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This study investigates the hydrological dynamics of the Pativilca Basin in the Southern Hemisphere using the SWAT (Soil and Water Assessment Tool) model. Seventy-seven watersheds across a mountainous region were analyzed using elevation data, land cover, soil type, and gridded meteorological products (RAIN4PE and PISCO) for hydrological simulations. Watershed delineation, aided by a Digital Elevation Model, enabled the identification of critical drainage points and the definition of Hydrological Response Units (HRUs). The model calibration and validation, performed using the SWAT-CUP with the SUFI-2 algorithm, achieved Nash–Sutcliffe Efficiency (NSE) values of 0.69 and 0.72, respectively. Cluster analysis categorized the watersheds into six distinct groups with unique hydrological and climatic characteristics. The results showed significant spatial variability in the precipitation and temperature, with pronounced seasonality influencing the daily flow patterns. The higher-altitude watersheds exhibited greater soil water storage and more effective aquifer recharge, whereas the lower-altitude watersheds, despite receiving less precipitation, displayed higher flows due to runoff from the upstream areas. These findings emphasize the importance of incorporating seasonality and spatial variability into water resource planning in mountainous regions and demonstrate the SWAT model's effectiveness in predicting hydrological responses in the Pativilca Basin, laying the groundwork for future research in mountain hydrology. [ABSTRACT FROM AUTHOR]

The concentrations and fluxes of arsenic (As) and cadmium (Cd) were examined in the hydrological cycle, litterfall and soils in a maquis broadleaved evergreen forest stand in western Greece. The concentrations of the metals in the hydrological cycle ranged from 0.076 to 0.306 μg L−1 and 0.040–0.050 μg L−1 for As and Cd, respectively. It was found that the enrichment of As in the atmosphere was both due to suspended geogenic material and long range transfer, whereas for Cd the long range transfer was the predominant way for deposition. Two models were assessed to find the volumes of percolation water to compare the fluxes with the estimated ones derived from soil solution measurements. The daily deterministic forest-hydrological model (WBS3) was found to perform better than the physically based 1D semi-distributed soil-plant-atmosphere model (BROOK90). When calculating the total amounts of the heavy metals in soils, the statistical uncertainty derived from every soil layer has to be taken into account. The residence time of Cd in the forest floor (L + FH) of the soils was found 6.4 years, whereas that of As 36.3 years. This difference shows the mobility of Cd in comparison to As in the soils of the area. [ABSTRACT FROM AUTHOR]

Evapotranspiration (ET) is a critical component of the hydrological cycle, and it has a decisive impact on the ecosystem balance in arid and semi-arid regions. The Yinchuan Plain, located in the Gobi of Northwest China, has a strong surface ET, which has a significant impact on the regional water resource cycle. However, there is a current lack of high-resolution evapotranspiration datasets and a substantial amount of time is required for long-time series remote sensing evapotranspiration estimation. In order to assess the ET pattern in this region, we obtained the actual ET (ETa) of the Yinchuan Plain between 1987 and 2020 using the Google Earth Engine (GEE) platform. Specifically, we used Landsat TM+/OLI remote sensing imagery and the GEE Surface Energy Balance Model (geeSEBAL) to analyze the spatial distribution pattern of ET over different seasons. We then reproduced the interannual variation in ET from 1987 to 2020, and statistically analyzed the distribution patterns and contributions of ET with regard to different land use types. The results show that (1) the daily ETa of the Yinchuan Plain is the highest in the central lake wetland area in spring, with a maximum value of 4.32 mm day−1; in summer, it is concentrated around the croplands and water bodies, with a maximum value of 6.90 mm day−1; in autumn and winter, it is mainly concentrated around the water bodies and impervious areas, with maximum values of 3.93 and 1.56 mm day−1, respectively. (2) From 1987 to 2020, the ET of the Yinchuan Plain showed an obvious upward and downward trend in some areas with significant land use changes, but the overall ET of the region remained relatively stable without dramatic fluctuations. (3) The ETa values for different land use types in the Yinchuan Plain region are ranked as follows: water body > cultivated land > impervious > grassland > bare land. Our results showed that geeSEBAL is highly applicable in the Yinchuan Plain area. It allows for the accurate and detailed inversion of ET and has great potential for evaluating long-term ET in data-scarce areas due to its low meteorological sensitivity, which facilitates the study of the regional hydrological cycle and water governance. [ABSTRACT FROM AUTHOR]

Thermokarst lake formation accelerates permafrost degradation due to climate warming, thereby releasing significant amounts of carbon into the atmosphere, complicating hydrological cycles, and causing environmental damage. However, the energy transfer mechanism from the surface to the sediment of thermokarst lakes remains largely unexplored, thereby limiting our understanding of the magnitude and duration of biogeochemical processes and hydrological cycles. Therefore, herein, a typical thermokarst lake situated in the center of the Qinghai–Tibet Plateau (QTP) was selected for observation and energy budget modeling. Our results showed that the net radiation of the thermokarst lake surface was 95.1, 156.9, and 32.3 W m−2 for the annual, ice-free, and ice-covered periods, respectively, and was approximately 76% of the net radiation consumed by latent heat flux. Alternations in heat storage in the thermokarst lake initially increased from January to April, then decreased from April to December, with a maximum change of 48.1 W m−2 in April. The annual average heat fluxes from lake water to sediments were 1.4 W m−2; higher heat fluxes occurred during the ice-free season at a range of 4.9–12.0 W m−2. The imbalance between heat absorption and release in the millennium scale caused the underlying permafrost of the thermokarst lake to completely thaw. At present, the ground temperature beneath the lake bottom at a depth of 15 m has reached 2.0 °C. The temperatures and vapor-pressure conditions of air and lake surfaces control the energy budget of the thermokarst lake. Our findings indicate that changes in the hydrologic regime shifts and biogeochemical processes are more frequent under climate warming and permafrost degradation.

This study investigates the specific circulation anomalies that have sustained the low Antarctic sea ice state since 2016. Firstly, we find a significant strengthening and southward shift in the Ferrel Cell (FC) during 2016–2022, resulting in a marked increase in southward transport of heat and moisture. Secondly, this enhanced FC is closely associated with a stronger mid‐latitude wave pattern. This pattern is zonally asymmetric and greatly amplifies the poleward advections of heat and moisture, leading to the increased downward longwave radiation, more liquid precipitation and sea ice retreat in specific regions, including the western Pacific and Indian Ocean sectors, Ross and northern Weddell Seas. The mechanism deduced from the short‐term period is further supported by the results of 40 ensemble members of simulations. The southward expansion of the FC and sea ice decline are closely linked to La Niña‐like conditions but may also be driven by anthropogenic global warming. Plain Language Summary: Following the sudden decline in 2016, the Antarctic sea ice extent has persisted at historically low levels. In 2023, it reached unprecedented record lows. However, the specific atmospheric circulation anomalies that have sustained the Antarctic sea ice at low levels are still unknown. It is well‐established that the Ferrel Cell, a mid‐latitude atmospheric meridional circulation, plays a pivotal role in the energy exchange between the high‐ and mid‐latitudes. Our findings indicate that the enhanced Ferrel Cell zonally intensified southward transport of heat and moisture over the sea ice regions, which sustains the overall low Antarctic sea ice state. Additionally, in the horizontal plane, the enhanced mid‐latitude wave pattern is responsible for the regional sea ice retreat over the western Pacific sector, Ross Sea, Indian Ocean sector, and northern Weddell Sea, and is also closely associated with the enhanced Ferrel Cell. The effects of the enhanced Ferrel Cell on Antarctic sea ice decline are further supported by the results of large ensemble simulations. Therefore, this study suggests that concurrent with the southward shifting of the Ferrel Cell, the stronger warm and moist air intrusions, and the increased liquid precipitation, restrict the Antarctic sea ice expansion following its sudden decline. Key Points: Since 2016, the low Antarctic sea ice extent has persisted, consistent with heat and moisture accumulation over the sea ice edgesThe Ferrel Cell was enhanced and shifted southward, leading to the increased southward heat/moisture advection, and liquid precipitationThe effects of the enhanced Ferrel Cell on Antarctic sea ice decline are further supported by the results of large ensemble simulations [ABSTRACT FROM AUTHOR]

We investigated changes in the hydrological cycle in inner East Asia during the boreal summer (June-to-August) from 1979 to 2020 by analyzing climate variables including latent heat flux, precipitation and soil moisture. We found that the correlation between latent heat flux (LH) and precipitation varies across East Asia, with a negative correlation dominating over the summer monsoon region. In contrast, LH is positively correlated with precipitation in inner East Asia. Additional analyses indicate a positive correlation of LH-precipitation, implying a coupling of land and atmosphere that becomes strong in inner East Asia across the late 1990s. Soil moisture is also consistently and significantly reduced in inner East Asia after the late 1990s, which could be associated with the strengthening of a positive correlation of LH-precipitation. A tripole-like sea surface temperature (SST) in the North Atlantic Ocean causes anomalous anticyclonic circulation in inner East Asia through zonal wave propagation, resulting in a significant reduction of soil moisture. Our results indicate that the hydrological cycle in inner East Asia is influenced remotely by North Atlantic SST through soil moisture – precipitation feedback. [ABSTRACT FROM AUTHOR]

Surface Water–Groundwater (SW–GW) interaction is a crucial aspect of the hydrological cycle and requires accurate modeling for reliable predictions. In many basic hydrological models and calculations, it is common to assume that the water density is constant. However, density-dependent flow, which accounts for changes in water density, plays a significant role in various hydrological processes. This study aims to quantify the effects of density-dependent flow on SW–GW interaction and evaluate the sensitivity of dominant hydrological drivers to density-dependent flow. Our simulations using the HydroGeoSphere model revealed that neglecting density-dependent flow in SW–GW interaction can lead to inaccurate estimations of water and solute balances. In particular, including density-dependent flow in the model yielded more realistic salinity distributions under gaining river scenarios and captured the gradual expansion of freshwater lenses under losing river scenarios. The results also indicated that under non-density-dependent flow, more saline groundwater is exposed to evapotranspiration, resulting in higher solute mass storage and a more saline unsaturated zone. Further, surface recharge and pumping rates played crucial roles in salinity distribution. This study highlights the critical importance of incorporating density-dependent flow in simulations, providing valuable insights for improving the accuracy of predictions and effectively managing water and solute balances in floodplain aquifers. [ABSTRACT FROM AUTHOR]

The article examines the effects of climate change on hydrological cycle, water availability, agricultural production, and energy usage in India. Topics discussed include explanation on hydrological cycle, how climate change affects water availability, and effect of climate on groundwater zones. Also mentioned are the effect of climate change on recharge and discharge and effects of climate change on groundwater quality.

In mountainous watersheds, floodplain sediments are typically characterized by gravel bed layers capped by an overlying soil unit that serves as a hotspot for biogeochemical reactivity. However, the influence of soil biogeochemistry on gravel bed underflow composition remains unclear, especially during hydrological transitions that alter the vertical connectivity between overlaying soils and the underlying gravel bed. This study investigates these dynamics by measuring hydraulic gradients and water compositions over three hydrological years in a typical mountainous, low‐order stream floodplain in the Upper Colorado River Basin. Results indicate that the timing of hydrological conditions strongly influences the vertical exchanges that control water quality. Specifically, during flooding events such as beaver ponding, that induce downward flushing of the soil, anoxic conditions prevalent in the biogeochemically active soil are transferred downstream via gravel bed underflow. Conversely, snowmelt and drought conditions increase oxic conditions in the gravel bed due to diminished hydrological connectivity with the overlying soil. To compare water quality response to hydrological transitions across similar floodplain environments, we propose a conceptual model that quantifies the inundation‐induced flushing of soil porewater to measure solute exchange efficiency with the gravel bed solute convergence efficiency (SCE). This model provides a framework for quantifying biogeochemical processes in hydrological underflow systems, which is critical for water and elemental budgets in these globally important mountainous ecosystems. Plain Language Summary: Mountains are important sources of freshwater for humans and ecosystems. They are however increasingly impacted by climate change. In this paper, we investigate how changes in water availability (droughts, snowmelt, inundations) can cascade into changes in water quality (concentrations of dissolved chemical elements). We show that in mountain valleys, the zone of contact between the soil and the underlying gravel bed aquifer is important for water quality. Chemical elements from the soil can be flushed down into the gravel bed aquifer, then transported by groundwater to the stream. Chemical elements from the soil can also react with chemical elements present in the aquifer. Our research can be used to better predict the water and elemental budgets in these important mountainous systems. Key Points: Soil / gravel bed connectivity in floodplains is important for water qualitySnowmelt and drought both reduce soil / gravel bed connectivityBeaver ponding increases downward flushing to gravel bed unit [ABSTRACT FROM AUTHOR]

Population growth, coupled with climate and social shifts, has resulted in a global phenomenon of water scarcity. Yet, the effect of social factors on water resources has been poorly studied. Hence, this study aimed to identify the key parameters in social systems that significantly impact hydrological system change and presents the best scenario for water management. The system dynamic (SD) approach was employed in this research to construct a combined framework of policies based on scenarios, which aimed to ensure social sustainability and coupled human-water systems. For this purpose, the SD model was simulated on the Gavshan Basin in the west of Iran for the long-term period 2020-2050. The results indicate that the water resources in the Gavshan Basin cannot meet the growth of the population. Meanwhile, about 20% of the water stored in the Gavshan Dam is not effectively used and flows out of the irrigation network as wastewater. The result of the sensitivity analysis showed that in scenarios 3 and 4, the policy of wastewater reuse in the agricultural sector significantly increases available water resources, has a major impact on water supply, and increases crop yields. These findings can be applied by policy-makers. Instead of making efforts only to change hydrological systems, policies need to first focus on socio-hydrology systems sustainability. It is suggested that national organizations' support should be implemented to prevent the adverse consequences of wastewater reuse in agriculture and reduce treated wastewater risks. [ABSTRACT FROM AUTHOR]

Quantitatively estimating the global mean precipitation (GMP) throughout Earth's history is crucial for enhancing our understanding of long-term climate evolution and the hydrological cycle. However, currently there is no established methodology for estimating global mean paleoprecipitation. Here, we present the first study that estimates GMP in the Phanerozoic era. The relationship between GMP and global mean surface temperature (GMST) is investigated by analyzing data from 23 models in the Coupled Model Intercomparison Project phase 6 (CMIP6). The result reveals consistent and significant impact of temperature on precipitation, with a sensitivity range of 2–3 % K−1. Additionally, we propose a method for accessing latitudinal variations in precipitation caused by land area distributions and paleo-Köppen climatic belts. These climatic belts are determined based on geological indicators such as coals, evaporites, and glacial deposits. The GMP is thus quantitatively estimated by combining variations in GMST, land area distributions, and paleo-köppen climatic belts, spanning from 540 Ma to the present day. The quantitative GMP curve demonstrates fluctuations in GMP about 500 mm yr−1, with values ranging from 948 to 1442 mm yr−1 over the Phanerozoic era. This curve aligns closely with findings derived from numerical simulations. The presented paleoprecipitation variations facilitate a more comprehensive understanding of the interconnected geological and paleoclimatic developments. [ABSTRACT FROM AUTHOR]

We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model version 4.1 (LM4.1), which builds on component and coupled model developments over 2013–2019 for the coupled carbon‐chemistry‐climate Earth System Model Version 4.1 (ESM4.1) simulation as part of the sixth phase of the Coupled Model Intercomparison Project. Analysis of ESM4.1/LM4.1 is focused on biophysical and biogeochemical processes and interactions with climate. Key features include advanced vegetation dynamics and multi‐layer canopy energy and moisture exchanges, daily fire, land use representation, and dynamic atmospheric dust coupling. We compare LM4.1 performance in the GFDL Earth System Model (ESM) configuration ESM4.1 to the previous generation component LM3.0 in the ESM2G configuration. ESM4.1/LM4.1 provides significant improvement in the treatment of ecological processes from GFDL's previous generation models. However, ESM4.1/LM4.1 likely overestimates the influence of land use and land cover change on vegetation characteristics, particularly on pasturelands, as it overestimates the competitiveness of grasses versus trees in the tropics, and as a result, underestimates present‐day biomass and carbon uptake in comparison to observations. Plain Language Summary: The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a new Land Model (LM4.1) as part of its 4th generation coupled model development. This model includes advances from the previous generation and introduces a new vegetation demography model, multi‐layer canopy, plant hydraulics, fire, and land use representation as well as dynamic atmospheric dust coupling. Coupled within an Earth System Model (ESM4.1), LM4.1 features an improved representation of many ecological processes from the previous generation of GFDL ESMs. Key Points: A new land model LM4.1 is developed at the Geophysical Fluid Dynamics Laboratory (GFDL) for the next‐generation Earth System Model (ESM) ESM4.1LM4.1 integrates age‐height structured vegetation dynamics, multi‐layer canopy‐soil‐snow energy exchanges, and prognostic fires and mineral dustESM4.1/LM4.1 improves patterns of land surface climate and carbon cycle compared to the previous generation GFDL model ESM2G/LM3.0 [ABSTRACT FROM AUTHOR]

Water budget over anthropized basins is profoundly altered by human interventions, among which irrigation prevails. This study investigates how such water use affects hydrological flux and state variables by comparing two SURFEX/ISBA (SURFace EXternalisée/Interaction Sol Biosphère Atmosphère) Land Surface Model simulations performed over a portion of the Ebro basin (Spain) that includes a heavily irrigated area. The first simulation is forced by atmospheric input only, while the second one is fed with atmospheric forcing plus remote sensing‐derived irrigation amounts added to the liquid precipitation. Four main hydrological output variables are compared under the two configurations: surface soil moisture (SM), total evaporative flux (i.e., the sum of components from bare soil and vegetation) (E), drainage, and runoff. Results indicate that SM and E are the variables mostly impacted during the main watering season. Considering mean daily rates per each month, a maximum increase of +30% and of +220% is found over the irrigation districts in July for SM and E, respectively. The reliability of the results is supported by comparisons with satellite evapotranspiration data from MODIS (MODerate‐resolution Imaging Spectroradiometer). The inclusion of irrigation reduces the RMSE between monthly anomalies of modeled E and of MODIS reference data by 41% during the main watering season. Further validation through wavelet coherence analysis is also proposed. This study sheds light on the potential of explicit satellite‐derived irrigation water use data that, when integrated into modeling platforms, allow us to track and quantify anthropogenic impacts on water resources. Key Points: Irrigation impacts on the water cycle are evaluated exploiting explicit satellite‐derived irrigation water use dataDuring the main watering season, surface soil moisture and evaporative flux show an increase up to +30% and +220%, respectively, due to irrigationComparison against an independent land evaporation data set and the wavelet coherence analysis support the physical robustness of the study [ABSTRACT FROM AUTHOR]

The Kuroshio‐Oyashio Extension and Gulf Stream oceanic frontal zones are characterized by enhanced activity of synoptic‐scale cyclones and anticyclones and vigorous air‐sea heat and moisture exchange in the cold season. However, the time‐mean air‐sea exchange attributed separately to cyclones and anticyclones has not been assessed. Here we quantify cyclonic and anticyclonic contributions around the frontal zones to surface turbulent heat fluxes, precipitation, and the associated hydrological cycle using atmospheric general circulation model experiments with observed and artificially smoothed sea‐surface temperature gradients. The evaluation reveals that precipitation exceeds evaporation climatologically within cyclonic domains while evaporation dominates within anticyclonic domains. These features as well as the net moisture transport from anticyclonic to cyclonic domains are all enhanced by the sharpness of the frontal zones. Oceanic frontal zones thus climatologically act to strengthen the hydrological cycle. These findings aid our understanding of the relationship between midlatitude air‐sea interactions on synoptic‐ and longer‐time scales. Plain Language Summary: Two regions with pronounced sea surface temperature gradients over the North Pacific and North Atlantic are known as major oceanic frontal zones that are important for air‐sea interactions with vigorous heat and moisture release from the ocean to the atmosphere. Recent studies found that day‐to‐day variations, due to migratory low‐ and high‐pressure systems, are essential for the time‐mean air‐sea interaction over these frontal zones. However, the relative importance of those low‐ and high‐pressure systems has not been quantified. Using atmospheric general circulation model experiments with observed and artificially smoothed sea‐surface temperature gradients, we show that contributions of high‐pressure systems are important for the enhanced heat and moisture supply from the ocean in response to realistic oceanic frontal zones, while contributions of low‐pressure systems are crucial for the changes in rainfall. We further demonstrate that the moisture transport from high‐ to low‐pressure systems is strengthened climatologically with the sharpness of midlatitude oceanic frontal zones, indicative of a strengthening of the hydrological cycle. Our findings indicate that synoptic‐scale migratory low‐ and high‐pressure systems play an important role in midlatitude air‐sea interactions. These results bridge the gap between our understanding of midlatitude air‐sea interactions from day‐to‐day to climatic time scales. Key Points: Cyclonic and anticyclonic contributions to air‐sea heat and moisture exchange are quantified around midlatitude oceanic frontal zonesOceanic frontal zones enhance surface turbulent heat fluxes within anticyclones and precipitation within cyclones, respectivelySharpness of midlatitude oceanic frontal zones strengthens the net moisture transport from anticyclones to cyclones [ABSTRACT FROM AUTHOR]

The projected changes in the hydrological cycle under global warming remain highly uncertain across current climate models. Here, we demonstrate that the observational past warming trend can be utilized to effectively co1nstrain future projections in mean and extreme precipitation on both global and regional scales. The physical basis for such constraints relies on the relatively constant climate sensitivity in individual models and the reasonable consistency of regional hydrological sensitivity among the models, which is dominated and regulated by the increases in atmospheric moisture. For the high-emission scenario, on the global average, the projected changes in mean precipitation are lowered from 6.9 to 5.2% and those in extreme precipitation from 24.5 to 18.1%, with the inter-model variances reduced by 31.0 and 22.7%, respectively. Moreover, the constraint can be applied to regions in middle-to-high latitudes, particularly over land. These constraints result in spatially resolved corrections that deviate substantially and inhomogeneously from the global mean corrections. This study provides regionally constrained hydrological responses over the globe, with direct implications for climate adaptation in specific areas. [ABSTRACT FROM AUTHOR]

Background and Objectives: In the hydrologic cycle, evaporation is the primary phenomenon that causes water loss. Evaporation takes into account various parts of the water balance under completely different climates, and its correct prediction is very important for water resources management. The importance of evaporation and its impact on surface water balance is highlighted through its relation to climate change and global warming. The latest outputs of meteorological models suggest that global warming has caused an increase in evaporation from the land surface and surface water bodies, which is anticipated to have a serious impact over time on water resources management and the global population. In arid and semi-arid regions, accurate prediction of evaporation is very important for decision-makers due to water scarcity. Estimating daily evaporation with the highest accuracy and in the fastest possible time is essential to determine the water needs of different products, design irrigation programs, and manage water resources in different areas, especially when there is insufficient meteorological information. Evaporation has complex and non-linear behavior. Also, the evaporation variable is not measured in some meteorological stations. Furthermore, meteorological stations are not correctly distributed in many developing countries including Iran. Since coastal areas have more evaporation than others, in many cases the amount of evaporation is higher than the global average. Despite the high importance of evaporation in coastal areas, very few studies have predicted this parameter in Iran. Moreover, accurate prediction of water loss in these areas leads to a better understanding of the hydrological cycle and is essential for optimal water management and agriculture. Thus, the purpose of this research is to predict daily evaporation values in four coastal stations of Abadan, Ramsar, Bandar Abbas, and Bandar Anzali. Methodology: The main meteorological variables including average relative humidity, minimum relative humidity, maximum relative humidity, average temperature, minimum temperature, maximum temperature, sunshine hours, and wind speed, under separate scenarios, as input for support vector regression (SVR) and SVR with firefly algorithm (SVR-FFA) for estimating evaporation values were used on a daily scale. Statistical parameters in the period of 1990-2021 were utilized as input to the mentioned models. To evaluate the performance of the implemented models, various statistical parameters were used, including correlation coefficient (R), root mean squared error (RMSE), Nash-Sutcliffe coefficient (NS), and Willmott's Index of Agreement (WI). To better estimate the daily evaporation values, eight different scenarios were used as the combinations of input parameters. Findings: Based on the obtained results for all studied stations, the SVR-FFA-8 showed the least error with RMSE = 2.843 (mm day-1) for Abadan station, RMSE = 1.13 (mm day-1) for Ramsar station, RMSE = 1.985 (mm day-1) for Bandar Abbas station and RMSE = 1.225 (mm day-1) for Bandar Anzali station. Therefore, the hybrid model was able to improve the accuracy of the standalone model and improve the prediction accuracy. Conclusion: For the indices of correlation coefficient, Nash-Sutcliffe coefficient, and Willmott's index of agreement, the SVR-FFA-8 model also indicated in the highest values between observed and predicted amounts. Also, the indices of correlation coefficient, Nash-Sutcliffe coefficient, and Willmott's index of agreement illustrated the highest accuracy in Abadan station for all combinations compared to other stations, which shows the high correlation of observed and predicted values in this station. After SVR-FFA-8, SVR-FFA-7 model in Abadan and Bandar Anzali stations and the SVR-FFA-6 in Ramsar and Bandar Abbas stations showed acceptable performance. Thus, the RMSE for Abadan and Bandar Anzali stations is 2.995 (mm day-1) and 1.272 (mm day-1), respectively, and for Ramsar and Bandar Abbas, 1.176 (mm day-1) and was obtained 1.993 (mm day-1). Comparing the results of SVR combinations also revealed that for Abadan, Ramsar, and Bandar Anzali stations, SVR-8 model and for Bandar Abbas station, SVR-6 model showed the highest accuracy among all SVR combinations in all four studied stations. Also, the Ramsar station presented the lowest RMSE compared to other stations. After the SVR-8 model for Abadan, Ramsar, and Bandar Anzali stations, the SVR-7 and SVR-6 models for the Bandar Abbas station showed a weaker performance due to having fewer input parameters. The comparison between the input parameters also concluded that the sunshine hours is the most important parameter in predicting the daily evaporation values in all four stations, thus increasing the accuracy of the models. [ABSTRACT FROM AUTHOR]

Study region: Sharavati River, Karnataka, India. Study focus: A small mountainous river system, Sharavati, was selected to study the impact of river damming on the hydrological cycle. Sharavati river flow is regulated by two dams, Linganamakki and Gersoppa. Despite the Western Ghats' global significance in controlling local and regional climates, the effects of damming on its hydrological cycles have received limited attention. A stable water isotopic approach was employed in the study. New hydrological insights for the region: The line-conditioned excess (lc-excess) was primarily negative across all seasons. Notably, the pre-monsoon season exhibited comparatively higher evaporation with high negative lc-excess, while the postmonsoon lc-excess values approached zero, indicating minimal evaporation. The sampling points from the dams exhibited very high evaporation signals, the evaporative loss during the pre-monsoon season from the Linganamakki reservoir was estimated as 10 %, and from the Gersoppa dam was 6 %. Consequently, groundwater sampled near the dams, plotted along the local evaporation line indicating recharge from the evaporated reservoir water. Damming has affected the hydrological cycle of the heavily regulated Sharavati River, transforming the entire catchment into a connected, narrow lake-like structure, especially during the pre-monsoon season. Since the Western Ghat river systems are regulated by many large and small dams, it is pertinent to study the impact of damming on the hydrological cycles of the entire system.

Abstract This study investigates the specific circulation anomalies that have sustained the low Antarctic sea ice state since 2016. Firstly, we find a significant strengthening and southward shift in the Ferrel Cell (FC) during 2016–2022, resulting in a marked increase in southward transport of heat and moisture. Secondly, this enhanced FC is closely associated with a stronger mid‐latitude wave pattern. This pattern is zonally asymmetric and greatly amplifies the poleward advections of heat and moisture, leading to the increased downward longwave radiation, more liquid precipitation and sea ice retreat in specific regions, including the western Pacific and Indian Ocean sectors, Ross and northern Weddell Seas. The mechanism deduced from the short‐term period is further supported by the results of 40 ensemble members of simulations. The southward expansion of the FC and sea ice decline are closely linked to La Niña‐like conditions but may also be driven by anthropogenic global warming.

The Paleocene-Eocene Thermal Maximum (PETM; 56 Ma) is one of our best geological analogs for understanding climate dynamics in a "greenhouse" world. However, proxy data representing the event are only available from select marine and terrestrial sedimentary sequences that are unevenly distributed across Earth's surface, limiting our view of the spatial patterns of climate change. Here, we use paleoclimate data assimilation (DA) to combine climate model and proxy information and create a spatially complete reconstruction of the PETM and the climate state that precedes it ("PETM-DA"). Our data-constrained results support strong polar amplification, which in the absence of an extensive cryosphere, is related to temperature feedbacks and loss of seasonal snow on land. The response of the hydrological cycle to PETM warming consists of a narrowing of the Intertropical Convergence Zone, off-equatorial drying, and an intensification of seasonal monsoons and winter storm tracks. Many of these features are also seen in simulations of future climate change under increasing anthropogenic emissions. Since the PETM-DA yields a spatially complete estimate of surface air temperature, it yields a rigorous estimate of global mean temperature change (5.6 ∘C; 5.4 ∘C to 5.9 ∘C, 95% CI) that can be used to calculate equilibrium climate sensitivity (ECS). We find that PETM ECS was 6.5 ∘C (5.7 ∘C to 7.4 ∘C, 95% CI), which is much higher than the present-day range. This supports the view that climate sensitivity increases substantially when greenhouse gas concentrations are high.

Rapid Arctic warming and sea-ice loss have intensified the Arctic hydrological cycle, increasing local evaporation and precipitation. Stable water isotopes as environmental tracers can provide useful insights into the Arctic hydrological cycle. However, the paucity of isotopic observations in the Arctic has limited our understanding of the hydrological changes. Here, we use an isotope-enabled atmospheric general circulation model (IsoGSM) combined with the Global Network of Isotopes in Precipitation (GNIP) observations to investigate the relationship between sea-ice changes and Arctic precipitation δ18O (δ18Op) to reveal the relative influence of local air temperature and evaporation on Arctic summer and winter δ18Op. We find that the Arctic δ18Op is negatively correlated with sea-ice concentration but positively with air temperature. Sea-ice loss leads to enriched Arctic δ18Op through enhanced local evaporation and warming, but the relative importance of these processes varies between seasons. During summer, both local evaporation and warming contribute equally to δ18Op changes. In contrast, winter δ18O is predominantly driven by air temperature. This work improves our understanding of how Arctic precipitation isotopes respond to sea-ice changes and has implications for the Arctic hydrological cycle and palaeotemperature reconstructions.

This study investigates the hydrological dynamics of the Pativilca Basin in the Southern Hemisphere using the SWAT (Soil and Water Assessment Tool) model. Seventy-seven watersheds across a mountainous region were analyzed using elevation data, land cover, soil type, and gridded meteorological products (RAIN4PE and PISCO) for hydrological simulations. Watershed delineation, aided by a Digital Elevation Model, enabled the identification of critical drainage points and the definition of Hydrological Response Units (HRUs). The model calibration and validation, performed using the SWAT-CUP with the SUFI-2 algorithm, achieved Nash–Sutcliffe Efficiency (NSE) values of 0.69 and 0.72, respectively. Cluster analysis categorized the watersheds into six distinct groups with unique hydrological and climatic characteristics. The results showed significant spatial variability in the precipitation and temperature, with pronounced seasonality influencing the daily flow patterns. The higher-altitude watersheds exhibited greater soil water storage and more effective aquifer recharge, whereas the lower-altitude watersheds, despite receiving less precipitation, displayed higher flows due to runoff from the upstream areas. These findings emphasize the importance of incorporating seasonality and spatial variability into water resource planning in mountainous regions and demonstrate the SWAT model’s effectiveness in predicting hydrological responses in the Pativilca Basin, laying the groundwork for future research in mountain hydrology.

Effects of global climate change on inland artisanal fisheries have received little attention from scientists. We investigated fisheries of the Lower Amazon in Brazil using a wavelet analysis of a 13‐year data series of fishery catches, environmental variables, and climatic indices, to determine how these variables affected catches of different species. The catch per unit of effort (CPUE) of gillnet and line fisheries increased over time. Relative humidity, latent heat, and sea surface temperatures influenced productivity of both fisheries. El Niño had a negative and immediate effect on gillnet CPUE, while its effect on line fishing was positive, with an eight month lag. Our findings indicate that fishery productivity was modulated by climate events, which influenced ecosystems and modified habitats and ecological niches, andthereby impacted fishery productivity. [ABSTRACT FROM AUTHOR]

The Early Eocene Climatic Optimum (EECO) may be a potentially useful analog for future global warming under high CO2 concentrations. However, a paucity of orbital‐scale terrestrial records limits our understanding of how the hydrological cycle responded during this protracted (∼4 Myr) interval of global warmth. In this study, we combine zircon U‐Pb dating and cyclostratigraphy to establish a high‐resolution astronomical timescale spanning the EECO (∼52.9 Ma to ∼49.9 Ma) through a >1 km fluviolacustrine succession from the Gonjo Basin, Southeast Tibet. Our results suggest that hydroclimate variability in the region during this interval was strongly controlled by eccentricity forcing (∼405 Kyr, ∼135–100 Kyr, and possibly ∼200 Kyr cycles). The dominance of eccentricity forcing in our record is consistent with coeval marine records, and indicates that modulation of low‐latitude summer insolation through nonlinear interactions with the global carbon cycle likely controlled hydroclimate and paleolake level in the Gonjo basin during the EECO. Our study offers new perspectives for the forcing mechanisms of terrestrial hydroclimate changes of East Asia in response to subtle changes in insolation during the EECO. Plain Language Summary: High resolution and relatively long‐duration terrestrial paleoenvironmental records across an interval of extreme warmth ∼50 million years ago (called the EECO) are rare, limiting our understanding of how the hydrological cycle operated during this time interval. In this study, we present high‐resolution geochemical data through a succession of continental sedimentary rocks from the Gonjo Basin (Southeast Tibet) spanning the EECO. Our data suggest that EECO hydroclimate was dominated by climate forcing controlled by variations in Earth's eccentricity, emphasizing the importance of eccentricity modulation of low‐latitude summer insolation through nonlinear interactions with the global carbon cycle. This finding is consistent with coeval marine records. Our study provides new insights into the forcing mechanisms of terrestrial paleoclimate change in East Asia under warm climate conditions and high CO2. Key Points: Astronomical timescale spanning the Early Eocene Climatic Optimum (EECO) established in East Asia terrestrial recordRelatively wetter and warmer climate during ∼52.9–52.39 Ma and ∼50.34–49.9 MaEccentricity forcing of EECO terrestrial climate, consistent with coeval marine records [ABSTRACT FROM AUTHOR]

The products from the Stable Water Isotope Intercomparison Group, Phase 2, are currently used for numerous studies, allowing water isotope model‐data comparisons with various isotope‐enabled atmospheric general circulation model (AGCMs) outputs. However, the simulations under this framework were performed using different parameterizations and forcings. Therefore, a uniform experimental design with state‐of‐the‐art AGCMs is required to interpret isotope observations rigorously. Here, we evaluate the outputs from three isotope‐enabled numerical models nudged by three different reanalysis products and investigate the ability of the isotope‐enabled AGCMs to reproduce the spatial and temporal patterns of water isotopic composition observed at the surface and in the atmospheric airborne water. Through correlation analyses at various spatial and temporal scales, we found that the model's performance depends on the model or reanalysis we use, the observations we compare, and the vertical levels we select. Moreover, we employed the stable isotope mass balance method to conduct decomposition analyses on the ratio of isotopic changes in the atmosphere. Our goal was to elucidate the spread in simulated atmospheric column δ18O, which is influenced by factors such as evaporation, precipitation, and horizontal moisture flux. Satisfying the law of conservation of water isotopes, this budget method is expected to explain various fractionation phenomena in atmospheric meteorological and climatic events. It also aims to highlight the spreads in modeled isotope results among different experiments using multiple models and reanalyses, which are primarily dominated by uncertainties in moisture flux and precipitation, respectively. Plain Language Summary: Our study focuses on surface and atmospheric water isotopes, which are crucial for understanding climate and environmental processes. We assessed the performance of different climate models that simulate water isotopes and compared them to real‐world observations. To accomplish this, we employed advanced atmospheric models that include isotopes and subjected them to different input data sets. We discovered that the accuracy of the simulations varied depending on the specific model and data used, as well as the vertical levels considered. By performing correlation analyses at different spatiotemporal scales, we obtained insights into how well the models align with the observed isotopic patterns in both surface and airborne water. Additionally, we utilized a stable isotope mass balance method to examine how various factors, such as evaporation, precipitation, and horizontal moisture flux, influence changes in the isotopic composition of the atmosphere. This method enabled us to identify the sources of uncertainty in the model results. Our research emphasizes the need for a standardized experimental design when studying water isotopes with climate models. By identifying the dominant sources of uncertainty, our findings will prove valuable for scientists from various disciplines and enhance the understanding of simulated future climate and water cycle studies. Key Points: We built a publicly available data set of isotope‐enabled nudged simulations from 1979 to 2020, utilizing three models and three reanalysesWe decomposed atmospheric processes impacting water isotopes, spanning evaporation to precipitation, leading to the first global estimationModeled water isotope spreads due to model and reanalysis choices were dominated by moisture flux and precipitation uncertainties [ABSTRACT FROM AUTHOR]

Research on how hydrological regimes affect the attributes of floodplain systems is fundamental to understanding ecosystem functioning. The shrimps Macrobrachium borellii (Nobili, 1896) and Palaemon argentinus (Nobili, 1901) are sympatric omnivorous species that are important components in the trophic webs of the Paraná River. This river presents a vast floodplain dominated by C3 plants, which in periods of high water (HW) are available as a trophic resource. We aimed to estimate the contribution of trophic resources to shrimp diets using stable isotope analysis of muscle and hepatopancreas and stomach content analysis. We expected that shrimp diets would vary between high and low water (LW) and that C3 plants would be an important food source for both shrimp during high water, especially for the larger and more agonistic shrimp species M. borellii. We also hypothesized that the two species consume prey at different trophic levels to avoid competition; due to the behavioral differences, we expect the niche of P. argentinus to be smaller than that of M. borellii. We found that both species consume C3 plants and that, unlike P. argentinus, M. borellii rapidly exploits the available C3 plants during HW. The niche breadth of M. borellii is generally larger than that of P. argentinus, while the niche overlap between both species is smaller during LW. The results obtained highlight the role of shrimp in aquatic food webs, as they can exploit C3 carbon sources during HW and make them available to other aquatic consumers. [ABSTRACT FROM AUTHOR]

The hydrological cycle, or water cycle, is one of the most important geochemical cycles on our planet. Normal functioning of its mechanisms (evaporation/evapotranspiration, condensation, and precipitation) is very important for the well-being of human beings. However, the acceleration of the hydrological cycle, mainly due to global warming, is increasing the frequency and intensity of extreme events (floods, droughts, and alterations in water resources) in many regions around the globe. This acceleration or intensification occurs because of rising temperature, which intensifies and speeds up evaporation (probable increase of 5.2%) and precipitation (probable increase of 6.5%); hence this scenario is escalating climate change. According to the datasets retrieved from the Global Land Data Assimilation System (GLDAS) of NASA, rain precipitation rate has shown changes in various regions of the world. Consequently, extreme and frequent events of heavy precipitation, floods, and droughts are also deteriorating the quality of water and preventing recharge of water reservoirs. Although some regions of the world will experience positive outcomes of this scenario in terms of water availability (due to frequent intense precipitation), most of the world’s regions are expected to face the daunting issue of water unavailability, as predicted by many researchers. [ABSTRACT FROM AUTHOR]

Abstract We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model version 4.1 (LM4.1), which builds on component and coupled model developments over 2013–2019 for the coupled carbon‐chemistry‐climate Earth System Model Version 4.1 (ESM4.1) simulation as part of the sixth phase of the Coupled Model Intercomparison Project. Analysis of ESM4.1/LM4.1 is focused on biophysical and biogeochemical processes and interactions with climate. Key features include advanced vegetation dynamics and multi‐layer canopy energy and moisture exchanges, daily fire, land use representation, and dynamic atmospheric dust coupling. We compare LM4.1 performance in the GFDL Earth System Model (ESM) configuration ESM4.1 to the previous generation component LM3.0 in the ESM2G configuration. ESM4.1/LM4.1 provides significant improvement in the treatment of ecological processes from GFDL's previous generation models. However, ESM4.1/LM4.1 likely overestimates the influence of land use and land cover change on vegetation characteristics, particularly on pasturelands, as it overestimates the competitiveness of grasses versus trees in the tropics, and as a result, underestimates present‐day biomass and carbon uptake in comparison to observations.

Abstract The Kuroshio‐Oyashio Extension and Gulf Stream oceanic frontal zones are characterized by enhanced activity of synoptic‐scale cyclones and anticyclones and vigorous air‐sea heat and moisture exchange in the cold season. However, the time‐mean air‐sea exchange attributed separately to cyclones and anticyclones has not been assessed. Here we quantify cyclonic and anticyclonic contributions around the frontal zones to surface turbulent heat fluxes, precipitation, and the associated hydrological cycle using atmospheric general circulation model experiments with observed and artificially smoothed sea‐surface temperature gradients. The evaluation reveals that precipitation exceeds evaporation climatologically within cyclonic domains while evaporation dominates within anticyclonic domains. These features as well as the net moisture transport from anticyclonic to cyclonic domains are all enhanced by the sharpness of the frontal zones. Oceanic frontal zones thus climatologically act to strengthen the hydrological cycle. These findings aid our understanding of the relationship between midlatitude air‐sea interactions on synoptic‐ and longer‐time scales.

The article presents the first comprehensive evaluation of the isotopic composition of precipitation and runoff in Georgia. It is based on data from the monitoring of δ18O and δ2H in monthly precipitation at eleven precipitation stations and monthly runoff from five large, mostly nested catchments in Georgia, collected between the years 2013 and 2022. The data represent different climatic and altitudinal conditions ranging from the wet western part of Georgia affected by the Black Sea to the dry eastern part of the county presumably affected also by the air masses from the Caspian Sea. Specific conditions affecting the isotopic composition of precipitation and river water seem to occur close to the main ridge of the Greater Caucasus Mountains. We defined the National Meteoric Water Line (NMWL) and the National River Water Line (NRWL) for Georgia as well as the Local Meteoric Water Lines (LMWLs) and River Water Lines (RWLs) for individual precipitation stations and river gauges and investigated their slopes and intercepts. Altitude gradients of δ18O and δ2H in the rivers were similar to those in precipitation (–0.16‰ and –1.2‰ per 100 m of altitude for δ18O and δ2H, respectively). The rivers were supplied from precipitation of the colder part of the year (November/December to March/April) while the isotopic composition of precipitation in the remaining warmer part of the year was clearly detached from that of the rivers. The isotopic signal in precipitation was dampened in the rivers by a factor of 5–6. The scaled line-conditioned excess (lc-excess*) did not indicate any significant deviations of river waters from meteorological conditions in the catchments.

Surface Water–Groundwater (SW–GW) interaction is a crucial aspect of the hydrological cycle and requires accurate modeling for reliable predictions. In many basic hydrological models and calculations, it is common to assume that the water density is constant. However, density-dependent flow, which accounts for changes in water density, plays a significant role in various hydrological processes. This study aims to quantify the effects of density-dependent flow on SW–GW interaction and evaluate the sensitivity of dominant hydrological drivers to density-dependent flow. Our simulations using the HydroGeoSphere model revealed that neglecting density-dependent flow in SW–GW interaction can lead to inaccurate estimations of water and solute balances. In particular, including density-dependent flow in the model yielded more realistic salinity distributions under gaining river scenarios and captured the gradual expansion of freshwater lenses under losing river scenarios. The results also indicated that under non-density-dependent flow, more saline groundwater is exposed to evapotranspiration, resulting in higher solute mass storage and a more saline unsaturated zone. Further, surface recharge and pumping rates played crucial roles in salinity distribution. This study highlights the critical importance of incorporating density-dependent flow in simulations, providing valuable insights for improving the accuracy of predictions and effectively managing water and solute balances in floodplain aquifers.

A good water budget involving four variables, including precipitation (P), evapotranspiration (ET), streamflow (R), and terrestrial water storage change (TWSC), is reflected in two aspects: a high accuracy against observations for each budget component and the low water budget closure residual error (ΔRes). Due to the lack of consideration of observations of budget components in existing water budget closure assessment methods (BCMs), when the ΔRes of budget components is low, their error against respective observations may still be high. In this study, we assess the water budget closure accuracy of satellite/reanalysis-based hydrological data products over mainland China based on six popular P products and multiple datasets of additional budget components (ET, R, and TWSC). The results indicated that the ΔRes changes between ±15 mm over mainland China. Satellite P products such as GPM IMERG showed better performance by comparing them with rain gauge-based observations. However, reanalysis P products such as GLDAS and FLDAS showed a better water budget closure since the selected datasets of additional budget components (ET and R) are also derived from reanalysis datasets. This indicates that these same data sources for budget components make it easier to close the water budget. The further development of satellite P products should consider the closure of the water budget with other water cycle variables. [ABSTRACT FROM AUTHOR]

Lacustrine rift basins are ideal continental settings for understanding organic matter (OM) accumulation mechanisms, because lacustrine deposits are most sensitive to variable climatic and palaeoenvironmental conditions. The Lunpola Basin is a Cenozoic lacustrine rift basin with widespread oil shale depositions that provide a valuable example of triggering factors that controlled OM accumulation. In the investigated interval of the basin, high Si, Ti, Zr, Th, Al and rare earth element concentrations suggest high clastic fluxes during the oil shale deposition. These high clastic fluxes could be attributed to warm and humid climates due to high values of ln(Al2O3/Na2O) and C‐value (Ʃ (Fe + Mn + Cr + Ni + V + Co)/Ʃ (Ca + Mg + Sr + Ba + K + Na)). The Lunpola shales and oil shales were deposited in saltwater environments under enhanced dysoxic to anoxic conditions with high palaeoproductivity. In the Lunpola Basin, climate warming can be subdivided into two different stages based on C‐value and ln(Al2O3/Na2O) variations. Each stage is consistent with an increase in the primary productivity of the palaeolake. Thus, a model for climate‐induced OM enrichment in lacustrine rift basins is proposed, which indicates that climate and primary productivity are the key factors controlling OM accumulation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sensitivity of land surface water cycle to the human activity is quantified. • Sensitivity varies widely among regions and hydrological variables. • Surface runoff was insensitive to human activity in main urban areas. • Lateral flow to stream flow was the most sensitive to human activity. While many studies have focused on the sensitivity of the water cycle to climate change, few have investigated this sensitivity to human activities. We don't know how sensitive the surface hydrological system is to human activity and which component of the water balance is most sensitive. This study performed model simulations to quantify the sensitivity of China's land surface water cycle to the human activity intensity (HAI). The results indicate that lateral flow to streamflow (LATQ) was the most sensitive to HAI (0.679), followed by base flow (GW, 0.291), actual evapotranspiration (ET, 0.203), and surface runoff (SURQ, 0.174), while the percolation amount (PERC) and water yield (WYLD) sensitivities were 0.172 and 0.081, respectively. Hotspots of the ET sensitivity to HAI occurred mainly in Western and Northeast China. The spatial sensitivity patterns of GW and PERC were similar, i.e., lower in the south than elsewhere. Notably, SURQ was insensitive to HAI changes in the urban areas of South China (sensitivity: 0–0.06). Although the HAI variation was the lowest in semiarid regions during 1980–2018 (0.07–0.82%), their water cycle sensitivity to HAI was the highest among all four climatic zones. The ET, SURQ, LATQ, PERC, GW and WYLD sensitivity coefficients to the HAI in the semiarid zone were 20.441, 41.292, 5.276, 24.431, 52.06 and 1.032, respectively. Hydrologically, semiarid areas, the Qinghai-Tibet Plateau and the region near the Karamay oilfield should be prioritized. Specifically, LATQ should receive the most attention. [ABSTRACT FROM AUTHOR]

Severe droughts of 2022 in Europe raise the question if one has sufficient knowledge and tools to predict and prevent such events. Analysis of 40 years of meteorological data for Hungary between 1981 and 2020 confirms significant upward trends with p = 0.05 in temperature, relative humidity, and net radiation. Shortwave net radiation has increased by 89.6 mm in water equivalent. Estimates of land evaporation and reference evapotranspiration also show a significantly increasing trend. The gap between them is widening with a significant trend in northeast Hungary. The demand (reference evapotranspiration) is increasing by 2.23 ± 0.3 mm yr--1, while supply (land evaporation) by 1.64 ± 0.2 mm yr--1. The opening gap can be interpreted as a decreasing stability of the energy distribution system, where water is the dominant energy transfer medium for climatic energy. This change could lead to an increased risk of droughts, a symptom of system underperformance. The climatic energy distribution process -- a key environmental service - is shifting to a new operating point where more water would be needed to transport energy. These trends call for a process capability approach. Desertification cannot be stopped by chasing water use efficiency and using less water, but vice versa: water scarcity can only be eliminated by ensuring that there is sufficient water available for evapotranspiration. This will raise the priority for initiatives to design and implement a wide range of preventing actions, such as nature-based solutions and water retention measures. [ABSTRACT FROM AUTHOR]

Droughts impact the water cycle, ecological balance, and socio-economic development in various regions around the world. The Orinoco River Basin is a region highly susceptible to droughts. The basin supports diverse ecosystems and supplies valuable resources to local communities. We assess the spatiotemporal patterns and impacts of droughts in the basin using remote sensing data and surface observations. We use monthly precipitation (P), air temperature near the surface (T2M), enhanced vegetation index (EVI) derived from Earth observations, and average daily flow (Q) data to quantify drought characteristics and impacts. We also investigated the association between drought and global warming by correlating the drought intensity and the percentage of dry area with sea surface temperature (SST) anomalies in the Pacific (Niño 3.4 index), Atlantic (North Atlantic Index [NATL]), and South Atlantic Index [SATL]) oceans. We evaluate the modulating effect of droughts on the hydrological regime of the most relevant tributaries by calculating the trend and significance of the regional standardized precipitation index (SPI) and percentage area affected by dry conditions. El Niño events worsen the region's drought conditions (SPI vs. Niño 3.4 index, r = −0.221), while Atlantic SST variability has less influence on the basin's precipitation regime (SPI vs. NATL and SATL, r = 0.117 and −0.045, respectively). We also found that long-term surface warming trends aggravate drought conditions (SPI vs. T2M anomalies, r = −0.473), but vegetation greenness increases despite high surface temperatures (SPI vs. EVI anomalies, r = 0.284). We emphasize the irregular spatial-temporal patterns of droughts in the region and their profound effects on the ecological flow of rivers during prolonged hydrological droughts. This approach provides crucial insights into potential implications for water availability, agricultural productivity, and overall ecosystem health. Our study underlines the urgent need for adaptive management strategies to mitigate the adverse effects of droughts on ecosystems and human populations. The insights derived from our study have practical implications for developing strategies to address the impacts of droughts and ensure the protection of this ecologically significant region. [ABSTRACT FROM AUTHOR]

The understanding of the spatiotemporal variability of the water molecule stable isotopes (δ18O and δ2H) in rain and groundwater has been used in hydrogeology to validate recharge zones. In this study, we analyzed the recharge of four aquifer systems located in northwestern Colombia. This region is characterized by highly complex topography, as the Andes Mountain range splits into three branches, two of them included in the study area. The relation in the variation of values of the δ18O‰ between Precipitation (P) and groundwater (GW) was analyzed through the arithmetic expression P/GW; when P/GW is equal or larger than 1, the recharge is direct, and values less than 1 indicate recharge from regional flows. For the purposes of this research, according to statistics criteria, values between 0.98 and 1.02 are considered as 1. It was found that on Bajo Cauca, Occidente and Urabá, the phreatic aquifers are recharged directly with rainwater or after slight evaporation processes, while the recharge of deep aquifers occur through regional flows. The Valle de Aburrá´s phreatic aquifer is also recharged from distant areas; this occurs because the surface has been impermeabilized by urban processes. The P/GW ratio seems to be useful in identifying recharge processes in regions with higher elevation gradients. In low-lying areas, the applicability of this method should be restricted.

The Tabu catchment, a typical-desertified steppe in China, was selected as the study area to qualitatively analyze the interaction between surface water (SW) and groundwater (GW), and an integration of hydrochemical analysis and isotopic techniques was applied. The results show that the ion contents in SW and GW increased from upstream to downstream, and the hydrochemical evolutions were both controlled by rock weathering and influenced by evaporation. The δD–δ18O lines of SW and GW were δD = 5.14δ18O − 24.68 and δD = 6.89δ18O − 5.81, respectively. Along the I–I′ profile, the contents of most indices, δD and δ18O in SW and GW both showed increasing tendencies. All of the similarities in the hydrochemical characteristics and isotopic techniques indicated that SW was recharged by GW. The δD–δ18O inconsistency in SW and GW samples from midstream and downstream areas indicated that SW did not recharge to GW in these areas and was consumed by evaporation or replenished the moisture in the vadose zones. The runoff decreased, which was mainly caused by excessive exploitation of GW and a decline in the GW level. This study deepens the understanding of the hydrological cycle and provides guidance for the optimal combined utilization of SW and GW. [ABSTRACT FROM AUTHOR]

This study assessed the phenology and functional structure of parental stocks of fish larvae, and evaluated how the species modulate their reproductive activities as a function of natural hydrological regimes in the lower Amazon River, Brazil.Larvae were collected using conical plankton nets monthly over two hydrological cycles between January and December 2013, and between May 2017 and April 2018. The collections were conducted around an alluvial island located near the municipality of Santarém, Pará, Brazil. After identification, individuals were classified into four functional groups (e.g., life‐history strategy, feeding habits, type of migration, and swimming‐performance/microhabitat‐use in the water column) based on species traits. The relationships between seasonal hydrology, environmental variables and larval assemblage structure were evaluated using multivariate analyses.A total of 31,347 fish larvae were collected, from nine taxonomic orders, 26 families, 40 genera and 53 species. The larval assemblages showed highly variable taxonomic and functional composition between different phases of the hydrological cycle (rising water, high water, receding water and low water) and were strongly influenced by environmental variables. Fish reproductive phenology in the lower Amazon is complex and distributed among the hydrological phases. The different ecological characteristics allow the species to withstand specific environmental conditions during each phase.Changes in the structure of larval assemblages indicate that hydrological variability is a key ecological factor influencing the reproductive dynamics of fish and reflects adaptive strategies used to reduce interspecific competition for space and food, maximising recruitment. The variation of fish reproduction across the hydrological cycle that we observed is a result of different ecological characteristics enabling species and functional groups to adapt to environmental conditions of each phase. [ABSTRACT FROM AUTHOR]

When projecting future monsoon changes by carbon dioxide (CO2) pathway, most studies have analyzed precipitation responses without considering monsoon area (total area of monsoon domain, MA) variations. However, how MA responds to CO2 removal remains uncertain. This study evaluates MA variations and impacts in idealized CO2 ramp‐up (toward CO2 quadrupling), ramp‐down, and stabilized simulations using the Community Earth System Model version 1. Global MA negatively overshoots (i.e., recovery with decreasing tendency beyond the original MA) during the ramp‐down period due to reduced or rapidly recovered MA in several regional monsoons, including Northern and Southern Africa, South and East Asia, and South America, showing hysteresis when comparing CO2 ramp‐up and ramp‐down periods despite similar global warming levels. These non‐linear regional MA variations come from distinct regional summer and winter precipitation variations, which are found to be mostly associated with Intertropical Convergence Zone movements and El Niño‐like response. Further, regional monsoon precipitation characteristics also vary through CO2 ramp‐up and ramp‐down periods consistently with overall hysteresis. Changes in total monsoon precipitation resemble the distinct responses of MA. Our results suggest that regions characterized by a monsoonal climate may experience reduced seasonal rainfall variations under net‐negative CO2 emissions. Plain Language Summary: Many studies have researched how increasing carbon dioxide (CO2) affects monsoon. Most have focused on precipitation changes without considering how monsoon area (i.e., total area of monsoon domain, MA) will change. Furthermore, it is unknown how global and regional MA responds to CO2 removal. In this study, we first investigate how global to regional MA changes under CO2 concentration increase, decrease and then equilibration using climate model simulations. After CO2 removal, the global MA is largely reduced below the present‐day state, as MA decreases over several monsoon regions. Further, when we compare MA at the same global warming level in CO2 ramp‐up and ramp‐down periods, many individual monsoon regions show reduced MA during the ramp‐down period. As MA is defined based on local contrast of summer and winter precipitation, we investigate hemispheric summer and winter precipitation responses and how they affect MA. The regional changes in MA are found to be due to distinct variations of local summer or winter precipitation, which are commonly associated with large‐scale atmospheric and oceanic variations. Further impacts of changes in MA with CO2 removal on the hydrological cycle more generally, are identified. Key Points: Non‐linear responses in global and regional monsoon areas to CO2 pathway are investigated using a coupled climate modelDistinct hysteresis in monsoon area variation is identified over global to regional monsoons between CO2 ramp‐up and ramp‐down periodThe hysteresis emerges due to distinct precipitation changes with large‐scale atmospheric and oceanic responses [ABSTRACT FROM AUTHOR]

It is of great importance to study the changes in reference evapotranspiration (ET0) and the factors that influence it to ensure sustainable and efficient water resource utilization. Daily ET0 data calculated using the Penman–Monteith method from 37 meteorological stations located within Guangdong Province in the humid zone of southern China from 1960 to 2020 were analyzed. The trend analysis and Mann–Kendall test were used to analyze the time series changes in ET0 and major climatic factors (air temperature (T), relative humidity (RH), sunshine duration (SD), and wind speed (u2)) for over 61 years. Sensitivity and contribution analyses were used to evaluate the driving factors of ET0. The main findings of the study are as follows: (1) the trend in average annual ET0 time series in Guangdong slightly increased at a trend rate of 1.61 mm/10a over the past 61 years, with most stations experiencing an increase in ET0. During the same period, air temperature significantly increased, while RH and SD decreased; u2 also decreased. (2) Sensitivity analysis showed that ET0 was more sensitive to RH and T than SD and u2, with ET0 being most sensitive to RH in spring and winter and T in summer and autumn. (3) The contribution analysis showed that T was the dominant factor for ET0 variation in Guangdong, followed by SD. SD was found to be the dominant factor in ET0 changes in areas where the "evaporation paradox" occurred, as well as in spring and summer. The study concludes that the climate in Guangdong became warmer and drier over the past 61 years, and if the current global warming trend continues, it will lead to higher evapotranspiration and drought occurrence in the future. [ABSTRACT FROM AUTHOR]

Aquatic ecosystems in arid regions are important habitats for aquatic wildlife and also support species biodiversity. Soda lakes in endorheic drainage basins, so‐called amplifier lakes, are sentinels of climate change and are of scientific significance.Here we present the results of a long‐term study (20 years) of the Torey lakes, Transbaikalia, Russia, including the period from the maximum capacity of the Torey lakes through to aridification, and then to the initial phase of refilling. Our aim was to determine the response of planktonic communities in the lakes to environmental change over this period.We conducted our studies during various parts of the hydrological cycle, including three phases for Lake Zun‐Torey (high [1999, 2003], intermediate [2007, 2011] and low [2014, 2016] water levels) and two phases for Lake Barun‐Torey (high level [1999, 2003] and the initial refilling phase [2014, 2016, 2018, 2020]). Lake Barun‐Torey dried up completely in summer 2009, and since 2013 has been undergoing initial refilling. Lake Zun‐Torey dried up in the autumn of 2017.Temporal variation in planktonic communities differs for each lake. We identified four stages of changes in the planktonic associations during the aridification of Lake Zun‐Torey (the phytoplankton experienced a decrease in abundance and biomass, and was dominated by green algae and diatoms; the zooplankton underwent a decrease in species diversity and an increase in total abundance and biomass, which were caused by an increase in the abundance of some crustacean species) and three stages of changes as the lake level fell and during the initial refilling of Lake Barun‐Torey (the plankton communities increased in species richness and abundance in the transient pools).These results show the complexity of long‐term community dynamics in arid‐zone soda lakes. They have revealed how phytoplankton and zooplankton community diversity and structure can vary with changing environmental conditions and may pass between distinct states related to the stages of the hydroperiod. [ABSTRACT FROM AUTHOR]

Rainforests play an important role in hydrological and carbon cycles, both at regional and global scales. They pump large quantities of moisture from the soil to the atmosphere and are major rainfall hotspots of the world. Satellite-observed stable water isotope ratios have played an essential role in determining sources of moisture in the atmosphere. Satellites provide information about the processes involving vapour transport in different zones of the world, identifying sources of rainfall and distinguishing moisture transport in monsoonal systems. This paper focuses on major rainforests of the world (Southern Amazon, Congo and Northeast India) to understand the role of continental evapotranspiration in influencing tropospheric water vapour. We have used satellite measurements of 1H2H16O/1H216O from Atmospheric InfraRed Sounder (AIRS), evapotranspiration (ET), solar-induced fluorescence (SIF), precipitation (P), atmospheric reanalysis-derived moisture flux convergence (MFC) and wind to discern the role of ET in influencing water vapour isotopes. A global map of the correlation between δ2Hv and ET-P flux indicates that densely vegetated regions in the tropics show the highest positive correlation (r > 0.5). Using mixing models and observations of specific humidity and isotopic ratio over these forested regions, we discern the source of moisture in pre-wet and wet seasons. [ABSTRACT FROM AUTHOR]

The hydrological cycle in irrigation areas under the influence of anthropogenic activities has emerged as a significant topic in the realm of water science research. The southern hilly irrigation area of China, characterized by its undulating terrain and myriad ponds, presents a unique opportunity for water storage and paddy field irrigation. However, these distinct landscape features have also given rise to a complex hydrological cycle with unclear exchange dynamics and a paucity of quantitative calculations on the reuse of return flow. To explore the fundamental laws governing water circulation in irrigation areas, we embarked on in-situ field trials over the 2a period (2021-2022), with the Yang-shu-dang watershed in the Zhanghe Irrigation System of Hubei Province, China, serving as our study area. Our approach entailed monitoring the groundwater depth of 4 typical paddy fields, the water storage levels of 5 typical ponds, as well as the flow of the bucket ditch outlet. The complexity of the hydrological cycle in southern hilly irrigation areas necessitates the development of a novel water balance formula for paddy fields, ponds, and ditches, respectively, that accurately captures their unique features. To this end, we conceived of a method for reviewing the water volume of the source and sink areas in paddy field irrigation water delivery, known as 'start-to-end comparison', which enables us to discern the locations in which reuse of return flow is taking place. Through the evaluation of the hydrological cycle and the extent of reuse of return flow during various reproductive stages, we obtained valuable insights into the significant ways in which human activities have disrupted the hydrological balance in southern hilly irrigation areas. The findings of our study revealed several valuable insights. First, the relative error for the 2-year period of paddy irrigation water amount computed by the water balance formula and the DRAINMOD model, as well as the total irrigation water volume from ponds, ditches, and channels were 4.65% and −2.74%, respectively, implying that 'start-to-end comparison' is a reliable and robust method that provides a new ideal for irrigation water volume simulation and review in multi-source irrigation areas. Further analysis revealed that the proportion of channel, ditch, and pond sources in different paddy irrigation sources were 9.77%, 71.81%, and 18.42%, respectively. Notably, due to the need for irrigation in paddy fields, 82.73% of initial pond water storage was consumed during whole reproductive stages. Moreover, canal supply, precipitation, and ditch supply accounted for 38.15%, 29.98%, and 14.60%, respectively. It transpired that the late tillering stage featured the highest reuse of water from the pond, with a gradual decline observed during the whole reproductive stage. While both the amount of return flow in the ditch and the amount of reused return flow water increased initially and then decreased, the ditch still emerged as the primary site exhibiting the reuse of return flow, with rates doubling that of ponds (80.86%). We also delved into the consumption pathways of irrigation water after channel irrigation, where in the first distribution of channel irrigation water accounted for only 29.44% into the paddy field. A second distribution facilitated the entrance of at least 72.10% of channel irrigation water into the paddy field. Intriguingly, only 3.44% of the water from channel irrigation was discharged out of the bucket ditch. The remaining 24.46% of the water lingered within the study area, with the potential for subsequent reuse. Examination of the hydrological cycle revealed that the paddy field system was characterized primarily by the vertical movement of irrigation-evaporative transpiration, while the pond system was characterized predominantly by the lateral movement of channel supply-pumping irrigation, and the ditch system was characterized primarily by the lateral movement of paddy drainage-pumping irrigation. Research showed that human activity is the main factor of effect ponds and ditch hydrological cycle. Strategies from irrigation district managers, ‘the main for ponds, the secondary for paddy’ met the needs of farmers and own high irrigation coefficient in southern hilly irrigated area. [ABSTRACT FROM AUTHOR]