing rates of recession are particularly common. Northern California and Oregon are especially impacted locations, with upwards of 60% of study gages exhibiting decreasing trends in late summer flow conditions. Detailed conceptual explanations for the connections between trends in recession form and indices of low-flow are also presented.

Drought Regionalization Based on Low Flow Trends in Selected River Basins in Slovakia

Drought Regionalization Based on Low Flow Trends in Selected River Basins in Slovakia

Seasonality Characteristics and Spatio-temporal Trends of 7-day Low Flows in a Large, Semi-arid Watershed

Seasonality characteristics and spatio-temporal trends of 7-day low flows were investigated for the 41,470 km2 semi-arid Karkheh watershed (western Iran), representing 12 stations with record length of 51 years (1958–2008). Hierarchical Cluster Analysis (HCA) was used to identify low-flow seasonality (clusters of low flow seasons). Monotonic trends were detected by the Mann-Kendall test. Breaks in low flow trends were detected by the Lombard’s change point test. Breaks in short-term trends were detected by the Lepage test. According to results seasonal clusters were different in most of the stations, indicating uniqueness in station low flow behaviour. Among the 12 stations studied, four stations showed seasonal monotonic trend and change points, some with abrupt change (change point in consecutive years). Two stations near watershed outlet with no monotonic trend showed abrupt change. Most seasonal change points (dates) compared well with documented droughts, highlighting drought impacts on low flow trends at affected stations. The Lepage test detected change point in seasonal low flow trends of most stations during 5- and 10-year periods. This research has shown that in semi-arid environments low flow seasonality characteristics may vary among stations, an indication that low flow trends should be evaluated individually for each station.

Analysis of trends of low flow in river stations in eastern Slovakia

The availability of using hypothesis test techniques to identify the long-term trends of hydrological time series is investigated in this study. The aim is to analyse trends of low flows at streams in eastern Slovakia, namely Poprad, Hornád, Bodva, Bodrog river basins. The article presents a methodology for prediction of hydrological drought based on statistical testing of low stream flows by non-parametric statistical test. The main objective is to identify low flow trends in the selected 63 river stations in eastern Slovakia. The stations with human impacts are also evaluated. The Mann-Kendall non-parametric test has been used to detect trends in hydrological time series. Statistically significant trends have been determined from the trend lines for the whole territory of eastern Slovakia. The results indicate that the observed changes in Slovakian river basins do not have a clearly defined trend.

Filling the white space on maps of European runoff trends: estimates from a multi-model ensemble

Abstract. An overall appraisal of runoff changes at the European scale has been hindered by "white space" on maps of observed trends due to a paucity of readily-available streamflow data. This study tested whether this white space can be filled using estimates of trends derived from model simulations of European runoff. The simulations stem from an ensemble of eight global hydrological models that were forced with the same climate input for the period 1963–2000. The derived trends were validated for 293 grid cells across the European domain with observation-based trend estimates. The ensemble mean overall provided the best representation of trends in the observations. Maps of trends in annual runoff based on the ensemble mean demonstrated a pronounced continental dipole pattern of positive trends in western and northern Europe and negative trends in southern and parts of eastern Europe, which has not previously been demonstrated and discussed in comparable detail. Overall, positive trends in annual streamflow appear to reflect the marked wetting trends of the winter months, whereas negative annual trends result primarily from a widespread decrease in streamflow in spring and summer months, consistent with a decrease in summer low flow in large parts of Europe. High flow appears to have increased in rain-dominated hydrological regimes, whereas an inconsistent or decreasing signal was found in snow-dominated regimes. The different models agreed on the predominant continental-scale pattern of trends, but in some areas disagreed on the magnitude and even the direction of trends, particularly in transition zones between regions with increasing and decreasing runoff trends, in complex terrain with a high spatial variability, and in snow-dominated regimes. Model estimates appeared most reliable in reproducing observed trends in annual runoff, winter runoff, and 7-day high flow. Modelled trends in runoff during the summer months, spring (for snow influenced regions) and autumn, and trends in summer low flow were more variable – both among models and in the spatial patterns of agreement between models and the observations. The use of models to display changes in these hydrological characteristics should therefore be viewed with caution due to higher uncertainty.

The contrasted evolution of high and low flows and precipitation indices in the Duero basin (Spain)

Trends in high and low flows are valuable indicators of hydrological change because they highlight changes in various parts of the frequency distribution of streamflow series. This enables improved assessment of water availability in regions with high seasonal and inter-annual variability. There has been a substantial reduction in water resources in the Duero basin (Iberian Peninsula, Spain) and other areas of the Mediterranean region during the last 50 years, and this is likely to continue because of climate change. In this study, we investigated the evolution and trends in high and low flows in the Spanish part of the Duero basin, and in equivalent or closely-related precipitation indices for the period 1961–2005. The results showed a general trend of decrease in the frequency and magnitude of high flows throughout most of the basin. Moreover, the number of days with low flows significantly increased over this period. No clear relationship was evident between the evolution of high/low flows and changes in the distribution frequencies of the precipitation series. In contrast to what was expected, the number of days with heavy precipitation and the mean annual precipitation did not show significant trends across the basin, and the number of days without rainfall decreased slightly. The divergence between precipitation and runoff evolution was more accentuated in spring and summer. In the absence of trends in precipitation, it is possible that reforestation processes in the region, and increasing temperatures in recent decades, could be related to the decreasing frequency of high flows and the increasing frequency of low flows. Editor Z.W. Kundzewicz; Associate editor S. Grimaldi Citation Morán-Tejeda, E., López-Moreno, J.I., Vicente-Serrano, S.M., Lorenzo-Lacruz, J. and Ceballos-Barbancho, A., 2012. The contrasted evolution of high and low flows and precipitation indices in the Duero basin (Spain). Hydrological Sciences Journal, 57 (4), 591–611.

Impact of climate change on the stream flow of the lower Brahmaputra: trends in high and low flows based on discharge-weighted ensemble modelling

Abstract. Climate change is likely to have significant effects on the hydrology. The Ganges-Brahmaputra river basin is one of the most vulnerable areas in the world as it is subject to the combined effects of glacier melt, extreme monsoon rainfall and sea level rise. To what extent climate change will impact river flow in the Brahmaputra basin is yet unclear, as climate model studies show ambiguous results. In this study we investigate the effect of climate change on both low and high flows of the lower Brahmaputra. We apply a novel method of discharge-weighted ensemble modeling using model outputs from a global hydrological models forced with 12 different global climate models (GCMs). Our analysis shows that only a limited number of GCMs are required to reconstruct observed discharge. Based on the GCM outputs and long-term records of observed flow at Bahadurabad station, our method results in a multi-model weighted ensemble of transient stream flow for the period 1961–2100. Using the constructed transients, we subsequently project future trends in low and high river flow. The analysis shows that extreme low flow conditions are likely to occur less frequent in the future. However a very strong increase in peak flows is projected, which may, in combination with projected sea level change, have devastating effects for Bangladesh. The methods presented in this study are more widely applicable, in that existing multi-model streamflow simulations from global hydrological models can be weighted against observed streamflow data to assess at first order the effects of climate change for specific river basins.

Observed changes in pan-arctic cold-season minimum monthly river discharge

Recent changes in pan-arctic land-surface hydrology may significantly affect ecosystems and the built environment. While spatial and temporal patterns in land-surface hydrology trends can be identified using in situ observations of river discharge, the existing studies of cold-season river discharge are often inconsistent and incomparable because they use different methodologies, time periods and geographic regions. By using a comprehensive dataset of river discharge and a trend analysis applying one algorithm over a range of temporal and spatial scales, a synthesis of pan-arctic cold-season low-flow was performed. Cold-season low-flow is increasing over most of the pan-arctic, with decreasing flow in eastern North America and unchanged flow in small basins in the late twentieth century in eastern Eurasia as the main exceptions. This study provides the first synthesis of spatially distributed cold-season low-flow trends in the pan-arctic and indicates that widespread changes in pan-arctic subsurface hydrology are occurring.

Identification of temporal trends in annual and seasonal low flows occurring in Canadian rivers: The effect of short- and long-term persistence

In order to investigate trends in time series of hydrological variables, observational records are generally assumed to be independent (IND) or having short-term persistence (STP), as opposed to long-term persistence (LTP). This study systematically investigates the effect of these assumptions for estimating significance of trends using simulated time series of known STP- and LTP-like serial structures and using observational records of annual, winter and summer 30-day low flows occurring in pristine rivers basins of Canada. For the analyses, the Mann–Kendall (MK) test and two modified versions of this test and a block bootstrap resampling based test is used to estimate significance of temporal trends under the assumptions of IND and STP. A semiparametric and a parametric procedure based on the fractional autoregressive integrated moving average modeling approach and a MK scaling test are used to estimate significance of temporal trends under the assumption of LTP. The results of the study suggest that for the majority of the time series of low flows analyzed, the assumption of IND or STP cannot be ruled out. On the contrary, the fluctuating behavior of trends revealed in selected time series of low flows, with longer records, using moving window technique favor the LTP hypothesis. However, only a modest evidence for the presence of LTP in low flow time series using estimates of the Hurst exponent could be revealed, perhaps because of small sample uncertainties. In general, the results of trend investigation suggest that the estimates of trend significance are highly sensitive to IND, STP and LTP assumptions, e.g. adopting IND assumption instead of LTP for a given set of hydrological time series exhibiting LTP, could result in incorrect estimation of trend significance. Also, substituting IND assumption for STP would result in incorrect estimates of trend significance. Therefore, for reliable trend investigation, satisfactory identification of STP- or LTP-like behavior in hydrological time series, which seldom exceed 100 years, is important and challenging and must be given adequate attention in all trend investigation studies.

The effect of climate and land use change on flow duration in the Maryland Piedmont region

AbstractThis paper describes the use of a continuous streamflow model to examine the effects of climate and land use change on flow duration in six urbanizing watersheds in the Maryland Piedmont region. The hydrologic model is coupled with an optimization routine to achieve an agreement between observed and simulated streamflow. Future predictions are made for three scenarios: future climate change, land use change, and jointly varying climate and land use. Future climate is modelled using precipitation and temperature predictions for the Canadian Climate Centre (CCC) and Hadley climate models. Results show that a significant increase in temperature under the CCC climate predictions produces a decreasing trend in low flows. A significant increasing trend in precipitation under the Hadley climate predictions produces an increasing trend in peak flows. Land use change by itself, as simulated by an additional 10% increase in imperviousness (from 20·5 to 30·5%), produces no significant changes in the simulated flow durations. However, coupling the effects of land use change with climate change leads to more significant decreasing trends in low flows under the CCC climate predictions and more significant increasing trends in peak flows under Hadley climate predictions than when climate change alone is employed. These findings indicate that combined land use and climate change can result in more significant hydrologic change than either driver acting alone. Copyright © 2008 John Wiley & Sons, Ltd.

Detection of Trends in Low Flows across Canada

A study of trends and variability of low flow characteristics was conducted for the Reference Hydrometric Basin Network (RHBN). A seven-day low-flow index from 57 hydrometric stations was extracted and examined to detect trends and changes in the timing of summer and winter seven-day low-flows. A modified Mann-Kendall (MK) nonparametric trend test was applied to the time series at a 0.05 significance level. The variance of the S statistic was modified if the absolute value of serial correlation was significant at a 0.1 significance level. Numerical analysis indicated that northern Canada (stations located above latitude 60oN) experienced an increasing significant trend in seven-day low-flows. A significant downward trend dominated the Atlantic Provinces and southern British Columbia. No evidence of significant trends in the Prairies and eastern Ontario was found. Summer seven-day low-flow shifted to arrive earlier in the year in the Atlantic Provinces and southern Ontario; however, it arrived later in the year in western and northwestern Canada. In 88% of significant trends, winter seven-day low-flow shifted to arrive earlier. Although both winter and summer low flows experienced a shift towards earlier dates in the eastern part of the country, they were in opposite direction in western Canada where winter seven-day low-flows were arriving earlier whereas summer seven-day low-flows were arriving later in the year.