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.
Rapidly rising river stages induced by flood events lead to considerable river water infiltration into aquifers and carry surface‐borne solutes into hyporheic zones which are widely recognized as an important place for the biogeochemical activity. Existing studies for surface‐groundwater exchanges induced by flood events usually limit to a river‐aquifer cross section that is perpendicular to river channels, and neglect groundwater flow in parallel with river channels. In this study, surface‐groundwater exchanges to a flood event are investigated with specific considerations of unconfined flow in direction that is in parallel with river channels. The groundwater flow is described by a two‐dimensional Boussinesq equation and the flood event is described by a diffusive‐type flood wave. Analytical solutions are derived and tested using the numerical solution. The results indicate that river water infiltrates into aquifers quickly during flood events, and mostly returns to the river within a short period of time after the flood event. However, the rest river water will stay in aquifers for a long period of time. The residual river water not only flows back to rivers but also flows to downstream aquifers. The one‐dimensional model of neglecting flow in the direction parallel with river channels will overestimate heads and discharge in upstream aquifers. The return flow induced by the flood event has a power law form with time and has a significant impact on the base flow recession at early times. The solution can match the observed hydraulic heads in riparian zone wells of Iowa during flood events.
Stream bank storage effects during floods have received limited attention, despite the significant role of such floods in aquifer water budgets. One reason is the complexity of geometry of the problem, which commonly has been treated numerically. Using a simple model in a domain with moving boundary, a semianalytical solution for bank storage effects is proposed to account for stream stage hydrograph, floodplain slope, and aquifer parameters. The results extend classic solutions by Cooper and Rorabaugh (1963, https://doi.org/10.3133/wsp1536J) for idealized vertical streambanks but applied to realistic floodplain cross sections. The accuracy of the semianalytical solution is verified by a one‐dimensional numerical method and compared to a vertical two‐dimensional variably saturated‐flow numerical model. Comparison indicates that a robust solution is valid for diagnostic analyses of modeling bank storage effects on floodplains. The semianalytical solution is applied to laboratory experiments as well. The results indicate that the present solution provides reasonable estimates of peak timing and head of groundwater flow response in the sloping bank during varying stream stage.
Unsaturated flow is an important process in base flow recessions and its effect is rarely investigated. A mathematical model for a coupled unsaturated‐saturated flow in a horizontally unconfined aquifer with time‐dependent infiltrations is presented. The effects of the lateral discharge of the unsaturated zone and aquifer compressibility are specifically taken into consideration. Semianalytical solutions for hydraulic heads and discharges are derived using Laplace transform and Cosine transform. The solutions are compared with solutions of the linearized Boussinesq equation (LB solution) and the linearized Laplace equation (LL solution), respectively. A larger dimensionless constitutive exponent κD (a smaller retention capacity) of the unsaturated zone leads to a smaller discharge during the infiltration period and a larger discharge after the infiltration. The lateral discharge of the unsaturated zone is significant when κD≤1, and becomes negligible when κD≥100. The compressibility of the aquifer has a nonnegligible impact on the discharge at early times. For late times, the power index b of the recession curve −dQ/dt∼ aQb, is 1 and independent of κD, where Q is the base flow and a is a constant lumped aquifer parameter. For early times, b is approximately equal to 3 but it approaches infinity when t→0. The present solution is applied to synthetic and field cases. The present solution matched the synthetic data better than both the LL and LB solutions, with a minimum relative error of 16% for estimate of hydraulic conductivity. The present solution was applied to the observed streamflow discharge in Iowa, and the estimated values of the aquifer parameters were reasonable.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.