Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Approaches to analyse and model changes in impacts: reply to discussions of “How to improve attribution of changes in drought and flood impacts”*
    Kreibich, Heidi ; Blauhut, Veit ; Aerts, Jeroen C.J.H. ; Bouwer, Laurens M. ; Lanen, Henny A.J. Van; Mejia, Alfonso ; Mens, Marjolein ; Loon, Anne F. Van - \ 2020
    Hydrological Sciences Journal 65 (2020)3. - ISSN 0262-6667 - p. 491 - 494.
    damage - dynamic risk - hydrological extremes - new data - projecting risk

    We thank the authors, Brunella Bonaccorso and Karsten Arnbjerg-Nielsen for their constructive contributions to the discussion about the attribution of changes in drought and flood impacts. We appreciate that they support our opinion, but in particular their additional new ideas on how to better understand changes in impacts. It is great that they challenge us to think a step further on how to foster the collection of long time series of data and how to use these to model and project changes. Here, we elaborate on the possibility to collect time series of data on hazard, exposure, vulnerability and impacts and how these could be used to improve e.g. socio-hydrological models for the development of future risk scenarios.

    The European 2015 drought from a hydrological perspective
    Laaha, Gregor ; Gauster, Tobias ; Tallaksen, Lena M. ; Vidal, Jean Philippe ; Stahl, Kerstin ; Prudhomme, Christel ; Heudorfer, Benedikt ; Vlnas, Radek ; Ionita, Monica ; Lanen, Henny A.J. Van; Loon, Anne F. Van - \ 2017
    Hydrology and Earth System Sciences 21 (2017)6. - ISSN 1027-5606 - p. 3001 - 3024.

    In 2015 large parts of Europe were affected by drought. In this paper, we analyze the hydrological footprint (dynamic development over space and time) of the drought of 2015 in terms of both severity (magnitude) and spatial extent and compare it to the extreme drought of 2003. Analyses are based on a range of low flow and hydrological drought indices derived for about 800 streamflow records across Europe, collected in a community effort based on a common protocol. We compare the hydrological footprints of both events with the meteorological footprints, in order to learn from similarities and differences of both perspectives and to draw conclusions for drought management. The region affected by hydrological drought in 2015 differed somewhat from the drought of 2003, with its center located more towards eastern Europe. In terms of low flow magnitude, a region surrounding the Czech Republic was the most affected, with summer low flows that exhibited return intervals of 100 years and more. In terms of deficit volumes, the geographical center of the event was in southern Germany, where the drought lasted a particularly long time. A detailed spatial and temporal assessment of the 2015 event showed that the particular behavior in these regions was partly a result of diverging wetness preconditions in the studied catchments. Extreme droughts emerged where preconditions were particularly dry. In regions with wet preconditions, low flow events developed later and tended to be less severe. For both the 2003 and 2015 events, the onset of the hydrological drought was well correlated with the lowest flow recorded during the event (low flow magnitude), pointing towards a potential for early warning of the severity of streamflow drought. Time series of monthly drought indices (both streamflow- and climate-based indices) showed that meteorological and hydrological events developed differently in space and time, both in terms of extent and severity (magnitude). These results emphasize that drought is a hazard which leaves different footprints on the various components of the water cycle at different spatial and temporal scales. The difference in the dynamic development of meteorological and hydrological drought also implies that impacts on various water-use sectors and river ecology cannot be informed by climate indices alone. Thus, an assessment of drought impacts on water resources requires hydrological data in addition to drought indices based solely on climate data. The transboundary scale of the event also suggests that additional efforts need to be undertaken to make timely pan-European hydrological assessments more operational in the future.

    Drought in a human-modified world : Reframing drought definitions, understanding, and analysis approaches
    Loon, Anne F. van; Stahl, Kerstin ; Baldassarre, Giuliano Di; Clark, Julian ; Rangecroft, Sally ; Wanders, Niko ; Gleeson, Tom ; Dijk, Albert I.J.M. van; Tallaksen, Lena M. ; Hannaford, Jamie ; Uijlenhoet, Remko ; Teuling, Ryan ; Hannah, David M. ; Sheffield, Justin ; Svoboda, Mark ; Verbeiren, Boud ; Wagener, Thorsten ; Lanen, Henny A.J. van - \ 2016
    Hydrology and Earth System Sciences 20 (2016)9. - ISSN 1027-5606 - p. 3631 - 3650.

    In the current human-modified world, or Anthropocene, the state of water stores and fluxes has become dependent on human as well as natural processes. Water deficits (or droughts) are the result of a complex interaction between meteorological anomalies, land surface processes, and human inflows, outflows, and storage changes. Our current inability to adequately analyse and manage drought in many places points to gaps in our understanding and to inadequate data and tools. The Anthropocene requires a new framework for drought definitions and research. Drought definitions need to be revisited to explicitly include human processes driving and modifying soil moisture drought and hydrological drought development. We give recommendations for robust drought definitions to clarify timescales of drought and prevent confusion with related terms such as water scarcity and overexploitation. Additionally, our understanding and analysis of drought need to move from single driver to multiple drivers and from uni-directional to multi-directional. We identify research gaps and propose analysis approaches on (1) drivers, (2) modifiers, (3) impacts, (4) feedbacks, and (5) changing the baseline of drought in the Anthropocene. The most pressing research questions are related to the attribution of drought to its causes, to linking drought impacts to drought characteristics, and to societal adaptation and responses to drought. Example questions include i) What are the dominant drivers of drought in different parts of the world? (ii) How do human modifications of drought enhance or alleviate drought severity? (iii) How do impacts of drought depend on the physical characteristics of drought vs. the vulnerability of people or the environment? (iv) To what extent are physical and human drought processes coupled, and can feedback loops be identified and altered to lessen or mitigate drought? (v) How should we adapt our drought analysis to accommodate changes in the normal situation (i.e. what are considered normal or reference conditions) over time? Answering these questions requires exploration of qualitative and quantitative data as well as mixed modelling approaches. The challenges related to drought research and management in the Anthropocene are not unique to drought, but do require urgent attention. We give recommendations drawn from the fields of flood research, ecology, water management, and water resources studies. The framework presented here provides a holistic view on drought in the Anthropocene, which will help improve management strategies for mitigating the severity and reducing the impacts of droughts in future.

    Impacts of European drought events : Insights from an international database of text-based reports
    Stahl, Kerstin ; Kohn, Irene ; Blauhut, Veit ; Loon, Anne F. Van; Melsen, Lieke A. ; Lanen, Henny A.J. van - \ 2016
    Natural Hazards and Earth System Sciences 16 (2016)3. - ISSN 1561-8633 - p. 801 - 819.

    Drought is a natural hazard that can cause a wide range of impacts affecting the environment, society, and the economy. Providing an impact assessment and reducing vulnerability to these impacts for regions beyond the local scale, spanning political and sectoral boundaries, requires systematic and detailed data regarding impacts. This study presents an assessment of the diversity of drought impacts across Europe based on the European Drought Impact report Inventory (EDII), a unique research database that has collected close to 5000 impact reports from 33 European countries. The reported drought impacts were classified into major impact categories, each of which had a number of subtypes. The distribution of these categories and types was then analyzed over time, by country, across Europe and for particular drought events. The results show that impacts on agriculture and public water supply dominate the collection of drought impact reports for most countries and for all major drought events since the 1970s, while the number and relative fractions of reported impacts in other sectors can vary regionally and from event to event. The analysis also shows that reported impacts have increased over time as more media and website information has become available and environmental awareness has increased. Even though the distribution of impact categories is relatively consistent across Europe, the details of the reports show some differences. They confirm severe impacts in southern regions (particularly on agriculture and public water supply) and sector-specific impacts in central and northern regions (e.g., on forestry or energy production). The protocol developed thus enabled a new and more comprehensive view on drought impacts across Europe. Related studies have already developed statistical techniques to evaluate the link between drought indices and the categorized impacts using EDII data. The EDII is a living database and is a promising source for further research on drought impacts, vulnerabilities, and risks across Europe. A key result is the extensive variety of impacts found across Europe and its documentation. This insight can therefore inform drought policy planning at national to international levels.

    Hydrology needed to manage droughts : The 2015 European case
    Lanen, Henny A.J. van; Laaha, Gregor ; Kingston, Daniel G. ; Gauster, Tobias ; Loon, Anne F. Van - \ 2016
    Hydrological Processes 30 (2016)17. - ISSN 0885-6087 - p. 3097 - 3104.
    Drought in the Anthropocene
    Loon, A.F. Van; Gleeson, Tom ; Clark, Julian ; Dijk, Albert I.J.M. Van; Stahl, Kerstin ; Hannaford, Jamie ; Baldassarre, Giuliano Di; Teuling, A.J. ; Tallaksen, Lena M. ; Uijlenhoet, Remko ; Hannah, David M. ; Sheffield, Justin ; Svoboda, Mark ; Verbeiren, Boud ; Wagener, Thorsten ; Rangecroft, Sally ; Wanders, Niko ; Lanen, H.A.J. Van - \ 2016
    Nature Geoscience 9 (2016)2. - ISSN 1752-0894 - p. 89 - 91.
    Drought management is inefficient because feedbacks between drought and people are not fully understood. In this human-influenced era, we need to rethink the concept of drought to include the human role in mitigating and enhancing drought.
    Hydrological classification, a practical tool for mangrove restoration
    Loon, Anne F. van; Brake, Bram te; Huijgevoort, Marjolein H.J. Van; Dijksma, Roel - \ 2016
    PLoS ONE 11 (2016)3. - ISSN 1932-6203 - 26 p.

    Mangrove restoration projects, aimed at restoring important values of mangrove forests after degradation, often fail because hydrological conditions are disregarded. We present a simple, but robust methodology to determine hydrological suitability for mangrove species, which can guide restoration practice. In 15 natural and 8 disturbed sites (i.e. disused shrimp ponds) in three case study regions in south-east Asia, water levels were measured and vegetation species composition was determined. Using an existing hydrological classification for mangroves, sites were classified into hydrological classes, based on duration of inundation, and vegetation classes, based on occurrence of mangrove species. For the natural sites hydrological and vegetation classes were similar, showing clear distribution of mangrove species from wet to dry sites. Application of the classification to disturbed sites showed that in some locations hydrological conditions had been restored enough for mangrove vegetation to establish, in some locations hydrological conditions were suitable for various mangrove species but vegetation had not established naturally, and in some locations hydrological conditions were too wet for any mangrove species (natural or planted) to grow. We quantified the effect that removal of obstructions such as dams would have on the hydrology and found that failure of planting at one site could have been prevented. The hydrological classification needs relatively little data, i.e. water levels for a period of only one lunar tidal cycle without additional measurements, and uncertainties in the measurements and analysis are relatively small. For the study locations, the application of the hydrological classification gave important information about how to restore the hydrology to suitable conditions to improve natural regeneration or to plant mangrove species, which could not have been obtained by estimating elevation only. Based on this research a number of recommendations are given to improve the effectiveness of mangrove restoration projects.

    Multiscale evaluation of the Standardized Precipitation Index as a groundwater drought indicator
    Kumar, Rohini ; Musuuza, Jude L. ; Loon, Anne F. Van; Teuling, Ryan ; Barthel, Roland ; Broek, Jurriaan Ten; Mai, Juliane ; Samaniego, Luis ; Attinger, Sabine - \ 2016
    Hydrology and Earth System Sciences 20 (2016)3. - ISSN 1027-5606 - p. 1117 - 1131.

    The lack of comprehensive groundwater observations at regional and global scales has promoted the use of alternative proxies and indices to quantify and predict groundwater droughts. Among them, the Standardized Precipitation Index (SPI) is commonly used to characterize droughts in different compartments of the hydro-meteorological system. In this study, we explore the suitability of the SPI to characterize local- and regional-scale groundwater droughts using observations at more than 2000 groundwater wells in geologically different areas in Germany and the Netherlands. A multiscale evaluation of the SPI is performed using the station data and their corresponding 0.5° gridded estimates to analyze the local and regional behavior of groundwater droughts, respectively. The standardized anomalies in the groundwater heads (SGI) were correlated against SPIs obtained using different accumulation periods. The accumulation periods to achieve maximum correlation exhibited high spatial variability (ranges 3-36 months) at both scales, leading to the conclusion that an a priori selection of the accumulation period (for computing the SPI) would result in inadequate characterization of groundwater droughts. The application of the uniform accumulation periods over the entire domain significantly reduced the correlation between the SPI and SGI (∼ 21-66%), indicating the limited applicability of the SPI as a proxy for groundwater droughts even at long accumulation times. Furthermore, the low scores of the hit rate (0.3-0.6) and a high false alarm ratio (0.4-0.7) at the majority of the wells and grid cells demonstrated the low reliability of groundwater drought predictions using the SPI. The findings of this study highlight the pitfalls of using the SPI as a groundwater drought indicator at both local and regional scales, and stress the need for more groundwater observations and accounting for regional hydrogeological characteristics in groundwater drought monitoring.

    Hydrological drought types in cold climates : Quantitative analysis of causing factors and qualitative survey of impacts
    Loon, A.F. Van; Ploum, S.W. ; Parajka, J. ; Fleig, A.K. ; Garnier, E. ; Laaha, G. ; Lanen, H.A.J. Van - \ 2015
    Hydrology and Earth System Sciences 19 (2015)4. - ISSN 1027-5606 - p. 1993 - 2016.

    For drought management and prediction, knowledge of causing factors and socio-economic impacts of hydrological droughts is crucial. Propagation of meteorological conditions in the hydrological cycle results in different hydrological drought types that require separate analysis. In addition to the existing hydrological drought typology, we here define two new drought types related to snow and ice. A snowmelt drought is a deficiency in the snowmelt discharge peak in spring in snow-influenced basins and a glaciermelt drought is a deficiency in the glaciermelt discharge peak in summer in glacierised basins. In 21 catchments in Austria and Norway we studied the meteorological conditions in the seasons preceding and at the time of snowmelt and glaciermelt drought events. Snowmelt droughts in Norway were mainly controlled by below-average winter precipitation, while in Austria both temperature and precipitation played a role. For glaciermelt droughts, the effect of below-average summer air temperature was dominant, both in Austria and Norway. Subsequently, we investigated the impacts of temperature-related drought types (i.e. snowmelt and glaciermelt drought, but also cold and warm snow season drought and rain-to-snow-season drought). In historical archives and drought databases for the US and Europe many impacts were found that can be attributed to these temperature-related hydrological drought types, mainly in the agriculture and electricity production (hydropower) sectors. However, drawing conclusions on the frequency of occurrence of different drought types from reported impacts is difficult, mainly because of reporting biases and the inevitably limited spatial and temporal scales of the information. Finally, this study shows that complete integration of quantitative analysis of causing factors and qualitative analysis of impacts of temperature-related droughts is not yet possible. Analysis of selected events, however, points out that it can be a promising research area if more data on drought impacts become available.

    Comparing low moisture availability and relative crop yields on the pan-European scale using the FAO water production function
    Oude Lenferink, K.J.B. ; Loon, A.F. Van; Huijgevoort, M.H.J. Van; Lanen, H.A.J. Van - \ 2015
    In: Drought: Research and Science-Policy Interfacing - Proceedings of the International Conference on Drought: Research and Science-Policy Interfacing. - CRC Press/Balkema - ISBN 9781138027794 - p. 105 - 112.

    This paper presents an approach for assessing low soil moisture availability and crop yields on the pan-European scale. It builds on the FAO water production function, linking evapotranspiration reduction simulated with a set of large-scale hydrological models to observed crop yields. Evapotranspiration data of almost 7000 grid cells (0.5° × 0.5°) covering Europe were transformed to 94 NUTS-1 regions to enable comparison with crop yield data and to account for cropland distribution. The multi-model ensemble median evapotranspiration reduction was calculated to accommodate differences between large-scale models. Obviously, Southern and Southeastern Europe showed higher reductions. The evapotranspiration reduction was connected to the relative crop yield (ratio of observed crop yield and simulated potential yield) for six major crops. More significant relationships occurred, for instance, for wheat, maize and potatoes than for rye. No significant relationships occurred in a substantial number of NUTS-1 regions in eastern and northern Europe. Maps show distinct geographical clustering in the strength of the links across Europe in other regions. The R2 of the regression equals 0.76 averaged over all crops, regions and models.

    Hydrological drought severity explained by climate and catchment characteristics
    Loon, A.F. Van; Laaha, G. - \ 2015
    Journal of Hydrology 526 (2015). - ISSN 0022-1694 - p. 3 - 14.
    Catchment characteristics - Climate - Drought deficit - Drought duration - Hydrological drought severity - Storage

    Impacts of a drought are generally dependent on the severity of the hydrological drought event, which can be expressed by streamflow drought duration or deficit volume. For prediction and the selection of drought sensitive regions, it is crucial to know how streamflow drought severity relates to climate and catchment characteristics. In this study we investigated controls on drought severity based on a comprehensive Austrian dataset consisting of 44 catchments with long time series of hydrometeorological data (on average around 50. year) and information on a large number of physiographic catchment characteristics. Drought analysis was performed with the variable threshold level method and various statistical tools were applied, i.e. bivariate correlation analysis, heatmaps, linear models based on multiple regression, varying slope models, and automatic stepwise regression. Results indicate that streamflow drought duration is primarily controlled by storage, quantified by the Base Flow Index or by a combination of catchment characteristics related to catchment storage and release, e.g. geology and land use. Additionally, the duration of dry spells in precipitation is important for streamflow drought duration. Hydrological drought deficit, however, is governed by average catchment wetness (represented by mean annual precipitation) and elevation (reflecting seasonal storage in the snow pack and glaciers). Our conclusion is that both drought duration and deficit are governed by a combination of climate and catchment control, but not in a similar way. Besides meteorological forcing, storage is important; storage in soils, aquifers, lakes, etc. influences drought duration and seasonal storage in snow and glaciers influences drought deficit. Consequently, the spatial variation of hydrological drought severity is highly dependent on terrestrial hydrological processes.

    Characteristics and drivers of drought in Europe-a summary of the DROUGHT-R&SPI project
    Tallaksen, Lena M. ; Stagge, James H. ; Stahl, Kerstin ; Gudmundsson, Lukas ; Orth, Rene ; Seneviratne, Sonia I. ; Loon, Anne F. van; Lanen, Henny A.J. van - \ 2015
    In: Drought: Research and Science-Policy Interfacing - Proceedings of the International Conference on Drought: Research and Science-Policy Interfacing. - CRC Press/Balkema - ISBN 9781138027794 - p. 15 - 22.

    A prerequisite to mitigate the wide range of drought impacts is to establish a good understanding of the drought generating mechanisms from their initiation as a meteorological drought through to their development as soil moisture and hydrological drought. The DROUGHT-R&SPI project has contributed to increased understanding of: (i) drought as a natural hazard through analysis of historical large-scale droughts in Europe, (ii) drought-controlling mechanisms for various scales, including large-scale climate drivers and local-scale processes, (iii) the potential for drought early warning and forecasting, and (iv) suitable drought indicators. The results provide new insight into key characteristics and drivers of major historical droughts as well as projected future drought. The paper summarises the main outcomes of these studies and identifies further research needs.

    Candidate Distributions for Climatological Drought Indices (SPI and SPEI)
    Stagge, James H. ; Tallaksen, Lena M. ; Gudmundsson, Lukas ; Loon, Anne F. Van; Stahl, Kerstin - \ 2015
    International Journal of Climatology 35 (2015)13. - ISSN 0899-8418 - p. 4027 - 4040.
    Drought index - Potential evapotranspiration - Probability distribution - SPEI - SPI - Standardized Precipitation Index

    The Standardized Precipitation Index (SPI), a well-reviewed meteorological drought index recommended by the World Meteorological Organization (WMO), and its more recent climatic water balance variant, the Standardized Precipitation-Evapotranspiration Index (SPEI), both rely on selection of a univariate probability distribution to normalize the index, allowing for comparisons across climates. Choice of an improper probability distribution may impart bias to the index values, exaggerating or minimizing drought severity. This study compares a suite of candidate probability distributions for use in SPI and SPEI normalization using the 0.5° × 0.5° gridded Watch Forcing Dataset (WFD) at the continental scale, focusing on Europe. Several modifications to the SPI and SPEI methodology are proposed, as well as an updated procedure for evaluating SPI/SPEI goodness of fit based on the Shapiro-Wilk test. Candidate distributions for SPI organize into two groups based on their ability to model short-term accumulation (1-2 months) or long-term accumulation (>3 months). The two-parameter gamma distribution is recommended for general use when calculating SPI across all accumulation periods and regions within Europe, in agreement with previous studies. The generalized extreme value distribution is recommended when computing the SPEI, in disagreement with previous recommendations.

    Investigation of variable threshold level approaches for hydrological drought identification
    Beyene, B.S. ; Loon, A.F. Van; Lanen, H.A.J. Van; Torfs, P.J.J.F. - \ 2014
    Hydrology and Earth System Sciences Discussions 11 (2014)11. - ISSN 1812-2108 - p. 12765 - 12797.

    Threshold level approaches are widely used to identify drought events in time series of hydrometeorological variables. However, the method used for calculating the threshold level can influence the quantification of drought events or even introduce artefact drought events. In this study, four methods of variable threshold calculation have been tested on catchment scale, namely (1) moving average of monthly quantile (M-MA), (2) moving average of daily quantile (D-MA), (3) thirty days moving window quantile (30D) and (4) fast Fourier transform of daily quantile (D-FF). The levels obtained by these methods were applied to hydrometeorological variables that were simulated with a semi-distributed conceptual rainfall-runoff model (HBV) for five European catchments with contrasting catchment properties and climate conditions. There are no physical arguments to prefer one method over the other for drought identification. The only way to investigate this is by applying the methods and visually inspecting the results. Therefore, drought statistics (i.e. number of droughts, mean duration, mean deficit) and time series plots were studied to compare drought propagation patterns determined by different threshold calculation methods. We found that all four approaches are sufficiently suitable to quantify drought propagation in contrasting catchments. Only the D-FF approach showed lower performance in two catchments. The 30D approach seems to be optimal in snow-dominated catchments, because it follows fast changes in discharge caused by snow melt more accurately. The proposed approaches can be successfully applied by water managers in regions where drought quantification and prediction are essential.

    How climate seasonality modifies drought duration and deficit
    Loon, A.F. van; Tijdeman, E. ; Wanders, N. ; Lanen, H.A.J. van; Teuling, A.J. ; Uijlenhoet, R. - \ 2014
    Journal of Geophysical Research: Atmospheres 119 (2014)8. - ISSN 2169-897X - p. 4640 - 4656.
    hydrological drought - low flows - catchment - reanalysis - indexes
    Drought propagation through the terrestrial hydrological cycle is associated with a change in drought characteristics (duration and deficit), moving from precipitation via soil moisture to discharge. Here we investigate climate controls on drought propagation with a modeling experiment in 1271 virtual catchments that differ only in climate type. For these virtual catchments we studied the bivariate distribution of drought duration and standardized deficit for the variables precipitation, soil moisture, and discharge. We found that for meteorological drought (below-normal precipitation), the bivariate distributions of drought characteristics have a linear shape in all climates and are thus not affected by seasonality in climate. Despite the linear shape of meteorological drought, soil moisture drought (below-normal storage in the unsaturated zone) and hydrological drought (below-normal water availability in aquifers, lakes, and/or streams) show strongly nonlinear shapes in drought characteristics in climates with a pronounced seasonal cycle in precipitation and/or temperature. These seasonality effects on drought propagation are found in monsoonal, savannah, and Mediterranean climate zones. In these regions, both soil moisture and discharge show deviating shapes in drought characteristics. The effect of seasonality on drought propagation is even stronger in cold seasonal climates (i.e., at high latitudes and altitudes), where snow accumulation during winter prevents recovery from summer hydrological drought, and deficit increases strongly with duration. This has important implications for water resources management in seasonal climates, which cannot solely rely on meteorology-based indices as proxies for hydrological drought duration and deficit and need to include seasonal variation in both precipitation and temperature in hydrological drought forecasting.
    Regional assessment of low flow processes and prediction methods across European regimes
    Laaha, G. ; Loon, A.F. van; Lang Delus, C. ; Koffler, D. - \ 2014
    How to distinguish between drought and water scarcity? Use an observation-modelling framework!
    Loon, A.F. van; Lanen, H.A.J. van - \ 2013
    In: Proc. 13th Int. Conf. on Environm. Sci. and Technology, 5-7 September 2013, Athens, Greece / Lekkas, T.D., Makri, C., Lekkas, D.F.,
    Making the distinction between water scarcity and drought using an observation-modeling framework
    Loon, A.F. van; Lanen, H.A.J. van - \ 2013
    Water Resources Research 49 (2013)3. - ISSN 0043-1397 - p. 1483 - 1502.
    decision-support-system - climate-change - hydrological drought - resource-management - central spain - runoff model - forcing data - groundwater - river - basin
    Drought and water scarcity are keywords for river basin management in water-stressed regions. “Drought” is a natural hazard, caused by large-scale climatic variability, and cannot be prevented by local water management. “Water scarcity” refers to the long-term unsustainable use of water resources, which water managers can influence. Making the distinction between drought and water scarcity is not trivial, because they often occur simultaneously. In this paper, we propose an observation-modeling framework to separate natural (drought) and human (water scarcity) effects on the hydrological system. The basis of the framework is simulation of the situation that would have occurred without human influence, the “naturalized” situation, using a hydrological model. The resulting time series of naturalized state variables and fluxes are then compared to observed time series. As second, more important and novel step, anomalies (i.e., deviations from a threshold) are determined from both time series and compared. We demonstrate the use of the proposed observation-modeling framework in the Upper-Guadiana catchment in Spain. Application of the framework to the period 1980–2000 shows that the impact of groundwater abstraction on the hydrological system was, on average, four times as high as the impact of drought. Water scarcity resulted in disappearance of the winter high-flow period, even in relatively wet years, and a nonlinear response of groundwater. The proposed observation-modeling framework helps water managers in water-stressed regions to quantify the relative impact of drought and water scarcity on a transient basis and, consequently, to make decisions regarding adaptation to drought and combating water scarcity.
    Evapotranspiration amplifies European summer drought
    Teuling, A.J. ; Loon, A.F. van; Seneviratne, S.I. ; Lehner, I. ; Aubinet, M. ; Heinesch, B. ; Bernhofer, C. ; Grünwald, T. ; Prasse, H. ; Spank, U. - \ 2013
    Geophysical Research Letters 40 (2013)10. - ISSN 0094-8276 - p. 2071 - 2075.
    forest - simulations - exchange - carbon - scale
    Drought is typically associated with a lack of precipitation, whereas the contribution of evapotranspiration and runoff to drought evolution is not well understood. Here we use unique long-term observations made in four headwater catchments in central and western Europe to reconstruct storage anomalies and study the drivers of storage anomaly evolution during drought. We provide observational evidence for the “drought-paradox” in that region: a consistent and significant increase in evapotranspiration during drought episodes, which acts to amplify the storage anomalies. In contrast, decreases in runoff act to limit storage anomalies. Our findings stress the need for the correct representation of evapotranspiration and runoff processes in drought indices.
    Hydrological drought across the world: impact of climate and physical catchment structure
    Lanen, H.A.J. van; Wanders, N. ; Tallaksen, L.M. ; Loon, A.F. van - \ 2013
    Hydrology and Earth System Sciences 17 (2013)5. - ISSN 1027-5606 - p. 1715 - 1732.
    water availability - european runoff - united-states - groundwater - flow - model - simulations - trends - classification - propagation
    Large-scale hydrological drought studies have demonstrated spatial and temporal patterns in observed trends, and considerable difference exists among global hydrological models in their ability to reproduce these patterns. In this study a controlled modeling experiment has been set up to systematically explore the role of climate and physical catchment structure (soils and groundwater systems) to better understand underlying drought-generating mechanisms. Daily climate data (1958-2001) of 1495 grid cells across the world were selected that represent Koppen-Geiger major climate types. These data were fed into a conceptual hydrological model. Nine realizations of physical catchment structure were defined for each grid cell, i.e., three soils with different soil moisture supply capacity and three groundwater systems (quickly, intermediately and slowly responding). Hydrological drought characteristics (number, duration and standardized deficit volume) were identified from time series of daily discharge. Summary statistics showed that the equatorial and temperate climate types (A-and C-climates) had about twice as many drought events as the arid and polar types (B-and E-climates), and the durations of more extreme droughts were about half the length. Selected soils under permanent grassland were found to have a minor effect on hydrological drought characteristics, whereas groundwater systems had major impact. Groundwater systems strongly controlled the hydrological drought characteristics of all climate types, but particularly those of the wetter A-, C-and D-climates because of higher recharge. The median number of droughts for quickly responding groundwater systems was about three times higher than for slowly responding systems. Groundwater systems substantially affected the duration, particularly of the more extreme drought events. Bivariate probability distributions of drought duration and standardized deficit for combinations of Koppen-Geiger climate, soil and groundwater system showed that the responsiveness of the groundwater system is as important as climate for hydrological drought development. This urges for an improvement of subsurface modules in global hydrological models to be more useful for water resources assessments. A foreseen higher spatial resolution in large-scale models would enable a better hydrogeological parameterization and thus inclusion of lateral flow.
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