Search for: All records

Abstract. We develop a new large-scale hydrological and water resources model, theCommunity Water Model (CWatM), which can simulate hydrology both globallyand regionally at different resolutions from 30 arcmin to 30 arcsec atdaily time steps. CWatM is open source in the Python programming environmentand has a modular structure. It uses global, freely available data in thenetCDF4 file format for reading, storage, and production of data in acompact way. CWatM includes general surface and groundwater hydrologicalprocesses but also takes into account human activities, such as water useand reservoir regulation, by calculating water demands, water use, andreturn flows. Reservoirs and lakes are included in the model scheme. CWatMis used in the framework of the Inter-Sectoral Impact Model IntercomparisonProject (ISIMIP), which compares global model outputs. The flexible modelstructure allows for dynamic interaction with hydro-economic and water qualitymodels for the assessment and evaluation of water management options.Furthermore, the novelty of CWatM is its combination of state-of-the-arthydrological modeling, modular programming, an online user manual andautomatic source code documentation, global and regional assessments atdifferent spatial resolutions, and a potential community to add to, change,and expand the open-source project. CWatM also strives to build a communitylearning environment which is able to freely use an open-source hydrologicalmodel and flexible coupling possibilities to other sectoral models, such asenergy and agriculture. 

Abstract. Global water models (GWMs) simulate the terrestrial watercycle on the global scale and are used to assess the impacts of climatechange on freshwater systems. GWMs are developed within different modellingframeworks and consider different underlying hydrological processes, leadingto varied model structures. Furthermore, the equations used to describevarious processes take different forms and are generally accessible onlyfrom within the individual model codes. These factors have hindered aholistic and detailed understanding of how different models operate, yetsuch an understanding is crucial for explaining the results of modelevaluation studies, understanding inter-model differences in theirsimulations, and identifying areas for future model development. This studyprovides a comprehensive overview of how 16 state-of-the-art GWMs aredesigned. We analyse water storage compartments, water flows, and humanwater use sectors included in models that provide simulations for theInter-Sectoral Impact Model Intercomparison Project phase 2b (ISIMIP2b). Wedevelop a standard writing style for the model equations to enhance modelintercomparison, improvement, and communication. In this study, WaterGAP2used the highest number of water storage compartments, 11, and CWatM used 10compartments. Six models used six compartments, while four models (DBH,JULES-W1, Mac-PDM.20, and VIC) used the lowest number, three compartments.WaterGAP2 simulates five human water use sectors, while four models (CLM4.5,CLM5.0, LPJmL, and MPI-HM) simulate only water for the irrigation sector. Weconclude that, even though hydrological processes are often based on similarequations for various processes, in the end these equations have beenadjusted or models have used different values for specific parameters orspecific variables. The similarities and differences found among the modelsanalysed in this study are expected to enable us to reduce the uncertaintyin multi-model ensembles, improve existing hydrological processes, andintegrate new processes. 

Abstract. Empirical evidence demonstrates that lakes and reservoirs are warming acrossthe globe. Consequently, there is an increased need to project futurechanges in lake thermal structure and resulting changes in lakebiogeochemistry in order to plan for the likely impacts. Previous studies ofthe impacts of climate change on lakes have often relied on a single modelforced with limited scenario-driven projections of future climate for arelatively small number of lakes. As a result, our understanding of theeffects of climate change on lakes is fragmentary, based on scatteredstudies using different data sources and modelling protocols, and mainlyfocused on individual lakes or lake regions. This has precludedidentification of the main impacts of climate change on lakes at global andregional scales and has likely contributed to the lack of lake water qualityconsiderations in policy-relevant documents, such as the Assessment Reportsof the Intergovernmental Panel on Climate Change (IPCC). Here, we describe asimulation protocol developed by the Lake Sector of the Inter-SectoralImpact Model Intercomparison Project (ISIMIP) for simulating climate changeimpacts on lakes using an ensemble of lake models and climate changescenarios for ISIMIP phases 2 and 3. The protocol prescribes lakesimulations driven by climate forcing from gridded observations anddifferent Earth system models under various representative greenhouse gasconcentration pathways (RCPs), all consistently bias-corrected on a0.5∘ × 0.5∘ global grid. In ISIMIP phase 2, 11 lakemodels were forced with these data to project the thermal structure of 62well-studied lakes where data were available for calibration underhistorical conditions, and using uncalibrated models for 17 500 lakesdefined for all global grid cells containing lakes. In ISIMIP phase 3, thisapproach was expanded to consider more lakes, more models, and moreprocesses. The ISIMIP Lake Sector is the largest international effort toproject future water temperature, thermal structure, and ice phenology oflakes at local and global scales and paves the way for future simulations ofthe impacts of climate change on water quality and biogeochemistry in lakes. 

Anthropogenic climate change is expected to affect global river flow. Here, we analyze time series of low, mean, and high river flows from 7250 observatories around the world covering the years 1971 to 2010. We identify spatially complex trend patterns, where some regions are drying and others are wetting consistently across low, mean, and high flows. Trends computed from state-of-the-art model simulations are consistent with the observations only if radiative forcing that accounts for anthropogenic climate change is considered. Simulated effects of water and land management do not suffice to reproduce the observed trend pattern. Thus, the analysis provides clear evidence for the role of externally forced climate change as a causal driver of recent trends in mean and extreme river flow at the global scale. 

Abstract Carbon fluxes at the land‐atmosphere interface are strongly influenced by weather and climate conditions. Yet what is usually known as “climate extremes” does not always translate into very high or low carbon fluxes or so‐called “carbon extremes.” To reveal the patterns of how climate extremes influence terrestrial carbon fluxes, we analyzed the interannual variations in ecosystem carbon fluxes simulated by the Terrestrial Biosphere Models (TBMs) in the Inter‐Sectoral Impact Model Intercomparison Project. At the global level, TBMs simulated reduced ecosystem net primary productivity (NPP; 18.5 ± 9.3 g C m⁻² yr⁻¹), but enhanced heterotrophic respiration (Rh; 7 ± 4.6 g C m⁻² yr⁻¹) during extremely hot events. TBMs also simulated reduced NPP (60.9 ± 24.4 g C m⁻² yr⁻¹) and reduced Rh (16.5 ± 11.4 g C m⁻² yr⁻¹) during extreme dry events. Influences of precipitation extremes on terrestrial carbon uptake were larger in the arid/semiarid zones than other regions. During hot extremes, ecosystems in the low latitudes experienced a larger reduction in carbon uptake. However, a large fraction of carbon extremes did not occur in concert with either temperature or precipitation extremes. Rather these carbon extremes are likely to be caused by the interactive effects of the concurrent temperature and precipitation anomalies. The interactive effects showed considerable spatial variations with the largest effects on NPP in South America and Africa. Additionally, TBMs simulated a stronger sensitivity of ecosystem productivity to precipitation than satellite estimates. This study provides new insights into the complex ecosystem responses to climate extremes, especially the emergent properties of carbon dynamics resulting from compound climate extremes.