Kerstin Stahl

Climate change and the institutional resilience of international river basins

In the existing 276 international river basins, the increase in water variability projected by most climate change scenarios may present serious challenges to riparian states. This research maps the institutional resilience to water variability in transboundary basins and combines it with both historic and projected variability regimes, with the objective of identifying areas at potential risk of future hydropolitical tension. To do so, it combs existing international treaties for sources of institutional resilience and considers the coefficient of variation of runoff as a measure of past and future water variability. The study finds significant gaps in both the number of people and area covered by institutional stipulations to deal with variability in South America and Asia. At present, high potential risk for hydropolitical tensions associated with water variability is identified in 24 transboundary basins and seems to be concentrated mainly in northern and sub-Saharan Africa. By 2050, areas at greatest potential risk are more spatially dispersed and can be found in 61 international basins, and some of the potentially large impacts of climate change are projected to occur away from those areas currently under scrutiny. Understanding when and where to target capacity-building in transboundary river basins for greater resilience to change is critical. This study represents a step toward facilitating these efforts and informing further qualitative and quantitative research into the relationship between climate change, hydrological variability regimes, and institutional capacity for accommodating variability.

Linking streamflow drought to the occurrence of atmospheric circulation patterns

The growing use of surface water resources calls for more intense research on low flow causes and characteristics. This paper presents an investigation linking regional streamflow drought to the occurrence of atmospheric circulation patterns (CPs). Streamflow drought events of 74 basins in southern Germany were determined using the threshold level approach. The regional classification of the basins through a cluster analysis of the drought series provided the basis for the definition of a regional drought index and the following investigation of the influence of CP occurrence on drought. Frequency cross-tabulation showed several high-pressure and anticyclonic CP types to be strongly associated with streamflow drought in southern Germany. The influence of these CPs on streamflow drought was quantified using a logistic regression model. The model results revealed important regional differences concerning the time lag of the drought response and the relevant CPs.

The Processes, Patterns and Impacts of Low Flows Across Canada

This paper provides an overview of low flow characteristics for six regions of Canada: the Arctic; the Mountains; the Prairies; southern Ontario; the Canadian Shield and the Atlantic. Processes that influence low flows are contrasted between the six regions examined. Data from a common analysis period for 51 gauging stations are used to evaluate flow duration curves and to explore the relationship between low flows and drainage area. The results reveal a diversity of processes influencing low flows and illustrate important regional differences in low flow characteristics and the impacts associated with low flows.

A drought index accounting for snow

The Standardized Precipitation Index (SPI) is the most widely used index to characterize droughts that are related to precipitation deficiencies. However, the SPI does not always deliver the relevant information for hydrological drought management particularly in snow-influenced catchments. If precipitation is temporarily stored as snow, then there is a significant difference between meteorological and hydrological drought because the delayed release of melt water to the stream. We introduce an extension to the SPI, the Standardized Snow Melt and Rain Index (SMRI), that accounts for rain and snow melt deficits, which effectively influence streamflow. The SMRI can be derived without snow data, using temperature and precipitation to model snow. The value of the new index is illustrated for seven Swiss catchments with different degrees of snow influence. In particular for catchments with a larger component of snowmelt in runoff generation, the SMRI was found to be a worthwhile complementary index to the SPI to characterize streamflow droughts.

Spatial and temporal patterns of large‐scale droughts in Europe: Model dispersion and performance

This study explores the performance of a suite of off-line, global (hydrological and land surface) models in mapping spatial and temporal patterns of large-scale hydrological droughts in Europe from simulated runoff in the period 1963–2000. Consistent model behavior was found for annual variability in mean drought area, whereas high model dispersion was revealed in the weekly evolution of contiguous area in drought and its annual maximum. Comparison with nearly three hundred catchment-scale streamflow observations showed an overall tendency to overestimate the number of drought events and hence underestimate drought duration, whereas persistence in drought-affected area (weekly mean) was underestimated, noticeable for one group of models. The high model dispersion in temporal and spatial persistence of drought identified implies that care should be taken when analyzing drought characteristics from only one or a limited number of models unless validated specifically for hydrological drought.

Streamflow Data from Small Basins: A Challenging Test to High-Resolution Regional Climate Modeling

AbstractLand surface models and large-scale hydrological models provide the basis for studying impacts of climate and anthropogenic changes on continental- to regional-scale hydrology. Hence, there is a need for comparison and validation of simulated characteristics of spatial and temporal dynamics with independent observations. This study introduces a novel validation framework that relates to common hydrological design measures. The framework is tested by comparing anomalies of runoff from a high-resolution climate-model simulation for Europe with a large number of streamflow observations from small near-natural basins. The regional climate simulation was performed as a “poor man’s reanalysis,” involving a dynamical downscaling of the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40) with the Danish “HIRHAM5” model. For 19 different anomaly levels, two indices evaluate the temporal agreement (i.e., the occurrence and frequency of dry and wet events based on daily anomalies), ...

Derivation of melt factors from glacier mass-balance records in western Canada

Melt factors for snow (k s ) and ice (k i ) were derived from specific mass-balance data and regionally interpolated daily air-temperature series at nine glaciers in the western Cordillera of Canada. Fitted k s and k i were relatively consistent across the region, with mean values (standard deviations) of 3.04 (0.38) and 4.59 (0.59) mm d -1 °C -1 respectively. The interannual variability of melt factors was investigated for two long-term datasets. Calculated annually, snow- and ice-melt factors were relatively stable from year to year; standard deviations for snowmelt factors were 0.48 (17%) and 0.42 (18%) at Peyto and Place Glaciers, respectively, while standard deviations of ice-melt factors were 1.17 (25%) and 0.81 (14%). While fitted values of k s are comparable to those presented in previous observational and modeling studies, fitted k i are substantially and consistently lower across the region. Fitted melt factors were sensitive to the choice of lapse rate used in the air-temperature interpolation. Melt factors fitted to mass-balance data from a single site (Place Glacier) provided reasonable summer balance predictions at most other sites representing both maritime and continental climates, although there was a tendency for under-prediction at several sites. The combination of regionally interpolated air temperatures and a degree-day model appears capable of generating first-order estimates of regional summer balance, which can provide a benchmark against which to judge the predictive ability of more complex (e.g. energy balance) models applied at a regional scale. Mass-balance sensitivity analyses indicate that a temperature increase of 1 K will increase summer ablation in the region by 0.51 m w.e. a -1 on average.

Towards pan-European drought risk maps: quantifying the link between drought indices and reported drought impacts

Drought in Europe is a hazard with a wide range of transboundary, environmental and socio-economic impacts on various sectors including agriculture, energy production, public water supply and water quality. Despite the apparent importance of this natural hazard, observed pan-European drought impacts have not yet been quantitatively related to the most important climatological drivers to map drought risk on a continental scale. This contribution approaches the issue by quantitatively assessing the likelihood of drought impact occurrence as a function of the standardized precipitation evapotranspiration index for four European macro regions using logistic regression. The resulting models allow mapping the sector-specific likelihood of drought impact occurrence for specific index levels. For the most severe drought conditions the maps suggest the highest risk of impact occurrence for ?Water Quality? in Maritime Europe, followed by ?Agriculture & Livestock Farming? in Western Mediterranean Europe and ?Energy & Industry? in Maritime Europe. Merely impacts on ?Public Water Supply? result in overall lower risk estimates. The work suggests that modeling and mapping for North- and Southeastern Europe requires further enhancement to the impact database in these regions. Such maps may become an essential component of drought risk management to foster resilience for this hazard at large scale.

Streamflow sensitivity to drought scenarios in catchments with different geology

Streamflow drought response depends to a large degree on groundwater recharge. To better predict and preempt streamflow droughts, the relationship between recharge deficit and streamflow response needs to be studied more systematically. We present a combined soil-vegetation-atmosphere transfer and conceptual groundwater model experiment that applies a novel set of recharge scenarios preceding drought events in humid-temperate catchments with different dominant aquifer types. The recharge scenarios are based on the permutation of historical time series and on modified time series matching extreme (50 year) drought events. Karstic and fractured aquifers show short-term sensitivity to drought with an event-specific relationship between recharge and streamflow response. Porous and complex aquifers show long-term sensitivity and a more catchment-controlled propagation of drought. The sensitivity of drought deficit and recovery time correlates with the water age distribution in baseflow, which is trackable in the groundwater model and a characteristic that should be exploited to improve streamflow drought prediction.

Catchment water storage variation with elevation

One of the most important functions of catchments is the storage of water. Catchment storagebuffers meteorological extremes and interannual streamflow variability, controls the partitioningbetween evaporation and runoff, and influences transit times of water. Hydrogeological data toestimate storage are usually scarce and seldom available for a larger set of catchments. This studyfocused on storage in prealpine and alpine catchments, using a set of 21 Swiss catchments comprising different elevation ranges. Catchment storage comparisons depend on storage definitions.This study defines different types of storage including definitions of dynamic and mobile catchment storage. We then estimated dynamic storage using four methods, water balance analysis,streamflow recession analysis, calibration of a bucket‐type hydrological model Hydrologiska Byrans Vattenbalansavdelning model (HBV), and calibration of a transfer function hydrographseparation model using stable isotope observations. The HBV model allowed quantifying the contributions of snow, soil and groundwater storages compared to the dynamic catchment storage.With the transfer function hydrograph separation model both dynamic and mobile storage wasestimated. Dynamic storage of one catchment estimated by the four methods differed up toone order of magnitude. Nevertheless, the storage estimates ranked similarly among the 21catchments. The largest dynamic and mobile storage estimates were found in high‐elevationcatchments. Besides snow, groundwater contributed considerably to this larger storage.Generally, we found that with increasing elevation the relative contribution to the dynamic catchmentstorage increased for snow, decreased for soil, but remained similar for groundwater storage.