Lukas Gudmundsson

Introducing a probabilistic Budyko framework

Water availability is of importance for a wide range of ecological, climatological, and socioeconomic applications. Over land, the partitioning of precipitation into evapotranspiration and runoff essentially determines the availability of water. At mean annual catchment scales, the widely used Budyko framework provides a simple, deterministic, first-order relationship to estimate this partitioning as a function of the prevailing climatic conditions. Here we extend the framework by introducing a method to specify probabilistic estimates of water availability that account for the nonlinearity of the underlying phase space. The new framework allows to evaluate the predictability of water availability that is related to varying catchment characteristics and conditional on the underlying climatic conditions. Corresponding results support the practical experience of low predictability of river runoff in transitional climates.

Assessing Global Water Storage Variability from GRACE: Trends, Seasonal Cycle, Subseasonal Anomalies and Extremes

Throughout the past decade, the Gravity Recovery and Climate Experiment (GRACE) has given an unprecedented view on global variations in terrestrial water storage. While an increasing number of case studies have provided a rich overview on regional analyses, a global assessment on the dominant features of GRACE variability is still lacking. To address this, we survey key features of temporal variability in the GRACE record by decomposing gridded time series of monthly equivalent water height into linear trends, inter-annual, seasonal, and subseasonal (intra-annual) components. We provide an overview of the relative importance and spatial distribution of these components globally. A correlation analysis with precipitation and temperature reveals that both the inter-annual and subseasonal anomalies are tightly related to fluctuations in the atmospheric forcing. As a novelty, we show that for large regions of the world high-frequency anomalies in the monthly GRACE signal, which have been partly interpreted as noise, can be statistically reconstructed from daily precipitation once an adequate averaging filter is applied. This filter integrates the temporally decaying contribution of precipitation to the storage changes in any given month, including earlier precipitation. Finally, we also survey extreme dry anomalies in the GRACE record and relate them to documented drought events. This global assessment sets regional studies in a broader context and reveals phenomena that had not been documented so far.

Predicting above normal wildfire activity in southern Europe as a function of meteorological drought

Wildfires are a recurrent feature of ecosystems in southern Europe, regularly causing large ecological and socio-economic damages. For efficient management of this hazard, long lead time forecasts could be valuable tools. Using logistic regression, we show that the probability of above normal summer wildfire activity in the 1985–2010 time period can be forecasted as a function of meteorological drought with significant predictability (p ) several months in advance. The results show that long lead time forecasts of this natural hazard are feasible in southern Europe, which could potentially aid decision-makers in the design of strategies for forest management.

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), ...

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.

The sensitivity of water availability to changes in the aridity index and other factors ‐ a probabilistic analysis in the Budyko‐space

One of the pending questions in the context of global change is whether climatic drivers or other factors have stronger influences on water availability. Here we present an approach that allows to estimate the probability that changes in the aridity index have a larger effect on water availability than equal relative changes in other factors. The analysis builds upon a probabilistic extension of the Budyko framework, which is subject to an analytical sensitivity assessment. The results show that changes in water availability are only dominated by changes in the aridity index in very humid climates. This implies that projected intensifications of aridity in drylands may have less influence on water availability than commonly assumed. Instead, other climatic or nonclimatic factors are dominating. The analysis does hence allow to map regions in which water availability is more sensitive to equal relative changes in either the aridity index or all other factors.

Anthropogenic climate change affects meteorological drought risk in Europe

Drought constitutes a significant natural hazard in Europe, impacting societies and ecosystems across the continent. Climate model simulations with increasing greenhouse gas concentrations project increased drought risk in southern Europe, and on the other hand decreased drought risk in the north. Observed changes in water balance components and drought indicators resemble the projected pattern. However, assessments of possible causes of the reported regional changes have so far been inconclusive. Here we investigate whether anthropogenic emissions have altered past and present meteorological (precipitation) drought risk. For doing so we first estimate the magnitude of 20 year return period drought years that would occur without anthropogenic effects on the climate. Subsequently we quantify to which degree the occurrence probability, i.e. the risk, of these years has changed if anthropogenic climate change is accounted for. Both an observational and a climate model-based assessment suggest that it is >95% likely that human emissions have increased the probability of drought years in the Mediterranean, whereas it is >95% likely that the probability of dry years has decreased in northern Europe. In central Europe the evidence is inconclusive. The results highlight that anthropogenic climate change has already increased drought risk in southern Europe, stressing the need to develop efficient mitigation measures.

Methods and model dependency of extreme event attribution: The 2015 European drought

Science on the role of anthropogenic influence on extreme weather events, such as heatwaves or droughts, has evolved rapidly in the past years. The approach of “event attribution” compares the occurrence-probability of an event in the present, factual, climate with its probability in a hypothetical, counterfactual, climate without human-induced climate change. Several methods can be used for event attribution, based on climate model simulations and observations, and usually researchers only assess a subset of methods and data sources. Here, we explore the role of methodological choices for the attribution of the 2015 meteorological summer drought in Europe. We present contradicting conclusions on the relevance of human influence as a function of the chosen data source and event attribution methodology. Assessments using the maximum number of models and counterfactual climates with pre-industrial greenhouse gas concentrations point to an enhanced drought risk in Europe. However, other evaluations show contradictory evidence. These results highlight the need for a multi-model and multi-method framework in event attribution research, especially for events with a low signal-to-noise ratio and high model dependency such as regional droughts.

Correspondence: Flawed assumptions compromise water yield assessment

In their recent study, Zhou et al.1 (hereafter Z15) aim at mapping global patterns of the relative contributions of climate and landcover change on the water yield coefficient. However, implicit and not discussed assumptions on ‘typical changes’ of the considered forcing factors2,3 raise questions on the physical integrity of substantial aspects of their analysis4. In fact, we show here that central findings of Z15 are reversed if physically and mathematically more justified assumptions are accounted for. Z15 analyse partial differentials of a rearranged version of Fu’s equations5,6 that predicts the long-term mean ratio of precipitation (P) and runoff (R), which they refer to as the water yield coefficient, as

A global reconstruction of climate‐driven subdecadal water storage variability

Since 2002, the Gravity Recovery and Climate Experiment (GRACE) mission has provided unprecedented observations of global mass redistribution caused by hydrological processes. However, there are still few sources on pre-2002 global terrestrial water storage (TWS). Classical approaches to retrieve past TWS rely on either land surface models (LSMs) or basin-scale water balance calculations. Here we propose a new approach which statistically relates anomalies in atmospheric drivers to monthly GRACE anomalies. Gridded subdecadal TWS changes and time-dependent uncertainty intervals are reconstructed for the period 1985–2015. Comparisons with model results demonstrate the performance and robustness of the derived data set, which represents a new and valuable source for studying subdecadal TWS variability, closing the ocean/land water budgets and assessing GRACE uncertainties. At midpoint between GRACE observations and LSM simulations, the statistical approach provides TWS estimates (doi:10.5905/ethz-1007-85) that are essentially derived from observations and are based on a limited number of transparent model assumptions.