Peter Molnar

Last Name

Molnar

First Name

Peter

ORCID

0000-0001-6437-4931

Organisational unit

09868 - Passalacqua, Paola / Passalacqua, Paola

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Publications1 - 10 of 127

Understanding flood regime changes in Europe: a state-of-the-art assessment
Hall, Julia; Arheimer, Berit; Borga, Marco; et al. (2014)
Hydrology and Earth System Sciences
There is growing concern that flooding is becoming more frequent and severe in Europe. A better understanding of flood regime changes and their drivers is therefore needed. The paper reviews the current knowledge on flood regime changes in European rivers that has traditionally been obtained through two alternative research approaches. The first approach is the data-based detection of changes in observed flood events. Current methods are reviewed together with their challenges and opportunities. For example, observation biases, the merging of different data sources and accounting for nonlinear drivers and responses. The second approach consists of modelled scenarios of future floods. Challenges and opportunities associated with flood change scenarios are discussed such as fully accounting for uncertainties in the modelling cascade and feedbacks. To make progress in flood change research, we suggest that a synthesis of these two approaches is needed. This can be achieved by focusing on long duration records and flood-rich and flood-poor periods rather than on short duration flood trends only, by formally attributing causes of observed flood changes, by validating scenarios against observed flood regime dynamics, and by developing low-dimensional models of flood changes and feedbacks. The paper finishes with a call for a joint European flood change research network.
Suspended sediment transport in a river network: testing signal propagation and modelling approaches
Agostini, Ludovico; Demmel, Sophia; Garipova, Sofia; et al. (2024)
EGUsphere
Using high-resolution stochastic climate ensembles to model the impacts and uncertainty of hydrology in mountainous catchments
Moraga, Jorge Sebastian; Peleg, Nadav; Molnar, Peter; et al. (2022)
IAHS Scientific Assembly 2022
Hydrological projections in the context of a changing climate may display high levels of uncertainty, particularly when examining small temporal and spatial scales. To project the response of hydrological processes to the increasing global temperatures, scientists and practitioners rely on chains of numerical models, each contributing some degree of uncertainty to the overall outputs. Furthermore, the randomness intrinsic to climate phenomena, known as internal climate variability, contributes to the uncertainty of the hydrological projections in the form of an irreducible stochasticity. In this work, we quantify the impacts and partition the uncertainty of hydrological processes emerging from climate models and internal variability for two mountainous catchments in the Swiss Alps and across a broad range of scales. To that end, we used high-resolution ensembles of climate and hydrological data produced using a two-dimensional stochastic weather generator (AWE-GEN-2d) and a distributed hydrological model (Topkapi-ETH). We quantified the uncertainty in hydrological projections towards the end of the century through the estimation of the values of signal-to-noise ratios (STNR). We found small STNR values (<-1) in the projection of annual streamflow for most sub-catchments in both study sites that are dominated by the large natural variability of precipitation (explains ~70% of total uncertainty). Furthermore, we investigated specific hydrological components that are critical in the model chain with detail. For example, snowmelt or liquid precipitation exhibits robust change signals, which translates into high STNR values for streamflow during warm seasons and at higher elevations, together with a larger contribution of climate model uncertainty, suggesting that an improvement of the involved models has the potential of significantly narrowing the uncertainty. In contrast, extreme flows show low STNR values due to large internal climate variability across all elevations, which limits the possibility of narrowing their estimation uncertainty due to a warming climate. This study demonstrates that high-resolution hydro-climatic ensembles enable the quantification of hydrological projections across spatial and temporal scales, which can be used to assess the potential for narrowing hydrological uncertainties.
Sediment Supply Effects in Hydrology-Sediment Modeling of an Alpine Basin
Battista, Giulia; Schlunegger, Fritz; Burlando, Paolo; et al. (2022)
Water Resources Research
In mountain river basins, sediment availability on hillslopes and in channels is key to predict the sediment response to hydrological forcing. However, quantification of sediment availability and its variability in time is challenging, because sediment supply is often strongly stochastic and dominated by mass wasting. In this paper, we introduce a variable landslide sediment supply as a function of topography, hydrology, and hillslope activity in the hydrology-sediment model TOPKAPI-ETH. We use the model to analyze the dynamics of sediment storage in a mesoscale pre-Alpine basin. We simulate a range of transport- and supply-limited conditions to quantify the variability of suspended sediment concentrations and load, and the seasonal dynamics of sediment storage. We show that supply limitation dampens the natural variability of the hydrological and sediment transport processes, and therefore reduces the scatter of the suspended sediment rating curve. By comparing the model results with observations, we demonstrate that alternation of low and high sediment availability favors sediment load variability at the outlet. The temporal dynamics of sediment storage depends on the hillslope activity, and the balance between sediment supply by landslides and evacuation by runoff. When the export flux is comparable or greater than the recharge flux, the storage shows seasonal fluctuations with a maximum in late winter or spring, and sediment starvation in summer. By representing the dynamics of both inputs and outputs of the sediment storage, the proposed model provides a physically based tool to isolate the effect of transport- and supply-limited conditions in basin sediment response.
Making stratigraphy in the Anthropocene: climate change impacts and economic conditions controlling the supply of sediment to Lake Geneva
Lane, Stuart N.; Bakker, Maarten; Costa, Anna; et al. (2019)
Scientific Reports
The Anthropocene has been proposed as a profound, globally synchronous rupture in the history of the Earth System with its current state fundamentally different to that of the Holocene and driven by the geological force of human activity. Here, we show how stratigraphy is being made in a lake that is heavily impacted upon by climate change and human activities. For one of the largest inner-Alpine catchments in the European Alps, we draw attention to how sedimentation rates are a product of non-stationary, reflexive, human actions. In Lake Geneva, we identify both a human-induced climate change (HCC) signature and the effects of a recent economic shock on sediment extraction upon sediment loading to and sedimentation rates in the lake. The HCC signature thus reflects the nature of climate change impacts in this basin, where sediment accumulation rates evolve with climate, but where economic conditions contribute to shifts in the supply of sediment to the lake. Following social theory, we call this glocalization because of the combined importance and inseparability of human impacts across different spatial scales. The nature of human impacts on sediment delivery to the lake mean that the influence of humans is unlikely to be captured in the long-term depositional record.
Uncertainty partition challenges the predictability of vital details of climate change
Fatichi, Simone; Ivanov, Valeriy Y.; Paschalis, Athanasios; et al. (2016)
Earth's Future
Decision makers and consultants are particularly interested in “detailed” information on future climate to prepare adaptation strategies and adjust design criteria. Projections of future climate at local spatial scales and fine temporal resolutions are subject to the same uncertainties as those at the global scale but the partition among uncertainty sources (emission scenarios, climate models, and internal climate variability) remains largely unquantified. At the local scale, the uncertainty of the mean and extremes of precipitation is shown to be irreducible for mid and end‐of‐century projections because it is almost entirely caused by internal climate variability (stochasticity). Conversely, projected changes in mean air temperature and other meteorological variables can be largely constrained, even at local scales, if more accurate emission scenarios can be developed. The results were obtained by applying a comprehensive stochastic downscaling technique to climate model outputs for three exemplary locations. In contrast with earlier studies, the three sources of uncertainty are considered as dependent and, therefore, non‐additive. The evidence of the predominant role of internal climate variability leaves little room for uncertainty reduction in precipitation projections; however, the inference is not necessarily negative, because the uncertainty of historic observations is almost as large as that for future projections with direct implications for climate change adaptation measures.
Simulating the Effects of Flow Regulation and Climate Change on Riparian Vegetation in an Alpine Gravel-Bed Braided River
Bergami, Gianluca; Molnar, Peter; Burlando, Paolo (2019)
AGU Fall Meeting Abstracts
Non-uniqueness in sediment transport in river network hydrology-sediment modelling
Molnar, Peter; Meierhans, Sascha; Battista, Giulia; et al. (2024)
EGUsphere
Gridded daily 2-m air temperature dataset for Ethiopia derived by debiasing and downscaling ERA5-Land for the period 1981–2010
Wakjira, Mosisa T.; Peleg, Nadav; Burlando, Paolo; et al. (2023)
Data in Brief
A gridded maximum and minimum (Tx and Tn) daily temperature dataset derived by spatial downscaling and bias correction of the ERA5-Land (ERA5L) for the period 1981–2010 is presented. Observed daily Tx and Tn at 154 stations in Ethiopia covering record lengths of 5–30 years were used as a reference. The statistics that define the Gaussian distribution (mean and standard deviation) of Tx and Tn from the station observations were interpolated in space to create a monthly climatology and interannual statistics at 0.05° × 0.05° resolution using a hybrid interpolation approach that combines linear regression with topographic and location attributes, and non-Euclidean inverse distance weighting interpolation. The interpolated monthly and interannual statistics were then used to debias the ERA5L Tx and Tn using a quantile mapping approach. Leave-one-out cross-validation showed that the mean absolute errors in the corrected and downscaled daily temperatures are about 0.7 °C for Tx and 1.1 °C for Tn, reducing the statistical biases in the ERA5L Tx and Tn by 68% and 25% respectively. For monthly climatology, 40–64% of the biases were removed for Tx while for Tn the reductions range from 19% to 32%. The correction also improved commonly used indices for extremes like the probability of warm days, cold days, and warm nights, but overestimated the probability of cold nights. The presented open-access Tx and Tn dataset is a substantial improvement over existing gridded temperature datasets for Ethiopia, such as ERA5L and the Climate Hazards Infrared Temperature with Station (CHIRTS), and we suggest it is suitable for a wide range of environmental applications, e.g. in the fields of hydrology, agriculture, and ecology.
Spatial organization in the step-pool structure of a steep mountain stream (Vogelbach, Switzerland)
Milzow, Christian; Molnar, Peter; McArdell, Brian W.; et al. (2006)
Water Resources Research
Spatial organization in the step-pool structure of a 1.58 km long section of the Vogelbach, a steep mountain stream (average channel bed slope 0.17 m/m) in the Alpthal basin, Switzerland, is analyzed to evaluate the extent of randomness in the occurrence of steps and to isolate the potential hydraulic and topographic controls on step placement. Statistics of the observed step sequence are compared with those of random sequences generated by permutation from the observed step height distribution. Hydraulic and topographic controls on step placement are analyzed from step length and steepness distributions and relations between mean step properties and step height and channel gradient for both observed and random sequences. Results show that (1) observed step length distributions are statistically significantly different from randomly generated sequences, (2) step steepness is significantly different in observed data because of a positive correlation between mean step length and height and it remains fairly constant for all step sizes, (3) spatial organization in steps does not extend far beyond the nearest step, and (4) the influence of channel gradient on step properties is insignificant and highly variable, indicating that hydraulic rather than topographic controls are dominant for step placement in this stream. Although the Vogelbach is a steep stream on the boundary between a cascading and step-pool morphology where we would expect randomness to dominate, spatial organization was nevertheless detected in many important aspects of the step-pool geometry.

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