Subsurface Topography Research Articles

Application of Geophysical Techniques for 3D Geohazard Mapping to Delineate Cavities and Potential Sinkholes in the Northern Part of Kuala Lumpur, Malaysia

This work describes the application of the electrical resistivity (ER) method to delineating subsurface structures and cavities in Kuala Lumpur Limestone within the Batu Cave area of Selangor Darul Ehsan, Malaysia. In all, 17 ER profiles were measured by using a Wenner electrode configuration with 2 m spacing. The field survey was accompanied by laboratory work, which involves taking resistivity measurements of rock, soil, and water samples taken from the field to obtain the formation factor. The relationship between resistivity and the formation factor and porosity for all the samples was established. The porosity values were plotted and contoured. A 2-dimensional and 3-dimensional representation of the subsurface topography of the area was prepared through use of commercial computer software. The results show the presence of cavities and sinkholes in some parts of the study area. This work could help engineers and environmental managers by providing the information necessary to produce a sustainable management plan in order to prevent catastrophic collapses of structures and other related geohazard problems.

Vegetation, fauna and groundwater interrelations in low nutrient temperate montane peat swamps in the upper Blue Mountains, New South Wales.

Newnes Plateau Shrub Swamps are a series of low nutrient temperate montane peat swamps around 1100 m elevation in the upper Blue Mountains, west of Sydney (lat 33° 23' S; long 150° 13'E). Transect-based vegetation studies show a closely related group of swamps with expanses of permanently moist, gently sloping peatlands. Vegetation patterns are related to surface hydrology and subsurface topography, which determine local peat depth. While there is evidence that a group of the highest elevation swamps on the western side of the Plateau are more dependent on rainwater, the majority of swamps, particularly those in the Carne Creek catchment, and east and south of it, may be considered primarily groundwater dependent with a permanently high watertable maintained by groundwater aquifers. An integral part of the swamps are a number of threatened groundwater dependent biota (plants-Boronia deanei subsp. deanei, Dillwynia stipulifera, dragonfly- Petalura gigantea, lizard- Eulamprus leuraensis), which are obligate swamp dwellers. This association of dependence leaves the entire swamp ecosystem highly susceptible to threats from any loss of groundwater, the current major one being the impact of damage to the confining aquicludes, aquitards, aquifers and peat substrates as a result of subsidence associated with longwall mining. Impacts on the swamps may also result from changes to hydrology through damming of creeks, mine waste water discharge, increased moisture competition from pine plantations, recreational motorbike and off-road vehicle tracks and climate change. If these groundwater dependent ecosystems do not receive protection from activities such as longwall mining subsidence, significant ecological damage is unlikely to be avoided or able to be mitigated even where provisions of the Commonwealth Environment Protection and Biodiversity Conservation and NSW Threatened Species Conservation Acts apply to groundwater dependent swamps and biota. The importance of the highest elevation part of the Plateau for a number of restricted (some endemic) plant species is also discussed.

Simulated effect of soil depth and bedrock topography on near‐surface hydrologic response and slope stability

ABSTRACTThis paper explores the effect of hillslope hydrological behavior on slope stability in the context of transient subsurface saturation development and landslide triggering. We perform a series of virtual experiments to address how subsurface topography affects the location and spatial pattern of slip surface development and pore pressure dynamics. We use a 3D Darcy–Richards equation solver (Hydrus 3‐D) combined with a cellular automata slope stability model to simulate the spatial propagation of the destabilized area. Our results showed that the soil–bedrock interface and in particular, bedrock depressions, played a key role in pore pressure dynamics, acting as an impedance for the downslope drainage of perched water. Filling and spilling of depressions in the bedrock surface microtopography induced localized zones of increased pressure head such that the development of pore‐pressure fields—not predictable by surface topography—lead to rapid landslide propagation. Our work suggests that landslide models should consider the subsurface topography in order to include a connectivity component in the mathematical description of hydrological processes operating at the hillslope scale. Copyright © 2012 John Wiley & Sons, Ltd.

Geophysical imaging of shallow subsurface topography and its implication for shallow landslide susceptibility in the Urseren Valley, Switzerland

Abstract Landslides and soil erosion are an ever present threat to water management, building construction, vegetation formation and biodiversity in the Swiss Alps. Improved understanding of the mechanics and causative factors of soil erosion is a key factor in mitigation of damage to Alpine natural resources. Recently, much progress has been achieved in the forecasting of landslides on Alpine slopes with a new generation of shallow landslide models. These models perform well in spatial predictions, but temporal control on the occurrence of shallow landslides is less successful. Realistic soil composition and geometry of interfaces are necessary input for better predictions. Geophysical methods have so far not been widely considered to obtain these parameters, in spite of their ability to cover much ground with high-resolution. In this study we successfully use such methods to derive adequate subsurface topography as input to dynamic spatially distributed hydrological and soil mechanical models. Trench, GPS, electrical resistivity tomography and ground penetrating radar data were collected, resulting in revealing images of the composition and geometry of past and future landslides. A conceptual model for the occurrence of local shallow landslides is derived, spanning from block-wise steady creep of detaching soil units to rapid sliding and downslope deposition of soil units via varying sliding planes. Significant topography was observed in the soil interfaces acting as sliding planes, leading to a more complex role of groundwater flow in the initiation of shallow landslides. Hydrogeologic models should be revised accordingly.

3D Visualizing of Volumetric Surface Movement at Karstic Limestone Area

Abstract Limestone bedrock topography has complex phenomena and highly relief subsurface topography due to the presence of karstic features. Geotechnical and environmental problems arise whenever foundations are established on the surface of the limestone bedrock or within the overburden soils. Geographical information system (GIS) and remote sensing are emerging as powerful techniques widely applicable in natural resources management to detect land use changes and devise strategies based on these changes. The study focuses on using aerial photography for the detection of changes and effects of mining on geomorphology, especially the use of sequential images that allows to detect changes taken place from time to time, by using. Volumetric Surface Movement Spatiotemporal Data Model (VSMSDM) application has been employed to create karst terrain surface movements and visualized 3D information in the Virtual Geographical Information Systems (VGIS). VSMSDM application was implemented by developing prototype of visualization system using with integrated time in the TIN structure. The data have been collected from aerial photography in 1981 and 2004, and the results displayed that the proposed data model is able to view the changing in karst topography to detect significant landscapes and landforms changes.

Terrestrial laser scanning and electrical resistivity tomography as combined tools for the geoarchaeological study of the Kritsa-Latô dolines (Mirambello, Crete, Greece)

A complex of enclosed karst depressions near Kritsa-Lato (Dikti Mountains, Eastern Crete) was analysed with regard to its surface and subsurface topography using a new multi-method approach. Based on a combination of terrestrial laser scanning, electrical resistivity tomography and geographic information systems, crucial geophysical and morphometrical parametres were derived and correlated with sedimentological results in order to provide first-time insights into the formation, size and archive function of dolines. High-resolution digital elevation models (0.25 m, 2 m) allowed conducting topographical analyses with special respect to micro-relief attributes and traces of former land use, as well as placing all findings into a 3D context for enhanced interpretation. As shown by the results, both limestone dissolution and local tectonics led to the development of an extensive sediment trap, whose subsurface is characterised by a distinct three-part segmentation. Above the bedrock, a highly fractured epikarst zone exhibits features typical of buried karst systems, such as subterraneous sinkholes and pinnacles. In contrast, the near-surface section completely consists of fine-grained, loamy substrates. Despite thicknesses of up to 25 m, the loose infilling entirely lacks stratification and therefore indicates permanent low energetic colluviation processes under rather stable geomorphodynamic conditions during the Quaternary. In respect to the local human impact, several geophysical anomalies suggest the existence of an ancient water harvesting system, which quite likely dates back to Minoan or Dorian times. As water supply must be considered essential for agricultural purposes in the karstified mountains of Crete, the Kritsa dolines must have served as favourable locations for settlement and land use purposes. With the the help of the multi-technique approach, both the geomorphological and the geoarchaeological characteristics of the study area could be captured. The findings help improve the understanding of the landscape evolution and the associated man-environment interactions in Eastern Crete. Combining terrestrial laser scanning and tomographical mapping proves to be particularly suitable for integral surface and subsurface prospecting and therefore promises substantial potential for future studies.

Possible Late Pleistocene volcanic activity on Nightingale Island, South Atlantic Ocean, based on geoelectrical resistivity measurements, sediment corings and 14C dating

Tristan da Cunha is a volcanic island group situated in the central South Atlantic. The oldest of these islands, Nightingale Island, has an age of about 18 Ma. In the interior of the island, there are several wetlands situated in topographic depressions. The ages of these basins have been unknown, and their genesis has been debated. Aiming towards the reconstruction of the geomorphological history of these basins, we conducted geoelectrical resistivity measurements to map the subsurface topography, extracted peat and sediment cores and dated the onset of sedimentation applying the radiocarbon method. The irregular shapes of the basins and the lack of clear erosional features indicate that they are not eruption craters and were not formed by erosion. Instead, we regard them as morphological depressions formed between ridges of trachytic lava flows and domes at a late stage of the formation of the volcanic edifice. The onset of sedimentation within these basins appears to have occurred between 24 and 37 ka with the highest situated wetland yielding the highest ages. These ages are very young compared to the timing of the main phase of the formation of the island, implying volcanic activity on the island during the Late Pleistocene.

Examining the role of throughfall patterns on subsurface stormflow generation

The effect of throughfall input patterns on the hydrological response of forested hillslopes is not well understood. While field studies have contributed to our understanding of subsurface stormflow generation at the hillslope scale, such work is still of limited value because of the small number of places and events that have been characterized to date and the uniqueness of each study hillslope. In recent years, virtual experiments have been used to investigate the role of topography, soil depth, bedrock permeability and storm size, on the generation of lateral subsurface flow. However, these studies have generally assumed spatially uniform rainfall, and the interaction between vegetation and its effect on the spatial structure of input (canopy interception, throughfall) for hillslope hydrologic response has not yet been explored. Here we present a number of virtual experiments that explore the interplay among hydrological inputs (temporal and spatial distribution of rainfall) and hillslope properties (subsurface topography, soil depth), i.e. physical phenomena that are sources of space and/or time variability. We address specifically the relative importance of fine-scale throughfall patterns for hillslope hydrologic response. Topography and hydrologic field observations from an existing study hillslope were used to calibrate and test a 3D Richards equation-based finite element model. Throughfall patterns were based on published throughfall patterns in an even age stand of young conifers in the Pacific Northwest. These patterns were then varied across the hillslope during the virtual experiments. Our results showed that, surprisingly, the effect of spatial input variability of throughfall on lateral subsurface stormflow generation was minimal. For our tested case, the bedrock topography control on flow generation was much greater than the fine-scale spatial variability of the input. Using a spatially uniform area-averaged “throughfall” (i.e. open rainfall reduced by some assumed fraction, which is the simplest and most common form of throughfall representations) yielded minimal differences in subsurface stormflow response. Nevertheless, using open rainfall as spatially uniform input strongly overestimated lateral subsurface stormflow, and thus, the average impact of throughfall is important for input estimation at the hillslope-scale. Overall, the effects of fine-scale throughfall patterns on subsurface stormflow generation appear to be of secondary importance compared to effects of temporal distribution of rainfall, subsurface topography and variable soil depths.

Stochastic characteristics of seismic excitations at a non-uniform (rock and soil) site

This study analyzes data recorded at the dense array in the Parkway Valley, Wainuiomata, New Zealand, a small alluvial valley surrounded by graywacke outcrops. The array consisted of stations on both the sediment basin and the surrounding soft rock, with station separation distances pertinent for earthquake engineering applications. The array's configuration renders itself uniquely for the study of the spatial variation of seismic ground motions at sites with irregular topography for bridge response evaluations: the locations of the soft-rock stations surrounding the valley may be viewed as the locations of the bridge's abutments, and the locations of the stations in the basin as those of the bridge's intermediate piers. A further-away station, north-east of the valley, provided information on firm-rock data. The analysis suggested that the motions in the valley were dominated by surface waves and considered two time windows for the evaluation of the stochastic characteristics (power spectral densities and lagged coherencies) of the data. During the first time window, at the onset of the excitation, the motions in the valley exhibited both low and high frequency energy that propagated down the valley. During the second window, high-amplitude, low-frequency, surface-wave energy controlled the horizontal motions in the valley. Very significant variabilities were observed in the amplitudes of the power spectra of the motions throughout the array during both windows indicating a complex wave propagation pattern affecting not only the valley but also the soft rock that surrounds the valley. Significant correlations in the motions appeared in the form of “hills” in the lagged coherency estimates when the energy of the power spectra of the station pairs peaked at similar or close-by frequency ranges, irrespective of whether the station pairs were located on the same or different ground types. The valley data indicated significant correlations in the dominant frequency ranges of the surface waves that control the motions during the two windows analyzed. Interestingly, motions at the soft-rock and soil stations, as well as those at the soft-rock stations and the firm-rock station, are significantly correlated at the onset of the excitation. This result may or may not depend on the event analyzed, as the spectra of its motions at the firm-rock, soft-rock and basin stations during this window peak at similar frequency ranges, and, in addition, the motions at the soft-rock and basin stations appear to be controlled by similar surface-wave energy at higher frequencies. On the other hand, during the later windows, there is tremendous amplification of the motions in the valley, whereas the spectral amplitudes of the soft-rock data are, comparatively, very low. The differences result from the fact that the seismic excitations within the valley have a significantly longer duration than the seismic ground motions recorded at the rock stations due to the formation of additional, low-frequency surface waves. This observation may have significant implications for the design of bridges supported at variable site conditions. Whereas the motions at rock (or excitations at the bridge abutments) have essentially ceased, the motions in the valley (or excitations at the bridge piers) undergo the severe part of the earthquake excitation. This scenario of seismic excitations is not currently considered in bridge response evaluations, but can adversely affect the structure's response. The analysis provides insight into the stochastic characteristics of seismic excitations at sites with irregular subsurface topography.

Imaging a shallow salt diapir using ambient seismic vibrations beneath the densely built-up city area of Hamburg, Northern Germany

Salt diapirs are common features of sedimentary basins. If close to the surface, they can bear a significant hazard due to possible dissolution sinkholes, karst formation and collapse dolines or their influence on ground water chemistry. We investigate the potential of ambient vibration techniques to map the 3-D roof morphology of shallow salt diapirs. Horizontal-to-vertical (H/V) spectral peaks are derived at more than 900 positions above a shallow diapir beneath the city area of Hamburg, Germany, and are used to infer the depth of the first strong impedance contrast. In addition, 15 small-scale array measurements are conducted at different positions in order to compute frequency-dependent phase velocities of Rayleigh waves between 0.5 and 25 Hz. The dispersion curves are inverted together with the H/V peak frequency to obtain shear-wave velocity profiles. Additionally, we compare the morphology derived from H/V and array measurements to borehole lithology and a gravity-based 3-D model of the salt diapir. Both methods give consistent results in agreement with major features indicated by the independent data. An important result is that H/V and array measurements are better suited to identify weathered gypsum caprocks or gypsum floaters, while gravity-derived models better sample the interface between sediments and homogeneous salt. We further investigate qualitatively the influence of the 3-D subsurface topography of the salt diapir on the validity of local 1-D inversion results from ambient vibration dispersion curve inversion.

Surficial geophysical deduction of the geomaterial and aquifer distributions at Ngor-Okpala local government area of Imo State, South Eastern Nigeria

AbstractTwenty seven vertical electrical sounding (VES) profiles surrounding four known traverses were obtained in Ngor-Okpala local government area of Imo state to examine the subsurface geomaterials and the associated groundwater potential. The VES data, constrained by borehole data, provided useful information about the subsurface hydrogeologic and lithologic conditions. From the validated interpretation, the area assessed has loamy soil, medium grained sands, well-sorted medium-grained/gravelly sands and river sand as the lithologic succession from top to the bottom of the depth penetrated. The aquifers in the area were found in the medium-grained sands and well-sorted medium-coarse-grained sands. The aquifer depth for all-season groundwater that would be devoid of draw-down can be found at a depth range of 42–50 m. The resistivity maps of selected depths exhibit sharp resistivity changes at depth due mainly to undulating subsurface topography. A map of the distribution of the kσ-values shows that good quality groundwater can be found in most parts of the area.

Mapping of Sand Layer Thickness in Deserts Using SAR Interferometry

This paper presents an interferometric synthetic aperture radar (InSAR) system to map the bedrock topography underneath the sand in deserts and arid areas. This is anticipated to greatly increase the efficiency of oil field and ground water exploration as well as environmental and archaeological studies. The proposed system consists of two InSAR subsystems, one operating at Ka-band to map the sand topography and the other operates in the VHF band to map the subsurface topography. The different issues associated with InSAR processing for subsurface mapping are discussed. It is shown that conventional InSAR processing produces unacceptable error in height estimation since it does not account for the refraction and the different propagation velocity in the sand. Thus, a new inversion algorithm is developed which can be used to accurately estimate the bedrock topography for arbitrary sand and bedrock geometries. A sensitivity analysis is then presented to show the effect of the different systematic and random errors. The inversion algorithm is verified experimentally for flat sand case using a scaled model that was implemented in the lab.

Wind‐controlled linear patterning and cyclic succession in Tasmanian Sphagnum mires

Summary 1. We investigated the characteristics and causes of striped patterning in minerotrophic Sphagnum cristatum mires on the western Central Plateau in Tasmania by surveying the mire vegetation, surface and subsurface topography, analysing their topographic exposure to the prevailing strong winds relative to non‐striped mires, stratigraphic analysis, measurements of wind speed and direction across and along ridges and swales in typical synoptic conditions, and measurement of growth rates and asymmetry of shrubs. 2. The stripes were orientated south‐west–north‐east, independent of the aspect of the general slope. The shrub Richea scoparia was dominant on the north‐west‐facing slopes of the low ridges that formed the stripes. Sphagnum cristatum was dominant on the south‐east‐facing ridge slopes and the restiad, Baloskion australe was dominant in the swales. The stripes were independent of the topography of the underlying block stream. Striped mires were more exposed to both north‐westerly and south‐westerly winds than non‐striped mires. 3. Strong north‐westerly winds formed rotors behind each ridge, with wind speed being highest on the north‐west‐facing ridge slope and lowest on the south‐east‐facing ridge slope. The annual growth of R. scoparia inversely reflected this variation in wind speed. South‐westerly winds were stronger in the swales than on the ridges and stronger on the south‐west extremity of the ridges than further to the north‐east. Asymmetry in R. scoparia on the ridges reflected this variation in wind speed. 4. We infer that the ridges slowly migrate in a cyclic successional pattern as the result of differential growth rates of R. scoparia in response to variation in the speed of drying north‐westerly winds. The stripes appear to result from training of shrubs by strong, cold south‐westerly winds, which also inhibit the growth of R. scoparia in the swales. 5. Synthesis. There are no other documented cases of wind‐controlled patterning and cyclic succession in mires, and no case of exposure to two distinct wind directions being responsible for patterning in any vegetation type. The distinctiveness of the process and the form merit recognition of these striped mires as outstanding in their universal value under the world heritage area criteria.

Connectivity at the hillslope scale: Identifying interactions between storm size, bedrock permeability, slope angle and soil depth

The links between soil water movement at the plot scale and runoff generation at the hillslope scale are highly non-linear and still not well understood. As such, a framework for the general characterization of hillslopes is still lacking. Here we present a number of virtual experiments with a 3D physically-based finite element model to systematically investigate the interactions between some of the dominant controls on subsurface stormflow generation. We used the well-studied Panola experimental hillslope to test our base case simulation and used the surface and subsurface topography and the stormflow data of this site as a framework for a subsequent series of virtual experiments. The parameterization of the soil and bedrock properties was based on field measurements of soil moisture and saturated hydraulic conductivity. After calibration and testing against multiple evaluation criteria including distributed trench flow data and internal tensiometric response, we varied slope angle, soil depth, storm size and bedrock permeability across multiple ranges to establish a set of response surfaces for several hillslope flow metrics. We found that connectivity of subsurface saturation was a unifying descriptor of hillslope behavior across the many combinations of slope type. While much of the interplay between our four hillslope variables was intuitive, several interactions in variable combinations were found. Our analysis indicated that, e.g. interactions between slope angle, soil depth and storm size that caused unexpected behavior of hydrograph peak times were the result of the interplay between subsurface topography and the overlying soil mantle with its spatially varying soil depth distribution. Those interactions led to new understanding of process controls on connectivity .

A subsurface criterion for predictive exploration of kimberlites from Bouguer gravity in the eastern Dharwar craton, India

The Archaean-Proterozoic Dharwar craton has many recorded occurrences of diamondiferous kimberlites. Reports of kimberlite emplacement in parts of the tectonically complex eastern Dharwar craton and a significant density contrast between kimberlites and the host peninsular gneisses motivated us to conduct gravity studies in the Narayanpet-Irladinne area of the eastern Dharwar craton. This region is contiguous with the Maddur-Narayanpet kimberlite that lies to its north, while the river Krishna lies to its south. From observed association of reported kimberlites in the Maddur-Narayanpet field with subsurface topography of the assumed three-layer earth section obtained by Bouguer gravity modelling, we developed a subsurface criterion for occurrence of kimberlites in the present study area. Using this criterion, five potential zones for kimberlite localization were identified in the Narayanpet-Irladinne region, eastern Dharwar craton.

광릉 산림 소유역에서의 대공극흐름율과 유효대공극부피분율의 공간 분포

The role of macropore in the hydrological processes is important at the hillslope scale. Developments and distribution of macropores have not been investigated in conjunction with the characteristics of the hillslope such as topography, soil property, and soil moisture. In this study, macropore properties, such as macropore flow and saturation hydraulic conductivity were measured at a hillslope located in Gwangneung Research Forest, Pochun-gun, Gyeonggi-do, South Korea. An intensive field survey provided a refined Digital Elevation Model (DEM) for surface and subsurface topography. Spatial distributions of upslope area and topographic index were obtained through the digital terrain analysis. The total number of monitoring points was 22, and the selected points were distributed along the transect of the digital contour map. Vertical fluxes through macropores were measured using a tension infiltrometer at the depth of 0.1 m from the surface. Spatial and temporal distributions of soil moisture were obtained using an on-line measurement system, TRASE, installed in the study area. Soil moisture for the aforementioned points was measured at 0.1 and 0.3m depths below the surface. The results from tension infiltrometer experiments present that the macropore flows ranged between 21 and 94%, and the measured macroporosities varied from 1.4 to 47%. Macropore flows and macroporosities tended to increase as the measurement location moved to downslope. The ability for water conduction through macropores becomes increasingly developed as the location approaches the outlet of the hillslope.

Towards simulating biogeochemical hot spots in the landscape: A geographic object-based approach

Physically based models may offer a means to improve our understanding of how biogeochemical hot spots form in the landscape, and a way to quantify the aggregated effects of their occurrences across scales [Burt, T.P., Pinay, G., 2005. Linking hydrology and biogeochemistry in complex landscapes. Progress in Physical Geography 29 (3), 297–316]. With this challenge in mind, a prototypical, geographic object-based simulation framework is presented to demonstrate methods for coupled hydrological and biogeochemical modelling. The framework integrates GIS-based spatial analysis and parameterization methods with object-oriented (OO) concepts and numerical methods for hydrological and biogeochemical modelling. As proof-of-concept, the object-model is used to demonstrate saturated, lateral subsurface flood-wave and solute propagation, as well as hypothetical solute reaction hot spots whereby loss rates are controlled by solute concentration and hydrologic residence times using a modified 1st order decay mechanism. For the steady-state examples, the spatial patterns of hot spot formation are governed by 1st (global) and 2nd (local) order variations in subsurface topography which influence hydrologic residence time in sediments of varying thickness and depth variable saturated hydraulic conductivity. However, in these simplified scenarios overall reduction efficiencies are more strongly governed by the slope of the subsurface and hydrogeomorphic configuration than by 1st order variations in topography. The sample simulations support a claim for models that are more capable of handling the spatially and temporally complex hydrological–biogeochemical linkages that drive the formation of hot spots in the landscape. Further development of this approach is advocated to provide a tool for exploratory analysis and hypothesis formulation and testing regarding landscape controls on hydrology and biogeochemistry.

On improving the simulation of Atlantic Water circulation in the Arctic Ocean

We performed a set of numerical experiments in order to better simulate the circulation of Atlantic Water in the Arctic Ocean by employing a coupled ice‐ocean Arctic regional model. We found that the inflow of Atlantic Water via Fram Strait is weak in the case of high viscosity and diffusivity coefficients (1 · 108 cm2/s and 1 · 107 cm2/s) and creates an anticyclonic circulation in the Eurasian basin. This flow increases significantly when both coefficients are scaled down (to 1 · 107 cm2/s and 0.5 · 106 cm2/s) but the current path is then unstable, and it becomes mostly anticyclonic after about a decade of integration. A further reduction of these coefficients leads to growing instabilities due to insufficient grid resolution. Short‐term integration of the model with doubled space resolution shows better agreement with observations, but long‐term calculation is still restricted by simulation time. Alternatively, a lower resolution version of this model benefits from parameterizations of subscale processes. Incorporation of the Neptune parameterization of eddy‐topography interaction into the model intensifies the current steered by subsurface topography and results in cyclonic circulation of the Atlantic Water in every subbasin and in the whole Arctic region. In the absence of salinity restoring there was strong salinity drift at the surface, resulting in a salinity increase in the central Arctic by 3 units from 1960 to 2005. The Arctic freshwater content is driven basically by year‐to‐year variations of convective mixing involving more saline water from lower layers. After cooling and increasing in salinity through brine rejection, the surface water becomes denser, leading to even stronger convective mixing. One‐dimensional model tests showed how model performance could be improved by implementing a lower vertical diffusion coefficient and higher vertical resolution.

Near-surface hydrologic response for a steep, unchanneled catchment near Coos Bay, Oregon: 1. sprinkling experiments

Sprinkling systems are frequently used to simulate rainfall for process-based investigations of near-surface hydrologic response without measuring or accounting for spatial variability. Data analyses from three sprinkling experiments at the Coos Bay 1 experimental catchment (CB1) demonstrate considerable spatial variability in sprinkling. Furthermore, simulated rainfall from sprinklers was found to be more heterogeneous than natural storms at CB1. Water balance calculations and evapotranspiration estimates indicate that evaporation of airborne droplets is a significant portion of applied sprinkling rates, although still less than the amount blown off the field site by strong winds. Incorporation of spatial variability in sprinkling input and soil-water storage did not significantly change water balance calculations. Saturation patterns within the near-surface soil profile and the timing of tensiometric response are affected by sprinkling heterogeneity. Pore-water pressure and saturation development at the soil-saprolite interface are primarily controlled by convergent surface / subsurface topography and bedrock fracture flow, but are also sensitive to sprinkling spatial variations. The analyses presented herein suggest that incorporating spatial variability in sprinkling rates is important when conducting hydrologic-response modeling of sprinkler experiments. This paper is the first-part of a two-part series focused on CB1. The data analyses in this paper are used to parameterize comprehensive physics-based hydrologic-response simulations of three CB1 sprinkling experiments reported in the companion paper.

Review of methods for mapping dryland salinity

Dryland salinity, a growing problem over much of Australia, can only be mapped and predicted with a thorough understanding of the landscape in three dimensions and the hydrological processes that operate within it. Hydrology is the key for understanding how salt stores are mobilised through the Earth, both in a vertical and horizontal sense. Mapping techniques have important roles in delineating soils, landforms, water flow and pathways through subsurface topography, as well as detecting or inferring the presence of salt itself. Satellite and airborne remote sensing techniques are useful in delineating existing surface and near-surface salt, and tracking changes over time. Airborne geophysical techniques, combined with ground and borehole control, are important tools in understanding salinity and hydrology at depth, and essential tools for predicting future changes in salinity. Salt is a hazard when it has the potential to be moved, usually by water, to a location where it can threaten an asset, which may include agriculture, infrastructure, water quality and biodiversity. Salinity risk is a measure of the chance that the salt hazard will cause harm to the asset at some time in the future. Cost-benefit analyses in salinity management should take into consideration total cost and total benefit in context with the value of the asset. The optimum strategy for salinity hazard and risk mapping depends on the scale (farm, community or catchment) and resources available to the user. The best approach makes use of existing information and then integrates a range of the available mapping methods in such a way to best address the specific question at hand. No one method has primacy, nor is there a `magic bullet' for salinity mapping or prediction. Effective use of mapping methods requires expert knowledge or access to trained personnel.