Publications by authors named "Walter Immerzeel"

The Asian water tower (AWT) serves as the source of 10 major Asian river systems and supports the lives of ~2 billion people. Obtaining reliable precipitation data over the AWT is a prerequisite for understanding the water cycle within this pivotal region. Here, we quantitatively reveal that the "observed" precipitation over the AWT is considerably underestimated in view of observational evidence from three water cycle components, namely, evapotranspiration, runoff, and accumulated snow.

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The Langtang catchment is a high mountain, third order catchment in the Gandaki basin in the Central Himalaya (28.2°N, 85.5°E), that eventually drains into the Ganges.

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Siloed-approaches may fuel the misguided development of hydropower and subsequent target-setting under the sustainable development goals (SDGs). While hydropower development in the Indus basin is vital to ensure energy security (SDG7), it needs to be balanced with water use for fulfilling food (SDG2) and water (SDG6) security. Existing methods to estimate hydropower potential generally focus on: only one class of potential, a methodological advance for either of hydropower siting, sizing, or costing of one site, or the ranking of a portfolio of projects.

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The land ice contribution to global mean sea level rise has not yet been predicted using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models, but primarily used previous-generation scenarios and climate models, and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios using statistical emulation of the ice sheet and glacier models.

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Glaciers are of key importance to freshwater supplies in the Himalayan region. Their growth or decline is among other factors determined by an interaction of 2-m air temperature (TAS) and precipitation rate (PR) and thereof derived positive degree days (PDD) and snow and ice accumulation (SAC). To investigate determining factors in climate projections, we use a model ensemble consisting of 36 CMIP5 general circulation models (GCMs) and 13 regional climate models (RCMs) of two Asian CORDEX domains for two different representative concentration pathways (RCP4.

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The riverine sediment flux (SF) is an essential pathway for nutrients and pollutants delivery and considered as an important indicator of land degradation and environment changes. With growing interest in environmental changes over the Tibetan Plateau (TP), this work investigated the variation of the SF in response to climate change in the headwater of the Yangtze River over the past 30 years. Annual time series of hydro-meteorological variables during 1986-2014 indicate significantly increasing trends of air temperature, precipitation, ground temperature, river discharge, suspended sediment concentration and SF.

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Melting snow and glacier ice in the Himalaya forms an important source of water for people downstream. Incoming longwave radiation (LW) is an important energy source for melt, but there are only few measurements of LW at high elevation. For the modelling of snow and glacier melt, the LW is therefore often represented by parameterizations that were originally developed for lower elevation environments.

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Glaciers around the world are shrinking, yet in a region in northwestern High Mountain Asia (HMA), glaciers show growth. A proposed explanation for this anomalous behavior is related to the variability of the "Western Tibetan Vortex" (WTV), which correlates well with near-surface temperatures in northwestern HMA. Using analytical formulations and ERA5 reanalysis data, we show that the WTV is the change of wind field resulting from changes in near-surface temperature gradients in geostrophic flow and that it is not unique to northwestern HMA.

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Article Synopsis
  • Global glacier shrinkage is a major indicator of climate change, but the Karakoram region shows different patterns, including stable or slightly positive glacier conditions.
  • Unlike most of High Mountain Asia that is experiencing glacier retreat, the Karakoram exhibits stable or advancing glaciers and increased ice flow speeds.
  • While various theories attempt to explain this "Anomaly" in glacier behavior, its long-term continuation is uncertain due to expected warming trends in the future.
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Accurate snow depth observations are critical to assess water resources. More than a billion people rely on water from snow, most of which originates in the Northern Hemisphere mountain ranges. Yet, remote sensing observations of mountain snow depth are still lacking at the large scale.

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Temperature index (TI) models are convenient for modelling glacier ablation since they require only a few input variables and rely on simple empirical relations. The approach is generally assumed to be reliable at lower elevations (below 3500 m above sea level, a.s.

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Many glaciers in the northwest of High Mountain Asia (HMA) show an almost zero or positive mass balance, despite the global trend of melting glaciers. This phenomenon is often referred to as the "Karakoram anomaly," although strongest positive mass balances can be found in the Kunlun Shan mountain range, northeast of the Karakoram. Using a regional climate model, in combination with a moisture-tracking model, we show that the increase in irrigation intensity in the lowlands surrounding HMA, particularly in the Tarim basin, can locally counter the effects of global warming on glaciers in Kunlun Shan, and parts of Pamir and northern Tibet, through an increase in summer snowfall and decrease in net radiance.

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Future hydrological extremes, such as floods and droughts, may pose serious threats for the livelihoods in the upstream domains of the Indus, Ganges, Brahmaputra. For this reason, the impacts of climate change on future hydrological extremes is investigated in these river basins. We use a fully-distributed cryospheric-hydrological model to simulate current and future hydrological fluxes and force the model with an ensemble of 8 downscaled General Circulation Models (GCMs) that are selected from the RCP4.

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Mountain ranges are the world's natural water towers and provide water resources for millions of people. However, their hydrological balance and possible future changes in river flow remain poorly understood because of high meteorological variability, physical inaccessibility, and the complex interplay between climate, cryosphere, and hydrological processes. Here, we use a state-of-the art glacio-hydrological model informed by data from high-altitude observations and the latest climate change scenarios to quantify the climate change impact on water resources of two contrasting catchments vulnerable to changes in the cryosphere.

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The analysis of climate change impact on the hydrology of high altitude glacierized catchments in the Himalayas is complex due to the high variability in climate, lack of data, large uncertainties in climate change projection and uncertainty about the response of glaciers. Therefore a high resolution combined cryospheric hydrological model was developed and calibrated that explicitly simulates glacier evolution and all major hydrological processes. The model was used to assess the future development of the glaciers and the runoff using an ensemble of downscaled climate model data in the Langtang catchment in Nepal.

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More than 1.4 billion people depend on water from the Indus, Ganges, Brahmaputra, Yangtze, and Yellow rivers. Upstream snow and ice reserves of these basins, important in sustaining seasonal water availability, are likely to be affected substantially by climate change, but to what extent is yet unclear.

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