Changes In Hydrological Cycle Research Articles

Runoff Change Characteristics and Response to Climate Variability and Human Activities Under a Typical Basin of Natural Tropical Rainforest Converted to Monoculture Rubber Plantations

Climate variability and human activities are major influences on the hydrological cycle. However, the driving characteristics of hydrological cycle changes and the potential impact on runoff in areas where natural forests have been converted to rubber plantations on a long-term scale remain unclear. Based on this, the Mann–Kendall (MK) and Pettitt breakpoint tests and the Double Mass Curve method were employed to identify the variation characteristics and breakpoints of precipitation (P), potential evapotranspiration (ET0), and runoff depth (R) in the Wanquan River Basin (WQRB) during the 1970–2016 period. The changes in runoff attributed to P, ET0, and the catchment characteristics parameter (n) were quantified using the elastic coefficient method based on the Budyko hypothesis. The results revealed that the P and R in the WQRB exhibited statistically insignificant decreasing trends, while ET0 displayed a significant increasing trend (p < 0.05). The breakpoint of runoff changes in the Jiabao and the Jiaji stations occurred in 1991 and 1983, respectively. The runoff changes show a negative correlation with both the n and ET0, while exhibiting a positive correlation with P. Moreover, it is observed that P and ET0 display higher sensitivity towards runoff changes compared to n. The decomposition analysis reveals that in the Dingan River Basin (DARB), human activities account for 53.54% of the runoff changes, while climate variability contributes to 46.46%. In the Main Wanquan River Basin (MWQRB), human activities contribute to 46.11%, whereas climate variability accounts for 53.89%. The research findings suggest that runoff is directly reduced by climate variability (due to decreased P and increased ET0), while human activities indirectly contribute to changes in runoff through n, exacerbating its effects. Rubber forest stands as the prevailing artificial vegetation community within the WQRB. The transformation of natural forests into rubber plantations constitutes the primary catalyst for the alteration of n in the WQRB. The research findings provide important reference for quantifying the driving force of hydrological changes caused by deforestation, which is of great significance for sustainable management of forests and water resources.

Biological and dust aerosols as sources of ice-nucleating particles in the eastern Mediterranean: source apportionment, atmospheric processing and parameterization

Abstract. Aerosol–cloud interactions in mixed-phase clouds (MPCs) are one of the most uncertain drivers of the hydrological cycle and climate change. A synergy of in situ, remote-sensing and modelling experiments were used to determine the source of ice-nucleating particles (INPs) for MPCs at Mount Helmos in the eastern Mediterranean. The influences of boundary layer turbulence, vertical aerosol distributions and meteorological conditions were also examined. When the observation site is in the free troposphere (FT), approximately 1 in ×106 aerosol particles serve as INPs around −25 °C. The INP abundance spans 3 orders of magnitude and increases in the following order: marine aerosols; continental aerosols; and, finally, dust plumes. Biological particles are important INPs observed in continental and marine aerosols, whereas they play a secondary, although important, role during Saharan dust events. Air masses in the planetary boundary layer (PBL) show both enriched INP concentrations and a higher proportion of INPs to total aerosol particles, compared with cases in the FT. The presence of precipitation/clouds enriches INPs in the FT but decreases INPs in the PBL. Additionally, new INP parameterizations are developed that incorporate the ratio of fluorescent-to-nonfluorescent or coarse-to-fine particles and predict >90 % of the observed INPs within an uncertainty range of a factor of 10; these new parameterizations exhibit better performance than current widely used parameterizations and allow ice formation in models to respond to variations in dust and biological particles. The improved parameterizations can help MPC formation simulations in regions with various INP sources or different regions with prevailing INP sources.

Spatio-temporal Dynamics of Land Use/Cover Change and Associated Carbon Stocks in Kanyabaha Wetland in Rukiga District, Uganda

Wetlands play an important ecological function of sequestering atmospheric carbon dioxide and thereby moderating adverse impacts of climate change. It is therefore important to understand the dynamics of carbon stocks in wetland vegetation and soils. This study investigated the spatio-temporal dynamics of aboveground, belowground, and total carbon stocks in Kanyabaha Wetland, located in Rukiga District, Uganda, spanning from 1990 to 2021. Through field sampling and laboratory analysis, aboveground carbon stocks were assessed by harvesting vegetation biomass and converting it to carbon stock using established conversion factors. Soil samples collected at different depths (0-20cm, 20-50cm, 50-100cm) were analyzed for soil organic carbon content to determine belowground carbon stocks. The study reveals variable spatio-temporal patterns of carbon stocks across land use types, with papyrus-dominated areas exhibiting the highest aboveground carbon stocks (49.66 tC/ha), followed by small-scale farmlands (33.73 tC/ha) and tree plantations (23.01 tC/ha). Conversely, built-up areas exhibit the lowest carbon stocks (1.29 tC/ha). Temporal analysis reveals fluctuating patterns in carbon stocks, with increases observed in built-up areas and small-scale farmlands, and decreases in grasslands and tree plantations that could be due to changes in hydrological cycle. Belowground carbon stocks follow similar trends, with papyrus areas maintaining the highest stocks (39.96 tC/ha), particularly at deeper soil depths that exhibit the highest carbon accumulation due to its extensive network of papyrus rhizome. Changes in land use, especially reclamation of the wetlands for farming and settlements affected carbon capture and storage in the wetland ecosystem. These findings highlight the importance of targeted conservation of natural wetlands and sustainable land management strategies in the Kanyabaha Wetland catchment for enhanced carbon sequestration. Further, in depth studies in the variability of carbon stocks due to various eco-climatic factors and anthropogenic activities are necessary to support sustainable wetland land management practices in Uganda.

Estimating freshwater flux amplification with ocean tracers via linear response theory

Abstract. Accurate estimation of changes in the global hydrological cycle over the historical record is important for model evaluation and understanding future trends. Freshwater flux trends cannot be accurately measured directly, so quantification of change often relies on ocean salinity trends. However, anthropogenic forcing has also induced ocean transport change, which imprints on salinity. We find that this ocean transport affects the surface salinity of the saltiest regions (the subtropics) while having little impact on the surface salinity in other parts of the globe. We present a method based on linear response theory which accounts for the regional impact of ocean circulation changes while estimating freshwater fluxes from ocean tracers. Testing on data from the Community Earth System Model large ensemble, we find that our method can recover the true amplification of freshwater fluxes, given thresholded statistical significance values for salinity trends. We apply the method to observations and conclude that from 1975–2019, the hydrological cycle has amplified by 5.04±1.27 % per degree Celsius of surface warming.

High-resolution reconstruction and correction of FY-4A precipitable water vapor in China using back propagation neural network

Precipitable water vapor (PWV) with high spatial and temporal resolution is essential for a deeper understanding of the study area's hydrological cycle and climate change. This study uses the European Center for Medium-Range Weather Forecasts Reanalysis 5 (ERA5) PWV as auxiliary verification data and proposes a reconstruction method based on back propagation neural network (BPNN) downscaling. Combining with high temporal resolution meteorological elements, digital elevation model (DEM), location, and time, reconstructed the Fengyun-4A (FY-4A) PWV data set with high spatial and temporal resolution in China was obtained. Subsequently, using the characteristics of FY-4A PWV with DEM, location, and time, a BPNN-based correction method is proposed to correct the reconstructed FY-4A PWV using the high-precision global navigation satellite system (GNSS) PWV. The results showed that the root mean square error (RMSE), Bias, and Pearson correlation coefficient (R) of the reconstructed model with respect to the FY-4A PWV are 1.32 mm, 0 mm, and 0.99, respectively. Compared with the auxiliary verification data ERA5 PWV, the RMSE, Bias, and R of the reconstructed FY-4A PWV are 0.85 mm, 0.18 mm, and 0.99, respectively. Compared with the GNSS PWV, the RMSE of the corrected FY-4A PWV is 2.44 mm, which is 28.86% lower than that of the reconstructed FY-4A PWV, the Bias is improved from −1.46 mm to 0 mm, and the R is enhanced from 0.98 to 0.99. The reconstruction and correction method of FY-4A PWV with high spatial and temporal resolution proposed in this study dramatically improves the completeness and accuracy of FY-4A PWV in China. It lays the foundation for studying rapid changes in China's hydrological cycle and climate.

Paleo±Dust: quantifying uncertainty in paleo-dust deposition across archive types

Abstract. Mineral dust aerosol concentrations in the atmosphere varied greatly on glacial–interglacial timescales. The greatest changes in global dust activity occurred in response to changes in orbital parameters (which affect dust emission intensity through glacial activity) and the lifetime of dust in the atmosphere (caused by changes in the global hydrological cycle). Long-term changes in the surface dust deposition rate are registered in geological archives such as loess, peats, lakes, marine sediments, and ice. Data provided by these archives are crucial for guiding simulations of dust and for better understanding the natural global dust cycle. However, the methods employed to derive paleo-dust deposition rates differ markedly between archives and are subject to different sources of uncertainty. Here, we present Paleo±Dust, an updated compilation of bulk and <10 µm paleo-dust deposition rates with quantitative 1σ uncertainties that are inter-comparable among archive types. Paleo±Dust incorporates a total of 285 pre-industrial Holocene (pi-HOL) and 209 Last Glacial Maximum (LGM) dust flux constraints from studies published until December 2022, including, for the first time, peat records. We also recalculate previously published dust fluxes to exclude data from the last deglaciation and thus obtain more representative constraints for the last pre-industrial interglacial and glacial end-member climate states. Based on Paleo±Dust, the global LGM:pi-HOL ratio of <10 µm dust deposition rates is 3.1 ± 0.7 (1σ). We expect Paleo±Dust to be of use for future paleoclimate dust studies and simulations using Earth system models of high to intermediate complexity. Paleo±Dust is publicly accessible at https://doi.org/10.1594/PANGAEA.962969 (Cosentino et al., 2024).

Retrieval of Surface Energy Fluxes Considering Vegetation Changes and Aerosol Effects

The exchange of moisture and energy between the land and the atmosphere plays a crucial role in terrestrial hydrological cycle and climate change. However, existing studies on the retrieval of surface water and heat flux tend to overlook the dynamic changes in surface vegetation and atmospheric aerosols, which directly affect surface energy and indirectly alter various meteorological factors, including cloud, precipitation, and temperature. In this study, we assess the machine-learning retrieval method for surface fluxes that takes into account vegetation changes and aerosol effects, using FLUXNET observations and remote sensing data to retrieve latent heat flux (LE) and sensible heat flux (H). We constructed four sets of deep neural network models: (a) The first set considers only meteorological factors, (b) the second set considers meteorological factors and aerosols, (c) the third set considers meteorological factors and vegetation changes, and (d) the fourth set comprehensively considers meteorological factors, aerosols, and vegetation changes. All model performances were evaluated using statistical indicators. ERA5 reanalysis and remote sensing data were used to drive the models and retrieve daily H and LE. The retrieved results were validated against ground observation sites that were not involved in model training or the FLUXCOM product. The results show that the model that considers meteorological factors, aerosols, and vegetation changes has the smallest errors and highest correlation for retrieving H and LE (RH = 0.85, RMSEH = 24.88; RLE = 0.88, RMSELE = 22.25). The ability of the four models varies under different vegetation types. In terms of seasons, the models that consider meteorological factors and vegetation changes, as well as those that comprehensively consider meteorological factors, aerosols, and vegetation changes, perform well in retrieving the surface fluxes. As for spatial distribution, when atmospheric aerosols are present in the region, the model that considers both meteorological factors and aerosols retrieves higher values of H compared to the model that considers only meteorological factors, while the LE values are relatively lower. The model that considers meteorological factors and vegetation changes, as well as the model that comprehensively considers meteorological factors, aerosols, and vegetation changes, retrieves lower values in most regions. Through the validation of independent observation sites and FLUXCOM products, we found that the model, considering meteorological factors, aerosols, and vegetation changes, was generally more accurate in the retrieval of surface fluxes. This study contributes to improving the retrieval and future prediction accuracy of surface fluxes in a changing environment.

Improved RepVGG ground-based cloud image classification with attention convolution

Abstract. Atmospheric clouds greatly impact Earth's radiation, hydrological cycle, and climate change. Accurate automatic recognition of cloud shape based on a ground-based cloud image is helpful for analyzing solar irradiance, water vapor content, and atmospheric motion and then predicting photovoltaic power, weather trends, and severe weather changes. However, the appearance of clouds is changeable and diverse, and their classification is still challenging. In recent years, convolution neural networks (CNNs) have made great progress in ground-based cloud image classification. However, traditional CNNs poorly associate long-distance clouds, making the extraction of global features of cloud images quite problematic. This study attempts to mitigate this problem by elaborating on a ground-based cloud image classification method based on the improved RepVGG convolution neural network and attention mechanism. Firstly, the proposed method increases the RepVGG residual branch and obtains more local detail features of cloud images through small convolution kernels. Secondly, an improved channel attention module is embedded after the residual branch fusion, effectively extracting the global features of cloud images. Finally, the linear classifier is used to classify the ground cloud images. Finally, the warm-up method is applied to optimize the learning rate in the training stage of the proposed method, making it lightweight in the inference stage and thus avoiding overfitting and accelerating the model's convergence. The proposed method is validated on the multimodal ground-based cloud dataset (MGCD) and the ground-based remote sensing cloud database (GRSCD) containing seven cloud categories, with the respective classification accuracy rate values of 98.15 % and 98.07 % outperforming those of the 10 most advanced methods used as the reference. The results obtained are considered instrumental in ground-based cloud image classification.

Does ERA5-land capture the changes in the terrestrial hydrological cycle across the globe?

Changes in the terrestrial hydrological cycle determine the future water availability across the globe with profound impacts in different facets of society. Precise estimation of such changes is vital for the effective implementation of water management policies. Among the numerous data products that describe the hydrological cycle components, ERA5-Land is one of the most increasingly used dataset. Still, there has been no assessment of its ability capacity to represent the water cycle shifts variability over land. This study endeavors to bridge this gap by comparing the magnitude and direction of change in precipitation minus evaporation (P—E) and runoff, as estimated globally by the ERA5-Land data product. Our findings reveal significant inconsistencies in the changes identified, with the climatological mean of P—E decreasing more substantially than runoff for numerous regions. Consequently, ERA5 presents a declining water availability for most of the regions, but the magnitude of change is incompatible to the change between P—E and runoff. To further validate, the estimates provided by the ERA5-Land product, two different hydrologic models (TerraClimate and Global Land Data Assimilation System, GLDAS-Noah) are also utilized. TerraClimate demonstrates a more reasonable alignment between changes in P—E and runoff, followed by GLDAS-Noah, particularly for the arid regions lying in the parts of Northern Africa and Southern Asia, the European continent, and the northern parts of Asia. Inconsistencies remain high for the tropical regions for both data products. Still, the estimates of change in water availability are better represented by the hydrologic model-based data sources for most parts of the globe, especially for the regions with low precipitation, such as the regions with arid and continental climates. Our results imply that ERA5-Land should be used with extreme caution when assessing the long-term changes in the terrestrial water cycle. Additionally, pinpointing the regions of the highest bias can help to improve the hydrological coupling of ERA5-Land in future versions of the reanalysis.

Uncertainty of the Simulated Mid-Pliocene Changes of Sahel Summer Rainfall in the PlioMIP2 Multimodel Ensemble

Abstract The mid-Pliocene (approximately 3.3–3.0 Ma) was one of the past geological warm periods. Since this past warmer climate was in many respects comparable to near future warming projections, how the regional monsoonal rainfall changed during the mid-Pliocene is an important scientific and socioeconomic concern. Based on phase 2 of the Pliocene Model Intercomparison Project (PlioMIP2), this work examines the simulated changes of Sahel summer rainfall during the mid-Pliocene. The results show the considerable intermodel uncertainty of the simulated mid-Pliocene Sahel rainfall changes in the PlioMIP2 multimodel ensemble, which is due to the uncertainties of both the simulated dynamic and thermodynamic responses to this past warmer climate. In particular, we find that the intermodel spread in the simulated northern North American warming is a major source of the uncertainty of the mid-Pliocene Sahel summer rainfall changes through two processes. One is a direct dynamic process: a stronger warming over northern North America could enhance the meridional temperature gradient between the extratropical and tropical regions, inducing an interhemisphere energy imbalance of the atmosphere. This could lead to a northward shift of the intertropical convergence zone, strengthening the Western African summer monsoon (WASM) circulation and Sahel summer rainfall. Another is an indirect thermodynamic process: the strengthened WASM circulation could further induce anomalous moisture convergence over the Sahel region, increasing local atmospheric moisture at the low-level troposphere, in favor of a wetter Sahel. Our results suggest that an improved warming simulation over northern North America is essential for the hydrological cycle simulation around the Sahel in the mid-Pliocene warmer climate. Significance Statement The Sahel summer rainfall change in a global warmer climate is a widespread scientific and socioeconomic issue because of its important impacts on regional agriculture, ecosystems, food security, water resources, and even cultural environment. However, the present study identifies a nonnegligible intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene (one of the most recent geological warm periods in Earth’s history) in the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) multimodel ensemble. In particular, such an intermodel uncertainty of the simulated Sahel rainfall changes during the mid-Pliocene is attributed to that of the simulated northern North America warming via both the direct dynamic process and the indirect thermodynamic process. This is quite different from the uncertainty source of near-future projections of Sahel summer rainfall changes. The present results improve our understanding of the underlying physics of the hydrological cycle change around the Sahel region in the mid-Pliocene warmer climate, with implications for the future projections of regional monsoonal rainfall.

ИЗМЕНЕНИЕ ОЛЕДЕНЕНИЯ СЕВЕРНОГО СКЛОНА ИЛЕ АЛАТАУ ЗА СЕМИДЕСЯТИЛЕТНИЙ ПЕРИОД

The problem of shortage of water resources, especially fresh water and runoff during the growing season, is relevant for all countries in arid regions. In Central Asia, including Kazakhstan, mountainous regions are one of the main suppliers of fresh water. The climate-caused degradation of mountain glaciation causes changes in mountain ecosystems, having direct impact to human life, causes changes in hydrological cycles, biogeochemistry of rivers and glacial lakes, affecting the quality and availability of water resources. The article presents the results of the seventy-year monitoring of changes in the area of glaciation of the northern slope of Ile Alatau. According to the satellite surveys of the Landsat 5 TM+ and Landsat 8, 9 OLI TIRS sensors for 1990, 2006, 2014 and 2022, the state of glaciation was assessed. To analyze the degradation of the glacial system were used the data from previous inventories of this region - as of 1955, 1974, 1979, 1990, 2008. For 67 years, the glaciation of the northern slope of Ile Alatau has decreased in area by 140.4 km2 (by 49%), losing 2.1 km2 or 0.72% of its area per year. The rate of degradation has not changed much in comparison with previous studies of the glaciation of this region. Based on the forecast, by the end of the 21st century, the glaciation of the northern slope of the Ile Alatau may disappear, of course, by the remaining conditions of degradation.

Impacts of tropical cyclones on the global water budget

Tropical cyclones (TCs) require substantial amounts of moisture for their genesis and development, acting as important moisture drivers from the ocean to land and from tropical to subtropical and extratropical regions. Quantifying anomalous moisture transport related to TCs is crucial for understanding long-term TC-induced changes in the global hydrological cycle. Our results highlight that, in terms of the global water budget, TCs enhance moisture transport from evaporative regions and precipitation over sink regions, leading to predominantly anomalous positive surface freshwater flux areas over the tropics and more regionally concentrated negative areas over the Intertropical Convergence Zone. Furthermore, we detected seasonal variability in the impact of TC on the hydrological cycle, which is closely related to the annual and seasonal TC frequency. Our analysis also revealed a global statistically significant drop (~40 mm year−1) in TC-induced surface freshwater fluxes from 1980 to 2018 in response to the increasing sea surface temperature and slightly decrease in global TC frequency and lifetime in the last two decades. These findings have important implications for predicting the impacts of TCs on the hydrological cycle under global warming conditions.

Temporal and Spatial Pattern Change in Evapotranspiration Over the High Plains: The Impact of and Guide on Extensive Groundwater‐Fed Irrigation

AbstractAquifer depletion due to extensive and intensive irrigation in the High Plains has threatened the environmental sustainability of the region. The change of crop evapotranspiration (ET), the major form of agriculture water consumption, presents a critical signature of hydrologic cycle change. This study evaluates the relative contributions of climate and groundwater‐fed irrigation to ET temporal and spatial pattern change over the High Plains Aquifer, one of the most severely depleted aquifers in the US. We developed a framework to extend the Budyko hypothesis to assess the impact of catchment storage change on long‐term ET. It is found that irrigation from groundwater pumping contributes more than half of the increase in ET (74.1 mm) from the period of 1940–1975 to 1976–2010, despite an increase of precipitation (35.0 mm) in the region. ET seasonal variance is decreased by the decline in precipitation variability (at −20.7 mm) and increase in irrigation (at +14.2 mm). As expected, irrigation decreases ET coefficient of variability (i.e., the ratio of standard deviation to mean). Spatially, we find that the human‐induced ET heterogeneity post‐1975 superimposes over the natural east‐to‐west gradient (in precipitation and ET) of this region. A correlation between the statistics (mean vs. coefficient of variation) on ET and crop yield provides promising signatures for understanding the coupled natural and human system of High Plains agriculture. Guides are discussed regarding how to handle the tradeoffs between agricultural development and natural resource sustainability under climate variability in the High Plains and other regions with similar conditions.

The seasonal origins and ages of water provisioning streams and trees in a tropical montane cloud forest

Abstract. Determining the sources of water provisioning streams, soils, and vegetation can provide important insights into the water that sustains critical ecosystem functions now and how those functions may be expected to respond given projected changes in the global hydrologic cycle. We developed multi-year time series of water isotope ratios (δ18O and δ2H) based on twice-monthly collections of precipitation, lysimeter, and tree branch xylem waters from a seasonally dry tropical montane cloud forest in the southeastern Andes mountains of Peru. We then used this information to determine indices of the seasonal origins, the young water fractions (Fyw), and the new water fractions (Fnew) of soil, stream, and tree water. There was no evidence for intra-annual variation in the seasonal origins of stream water and lysimeter water from 1 m depth, both of which were predominantly comprised of wet-season precipitation even during the dry seasons. However, branch xylem waters demonstrated an intra-annual shift in seasonal origin: xylem waters were comprised of wet-season precipitation during the wet season and dry-season precipitation during the dry season. The young water fractions of lysimeter (< 15 %) and stream (5 %) waters were lower than the young water fraction (37 %) in branch xylem waters. The new water fraction (an indicator of water ≤ 2 weeks old in this study) was estimated to be 12 % for branch xylem waters, while there was no significant evidence for new water in stream or lysimeter waters from 1 m depth. Our results indicate that the source of water for trees in this system varied seasonally, such that recent precipitation may be more immediately taken up by shallow tree roots. In comparison, the source of water for soils and streams did not vary seasonally, such that precipitation may mix and reside in soils and take longer to transit into the stream. Our insights into the seasonal origins and ages of water in soils, streams, and vegetation in this humid tropical montane cloud forest add to understanding of the mechanisms that govern the partitioning of water moving through different ecosystems.

Changes in the Urban Hydrological Cycle of the Future Using Low-Impact Development Based on Shared Socioeconomic Pathway Scenarios

Representative Concentration Pathway (RCP) scenarios have been used for various studies in the field of climate change. In this regard, the Shared Socioeconomic Pathway (SSP) scenario has been newly introduced to examine climate change impacts, but relevant research is still insufficient. For this reason, new SSP scenarios with a combination of Low-Impact Development (LID) techniques are applied to predict rainfall-runoff efficiency and hydrological variation. The inter-model variability in the monthly average precipitation for each GCM according to new SSP scenarios under future climate was investigated. Based on the RCP 4.5 and RCP 8.5 scenarios, the results show precipitation changes with an increase of 4.8% and 12.3%, respectively. Furthermore, precipitation projections under SSP2-4.5 and SSP5-8.5 scenarios are predicted to increase by 13.9% and 20.6%, respectively, indicating that the magnitude of precipitation increases with new climate change scenarios. The Storm Water Management Model (SWMM) during the future period indicated that LID applications will reduce runoff compared with scenarios with no LID application. In particular, the introduction of permeable pavement and infiltration trenches revealed the best runoff reduction performance among the combinations of LID techniques considered. In addition, this study projected changes in the urban hydrological cycle for the climate over the next 30 years to reflect the implementation of urban hydrological cycle plans, which take approximately 10 years. Overall, it was found that, in the future, LID applications will contribute to improving the sustainability of the urban hydrological cycle of the study area. The results of our study can provide future directions for water management strategies in Korea.

Seasonally distinct contributions of greenhouse gases and anthropogenic aerosols to historical changes in Arctic moisture budget

Arctic hydrological cycle has important global implications through modifying deep ocean circulation via fresh water fluxes. However, attribution of the observed changes in Arctic moisture budget remains challenging due to the lack of reliable observations. Here, as a first step, past changes in hydrological cycle over the Arctic were evaluated using CMIP6 historical simulations. To examine possible influences of individual external forcings, CMIP6 multi-model simulations performed under natural-plus-anthropogenic (ALL), greenhouse gas (GHG), natural (NAT), and aerosol (AER) forcings were compared. Results indicate that Arctic precipitation and evaporation increase in ALL and GHG but decrease in AER. In ALL and GHG, Arctic precipitation increases in summer mainly due to enhanced poleward moisture transport whereas Arctic moistening during winter is affected more by increased surface evaporation over sea-ice retreat areas. In AER, Arctic precipitation tends to decrease due to reduced evaporation over sea-ice advance areas during cold months. Poleward meridional moisture flux (MMF) across 70°N is the strongest in summer due to the highest moisture. GHG has stronger MMF than other forcing simulations due to larger increase in moisture in line with stronger warming. The MMF response is found to be largely determined by variations in transient eddies with distinct seasonal and regional contributions.

Stable isotope hydrology of surface and ground waters in King George Island, Antarctica

ABSTRACT The region around the tip of the Antarctic Peninsula is warming fast, a situation that will lead to widespread changes in local hydrological cycles. King George Island (KGI) hosts a complex network of lakes and rivers, fed by glaciers, snow and rain, and underlain by thick permafrost. We present here the first study of the stable isotope composition of the surface waters in the ice-free southern peninsulas of KGI. Permafrost samples had the highest δ 18O and δ 2H values (–6.7 and –50 ‰, respectively), and river waters the lowest (–9.1 and –70 ‰, respectively), with groundwater (–8.2 and –62.7 ‰, respectively), lakes (–8.6 and –66.8 ‰, respectively) and (summer) meltwater (–9 and –69.5 ‰, respectively) having intermediary values. Our results suggest that a clear separation of the various water bodies (permafrost, snow, meltwater, lakes) based on the δ 18Owater and δ 2Hwater is possible. Further, water in lakes on a W–E transect (i.e. with increased distance from the Bellingshausen Sea) have a general tendency towards lower δ 18O (and δ 2H) values. The results allow for the establishment of a baseline against which ongoing and future changes of the hydrological cycle could be analysed, and past climate changes be reconstructed.

Why Has the Summertime Central U.S. Warming Hole Not Disappeared?

Abstract A cooling trend in summer (May–August) daytime temperatures since the mid-twentieth century over the central United States contrasts with strong warming of the western and eastern United States. Prior studies based on data through 1999 suggested that this so-called warming hole arose mainly from internal climate variability and thus would likely disappear. Yet it has prevailed for two more decades, despite accelerating global warming, compelling reexamination of causes that in addition to natural variability could include anthropogenic aerosol–induced cooling, hydrologic cycle intensification by greenhouse gas increases, and land use change impacts. Here we present evidence for the critical importance of hydrologic cycle change resulting from ocean–atmosphere drivers. Observational analysis reveals that the warming hole’s persistence is consistent with unusually high summertime rainfall over the region during the first decades of the twenty-first century. Comparative analysis of large ensembles from four different climate models demonstrates that rainfall trends since the mid-twentieth century as large as observed can arise (although with low probability) via internal atmospheric variability alone, which induce warming-hole-like patterns over the central United States. In addition, atmosphere-only model experiments reveal that observed sea surface temperature changes since the mid-twentieth century have also favored central U.S cool/wet conditions during the early twenty-first century. We argue that this latter effect is symptomatic of external radiative forcing influences, which, via constraints on ocean warming patterns, have likewise contributed to persistence of the U.S. warming hole in roughly equal proportion to contributions by internal variability. These results have important ramifications for attribution of extreme events and predicting risks of record-breaking heat waves in the region. Significance Statement Our paper makes a significant contribution to analysis of a cooling trend in summer (May–August) daytime temperatures since the mid-twentieth century over the central United States, contrasting with strong warming over the remainder of the United States and having important ramifications for assigning cause to and predicting record-breaking heat waves in the region. Observations and model simulations reveal the critical importance of hydrologic cycle change resulting from ocean–atmosphere impacts. Precipitation has increased substantially over the region as a result of atmospheric circulation trends consisting of generally lower pressure and cooler air advection into the region. The persistence of this pattern of increased rainfall/lower temperatures is likely due to near-equal contributions of external forcing (climate change) and internal climate variability.

A review of the effects of solar radiation management on hydrological extremes

Solar radiation management (SRM) is one of the proposed climate mitigation strategies to cool the planet rapidly. The injection of aerosol particles into the stratosphere for reflecting solar radiation back to the space is one of the SRM methods that are widely discussed. Theoretically, SRM might lower the earth’s temperature within a few months of deployment, reducing the impacts of climate change on natural disasters, i.e., floods and drought, which lead to huge losses in economic and human life. Solar radiation variability was identified to be a substantial factor that induced the hydrological changes, particularly in precipitation extreme. The effects of SRM on hydrological cycles, however, fluctuate depending on the location and environment. Hence, this article reviews the past SRM studies that related to the analysis of the hydrological cycle changes. A total of 17 articles were identified and collected from the Web of Science and Scopus databases. The results show that there have been an increasing number of articles in recent years studying the effects of SRM on the hydro-climatic changes. The Geoengineering Model Intercomparison Project (GeoMIP) and the Geoengineering Large Ensemble (GLENS) are two commonly used SRM-based general circulation models. In general, SRM is projected to slow down the global hydrological cycle. In comparison to the RCP 8.5 scenario, SRM generally tends to lower flood risk in many parts of the world. However, the majority of SRM research in hydrology has been conducted on a global scale, which results in a lack of robust basin-scale assessment needed for flood control policy formulation. In addition, more SRM climate models and scenario experiments should be considered to minimize the uncertainty in the framework for hydro-climatic modelling framework.

Detecting the main driving force of runoff change in the Beiluo River Basin, China.

Understanding the evolution of runoff and identifying the main driving force of hydrological cycle changes are essential for water resource management. In this study, the spatial and temporal patterns of climate variables and hydrological factors were explored by applying geostatistical analysis and trend analysis in the Beiluo River Basin (BRB), China, and conversions of land use/cover change (LUCC) were assessed using chord diagrams. Contributions of climate change and human activities to runoff change were quantified employing multiple methods. The results show that annual precipitation and actual evapotranspiration increased significantly during the impact period (2004-2014) (p < 0.05), at rates of 19.3 mm/a and 11 mm/a, respectively, and there was a minor upward trend in annual runoff, at a rate of 0.38 mm/a, while annual potential evapotranspiration decreased slightly at a rate of - 3.3 mm/a. Climate variables were the primary contributor to runoff decrease from 2004 to 2011, with an average contribution of - 79% according to the three methods. Human activities were estimated to account for - 81% of runoff change from 2012 to 2014, which was inextricably linked to the increasing LUCC. The results of this study can provide a theoretical basis for regional water resource management under the influence of climate change and human activities.