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Glacio‐hydrological model calibration and evaluation

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Abstract Glaciers are essential for downstream water resources. Hydrological modeling is necessary for a better understanding and for future projections of the water resources in these rapidly changing systems, but modeling glacierized catchments is especially challenging. Here we review a wealth of glacio‐hydrological modeling studies (145 publications) in catchments around the world. Major model challenges include a high uncertainty in the input data, mainly precipitation, due to scarce observations. Consequently, the risk of wrongly compensating input with model errors in competing snow and ice accumulation and melt process parameterization is particularly high. Modelers have used a range of calibration and validation approaches to address this issue. The review revealed that while a large part (~35%) of the reviewed studies used only streamflow data to evaluate model performances, most studies (~50%) have used additional data related to snow and glaciers to constrain model parameters. These data were employed in a variety of calibration strategies, including stepwise and multi‐signal calibration. Although the primary aim of glacio‐hydrological modeling studies is to assess future climate change impacts, long‐term changes have rarely been taken into account in model performance evaluations. Overall, a more precise description of which data are used how for model evaluation would facilitate the interpretation of the simulation results and their uncertainty, which in turn would support water resources management. Moreover, there is a need for systematic analyses of calibration approaches to disentangle what works best and why. Addressing this need will improve our system understanding and model simulations of glacierized catchments. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Methods
conceptual glacierized catchment with an ideal measurement network and ideally known fluxes. The catchment has one main glacier and a second smaller glacier (which in reality often lacks observations, in case the main glacier has observations). The catchment has meteorological stations at different elevations and one on the glacier. Snow and ice melt and accumulation are monitored with stakes and snow pits. Ice thickness is measured with ground‐penetrating radar. Streamflow is measured at multiple locations in the catchments, and water samples are taken to analyze the stable water isotopes composition to infer information on the different streamflow contributions (rain, snow, groundwater, and glacier). Several satellite observations are available for information on catchment elevation, glacier volume changes, snow‐covered area and water storage changes (GRACE). In the ideal case, also information of precipitation falling as snow or rain is available, as well as evapotranspiration measurements and groundwater fluxes
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Simulation results obtained by calibrating against different data sources and objective functions. Each calibration was performed five times, the resulting range in streamflow regime of these five parameter sets is shown in (a) the best calibration run is shown with a line, the inset in (a) shows the modeled cumulative volume change of the glacier(s) in the catchment compared to the observed volume change (black) dot. (b) and (c) show comparison between calibration on streamflow (red) and on annual glacier mass balances (blue) regarding annual streamflow (b) and simulated annual mass balances (c). The colors indicate the different calibration options
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Data used in glacio‐hydrological modeling calibration for different catchment glacier cover fractions. The circle size indicates the catchment size and the colors refer to different geographical regions. The numbers on the right‐hand side indicate the number of studies in each category. Q, streamflow‐related data; S, snow‐related data; G, glacier‐related data; HMA, High Mountain Asia; EU, Europe; NAM, North America; SAM, South America; CAU, Caucasus; NZ, New Zealand. Note that each study was assigned to one category, meaning that “testing” studies (one data source against multi‐data source calibration) were classified in the most complex category and if multiple catchments were modeled, the catchment with largest glacier cover fraction was taken
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Overview of calibration approaches that were used in the reviewed studies. The number of studies is given in brackets. The calibration approaches are described by the optimization strategy (first row), data criteria (second row), the calibration technique (third row), and the objective functions used for model calibration (fourth row). Orange arrows indicate the options for stepwise calibration and blue for simultaneous calibration. In case of stepwise optimization strategy, each step is iterated through row three and four. The boxes left show (upper box) data that are used in the reviewed studies for calibration (lower box) objective functions
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Characteristics of the modeled catchments. (a) shows the glacier cover fractions of the catchments in the different regions, (b) the catchment areas in relation to the glacier cover fractions, and (c) the minimum and maximum elevation range from the catchments. The inset in (b) shows catchments larger than 5,000 km2, three catchments that were larger than 200,000 km2 are not plotted. The gray line in (c) shows the 1:1 line; the larger the distance from this line, the larger the elevation range (also indicated by size of circles). The colors indicate the regions. HMA, High Mountain Asia; EU, Europe; NAM, North America; SAM, South America; NZ, New Zealand; CAU, Caucasus are not indicated in (a) because only one study in each of these regions was reviewed. In (b) and (c) NZ is indicated in yellow and CAU in pink
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Science of Water > Hydrological Processes

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