Responses of groundwater levels to Earth tides have been widely studied to evaluate aquifer properties because it has been shown to be an economical and effective approach to estimate aquifer parameters. Existing studies suggest that unsaturated zones may have non‐negligible effects on groundwater responses to Earth tides in an unconfined aquifer. However, an analytical model for these effects is unavailable; as a result, the impacts of the unsaturated flow on the tidal response of water levels has not been fully explored. Here we present an analytical solution for the coupled unsaturated‐saturated flow equation, which is linearized by the perturbation method, to study the hydraulic head responses to Earth tides. The solutions are compared with that from numerical simulation using the finite‐element method built in COMSOL Multiphysics and an existing numerical model. The results indicate that the unsaturated zone has significant impacts on the hydraulic head responses to Earth tides. The traditional model for an unconfined aquifer that neglects the effects of an unsaturated zone commonly underestimates the amplitude ratio and overestimates the phase shift. The fluctuations of the water table near the ground surface (<1.0 m) causes dramatic variations of both amplitude ratio and phase shift, which in turn cause the traditional model to fail in estimating aquifer parameters. The solutions are applied to field data to interpret large seasonal variations in the tidal responses. The solutions derived in this study should be an important addition to existing analytical models for tidal analysis.

Lakes are important natural resources and carbon gas emitters and are undergoing rapid changes worldwide in response to climate change and human activities. A detailed global characterization of lakes and their long-term dynamics does not exist, which is however crucial for evaluating the associated impacts on water availability and carbon emissions. Here, we map 3.4 million lakes on a global scale, including their explicit maximum extents and probability-weighted area changes over the past four decades. From the beginning period (1984–1999) to the end (2010–2019), the lake area increased across all six continents analyzed, with a net change of +46,278 km2, and 56% of the expansion was attributed to reservoirs. Interestingly, although small lakes (<1 km2) accounted for just 15% of the global lake area, they dominated the variability in total lake size in half of the global inland lake regions. The identified lake area increase over time led to higher lacustrine carbon emissions, mostly attributed to small lakes. Our findings illustrate the emerging roles of small lakes in regulating not only local inland water variability, but also the global trends of surface water extent and carbon emissions.

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