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Life on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of essential climate variables (ECVs), many related to the water cycle, required to systematically monitor Earth’s climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, and resolution, to consistently characterize water cycle variability at multiple spatial and temporal scales. Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model–data synthesis capabilities, particularly at regional to local scales.

Central Asia (CA; 35°–55°N, 55°–90°E) has been experiencing a significant warming trend during the past five decades, which has been accompanied by intensified local hydrological changes. Accurate identification of variations in hydroclimatic conditions and understanding the driving mechanisms are of great importance for water resource management. Here, we attempted to quantify dry/wet variations by using precipitation minus evapotranspiration (P–E) and attributed the variations based on the atmosphere and surface water balances. Our results indicated that the dry season became drier while the wet season became wetter in CA for 1982–2019. The land surface water budget revealed precipitation (96.84%) and vapor pressure deficit (2.26%) as the primary contributing factors for the wet season. For the dry season, precipitation (95.43%), net radiation (3.51%), and vapor pressure deficit (−2.64%) were dominant factors. From the perspective of the atmospheric water budget, net inflow moisture flux was enhanced by a rate of 72.85 kg m−1 s−1 in the wet season, which was mainly transported from midwestern Eurasia. The increase in precipitation induced by the external cycle was 11.93 mm (6 months)−1. In contrast, the drying trend during the dry season was measured by a decrease in the net inflow moisture flux (74.41 kg m−1 s−1) and reduced external moisture from midwestern Eurasia. An increase in precipitation during the dry season can be attributed to an enhancement in local evapotranspiration, accompanied by a 4.69% increase in the recycling ratio. The compounding enhancements between wet and dry seasons ultimately contribute to an increasing frequency of both droughts and floods.

The increasing availability of remote sensing products for all components of the terrestrial water cycle makes it now possible to evaluate the potential of water balance closure purely from remote sensing sources. We take precipitation (P) from the TMPA and CMORPH products, a Penman‐Monteith based evapotranspiration (E) estimate derived from NASA Aqua satellite data and terrestrial water storage change (ΔS) from the GRACE satellite. Their combined ability to close the water budget is evaluated over the Mississippi River basin for 2003–5 by estimating streamflow (Q) as a residual of the water budget and comparing to streamflow measurements. We find that Q is greatly overestimated due mainly to the high bias in P, especially in the summer. Removal of systematic biases in P reduces the error significantly. However, uncertainties in the individual budget components due to simplifications in process algorithms and input data error are generally larger than the measured streamflow.

Modern rates of water discharge often exceed groundwater recharge in arid catchments. This apparent mass imbalance within a catchment may be reconciled through either groundwater flow between topographic drainages and/or the draining of stored groundwater recharged during pluvial periods. This study investigates discrepancies in the modern hydrologic budget of catchments along the west flank of the Andes in northern Chile (21–25° S), focusing on the endorheic Salar de Atacama basin and adjacent basins. Uncertainty-bounded estimates of modern recharge rates are presented, which do not come close to balancing observed modern groundwater discharge within topographic catchments. Two conceptualizations of hydrogeologic catchments discharging to Salar de Atacama were explored with a simplified two-dimensional groundwater model. Results from the models support the interpretation that subsurface interbasin flow and transient drainage of groundwater from storage are required to balance water budgets along the plateau margin. The models further examine whether this system is still responding to climatic forcing (on paleoclimatic time scales) from pluvial periods and highlight general characteristics for similar plateau margin systems including: (1) water level changes at the plateau margin that are highly sensitive to long-term (100–1,000 years) changes in recharge on the plateau, (2) extent and magnitude of the changes in water table, which are controlled by the distribution of hydraulic conductivity at the margin, and (3) the contributing area to the lower-elevation catchment, which is itself dynamic and not coincident with the topographic boundary.

Observation-based climate model evaluation and future projections help policymakers in developing action plans for efficient management of water resources and mitigation of the impacts of hazardous extremes. A part from this socioeconomic importance, the scientific value cannot be overstated, especially in light of the upcoming Fifth U.S. National Climate Assessment (NCA) report. In this study, we evaluate the realism of hydroclimate variability in the historical simulations of a suite of coupled general circulation models (CGCMs) participating in the fifth and sixth phases of the Coupled Model Intercomparison Project (CMIP5 and CMIP6). Our results demonstrate systematic biases in the simulated seasonal precipitation—most prominently, wet bias over the mountainous western United States in winter, and dry bias over the U.S. central plains in summer. A distinctive feature of this work is our focus on the examination of the atmospheric water budget, in particular the relative importance of remote and local contributions—convergence of moisture fluxes and local land surface processes (evapotranspiration) respectively—in helping produce precipitation. This diagnosis reveals that the leading contribution of the remote influence in winter is overestimated by the CMIP6 multimodel mean (MMM), whereas the local influence, which is more influential in summer, is underestimated. Our results aid in understanding the drivers of seasonal precipitation over the United States, where precipitation will likely increase by the end of the century but with significant model disagreement for the summer and fall. In support of ongoing NCA efforts, our study aims to contribute a comprehensive, regional-level analysis of the moisture budget and emphasizes the importance of realistically simulating its major components in CGCMs.

Characterization of groundwater-surface water interaction in the Sapanca Lake basin (eastern Marmara region of Turkey) is critical for sustainable management of freshwater resources in the region. In this study, the groundwater-surface water interaction in the Sapanca Lake basin has been evaluated using multi-method approaches including hydrogeological, hydrogeochemical and environmental isotope analyses, seepage meter measurements and lake water budget analysis. Alluvial and karst aquifer systems represent the most productive aquifer systems in the Sapanca lake basin. Groundwater flow in the karst aquifer is relatively fast and primarily governed by NE and NW trending fractures developed by active tectonism in the region. The alluvial aquifer system at the south of the lake basin acts as a transition zone between the lake and the karst aquifer. It is recharged not only by infiltration but also periodically from streams and significantly from mountain-front recharge mechanisms through the karst aquifer. Although the existence of direct discharges into lake from the marbles of karst aquifer were not noted in field studies, both lake water budget calculations and seepage measurements on the lake bed revealed that groundwater contribution in lake water budget was about 34% of total inflow. Seepage measurements showed that the contribution of the alluvial aquifer to the recharge of the lake remained negligible. This study demonstrates the existence of karstic discharges into the lake occurring through deep fracture-controlled flow paths developed in karst aquifer, and their importance in the groundwater recharge mechanism of Sapanca Lake.

Accurate estimates of terrestrial water and energy cycle components are needed to better understand climate processes and improve models’ ability to simulate future change. Various observational estimates are available for the individual budget terms; however, these typically show inconsistencies when combined in a budget. In this work, a Conserving Land–Atmosphere Synthesis Suite (CLASS) of estimates of simultaneously balanced surface water and energy budget components is developed. Individual CLASS variable datasets, where possible, 1) combine a range of existing variable product estimates, and hence overcome the limitations of estimates from a single source; 2) are observationally constrained with in situ measurements; 3) have uncertainty estimates that are consistent with their agreement with in situ observations; and 4) are consistent with each other by being able to solve the water and energy budgets simultaneously. First, available datasets of a budget variable are merged by implementing a weighting method that accounts both for the ability of datasets to match in situ measurements and the error covariance between datasets. Then, the budget terms are adjusted by applying an objective variational data assimilation technique (DAT) that enforces the simultaneous closure of the surface water and energy budgets linked through the equivalence of evapotranspiration and latent heat. Comparing component estimates before and after applying the DAT against in situ measurements of energy fluxes and streamflow showed that modified estimates agree better with in situ observations across various metrics, but also revealed some inconsistencies between water budget terms in June over the higher latitudes. CLASS variable estimates are freely available via https://doi.org/10.25914/5c872258dc183.

The construction of a surface reservoir in a karst area will inevitably change the regional hydrogeological conditions and impact the groundwater circulation process and recharge dynamics. In this study, a representative synclinal basin-type karst groundwater system in the Yangzhuang Basin, northern China, was selected to analyze the influences of surface reservoir construction on the groundwater circulation mechanism and recharge conditions through multi-factor regime monitoring, multiple regression analysis, and the groundwater balance equation. Following the filling of the Zhuangli Reservoir, the hydraulic gradient in the karst groundwater system gradually decreased from upstream to downstream, accompanied by a decrease in the groundwater depression cone area. The flow of a downstream karst spring increased, while the responsiveness of karst groundwater levels to surface water runoff significantly decreased due to the influence of artificial storage. Karst groundwater levels remained significantly influenced by precipitation. Comparative analysis of conditions before and after the reservoir construction of the Zhuangli Reservoir showed that the amount of recharge to karst aquifers increased from 8598.56 × 10 4 m 3 /year to 11,731.89 × 10 4 m 3 /year, with seepage from the reservoir becoming one of the main sources of karst groundwater recharge in the Yangzhuang Basin. Concurrently, the discharge of karst groundwater resources decreased from 10,443.29 × 10 4 m 3 /year to 9994.42 × 10 4 m 3 /year, and the water budget for the karst groundwater system changed from negative to positive.

In this study, the net water flux (precipitation minus evaporation) over the Tibetan Plateau (TP) and its 12 drainage basins is estimated using ERA5. The terrestrial branch of the water cycle is investigated using the total water storage anomalies (TWSAs) derived from GRACE (Gravity Recovery and Climate Experiment) data and daily streamflow records collected in Zhimenda and Tangnaihai (two hydrological stations located in the upper Yangtze River Basin and upper Yellow River Basin). This work provides a preliminary assessment of discrepancies between model-derived and space-based observations in the atmospheric–terrestrial water cycle over the TP and its drainage basins. The results show that the net water fluxes occurring over the TP and the scale of its drainage basins are closely tied to local dynamics and physical processes and to large-scale circulation and atmospheric water vapor. ERA5 maintains the atmospheric water balance over the TP. ERA5-derived net water flux anomalies constitute a major component of the water cycle and correspond to GRACE-derived TWSAs. The water budget–based approach with the ERA5 and ITSG-Grace2018 datasets constrains the atmospheric–terrestrial water cycle over the TP and its drainage basins. Both the ERA5- and GRACE-derived estimates contain consistent long- and short-term variations over the TP. Discrepancies are evident at the drainage basin, while the ratio of signal to noise in both the ERA5 and GRACE datasets might cause discrepancies between estimates over relatively small or arid basins. Nevertheless, the observed good correspondence between ERA5- and GRACE-derived atmospheric–terrestrial water cycles over the TP highlights the potential value of the rational application of water resource information.

With an extent of ~1,860,000 km2, the Upper Mega Aquifer System on the Arabian Platform forms one of the largest aquifer systems of the world. It is built up by several bedrock aquifers (sandstone and karstified limestone aquifers), which are imperfectly hydraulically connected to each other. The principal aquifers are the Wasia-Biyadh sandstone aquifer, and the karstified Umm Er Radhuma and Dammam limestone aquifers. The stored groundwater is mainly fossil. Groundwater recharge took place in the geologic past under more humid climatic conditions. Due to the good water quality and high yield, the aquifers are intensively exploited, which has caused depletion of the groundwater resources. The presented qualitative and semi-quantitative description of the hydrogeology and the groundwater budget is the basis for integrated groundwater management of the aquifer system.