[1] The water table fluctuation method for determining recharge from precipitation and water table measurements was originally developed on an event basis. Here a new multievent time series approach is presented for inferring groundwater recharge from longterm water table and precipitation records. Additional new features are the incorporation of a variable specific yield based upon the soil moisture retention curve, proper accounting for the Lisse effect on the water table, and the incorporation of aquifer drainage so that recharge can be detected even if the water table does not rise. A methodology for filtering noise and non-rainfall-related water table fluctuations is also presented. The model has been applied to 2 years of field data collected in the Tomago sand beds near Newcastle, Australia. It is shown that gross recharge estimates are very sensitive to time step size and specific yield. Properly accounting for the Lisse effect is also important to determining recharge.Citation: Crosbie, R. S., P. Binning, and J. D. Kalma (2005), A time series approach to inferring groundwater recharge using the water table fluctuation method, Water Resour. Res., 41, W01008,

Fluoride contamination of groundwater, both anthropogenic and natural, is a major problem worldwide. In this study, fluoride removal by crushed limestone (99% pure calcite) was investigated by batch studies and surface-sensitive techniques from solutions with fluoride concentrations from 150 micromol/L (3 mg/L) to 110 mM (approximately 2100 mg/L). Surface-sensitive techniques, including atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) as well as zeta potential measurements, confirm that, in addition to precipitation reactions, adsorption of fluoride also occurs. Results indicate that fluoride adsorption occurs immediately over the entire calcite surface with fluorite precipitating at step edges and kinks, where dissolved Ca2+ concentration is highest. The PHREEQ geochemical model was applied to the observed data and indicates that existing models, especially at low fluoride concentrations and high pH (>7.5) are not equipped to describe this complex system, largely because the PHREEQ model includes only precipitation reactions, whereas a combination of adsorption and precipitation parameters are required.

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Urban water supplies are traditionally based on limited freshwater resources located outside the cities. However, a range of concepts and techniques to exploit alternative water resources has gained ground as water demands begin to exceed the freshwater available to cities. Based on 113 cases and 15 in-depth case studies, solutions used to increase water self-sufficiency in urban areas are analyzed. The main drivers for increased self-sufficiency were identified to be direct and indirect lack of water, constrained infrastructure, high quality water demands and commercial and institutional pressures. Case studies demonstrate increases in self-sufficiency ratios to as much as 80% with contributions from recycled water, seawater desalination and rainwater collection. The introduction of alternative water resources raises several challenges: energy requirements vary by more than a factor of ten amongst the alternative techniques, wastewater reclamation can lead to the appearance of trace contaminants in drinking water, and changes to the drinking water system can meet tough resistance from the public. Public water-supply managers aim to achieve a high level of reliability and stability. We conclude that despite the challenges, self-sufficiency concepts in combination with conventional water resources are already helping to reach this goal.

SUMMARYAn automatic time stepping scheme with embedded error control is developed and applied to the moisture-based Richards equation. The algorithm is based on the ÿrst-order backward Euler scheme, and uses a numerical estimate of the local truncation error and an e cient time step selector to control the temporal accuracy of the integration. Local extrapolation, equivalent to the use of an unconditionally stable Thomas-Gladwell algorithm, achieves second-order temporal accuracy at minimal additional costs. The time stepping algorithm also provides accurate initial estimates for the iterative non-linear solver. Numerical tests conÿrm the ability of the scheme to automatically optimize the time step size to match a user prescribed temporal error tolerance.An important merit of the proposed method is its conceptual and computational simplicity. It can be directly incorporated into existing or new software based on the backward Euler scheme (currently prevalent in subsurface hydrologic modelling), and markedly improves their performance compared with simple ÿxed or heuristic time step selection. The generality of the approach also makes possible its use for solving PDEs in other engineering applications, where strong non-linearity, stability or implementation considerations favour a simple and robust low-order method, or where there is a legacy of backward Euler codes in current use.

A numerical algorithm for simulation of two‐phase flow in porous media is presented. The algorithm is based on a modified Picard linearization of the governing equations of flow, coupled with a lumped finite element approximation in space and dynamic time step control. Numerical results indicate that the algorithm produces solutions that are essentially mass conservative and oscillation free, even in the presence of steep infiltrating fronts. When the algorithm is applied to the case of air and water flow in unsaturated soils, numerical results confirm the conditions under which Richards's equation is valid. Numerical results also demonstrate the potential importance of air phase advection when considering contaminant transport in unsaturated soils. Comparison to several other numerical algorithms shows that the modified Picard approach offers robust, mass conservative solutions to the general equations that describe two‐phase flow in porous media.

[1] Noniterative implicit time stepping schemes with adaptive temporal truncation error control are developed for the solution of the pressure form of Richards equation. First-and second-order linearizations of an adaptive backward Euler/Thomas-Gladwell formulation are introduced and are shown to constrain the temporal truncation errors near a userprescribed tolerance and maintain adequate mass balance. Numerical experiments demonstrate that accurate noniterative linearizations achieve cost-effective solutions of problems where soils are described by highly nonlinear and nonsmooth constitutive functions. For these problems many conventional iterative solvers fail to converge. The noniterative formulations are considerably more efficient than analogous time stepping schemes with iterative solvers. The second-order noniterative scheme is found to be more efficient than the first-order noniterative scheme. The proposed adaptive noniterative algorithms can be easily incorporated into existing backward Euler software, which is widely used for Richards equation and other nonlinear PDEs.