Martin S. Andersen

Assessing the accuracy of 1-D analytical heat tracing for estimating near-surface sediment thermal diffusivity and water flux under transient conditions

Amplitude decay and phase delay of oscillating temperature records measured at two vertical locations in near-surface sediments can be used to infer water fluxes, thermal diffusivity, and sediment scour/deposition. While methods that rely on the harmonics-based analytical heat transport solution assume a steady state water flux, many applications have reported transient fluxes but ignored the possible violation of this assumption in the method. Here we use natural heat tracing as an example to investigate the extent to which changes in the water flux, and associated temperature signal nonstationarity, can be separated from other influences. We systematically scrutinize the assumption of steady state flow in analytical heat tracing and test the capabilities of the method to detect the timing and magnitude of flux transients. A numerical model was used to synthesize the temperature response to different step and ramp changes in advective thermal velocity magnitude and direction for both a single-frequency and multifrequency temperature boundary. Time-variable temperature amplitude and phase information were extracted from the model output with different signal-processing methods. We show that a worst-case transient flux induces a temperature nonstationarity, the duration of which is less than 1 cycle for realistic sediment thermal diffusivities between 0.02 and 0.13 m2/d. However, common signal-processing methods introduce erroneous temporal spreading of advective thermal velocities and significant anomalies in thermal diffusivities or sensor spacing, which is used as an analogue for streambed scour/deposition. The most time-variant spectral filter can introduce errors of up to 57% in velocity and 33% in thermal diffusivity values with artifacts spanning ±2 days around the occurrence of rapid changes in flux. Further, our results show that analytical heat tracing is unable to accurately resolve highly time-variant fluxes and thermal diffusivities and does not allow for the inference of scour/depositional processes due to the limitations of signal processing in disentangling flux-related signal nonstationarities from those stemming from other sources. To prevent erroneous interpretations, hydrometric data should always be acquired in combination with temperature records.

River‐aquifer interactions in a semiarid environment investigated using point and reach measurements

A critical hydrological process is the interaction between rivers and aquifers. However, accurately determining this interaction from one method alone is difficult. At a point, the water exchange in the riverbed can be determined using temperature variations over depth. Over the river reach, differential gauging can be used to determine averaged losses or gains. This study combines these two methods and applies them to a 34 km reach of a semiarid river in eastern Australia under highly transient conditions. It is found that high and low river flows translate into high and low riverbed Darcy fluxes, and that these are strongly losing during high flows, and only slightly losing or gaining for low flows. The spatial variability in riverbed Darcy fluxes may be explained by riverbed heterogeneity, with higher variability at greater spatial scales. Although the river-aquifer gradient is the main driver of riverbed Darcy flux at high flows, considerable uncertainty in both the flux magnitude and direction estimates were found during low flows. The reach-scale results demonstrate that high-flow events account for 64% of the reach loss (or 43% if overbank events are excluded) despite occurring only 11% of the time. By examining the relationship between total flow volume, river stage and duration for in-channel flows, we find the loss ratio (flow loss/total flow) can be greater for smaller flows than larger flows with similar duration. Implications of the study for the modeling and management of connected water resources are also discussed. Key Points Losing riverbed fluxes under high flows and approximately neutral under low flows Event driven riverbed fluxes dominate reach losses Smaller events can have higher loss ratio than larger events

Evaporative cooling of speleothem drip water

This study describes the first use of concurrent high-precision temperature and drip rate monitoring to explore what controls the temperature of speleothem forming drip water. Two contrasting sites, one with fast transient and one with slow constant dripping, in a temperate semi-arid location (Wellington, NSW, Australia), exhibit drip water temperatures which deviate significantly from the cave air temperature. We confirm the hypothesis that evaporative cooling is the dominant, but so far unattributed, control causing significant disequilibrium between drip water and host rock/air temperatures. The amount of cooling is dependent on the drip rate, relative humidity and ventilation. Our results have implications for the interpretation of temperature-sensitive, speleothem climate proxies such as δ18O, cave microecology and the use of heat as a tracer in karst. Understanding the processes controlling the temperature of speleothem-forming cave drip waters is vital for assessing the reliability of such deposits as archives of climate change.

Association of Arsenic and Phosphorus with Iron Nanoparticles between Streams and Aquifers: Implications for Arsenic Mobility.

The microbial oxidation of organic matter coupled to reductive iron oxide dissolution is widely recognized as the dominant mechanism driving elevated arsenic (As) concentrations in aquifers. This paper considers the potential of nanoparticles to increase the mobility of As in aquifers, thereby accounting for discrepancies between predicted and observed As transport reported elsewhere. Arsenic, phosphorus, and iron size distributions and natural organic matter association were examined along a flow path from surface water via the hyporheic zone to shallow groundwater. Our analysis demonstrates that the colloidal Fe concentration (>1 kDa) correlates with both colloidal P and colloidal As concentrations. Importantly, increases in the concentration of colloidal P (>1 kDa) were positively correlated with increases in the concentration of nominally dissolved As (<1 kDa), but no correlation was observed between colloidal As and nominally dissolved P. This suggests that P actively competes for adsorption sites on Fe nanoparticles, displacing adsorbed As, thus mirroring their interaction with Fe oxides in the aquifer matrix. Dynamic redox fronts at the interface between streams and aquifers may therefore provide globally widespread conditions for the generation of Fe nanoparticles, a mobile phase for As adsorption currently not a part of reactive transport models.

Understanding connected surface-water/groundwater systems using Fourier analysis of daily and sub-daily head fluctuations

The long-term monitoring records of hydraulic heads frequently contain fluctuations originating from different cyclic drivers. Fourier analysis applied to these records can reveal connected surface-water/groundwater system characteristics. The various components of the atmospheric tides, the earth tides and the presence of diurnal responses to evapotranspiration are identified and isolated through band-pass filtering of data recorded from both vented and absolute gauge transducers. The signature of the different cyclic drivers is contained in amplitude and phase of the various signal components and can be used to determine the degree of system confinement. A methodology is described for the calculation of barometric efficiency in confined aquifers based upon the amplitude of the M2 and S2 components of the earth and atmospheric tides. It is demonstrated that Fourier analysis of water-level fluctuations is a simple but underused tool that can help to characterise shallow groundwater systems.RésuméLes enregistrements de suivi à long terme des charges hydrauliques renferment fréquemment des fluctuations ayant pour origine différents facteurs cycliques. L’analyse de Fourier appliquée à ces enregistrements peut révéler les caractéristiques de systèmes connectés eaux de surface/eaux souterraines. Les différentes composantes des marées atmosphériques, des marées terrestres et la présence de réponses diurnes à l’évapotranspiration sont identifiées et isolées par filtration des bandes passantes des données, enregistrées tant au moyen de capteurs absolus que compensés en pression atmosphérique. La signature des différents facteurs cycliques est contenue dans l’amplitude et la phase des différentes composantes du signal et peut être utilisée pour déterminer le degré de captivité du système. Une méthodologie est décrite pour le calcul de l’efficacité barométrique des aquifères captifs, basée sur l’amplitude des composantes M2 and S2 des marées terrestres et atmosphériques. Il est démontré que l’analyse de Fourier des fluctuations de niveaux d’eau est un outil simple mais sous-utilisé qui peut aider à caractériser les systèmes aquifères peu profonds.ResumenLos registros de monitoreo a largo plazo de las cargas hidráulicas frecuentemente contienen fluctuaciones procedentes de diferentes forzantes cíclicos. El análisis de Fourier aplicado a estos registros puede revelar las características de la conexión de los sistemas agua superficial / aguas subterráneas. Se identificaron y aislaron los distintos componentes de las mareas atmosféricas, las mareas terrestres y la presencia de respuestas diurnas a la evapotranspiración a través de filtros pasa banda de datos provenientes de sensores de presión ventilados y absolutos. Las peculiaridades de los diferentes forzantes cíclicos están contenidas en amplitud y fase de las distintas señales componentes y pueden ser usados para determinar el grado de confinamiento del sistema. Se describe una metodología para el cálculo de la eficiencia barométrica en acuíferos confinados basado en la amplitud de las componentes M2 y S2 de las mareas terrestre y atmosférica. Se demuestra que el análisis de Fourier de las fluctuaciones de los niveles de agua es una herramienta simple pero poco utilizada que puede ayudar a caracterizar sistemas de agua subterránea someros.摘要期水头监测记录经常包括不同循环驱动产生的波动。对这些记录进行傅里叶分析可揭示相互连接的地表水/地下水系统的特征。通过对排放的和绝对的计量传感器记录的资料进行带通滤波,确定了大气潮汐、地球潮汐各种各样的成分及存在着对蒸发蒸腾一日间的响应,并对每个因素进行了单独的分析。不同循环驱动的特征码包含在各种各样信号成分的幅相中,可用来确定系统限制的程度。根据地球潮汐和大气潮汐M2 和 S2成分的振幅,描述了计算承压含水层中气压效率的方法。研究表明,水位波动的傅里叶分析法是一个简单而又未充分利用的工具,可有助于描述浅层地下水系统的特征。ResumoOs registros de monitorização de níveis piezométricos a longo prazo contêm frequentemente oscilações causadas por fatores cíclicos. Uma análise de Fourier aplicada a esses registros pode revelar caraterísticas do sistema de interações água superficial/água subterrânea. As várias componentes das marés atmosféricas e terrestres e a presença de respostas diurnas à evapotranspiração são identificadas e isoladas por métodos de filtragem passa-banda de dados registados a partir de transdutores de pressão absoluta ou ventilados. A assinatura dos diferentes fatores cíclicos está incluída na amplitude e fase dos vários componentes de sinal e pode ser usada para determinar o grau de confinamento do sistema. É descrita uma metodologia para o cálculo da eficiência barométrica em aquíferos confinados, com base na amplitude das componentes M2 e S2 das marés atmosféricas e terrestres. Demonstra-se que a análise de Fourier das flutuações de nível de água é uma ferramenta simples, mas subutilizada, que pode ajudar a caraterizar sistemas hidrogeológicos subsuperficiais.

A reassessment of the Lower Namoi Catchment aquifer architecture and hydraulic connectivity with reference to climate drivers

We demonstrate the need for better representations of aquifer architecture to understand hydraulic connectivity and manage groundwater allocations for the ∼140 m-thick alluvial sequences in the Lower Namoi Catchment, Australia. In the 1980s, an analysis of palynological and groundwater hydrograph data resulted in a simple three-layer stratigraphic/hydrostratigraphic representation for the aquifer system, consisting of an unconfined aquifer overlying two semi-confined aquifers. We present an analysis of 278 borehole lithological logs within the catchment and show that the stratigraphy is far more complex. The architectural features and the net-to-gross line-plot of the valley-filling sequence are best represented by a distributive fluvial system, where the avulsion frequency increases at a slower rate than the aggradation rate. We also show that an improved understanding of past climates contextualises the architectural features observable in the valley-filling sequence, and that the lithofacies distribution captures information about the impact of climate change during the Neogene and Quaternary. We demonstrate the correlation between climate and the vertical lithological succession by correlating the sediment net-to-gross ratio line-plot with the marine benthic oxygen isotope line-plot – a climate change proxy. Pollens indicate that there was a transition from a relatively wet climate in the mid–late Miocene to a drier climate in the Pleistocene, with a continuing drying trend until present. Groundwater is currently extracted from the sand and gravel belts associated with the high-energy wetter climate. However, some of these channel belts are disconnected from the modern river and flood zone. We show that the cutoff between the hydraulically well- and poorly connected portions of the valley-filling sequence matches the connectivity threshold expected from a fluvial system.

Aquifer heterogeneity and response time: the challenge for groundwater management

Abstract. Groundwater is an important contributor to irrigation water supplies. The time lag between withdrawal and the subsequent impacts on the river corridor presents a challenge for water management. We highlight aspects of this challenge by examining trends in the groundwater levels and changes in groundwater management goals for the Namoi Catchment, which is within the Murray–Darling Basin, Australia. The first high-volume irrigation bore was installed in the cotton-growing districts in the Namoi Catchment in 1966. The development of high-yielding bores made accessible a vast new water supply, enabling cotton growers to buffer the droughts. Prior to the development of a groundwater resource it is difficult to accurately predict how the water at the point of withdrawal is hydraulically connected to recharge zones and nearby surface-water features. This is due to the heterogeneity of the sediments from which the water is withdrawn. It can take years or decades for the impact of groundwater withdrawal to be transmitted kilometres through the aquifer system. We present the analysis of both historical and new groundwater level and streamflow data to quantify the impacts of extensive groundwater withdrawals on the watertable, hydraulic gradients within the semi-confined aquifers, and the movement of water between rivers and aquifers. The results highlight the need to monitor the impacts of irrigated agriculture at both the regional and local scales, and the need for additional research on how to optimise the conjunctive use of both surface-water and groundwater to sustain irrigated agriculture while minimising the impact on groundwater-dependent ecosystems.

A New Method for Estimating Recharge to Unconfined Aquifers Using Differential River Gauging

In semiarid and arid environments, leakage from rivers is a major source of recharge to underlying unconfined aquifers. Differential river gauging is widely used to estimate the recharge. However, the methods commonly applied are limited in that the temporal resolution is event-scale or longer. In this paper, a novel method is presented for quantifying both the total recharge volume for an event, and variation in recharge rate during an event from hydrographs recorded at the upstream and downstream ends of a river reach. The proposed method is applied to river hydrographs to illustrate the method steps and investigate recharge processes occurring in a sub-catchment of the Murray Darling Basin (Australia). Interestingly, although it is the large flood events which are commonly assumed to be the main source of recharge to an aquifer, our analysis revealed that the smaller flow events were more important in providing recharge.

Understanding and quantifying focused, indirect groundwater recharge from ephemeral streams using water table fluctuations

Understanding and managing groundwater resources in drylands is a challenging task, but one that is globally important. The dominant process for dryland groundwater recharge is thought to be as focused, indirect recharge from ephemeral stream losses. However, there is a global paucity of data for understanding and quantifying this process and transferable techniques for quantifying groundwater recharge in such contexts are lacking. Here we develop a generalized conceptual model for understanding water table and groundwater head fluctuations due to recharge from episodic events within ephemeral streams. By accounting for the recession characteristics of a groundwater hydrograph, we present a simple but powerful new water table fluctuation approach to quantify focused, indirect recharge over both long term and event time scales. The technique is demonstrated using a new, and globally unparalleled, set of groundwater observations from an ephemeral stream catchment located in NSW, Australia. We find that, following episodic streamflow events down a predominantly dry channel system, groundwater head fluctuations are controlled by pressure redistribution operating at three time scales from vertical flow (days to weeks), transverse flow perpendicular to the stream (weeks to months), and longitudinal flow parallel to the stream (years to decades). In relative terms, indirect recharge decreases almost linearly away from the mountain front, both in discrete monitored events as well as in the long-term average. In absolute terms, the estimated indirect recharge varies from 80 to 30 mm/a with the main uncertainty in these values stemming from uncertainty in the catchment-scale hydraulic properties.

Social tipping points in global groundwater management

Groundwater is critical to global food security, environmental flows, and millions of rural livelihoods in the face of climate change1. Although a third of Earth’s largest groundwater basins are being depleted by irrigated agriculture2, little is known about the conditions that lead resource users to comply with conservation policies. Here we developed an agent-based model3,4 of irrigated agriculture rooted in principles of cooperation5,6 and collective action7 and grounded on the World Values Survey Wave 6 (n = 90,350). Simulations of three major aquifer systems facing unsustainable demands reveal tipping points where social norms towards groundwater conservation shift abruptly with small changes in cultural values and monitoring and enforcement provisions. These tipping points are amplified by group size and best invoked by engaging a minority of rule followers. Overall, we present a powerful tool for evaluating the contingency of regulatory compliance upon cultural, socioeconomic, institutional and physical conditions, and its susceptibility to change beyond thresholds. Managing these thresholds may help to avoid unsustainable groundwater development, reduce enforcement costs, better account for cultural diversity in transboundary aquifer management and increase community resilience to changes in regional climate. Although we focus on groundwater, our methods and findings apply broadly to other resource management issues.Global groundwater resources are threatened by over-extraction. An agent-based model is presented, incorporating cooperative and collective action theory that reveals tipping points in social attitudes toward conservation in three at-risk regions.