Adrian D. Werner

Impact of Sea-Level Rise on Sea Water Intrusion in Coastal Aquifers

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.

Hydrodynamic and Hydrological Modeling of the Poyang Lake Catchment System in China

AbstractIn this paper, a distributed catchment-hydrology model and a physically based lake hydrodynamic model were used to simulate the large-scale and highly dynamic lake catchment system of Poyang Lake, in the middle reach of the Yangtze River basin, China. The simulation of the hydrodynamics of the lake is a significant extension to previous efforts to simulate Poyang Lake’s considerable variability in lake extent and flow rates. Further, the combination of the distributed catchment-hydrology model and the lake-hydrodynamic model, applied to a highly dynamic and large-scale system, is a rare attempt to develop a physically based management model of this complexity and scale. Model calibration and validation were undertaken to evaluate the model’s performance and to enhance its effectiveness in simulating catchment discharges, lake water levels, lake water surface areas, and lake flow patterns. The results showed a satisfactory agreement with field observations, with Nash-Sutcliffe efficiencies of 0.71–...

A hydraulic mixing-cell method to quantify the groundwater component of streamflow within spatially distributed fully integrated surface water-groundwater flow models

The complexity of available hydrological models continues to increase, with fully integrated surface water-groundwater flow and transport models now available. Nevertheless, an accurate quantification of streamflow generation mechanisms within these models is not yet possible. For example, such models do not report the groundwater component of streamflow at a particular point along the stream. Instead, the groundwater component of streamflow is approximated either from tracer transport simulations or by the sum of exchange fluxes between the surface and the subsurface along the river. In this study, a hydraulic mixing-cell (HMC) method is developed and tested that allows to accurately determine the groundwater component of streamflow by using only the flow solution from fully integrated surface water-groundwater flow models. By using the HMC method, the groundwater component of streamflow can be extracted accurately at any point along a stream provided the subsurface/surface exchanges along the stream are calculated by the model. A key advantage of the HMC method is that only hydraulic information is used, thus the simulation of tracer transport is not required. Two numerical experiments are presented, the first to test the HMC method and the second to demonstrate that it quantifies the groundwater component of streamflow accurately.

Bias of apparent tracer ages in heterogeneous environments.

The interpretation of apparent ages often assumes that a water sample is composed of a single age. In heterogeneous aquifers, apparent ages estimated with environmental tracer methods do not reflect mean water ages because of the mixing of waters from many flow paths with different ages. This is due to nonlinear variations in atmospheric concentrations of the tracer with time resulting in biases of mixed concentrations used to determine apparent ages. The bias of these methods is rarely reported and has not been systematically evaluated in heterogeneous settings. We simulate residence time distributions (RTDs) and environmental tracers CFCs, SF6 , (85) Kr, and (39) Ar in synthetic heterogeneous confined aquifers and compare apparent ages to mean ages. Heterogeneity was simulated as both K-field variance (σ(2) ) and structure. We demonstrate that an increase in heterogeneity (increase in σ(2) or structure) results in an increase in the width of the RTD. In low heterogeneity cases, widths were generally on the order of 10 years and biases generally less than 10%. In high heterogeneity cases, widths can reach 100 s of years and biases can reach up to 100%. In cases where the temporal variations of atmospheric concentration of individual tracers vary, different patterns of bias are observed for the same mean age. We show that CFC-12 and CFC-113 ages may be used to correct for the mean age if analytical errors are small.

Is Decoupling GDP Growth from Environmental Impact Possible

The argument that human society can decouple economic growth—defined as growth in Gross Domestic Product (GDP)—from growth in environmental impacts is appealing. If such decoupling is possible, it means that GDP growth is a sustainable societal goal. Here we show that the decoupling concept can be interpreted using an easily understood model of economic growth and environmental impact. The simple model is compared to historical data and modelled projections to demonstrate that growth in GDP ultimately cannot be decoupled from growth in material and energy use. It is therefore misleading to develop growth-oriented policy around the expectation that decoupling is possible. We also note that GDP is increasingly seen as a poor proxy for societal wellbeing. GDP growth is therefore a questionable societal goal. Society can sustainably improve wellbeing, including the wellbeing of its natural assets, but only by discarding GDP growth as the goal in favor of more comprehensive measures of societal wellbeing.

How important is the impact of land-surface inundation on seawater intrusion caused by sea-level rise?

The influence of sea-level rise (SLR) on seawater intrusion (SWI) has been the subject of several publications, which consider collectively a range of functional relationships within various hydrogeological and SLR settings. Most of the recent generalized analyses of SWI under SLR neglect land-surface inundation (LSI) by seawater. A simple analytical method is applied to quantitatively assess the influence and importance of LSI on SLR–SWI problems under idealized conditions. The results demonstrate that LSI induces significantly more extensive SWI, with inland penetration up to an order of magnitude larger in the worst case, compared to the effects of pressure changes at the shoreline in unconfined coastal aquifers with realistic parameters. The study also outlines some of the remaining research challenges in related areas, concluding that LSI impacts are among other important research questions regarding the SLR–SWI problems that have not been addressed, including the effects of aquifer heterogeneities, real-world three dimensionality, and mitigation measures.RésuméL’influence de l’élévation du niveau marin sur l’intrusion marine a fait l’objet de plusieurs publications, qui considèrent ensemble une série de relations fonctionnelles entre diverses configurations hydrogéologiques et marines. La plupart des analyses générales récentes portant sur les intrusions marines liées à la fluctuation du niveau marin négligent l’inondation des terres par la mer. Une méthode analytique simple est appliquée pour évaluer quantitativement, dans des conditions théoriques, l’influence et l’importance de l’inondation des terres sur l’élévation du niveau de la mer et l’intrusion marine. Les résultats montrent que l’inondation des terres induit de façon significative une intrusion marine plus étendue et, dans le pire des cas, une pénétration dans l’arrière pays plus importante que celle due aux effets des variations de pression sur les aquifères côtiers libres le long la ligne de rivage, calculée avec des paramètres réalistes. L’article souligne également quelques uns des défis encore posés à la recherche dans les domaines de ce genre, concluant que les impacts de l’inondation des terres figurent parmi les autres questions importantes de la recherche concernant les problèmes d’élévation du niveau de la mer et d’intrusion de l’eau de mer à n’avoir pas été abordées et qui comprennent l’incidence des hétérogénéités de l’aquifère, le caractère tridimensionnel du monde réel et les approches simplificatrices.ResumenLa influencia del ascenso del nivel del mar (SLR) en la intrusión de agua marina (SWI) ha sido objeto de varias publicaciones, que consideran colectivamente un rango de relaciones funcionales dentro de varias configuraciones hidrogeológicas y de SLR. La mayor parte de los recientes análisis generalizados de SWI bajo SLR desprecian la inundación de la superficie del terreno (LSI) por el agua de mar. Se aplica un método analítico simple para evaluar cuantitativamente la influencia y la importancia de LSI sobre los problemas de SLR–SWI bajo condiciones idealizadas. Los resultados demuestran que la LSI induce significativamente en SWI más extensas, con penetración tierra adentro de hasta un orden de magnitud más grande que en el peor de los casos, comparado con los efectos de los cambios de presión en la línea de costa en acuíferos costeros no confinados con parámetros realistas. El trabajo también esboza algunos de los desafíos de investigación que restan en áreas relacionados, concluyendo que los impactos de LSI, entre otras cuestiones importantes en relación a los problemas SLR–SWI, no han sido evaluados, incluyendo los efectos de las heterogeneidades del acuífero, tridimensionalidad del mundo real y las medidas de mitigación.ResumoA influência da elevação do nível do mar (ENM) na intrusão salina (IS) tem sido objeto de várias publicações, as quais consideram coletivamente uma gama de relações funcionais dentro de vários cenários hidrogeológicos e de ENM. A maior parte das análises recentes mais generalizadas de IS sob ENM negligenciam a inundação da superfície do solo (ISS) pela água marinha. É aplicado um método analítico simples para calcular quantitativamente a influência e importância da ISS nos problemas de ENM-IS sob condições ideais. Os resultados demonstram que a ISS induz uma IS significativamente mais extensa, com uma penetração continental até uma ordem de magnitude maior no pior dos casos, quando comparada com os efeitos das alterações de pressão na linha de costa em aquíferos livres costeiros com parâmetros realistas. O documento também enfatiza alguns dos restantes desafios da investigação em áreas similares, concluindo que os impactes da ISS estão entre outras questões importantes para a investigação relacionada com os problemas de ENM-IS que não foram ainda abordados, incluindo os efeitos das heterogeneidades dos aquíferos, a tridimensionalidade do mundo real e as medidas de mitigação.

GRACE-Based Hydrological Drought Evaluation of the Yangtze River Basin, China

AbstractIn this study, hydrological drought in the Yangtze River basin (YRB) is characterized based on Gravity Recovery and Climate Experiment (GRACE) total water storage (TWS). An artificial neural network approach is applied to extend the GRACE TWS observations (2003–12) to a longer TWS time series (1979–2012), which is well matched (Nash–Sutcliff efficiency of 0.83) to the GRACE data. Hydrological drought is identified by water storage deficit (WSD; the shortfall in TWS from the average value) in three consecutive months. The method builds on previous research by considering potentially ineffective interdrought events and by characterizing drought recovery time from a multidecadal TWS time series. The results show that the YRB was in hydrological drought 29 times during 1979–2012, and the three subbasins of the YRB (upper, middle, and lower YRB) experienced between 21 and 28 hydrological drought events during the same period. The drought recovery time, defined as the time required for WSD to recover to...

Heat and solute tracers: how do they compare in heterogeneous aquifers?

A comparison of groundwater velocity in heterogeneous aquifers estimated from hydraulic methods, heat and solute tracers was made using numerical simulations. Aquifer heterogeneity was described by geostatistical properties of the Borden, Cape Cod, North Bay, and MADE aquifers. Both heat and solute tracers displayed little systematic under- or over-estimation in velocity relative to a hydraulic control. The worst cases were under-estimates of 6.63% for solute and 2.13% for the heat tracer. Both under- and over-estimation of velocity from the heat tracer relative to the solute tracer occurred. Differences between the estimates from the tracer methods increased as the mean velocity decreased, owing to differences in rates of molecular diffusion and thermal conduction. The variance in estimated velocity using all methods increased as the variance in log-hydraulic conductivity (K) and correlation length scales increased. The variance in velocity for each scenario was remarkably small when compared to σ2 ln(K) for all methods tested. The largest variability identified was for the solute tracer where 95% of velocity estimates ranged by a factor of 19 in simulations where 95% of the K values varied by almost four orders of magnitude. For the same K-fields, this range was a factor of 11 for the heat tracer. The variance in estimated velocity was always lowest when using heat as a tracer. The study results suggest that a solute tracer will provide more understanding about the variance in velocity caused by aquifer heterogeneity and a heat tracer provides a better approximation of the mean velocity.

The influence of river-to-lake backflow on the hydrodynamics of a large floodplain lake system (Poyang Lake, China)

Backflow, the temporary reversal of discharge at the outlet of a lake, is an important mechanism controlling flow and transport in many connected river–lake systems. This study used statistical methods to examine long-term variations and primary causal factors of backflow from the Yangtze River to a laterally connected, large floodplain lake (Poyang Lake, China). Additionally, the effects of backflow on the lake hydrology were explored using a physically based hydrodynamic model and a particle-tracking model. Although backflow into Poyang Lake occurs frequently, with an average of 16 backflow events per year, and varies greatly in magnitude between years, statistical analysis indicates that both the frequency and magnitude of backflow reduced significantly during 2001–2010 relative to the previous period of 1960–2000. The ratio of Poyang Lake catchment inflows to Yangtze River discharge can be used as an indication of the daily occurrence of backflow, which is most likely to occur during periods when this ratio is lower than 5%. Statistical analysis also indicates that the Yangtze River discharge is the main controlling factor of backflow during July to October, rather than catchment inflows to the lake. Hydrodynamic modelling reveals that, in general, backflow disturbs the normal northward water flow direction in Poyang Lake and transports mass ~20 km southward into the lake. The effects of backflow on flow direction, water velocities and water levels propagate to virtually its upstream extremity. The current study represents a first attempt to explore backflow and causal factors for a highly dynamic floodplain lake system. An improved understanding of Poyang Lake backflow is critical for guiding future strategies to manage the lake, its water quality and ecosystem value, given proposals to modify the lake–river connectivity. Copyright

Variable density groundwater flow: From modelling to applications

Arid and semi-arid climates are mainly characterised as those areas where precipitation is less (and often considerably less) than potential evapotranspiration. These climate regions are ideal environments for salt to accumulate in natural soil and groundwater settings since evaporation and transpiration essentially remove freshwater from the system, leaving residual salts behind. Similarly, the characteristically low precipitation rates reduce the potential for salt to be diluted by rainfall. Thus arid and semi-arid regions make ideal ‘salt concentrator’ hydrologic environments. Indeed, salt flats, playas, sabkhas and saline lakes, for example, are ubiquitous features of arid and semi-arid regions throughout the world (Yechieli and Wood,2002). In such settings, variable density flow phenomena are expected to be important, especially where hypersaline brines overlie less dense groundwater at depth. In contrast, seawater intrusion in coastal aquifers is a global phenomenon that is not constrained to only arid and semi-arid regions of the globe and is inherently a variable density flow problem by its very nature. These two examples make it clear that variable density flow problems occur in, but importantly extend beyond, arid and semi-arid regions of the globe. The intention of this chapter is therefore not to limit ourselves to modelling arid zone hydrological systems, but rather to present a more general treatment of variable density groundwater flow and solute transport phenomena and modelling. The concepts presented in this chapter are therefore not climatologically constrained to arid or semi-arid zones of the world, although they do apply equally there.