the 1979 2005 greenland ice sheet melt extent from passive microwave data using an improved version of the melt retrieval xpgr algorithm analysis of passive microwave satellite observations over the greenland ice sheet reveals a significant increase in surface melt over the period 1979 2005 since 1979 the total melt area was found to have increased by 1 22 x 10 7 km 2 an improved version of the cross polarized gradient ratio xpgr technique is used to identify the melt from the brightness temperatures the improvements in the melt retrieval xpgr algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere snow regional climate model from 1979 to 2005 the ablation period has been increasing everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall indeed more snowfall in summer decreases the liquid water content of the snowpack raises the albedo and therefore reduces the melt finally the observed melt acceleration over the greenland ice sheet is highly correlated with both greenland and global warming suggesting a continuing surface melt increase in the future

holocene deceleration of the greenland ice sheet recent peripheral thinning of the greenland ice sheet is partly offset by interior thickening and is overprinted on its poorly constrained holocene evolution on the basis of the ice sheet s radiostratigraphy ice flow in its interior is slower now than the average speed over the past nine millennia generally higher holocene accumulation rates relative to modern estimates can only partially explain this millennial scale deceleration the ice sheet s dynamic response to the decreasing proportion of softer ice from the last glacial period and the deglacial collapse of the ice bridge across nares strait also contributed to this pattern thus recent interior thickening of the greenland ice sheet is partly an ongoing dynamic response to the last deglaciation that is large enough to affect interpretation of its mass balance from altimetry

spatial and temporal monitoring of soil moisture using surface electrical resistivity tomography in mediterranean soils abstracts ert techniques are especially promising in semi arid areas with shallow and rocky soils where other methods fail to produce soil moisture maps and to obtain soil profile information electrical resistivity tomography ert was performed in the peyne catchment in southern france at four sites located on four different geological substrates the main objective was to test the usefulness of such geoelectrical method for the assessment of spatial and temporal variability of soil water content and to demarcate subsurface horizontal zones topsoil active zone and bedrock we used time lapse ert to separate lithological variability from soil moisture content and to evaluate its potential to demarcate subsurface soil horizons three measurement campaigns were carried out and field data were collected using the sting r1 ip advanced resistivity meter with one channel receiver we used an array of 28 electrodes with 1 m spacing which provided reliable resistivity information to a depth of approximately 5 5 m ert were followed by soil sampling carried out with a hand auger directly below the respective geoelectrical profiles empirical linear regression was used to determine the specific relationships between volumetric soil moisture of the field samples and electrical resistivity data obtained in situ our conclusions are that ert is a useful technique for providing information on the spatial temporal variability of water content in semi arid areas and for reaching depths otherwise inaccessible in this study we proposed a method to delineate three important different soil zones topsoil active zone bedrock based on resistivity data these three layers are essential information in understanding and modelling the interaction between subsurface and vegetation water uptake particularly in semi arid regions

growth of greenland ice sheet measurement measurements of ice sheet elevation change by satellite altimetry show that the greenland surface elevation south of 72 degrees north latitude is increasing the vertical velocity of the surface is 0 20 0 06 meters per year from measured changes in surface elevations at 5906 intersections between geosat paths in 1985 and seasat in 1978 and 0 28 0 02 meters per year from 256 694 intersections of geosat paths during a 548 day period of 1985 to 1986

continental climate gradients in north america and western eurasia before and after the closure of the central american seaway abstract the gulf stream as part of the atlantic meridional overturning circulation amoc is known as a major driver of latitudinal energy transport in the north atlantic presently causing mild winters over northwestern eurasia the intensity of the amoc throughout the neogene prior to the final closure of the central american seaway cas in the early pliocene is still poorly known but most authors assume that the circulation was considerably weaker than present here we address this issue from a continental point of view we studied the past amoc intensity by analyzing neogene continental climate patterns along north american and western eurasian transects based on a total of 317 palaeofloras thermal latitudinal gradients are reconstructed for three neogene time slices namely the middle miocene late miocene and late pliocene using the coexistence approach to obtain quantitative climate data the obtained proxy based continental temperature gradients are evaluated against data from a selection of published general circulation model gcm simulations for the three time slices studied our study suggests that shallow thermal latitudinal gradients existed in north america and western eurasia throughout the miocene but became strongly steepened in the late pliocene in both miocene time slices studied the higher latitudes were by up to 30 c warmer than present cold month mean also at times with presumed pre industrial co2 such as the late miocene in the late pliocene high latitude the temperature difference with respect to the present had decreased by up to 10 c cold month mean both mean annual temperatures and cold month means of the lower mid and low latitudes were at the present day level throughout all three time slices or even slightly below in both miocene time slices zonal temperature means at both continental transects were similar in the mid and higher latitudes however several northwest european sites reveal very mild winter condition suggesting the early existence of a probably less intense palaeo gulf stream the distinct thermal anomaly annual and cold month means today existing between north america and western eurasia appeared for the first time in the late pliocene attaining about 50 of the present day magnitude this supports the assumption that the amoc intensified after the final closure of the cas during the early pliocene the results obtained from the palaeobotanical proxies are in line with data from coeval marine archives particularly with north atlantic sea surface temperatures ssts inferred from oxygen isotopes however the proxy based thermal gradients are not well reproduced by a selection of gcm simulations due to a well known systematic underestimation of high latitude warming by gcms for the miocene and pliocene time slices

recent sea level contributions of the antarctic and greenland ice sheets after a century of polar exploration the past decade of satellite measurements has painted an altogether new picture of how earth s ice sheets are changing as global temperatures have risen so have rates of snowfall ice melting and glacier flow although the balance between these opposing processes has varied considerably on a regional scale data show that antarctica and greenland are each losing mass overall our best estimate of their combined imbalance is about 125 gigatons per year of ice enough to raise sea level by 0 35 millimeters per year this is only a modest contribution to the present rate of sea level rise of 3 0 millimeters per year however much of the loss from antarctica and greenland is the result of the flow of ice to the ocean from ice streams and glaciers which has accelerated over the past decade in both continents there are suspected triggers for the accelerated ice discharge surface and ocean warming respectively and over the course of the 21st century these processes could rapidly counteract the snowfall gains predicted by present coupled climate models

envisat derived elevation changes of the greenland ice sheet and a comparison with icesat results in the accumulation area abstract we show for the first time over the greenland ice sheet that an along track method for deriving rates of elevation change can successfully be applied to envisat radar altimetry data 2002 2010 the results provide improved resolution and coverage compared to previous results obtained from cross over methods also we find that temporal changes in the elevation change rate can be derived from envisat data and show clear examples of this by generating five year running means for selected areas of the greenland ice sheet for a period between 2003 and 2009 the elevation of the ice sheets was measured by both the laser altimeter on board icesat and the radar altimeter on board envisat we compare rates of elevation change derived from icesat and envisat for this time span in which both sensors were operating we focus on the area above the equilibrium line altitude in order to specifically derive information on snow parameters a comparison of the elevation changes observed by the two sensors shows a complex pattern which can be explained regionally by model output describing the changes in both firn air content and accumulation rates

estimation of flow properties using surface deformation and head data a trajectory based approach 1 a trajectory based algorithm provides an efficient and robust means to infer flow properties from surface deformation and head data the algorithm is based upon the concept of an arrival time of a drawdown front which is defined as the time corresponding to the maximum slope of the drawdown curve the technique involves three steps the inference of head changes as a function of position and time the use of the estimated head changes to define arrival times and the inversion of the arrival times for flow properties trajectories computed from the output of a numerical simulator are used to relate the drawdown arrival times to flow properties the inversion algorithm is iterative requiring one reservoir simulation for each iteration the method is applied to data from a set of 14 tiltmeters located at the raymond quarry field site in california using the technique i am able to image a high conductivity channel which extends to the south of the pumping well the presence of this permeable pathway is supported by an analysis of earlier cross well transient pressure test data

a new ice thickness and bed data set for the greenland ice sheet 2 relationship between dynamics and basal topography new data sets of bed and surface elevation have been combined with balance velocities bed topography and radar reflectivity to investigate the relationship between ice flow and bed conditions it has been found that the bed topography affects the local driving stresses and initiates the onset of streaming flow in specific areas where there are large undulations this onset region only appears significantly inland in three areas the northeast greenland ice stream kangerdlugssuaq gletscher and a region in the southwest associated with abnormally low driving stresses and an apparently unnamed but active glacier bed reflectance values below the northeast ice stream suggest that basal melting could be occurring along its length and it seems likely therefore that the enhanced flow observed inland from the onset of streaming may be due in part to basal sliding

gps measurements of vertical crustal motion in greenland we have analyzed 5 years of continuous global positioning system gps measurements taken at kellyville just off the western margin of the ice sheet in southern greenland a fit to the vertical component gives a negative secular uplift rate of 5 8 1 0 mm yr a negative rate i e a subsidence is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr however it is inconsistent with estimates of the earth s continuing viscoelastic response to melting ice prior to 4 ka years ago which predict that kellyville should be uplifting rather than subsiding by 2 0 3 5 mm yr the resulting 7 8 3 6 mm yr discrepancy is too large to be the result of loading from present day changes in nearby ice we show instead that it is consistent with independent suggestions that the western ice sheet margin in this region of greenland may have advanced by 50 km during the past 3 4 kyr

a reconciled estimate of ice sheet mass balance we combined an ensemble of satellite altimetry interferometry and gravimetry data sets using common geographical regions time intervals and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of earth s polar ice sheets we find that there is good agreement between different satellite methods especially in greenland and west antarctica and that combining satellite data sets leads to greater certainty between 1992 and 2011 the ice sheets of greenland east antarctica west antarctica and the antarctic peninsula changed in mass by 142 49 14 43 65 26 and 20 14 gigatonnes year 1 respectively since 1992 the polar ice sheets have contributed on average 0 59 0 20 millimeter year 1 to the rate of global sea level rise

the application of time lapse electrical tomography in groundwater studies recently various techniques of electrical tomography have been refined and fast efficient commercial systems are now available new inversion algorithms produce electrical images that more accurately model a two dimensional or three dimensional subsurface than ever before over the past three years electrical tomography surveys in germany the u k and the u s have produced remarkable results

the non invasive characterization of pumping induced dewatering using ground penetrating radar abstract ground penetrating radar gpr profiling is a non invasive geophysical technique that has been used by endres et al ground water 38 2000 566 to successfully image pumping induced drainage in an unconfined aquifer however the drained water volume calculated from the gpr data was significantly less than the actual pumped volume to investigate the reasons for this discrepancy a seven day pumping test and five day recovery test was performed at canadian forces base borden in ontario canada a dense spatial coverage of gpr profiles was used to better quantify variations in drainage due to small scale aquifer heterogeneity in addition a neutron moisture content probe was used to directly observe drainage and the soil moisture profile at a sealed well near the pumping well neutron logging indicated that the transition zone translated downward during pumping without undergoing significant extension comparison of the gpr and neutron derived transition zone drawdowns show nearly equal responses both of these observations support the hypothesis that the behaviour of the gpr reflection is an accurate measure of the transition zone response in contrast transition zone drawdown obtained from both gpr and neutron logging are significantly delayed relative to potentiometric drawdown resulting in an extended capillary fringe the drained water volume was determined from the gpr derived transition zone drawdown data using a number of different approaches methods that incorporated information about spatial variations in drainage gave larger estimates of drained water volume however those estimates were still lower than the actual pumped volume the unaccountable volume of water could be a result of several factors aquifer heterogeneity may still not be adequately represented by the increased gpr coverage and or leakage from the underlying aquitard may be providing a significant volume of water

integration of electrical resistivity imaging eri and ground penetrating radar gpr methods to identify soil profile around necatibey subway station ankara turkey electrical resistivity imaging eri and ground penetrating radar gpr methods were utilized in this study to identify soil profiles around necatibey subway station of kizilay cayyolu metro line ankara turkey the necatibey metro station is located within the alluvial deposits of dikmen stream and the so called ankara clay at the metro station a number of boreholes were drilled however due to the spacing of the boreholes the boundary between alluvium and ankara clay deposits could not be separated precisely thus in this study eri and gpr methods were utilized to distinguish soil types at the study area gpr measurements were taken from a total of 14 profiles and total length of the profiles was about 320 m for every eri measurement section schlumberger dipole dipole dipole dipole schlumberger and wenner arrays were used results from the geophysical measurements identified that the fill materials are underlain by the dikmen stream channel deposits which consist of silty clay and gravelly sand units the study also shows that the dikmen stream channel deposits are underlain by the ankara clay unit the meaningful range of resistivity values was between 1 and 15 m and the gpr signals were strong in sandy units while they attenuated in clayey environments based on borehole logs eri and gpr data three dimensional lithological subsurface model of the survey area was constructed the resultant three dimensional diagrams may serve engineers as a practical tool during different construction stages groundwater surface water interactions within short and long term and probable remedial measures

sensitivity of the modeled present day greenland ice sheet to climatic forcing and spin up methods and its influence on future sea level projections 1 the accuracy of the modeled present day greenland ice sheet gris is crucial for future projections of gris changes in this study we present a detailed evaluation of the modeled present day gris sensitivity to different present day climatology data sets and spin up methods and further assess the influence of the modeled present day gris on future sea level projections our study demonstrates that with present day climatology data sets derived from the regional atmospheric climate model racmo2 the modeled ice volume area and elevation of the gris agree substantially better with observations compared to simulations with present day climatology data sets from temperature parameterizations and era interim reanalysis with transient spin up the simulated rates of ice elevation changes for the 1993 2010 period are closer to observations than those with steady state spin up these results indicate that the racmo2 forcing and transient spin up may be preferable for use in the ice sheet model moreover our results reveal that the present day climatology data sets and spin up methods affect future sea level projections using transient spin up the ice sheet model estimates a range of 26 mm in projected sea level rise by 2098 under the a1b emissions scenario due to different present day climatology data sets compared to the results with transient spin up the estimated sea level rise by 2098 is reduced by 5 21 mm with steady state spin up this discrepancy is attributed mainly to differences in ice thickness and ice velocity between the modeled present day gris with transient and steady state spin ups and the effect of paleoclimatic changes with transient spin up

greenland ice sheet melt extent 1979 1999 analysis of melt extent on the greenland ice sheet is updated to span the time period 1979 1999 and examined along with its spatial and temporal variability using passive microwave satellite data to acquire the full record the issue of continuity between the previous passive microwave sensors smmr ssm i f 8 and ssm i f 11 and the most recent ssm i f 13 sensor is addressed the f 13 cross polarized gradient ratio melt classification threshold is determined to be 0 0154 results show that for the 21 year record a positive melt trend of nearly 1 yr is observed but this trend falls just below the 90 significance level the observed melt increase does appear to be driven by conditions in the western portion of the ice sheet rather than the east where melt appears to have decreased slightly moreover the eruption of mount pinatubo in 1991 is likely to have had some impact on the melt but not so much as previously suspected the 1992 melt anomaly is 1 7 standard deviations from the mean finally the relationship between coastal temperatures and melt extent suggests an increase in surface runoff contribution to sea level of 0 31 mm yr for a 1 c temperature rise

hydrological response of weathered clay shale slopes water infiltration monitoring with time lapse electrical resistivity tomography this work presents an attempt to monitor water infiltration and subsurface flow within a clay shale landslide using time lapse electrical resistivity tomography ert a rainfall experiment was carried out on a plot of 100 m2 at the laval landslide in the draix experimental catchments ore draix south french alps in order to characterize the spatial and temporal development of water circulation in the soil and to identify when steady state flow conditions are reached the experiment was conducted during 67 h with initial unsaturated conditions in the slope the apparent electrical resistivity values were inverted with a time lapse approach using several cross models the results indicate a significant decrease in resistivity 18 compared to the initial state in the rain plot downslope progression of negative resistivity anomalies is imaged suggesting that vertical and subsurface lateral flows have developed about 21 h after the start of the rain experiment a constant level of resistivity values is observed indicating that the hydrological system reached steady state flow conditions this observation is consistent with ground water level observations and chemical tracer analysis computed differences in time of steady state conditions highlight possible preferential flows near the landslide toe a hydrological concept of functioning of the slope is proposed and apparent saturated hydraulic conductivity ks of 1 7 10 4 m s 1 is computed from the steady state times this study demonstrates the potentiality of ert monitoring to monitor water infiltration in clay shale slopes and the high water transfer capacity of reworked clay shale material

integrated geoelectrical survey for groundwater and shallow subsurface evaluation case study at siliyin spring el fayoum egypt siliyin spring is one of the many natural fresh water springs in the western desert of egypt it is located at the central part of el fayoum delta which is a potential place for urban developments and touristic activities integrated geoelectrical survey was conducted to facilitate mapping the groundwater resources and the shallow subsurface structures in the area twenty eight transient electromagnetic tem soundings three vertical electrical soundings ves and three electrical resistivity tomography ert profiles were carried out around the siliyin spring location the dense cultivation the rugged topography and the existence of infra structure in the area hindered acquiring more data the tem data were inverted jointly with the ves and ert and constrained by available geological information based on the inversion results a set of geoelectrical cross sections have been constructed the shallow sand to sandy clay layer that forms the shallow aquifer has been completely mapped underneath and around the spring area flowing of water from the siliyin spring is interconnected with the lateral lithological changes from clay to sand soil exploration of the extension of siliyin spring zone is recommended the interpretation emphasizes the importance of integrating the geoelectrical survey with the available geological information to obtain useful cheap and fast lithological and structural subsurface information

using remotely sensed data from airs to estimate the vapor flux on the greenland ice sheet comparisons with observations and a regional climate model mass loss from the greenland ice sheet gris in recent years has been dominated by runoff from surface melt it is currently being studied extensively while little interest has been given to the smallest component of surface mass balance smb the vapor flux although poorly understood it is not negligible and could potentially play a larger role in smb in a warming climate where temperature relative humidity and precipitation changes remain uncertain here we present an innovative approach to estimate the vapor flux using the atmospheric infrared sounder airs version 6 data and a modified vapor flux model bmf13 over the gris between 2003 and 2014 one modification to the bmf13 model includes a new multiangle imaging spectroradiometer surface aerodynamic roughness product which likely produces more accurate estimates of the drag coefficient on the ice sheet when comparing airs data with gc net and programme for monitoring of the greenland ice sheet automatic weather station observations of skin temperature near surface air temperature and humidity they agree within 2 k 2 68 k and 0 34 g kg 1 largest differences occur in the ablation zone where there is significant subgrid heterogeneity overall the average vapor flux from the gris between 2003 and 2014 was found to be 14 6 3 6 gt yr 1 no statistically significant trends were found during the data record this data set is compared to the regional atmospheric climate model racmo2 3 vapor flux and bmf13 produced smaller vapor fluxes in the summer 0 05 gt d 1 and slightly more deposition in the winter 9 4 10 3 gt d 1 annually differences between bmf13 and racmo2 3 were only 30 15

progressive increase in ice loss from greenland 1 laser altimeter measurements over greenland show increasing thickening rates above 2000 m reflecting increasing snowfall in a warming climate but near coastal thinning rates have increased substantially since the mid 1990s and net mass loss more than doubled from an average of 4 50 gt yr 1 between 1993 4 and 1998 9 to 57 105 gt yr 1 between 1998 9 and 2004 this increasing trend is very similar to findings from independent mass budget studies but differs widely from ers radar altimeter results this may result from limitations associated with the large ers footprint over sloping and undulating surfaces that typify fast narrow glaciers where thinning is most pronounced

elevation change of the southern greenland ice sheet from 1978 to 1988 interpretation the spatial pattern of elevation change estimates derived from satellite radar altimeter data for the period from 1978 to 1988 for the southern greenland ice sheet is examined as reported previously the results from 12 ice cores widely distributed in the study area indicate that much of the spatial variability in the elevation change estimates can be explained by temporal variations in accumulation most notably the areas of largest thickening and thinning east and west of the ice divide around 66 n recently experienced substantial decadal fluctuations in accumulation sufficient to explain the observed elevation change rates that span the range of 24 cm yr 1 elevation change estimates in the nw of the study area around the 2000 m elevation contour were found to be inconsistent with both short and long term changes in accumulation fluctuations examination of stratigraphy in several shallow ice cores in the nw indicates that significant melting took place in 1977 and 1978 and this was followed by many years of very little melt activity this likely resulted in a significant downward bias in the 1978 1988 elevation change estimates in the nw comparison of measured surface velocities with those calculated assuming steady state balance show very good agreement overall but there are many instances in the southeast where the steady state velocities are significantly smaller than measured surface velocities we cannot rule out the possibility that increased ice flow in lower elevation outlet glaciers has migrated farther upstream thereby causing negative long term mass imbalance in southeast greenland

a salt tracer test monitored with surface ert to detect preferential flow and transport paths in fractured karstified limestones in hard rock aquifers fractured zones constitute adequate drinking water exploitation areas but also potential contamination paths one critical issue in hydrogeological research is to identify characterize and monitor such fractured zones at a representative scale a tracer test monitored with surface electrical resistivity tomography ert could help by delineating such preferential flow paths and estimating dynamic properties of the aquifer however multiple challenges exist including the lower resolution of surface ert compared with crosshole ert the finite time that is needed to complete an entire data acquisition and the strong dilution effects we conducted a natural gradient salt tracer test in fractured limestones to account for the high transport velocity we injected the salt tracer continuously for four hours at a depth of 18 m we monitored its propagation with two parallel ert profiles perpendicular to the groundwater flow direction concerning the data acquisition we always focused

evolution of soil wetting patterns preceding a hydrologically induced landslide inferred from electrical resistivity survey and point measurements of volumetric water content and pore water pressure 1 the hydrological state of a hillslope prior to a sprinkling induced shallow landslide was monitored using electrical resistivity tomography ert along a 47 m long transect supplemented by local time domain reflectometry tdr and tensiometer measurements the spatial and temporal evolution of wetting patterns in the soil material indicated attainment of a stationary fully saturated profile in a slope region underlain by shallow sandstone bedrock the significant decrease in spatially averaged standard deviation of water saturation has not been observed during an earlier failed attempt to trigger a landslide by intense sprinkling while for the stable experiment no landslide was triggered water saturation and soil moisture variability were still increasing with time the unstable experiment reached a time invariant state of high pore water pressures and saturations until it finally failed the results indicate that when large and interconnected regions of hillslope are saturated as confirmed by high volumetric water content and low standard deviation of water saturation additional water cannot be redistributed to empty drier regions and may eventually enhance local pore water pressure and seepage force initiating large shear deformation and failure accordingly a transition to such a critical steady state of high average water saturation associated with low and constant spatial standard deviation may serve as additional hydro geophysical indicator for the imminence of a landslide release

acceleration of greenland ice mass loss in spring 2004 in 2001 the intergovernmental panel on climate change projected the contribution to sea level rise from the greenland ice sheet to be between 0 02 and 0 09 m from 1990 to 2100 ref 1 however recent work has suggested that the ice sheet responds more quickly to climate perturbations than previously thought particularly near the coast here we use a satellite gravity survey by the gravity recovery and climate experiment grace conducted from april 2002 to april 2006 to provide an independent estimate of the contribution of greenland ice mass loss to sea level change we detect an ice mass loss of 248 36 km3 yr 1 equivalent to a global sea level rise of 0 5 0 1 mm yr 1 the rate of ice loss increased by 250 per cent between the periods april 2002 to april 2004 and may 2004 to april 2006 almost entirely due to accelerated rates of ice loss in southern greenland the rate of mass loss in north greenland was almost constant continued monitoring will be needed to identify any future changes in the rate of ice loss in greenland

ocean freshening sea level rising sea ice melting 1 estimates of 20th century sea level rise are typically 1 5 to 2 mm y with a steric contribution of 0 5 0 2 mm y estimates of the eustatic contribution vary widely between 1 1 and 1 3 mm y we attempt an independent estimate of eustatic sea level rise based on the measured freshening of the global ocean and with attention to the contribution from melting of sea ice which affects freshening but not sea level our estimate is based on a secular decrease in global average salinity estimated by antonov et al 2002 which if assumed due entirely to run off would produce a eustatic rise of 1 8 0 7 mm y and would correspond to a run off volume of 650 cu km y measurements with upward looking sonars mounted on submarines have suggested a historical thinning of the arctic ice sheet equivalent to 525 105 cu km y allowing for some growth in antarctic sea ice a reduced figure of 430 130 cu km y is obtained allowing about 220 cu km y of run off from land sources such as glaciers this would produce a eustatic rise of only 0 6 mm y for a total of 1 1 mm y somewhat less than ipcc estimates this also has implicationsforourunderstandingofglacialretreatforatotal of 1 1 mm y index terms 1635 global change oceans 4203 4207 oceanography general arctic and antarctic oceanography 4540 oceanography physical ice mechanics and air sea ice exchange processes 4556 oceanography physical sea level variations citation wadhams p and w munk 2004

greenland 2012 melt event effects on cryosat 2 radar altimetry cryosat 2 data are used to study elevation changes over an area in the interior part of the greenland ice sheet during the extreme melt event in july 2012 the penetration of the radar signal into dry snow depends heavily on the snow stratigraphy and the rapid formation of refrozen ice layers can bias the surface elevations obtained from radar altimetry we investigate the change in cryosat 2 waveforms and elevation estimates over the melt event and interpret the findings by comparing in situ surface and snow pit observations from the north greenland eemian ice drilling project camp the investigation shows a major transition of scattering properties around the area and an apparent elevation increase of 56 26 cm is observed in reprocessed cryosat 2 data we suggest that this jump in elevation can be explained by the formation of a refrozen melt layer that raised the reflective surface introducing a positive elevation bias

greenland ice sheet mass balance distribution of increased mass loss with climate warming 2003 07 versus 1992 2002 we derive mass changes of the greenland ice sheet gis for 2003 07 from icesat laser altimetry and compare them with results for 1992 2002 from ers radar and airborne laser altimetry the gis continued to grow inland and thin at the margins during 2003 07 but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s the net balance changed from a small loss of 7 3g t a 1 in the 1990s to 171 4g t a 1 for 2003 07 contributing 0 5 mm a 1 to recent global sea level rise we divide the derived mass changes into two components 1 from changes in melting and ice dynamics and 2 from changes in precipitation and accumulation rate we use our firn compaction model to calculate the elevation changes driven by changes in both temperature and accumulation rate and to calculate the appropriate density to convert the accumulation driven changes to mass changes increased losses from melting and ice dynamics 17 206 gt a 1 are over seven times larger than increased gains from precipitation 10 35 gt a 1 during a warming period of 2 k 10 a 1 over the gis above 2000 m elevation the rate of gain decreased from 44 to 28 gt a 1 while below 2000 m the rate of loss increased from 51 to 198 gt a 1 enhanced thinning below the equilibrium line on outlet glaciers indicates that increased melting has a significant impact on outlet glaciers as well as accelerating ice flow increased thinning at higher elevations appears to be induced by dynamic coupling to thinning at the margins on decadal timescales

characterization of seasonal local recharge using electrical resistivity tomography and magnetic resonance sounding a groundwater recharge process of heterogeneous hard rock aquifer in the moole hole experimental watershed south india is being studied to understand the groundwater flow behaviour significant seasonal variations in groundwater level are observed in boreholes located at the outlet area indicating that the recharge process is probably taking place below intermittent streams in order to localize groundwater recharge zones and to optimize implementation of boreholes a geophysical survey was carried out during and after the 2004 monsoon across the outlet zone magnetic resonance soundings mrs have been performed to characterize the aquifer and measure groundwater level depletion the results of mrs are consistent with the observation in boreholes but it suffers from degraded lateral resolution a better resolution of the regolith bedrock interface is achieved using electrical resistivity tomography ert ert results are confirmed by resistivity logging in the boreholes ert surveys have been carried out twice before and during the monsoon across the stream area the major feature of recharge is revealed below the stream with a decrease by 80 of the calculated resistivity the time lapse ert also shows unexpected variations at a depth of 20 m below the slopes that could have been interpreted as a consequence of a deep seasonal water flow however in this area time lapse ert does not match with borehole data numerical modelling shows that in the presence of a shallow water infiltration an inversion artefact may take place thus limiting the reliability of time lapse ert a combination of ert with mrs provides valuable information on structure and aquifer properties respectively giving a clue for a conceptual model of the recharge process infiltration takes place in the conductive fractured fissured part of the bedrock underlying the stream and clayey material present on both sides slows down its lateral dissipation

recent ice sheet growth in the interior of greenland a continuous data set of greenland ice sheet altimeter height from european remote sensing satellites ers 1 and ers 2 1992 to 2003 has been analyzed an increase of 6 4 0 2 centimeters per year cm year is found in the vast interior areas above 1500 meters in contrast to previous reports of high elevation balance below 1500 meters the elevation change rate is 2 0 0 9 cm year in qualitative agreement with reported thinning in the ice sheet margins averaged over the study area the increase is 5 4 0 2 cm year or 60 cm over 11 years or 54 cm when corrected for isostatic uplift winter elevation changes are shown to be linked to the north atlantic oscillation

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

changes in greenland ice sheet elevation attributed primarily to snow accumulation variability the response of grounded ice sheets to a changing climate critically influences possible future changes in sea level recent satellite surveys over southern greenland show little overall elevation change at higher elevations but large spatial variability using satellite studies alone it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability here we derive changes in ice sheet elevation in southern greenland for the years 1978 88 using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation our patterns of accumulation driven elevation change agree closely with contemporaneous satellite measurements of ice sheet elevation change and we therefore attribute the changes observed in 1978 88 to variability in snow accumulation similar analyses of longer ice core records show that in this decade the greenland ice sheet exhibited typical variability at high elevations well within the long term natural variability our results indicate that a better understanding of ice sheet mass changes will require long term measurements of both surface elevation and snow accumulation

greenland ice sheet increased coastal thinning repeated laser altimeter surveys and modelled snowfall summer melt show average ice loss from greenland between 1997 and 2003 was 80 12 km3 yr 1 compared to about 60 km3 yr 1 for 1993 4 1998 9 half of the increase was from higher summer melting with the rest caused by velocities of some glaciers exceeding those needed to balance upstream snow accumulation velocities of one large glacier almost doubled between 1997 and 2003 resulting in net loss from its drainage basin by about 20 km3 of ice between 2002 and 2003 copyright 2004 by the american geophysical union

time lapse electrical resistivity monitoring of salt affected soil and groundwater summary time lapse electrical geophysics is used to monitor the remediation of oilfield brines in soil and groundwater challenges with comparing geophysical surveys done during different seasons at different temperatures were overcome through temperature monitoring and geophysical processing techniques the background electrical conductivity structure captured by the geophysics compared well with push tool conductivity logs collected and had strong correlation with salt samples taken however the changes in electrical conductivity observed in the timelapse geophysics provided information about the remediation effectiveness that was not available from the logs and point measurements from the observed changes in the geophysics we interpret that depression focused recharge has a large impact on remediation effectiveness this research indicates that time lapse electrical geophysics is an effective and low cost remediation monitoring method at appropriate sites

merging and analysis of elevation time series over greenland ice sheet from satellite radar altimetry spatial temporal variability and changes of greenland ice sheet elevation from 1992 to 2008 are analyzed from merged ers 1 ers 2 and envisat satellite radar altimeter data a methodology for determining intersatellite biases was developed and applied in order to merge measurements from these different satellites and to create continuous and consistent time series intersatellite biases of elevation and backscatter coefficient have shown to be significantly affected by the bias between measurements in ascending and descending orbits adjustment of elevation time series for its dependence on backscatter coefficient and other waveform parameters performed in this paper substantially reduced the amplitude of elevation seasonal variations and locally corrected elevation change rate estimates by up to several centimeters per year it was found that the correction depends not only on the variations in the waveform parameters but also on the temporal variations of the correlation gradients which represent the sensitivity of the elevation change to the change in the waveform parameters an elevation change rate of 2 8 0 2 cm year from 1992 to 2008 over 76 of the greenland ice sheet area was found increases in surface elevation from 1995 observed over the high elevation regions of greenland were followed by an elevation decrease from 2006 for the whole period of 1992 2008 the elevation increase is 4 0 0 2 cm year over 87 of the area above 1500 m in contrast over 38 of the low elevation areas below 1500 m the rate of elevation change is 7 0 1 0 cm year and the surface elevation decrease that started from 2000 has continued

a tracer test in a shallow heterogeneous aquifer monitored via time lapse surface electrical resistivity tomography we illustrate a case study of a saline tracer test in a shallow highly heterogeneous aquifer monitored by means of surface time lapse ert the test was aimed at identifying the system s hydraulic properties some of the expected limitations of the method particularly caused by the strong decrease in ert resolution with depth and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location the vast heterogeneity of the system ranging in lithology from clay to gravel at a scale of meters to tens of meters reflects itself in the tracer migration and distribution over time the tracer is trapped in the low permeability regions and from these it is slowly released over time high resolution surface ert proves effective at picturing this system behavior over time the extreme heterogeneity is also a challenge in the attempt to translate bulk electrical

mass balance of higher elevation parts of the greenland ice sheet satellite radar and aircraft laser altimeter data and a volume budget comparison of total snow accumulation with total ice discharge give three independent estimates of the recent mass balance of parts of the greenland ice sheet above 2000 m elevation results show the entire region on average to be in balance to within 10 mm yr 1 with very low rates of regional thickness change dh dt in the northeast but high rates with large spatial variability in the south only the volume budget estimates show significant thinning in the northwest but thinning in this area is also inferred from local measurements of ice vertical velocities down boreholes south of latitude 68 n there has been rapid thickening west of the ice divide with equally rapid thinning in the southeast but with large differences between the three estimates of dh dt throughout this region the radar data apply to the period 1978 1988 the laser data are for 1993 1994 1998 1999 and the volume budget calculations represent conditions over at least the last few decades consequently many of the differences between results could be caused by temporal changes particularly in snow accumulation rates that occurred since the 1970s however taken with other information our results suggest long term thickening in the southwest and possibly quite recent onset of rapid thinning in the southeast

meltwater flux and runoff modeling in the ablation area of jakobshavn isbrae west greenland the temporal variability in surface snow and glacier melt flux and runoff were investigated for the ablation area of lakobshavn isbrae west greenland high resolution meteorological observations both on and outside the greenland ice sheet gris were used as model input realistic descriptions of snow accumulation snow and glacier ice melt and runoff are essential to understand trends in ice sheet surface properties and processes snowmodel a physically based spatially distributed meteorological and snow evolution modeling system was used to simulate the temporal variability of lakobshavn isbrre accumulation and ablation processes for 2000 01 2006 07 winter snow depth observations and modis satellite derived summer melt observations were used for model validation of accumulation and ablation simulations agreed well with observed values simulated annual surface melt varied from as low as 3 83 x 10 sup 9 m sup 3 2001 02 to as high as 8 64 x 10 sup 9 m sup 3 2004 05 modeled surface melt occurred at elevations reaching 1 870 m a s l for 2004 05 while the equilibrium line altitude ela fluctuated from 990 to 1 210 m a s l during the simulation period the snowmodel meltwater retention and refreezing routines considerably reduce the amount of meltwater available as ice sheet runoff without these routines the lakobshavn surface runoff would be overestimated by anmore average of 80 from september october through may june no runoff events were simulated the modeled interannual runoff variability varied from 1 81 x 10 sup 9 m sup 3 2001 02 to 5 21 x 10 sup 9 m sup 3 2004 05 yielding a cumulative runoff at the jakobshavn glacier terminus of approx 2 25 m w eq to approx 4 5 m w eq respectively the average modeled lakobshavn runoff of approx 3 4 km sup 3 y sup 1 was merged with previous estimates of jakobshavn ice discharge to quantify the freshwater flux to illulissat icefiord for both runoff and ice discharge the average trends are similar indicating increasing insignificant influx of freshwater to the illulissat icefiord for the period 2000 01 2006 07 this study suggests that surface runoff forms a minor part of the overall jakobshavn freshwater flux to the fiord around 7 approx 3 4 km sup 3 y sup 1 of the average annual freshwater flux of approx 51 0 km sup 3 y sup 1 originates from the surface runoff less

the oldest managed aquifer recharge system in europe new insights from the espino recharge channel sierra nevada southern spain abstract in sierra nevada southern spain the highest mountain range in southern europe the application of an ancestral integrated water resources management system iwrm based on the conjunctive use of groundwater and surface water provides water resources for irrigation and supply in the driest months of the year in this semiarid mountain region meltwater is derived from the headwaters of the mountain streams and rivers through a set of uncoated channels excavated in the ground locally known as acequias de careo to infiltrate at the upper part of the valleys water infiltrated along the acequias de careo slowly flows down the hillsides through the weathered zone of the hard rock aquifer and the glacial and periglacial sediments the recharge accomplished through this managed aquifer recharge technique mar activates numerous springs located halfway down the hillside and increases the base flow of the rivers in this study focused on a careo channel located on the southern slope of sierra nevada called acequia de el espino different archaeological sedimentological geophysical and hydrogeological techniques are applied to determine the age and the efficiency of this ancestral example of a mar and iwrm system results suggest that the acequias de careo may be the oldest mar system in europe and that this mar technique could be applied in other high mountain alpine watersheds to mitigate the effects of climate change

a shallow geothermal experiment in a sandy aquifer monitored using electric resistivity tomography groundwater resources are increasingly used around the world for geothermal exploitation systems to monitor such systems and to estimate their governing parameters we rely mainly on borehole observations of the temperature field at a few locations bulk electric resistivity variations can bring important information on temperature changes in aquifers we have used surface electric resistivity tomography to monitor spatially temperature variations in a sandy aquifer during a thermal injection test heated water 48 c was injected for 70 hours at the rate of 87 l h in a 10 5 c aquifer temperature changes derived from time lapse electric images were in agreement with laboratory water electric conductivity temperature measurements in parallel a coupled hydrogeologic saturated flow and heat transport model was calibrated on geophysical data for the conceptual model and on hydrogeologic and temperature data for the parameters the resistivity images showed an upper flow of heated water along the well above the injection screens and led to a new conceptualization of the hydrogeologic source term the comparison between the temperature models derived from resistivity images and from the simulations was satisfactory quantitatively resistivity changes allowed estimating temperature changes within the aquifer and qualitatively the heated plume evolution was successfully monitored this work demonstrates the ability of electric resistivity tomography to study heat and storage experiments in shallow aquifers these results could potentially lead to a number of practical applications such as the monitoring or the design of shallow geothermal systems

implications of changing scattering properties on greenland ice sheet volume change from cryosat 2 altimetry abstract long term observations of surface elevation change of the greenland ice sheet gris is of utmost importance when assessing the state of the ice sheet satellite radar altimetry offers a long time series of data over the gris starting with ers 1 in 1991 esa s cryosat 2 mission launched in 2010 provides an invaluable radar altimetry dataset for monitoring the current changes of the ice sheets due to its dense spatial and temporal coverage of these areas here we investigate the effects of including different parameters which describe the shape of the return radar waveform waveform parameters in the elevation change algorithm to correct for temporal changes in the ratio between surface and volume scatter in cryosat 2 observations we present elevation and volume changes for the greenland ice sheet in the period from 2010 until 2014 the waveform parameters considered here are the backscatter coefficient and the leading edge width which are both available in the esa cryosat 2 level 2i data product investigations into relocation of radar reflection points are also included inter comparison of the cryosat 2 derived elevation changes with those derived from operation icebridge laser data suggests waveform parameters to be applicable for correcting for changes in volume scattering the best results in the synthetic aperture radar interferometric mode area of the gris are found when applying only the backscatter correction whereas the best result in the low resolution mode area is obtained by only applying a leading edge width correction using this approach to correct for the scattering properties a volume loss of 292 38 km 3 yr 1 is found for the gris for the time span november 2010 until november 2014 the inclusion of waveform parameter corrections and improved relocation for the gris helps to reconcile the satellite derived elevation changes with those observed by operation icebridge however the bias of temporal changes in the scattering horizons of cryosat 2 is not entirely removed and suggests that future improvements could be made by including climate data and or additional waveform parameters to make additional corrections in the cryosat 2 radar altimetry

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

remote sensing of greenland ice sheet using multispectral near infrared and visible radiances 1 we present the physical basis of and validate a new remote sensing algorithm that utilizes reflected visible and near infrared radiation to discriminate between dry and wet snow when applied to the moderate resolution imaging spectroradiometer modis satellite data our discrimination algorithm has the potential to retrieve melting regions of the ice sheet at a spatial resolution of 0 25 km 2 over three orders of magnitude higher than the resolution of current microwave methods the method should be useful for long term monitoring of the melt area of the greenland ice sheet especially regions close to ice sheet margins and of the outflow glaciers our analysis of modis retrievals of the western portion of the greenland ice sheet over the period 2000 to 2006 indicates significant interannual variability with a maximum melt extent in 2005 collocated in situ meteorological data reveal a high correlation 0 80 between the modis melt day area and the average summer temperature our analysis suggests that it is the magnitude of the summer temperature that dominates the melting not the variability of the length of the melting season furthermore we find that the melt day area increases by about 3 8 for each 0 1 k increase in the average surface air summer temperature we combine this empirical relationship with historic temperature data to infer that the melt day area of the western part of the ice sheet doubled between the mid 1990s and mid 2000s and that the largest ice sheet surface melting probably occurred between 1920s and 1930s concurrent with the warming in that period

the melt anomaly of 2002 on the greenland ice sheet from active and passive microwave satellite observations 1 active and passive microwave satellite data are used to map snowmelt extent and duration on the greenland ice sheet the passive microwave pm data reveal the extreme melt extent of 690 000 km2 in 2002 as compared with an average extent of 455 000 km2 from 1979 2003 a statistical analysis of the melt time series affirms an increased melt extent earlier in the melt season the quikscat qscat or am analysis for 1999 to 2004 confirms the extreme melt situation in 2002 qscat data show a significant increase in melt season length over several areas in 2003 ssm i and qscat melt data reveal that the later detects melt earlier and is more sensitive to surface melt qscat detects an earlier stage of melt and ssm i detects a later stage of melt the ssm i xgpr melt extent is approximately confined to the qscat melt areas experiencing 2 weeks or more of melting time

compensating changes in the penetration depth of pulse limited radar altimetry over the greenland ice sheet changes in firn properties affect the shape of pulse limited radar altimeter echoes acquired over the polar ice sheets we apply a waveform deconvolution model to cryosat 2 low resolution mode echoes to determine the depth distribution of radar backscattering across the greenland ice sheet the deconvolution allows us to calculate the relative contributions of surface and volume scattering and the effective penetration depth of the radar echoes into the snowpack the most prominent signal is associated with the extreme surface melting of summer 2012 which resulted in a shift of the dominant radar scattering horizon toward the snow surface in the accumulation zone at locations above 2000 m the average penetration depth in july 2012 prior to the melt event was 3 79 1 12 m following the melt event there was an abrupt reduction in the average penetration depth across the same region to 1 45 0 94 m the average penetration depth then gradually increased to 3 28 1 13 m by the end of 2017 as fresh snow accumulated on the ice sheet surface although the variation in penetration is evident in surface height estimates derived from the cryosat 2 echoes the magnitude of the effect is reduced by waveform retracking using airborne laser altimeter data recorded over the same time period we show that the penetration variation can be compensated effectively by incorporating the deconvolution penetration depth into the surface height retrieval

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

calibrating a surface mass balance model for austfonna ice cap svalbard austfonna 8120 km 2 is by far the largest ice mass in the svalbard archipelago there is considerable uncertainty about its current state of balance and its possible response to climate change over the 2004 05 period we collected continuous meteorological data series from the ice cap performed mass balance measurements using a network of stakes distributed across the ice cap and mapped the distribution of snow accumulation using ground penetrating radar along several profile lines these data are used to drive and test a model of the surface mass balance the spatial accumulation pattern was derived from the snow depth profiles using regression techniques and ablation was calculated using a temperature index approach model parameters were calibrated using the available field data parameter calibration was complicated by the fact that different parameter combinations yield equally acceptable matches to the stake data while the resulting calculated net mass balance differs considerably testing model results against multiple criteria is an efficient method to cope with non uniqueness in doing so a range of different data and observations was compared to several different aspects of the model results we find a systematic underestimation of net balance for parameter combinations that predict observed ice ablation which suggests that refreezing processes play an important role to represent these effects in the model a simple pmax approach was included in its formulation used as a diagnostic tool the model suggests that the surface mass balance for the period 29 april 2004 to 23 april 2005 was negative 318 mm w e

estimation of volume change rates of greenland s ice sheet from icesat data using overlapping footprints abstract data from the icesat glas laser altimetry mission is used to obtain an estimate of the volume change of greenland s ice sheet over the time span of february 2003 to april 2007 a novel processing strategy is developed and applied it uses approximately 1 million icesat elevation differences at geometrically overlapping footprints of both crossing and repeated tracks the data are edited using quality flags defined by the icesat glas science team as well as other additional criteria in order to reduce the influence of surface slope we propose a correction based on the icesat glas laser altimetry digital elevation model three slightly different processing strategies to convert the observed temporal elevation differences to elevation volume changes are compared for 6 different drainage systems further divided into regions above and below 2000 m in elevation the final chosen strategy includes the correction for surface slopes but does not include the removal of outlying elevation changes for the region above 2000 m a positive elevation change rate of 2 cm year is obtained which corresponds to a volume change rate of 21 km3 year for the region below 2000 m the estimated elevation change rate is 24 cm year which corresponds to a volume loss of 168 km3 year in general the obtained results are in agreement with trends discovered by other authors that were also derived from laser altimetry nevertheless the estimation obtained in this study suggests a more negative trend than those obtained previously the differences can be explained by differences in the sampling of the region below 2000 m and to a certain extent by different time spans of the datasets used a representative sampling of coastal areas is identified as the most critical issue for an accurate estimation of volume change rates in greenland

study of infiltration in a sahelian gully erosion area using time lapse resistivity mapping abstract observing that concentrated runoff destroys indurate and impermeable surface horizons to form gullies on sahelian slopes we investigated whether these gullies are preferential places for deep infiltration and groundwater recharge processes the primary aim of this study is to determine if resistivity mapping is an appropriate method to use for locating recharge zones from the surface the study area in northern burkina faso is a typical 1 ha gully erosion area located at the outlet of an 82 ha catchment with solonetz soils and a crystalline basement taking advantage of a long dry season followed by a short rainy season we made use of a time lapse approach to carry out electrical resistivity mapping and monitor apparent resistivity variations that occurred in the soils during the rainy season between june and september we made nine apparent resistivity maps in the year 2000 and two in january and march 2001 to monitor expected infiltration and percolation to depths of 5 m or more we laid out wenner array profiles with an inter electrode spacing of 5 m the time lapse mapping was also controlled with i neutron probe measurements ii resistivity measurements on outcrops during infiltration tests iii electrical resistivity logging in auger holes geophysical results showed that the apparent resistivity parameter can either decrease typical case or increase unexpected case after a rain neutron probe measurements indicated that infiltration varies within a few decimeters even at the centre of the main gully using one dimensional 1d modelling based on resistivity variations monitored during infiltration tests we concluded that apparent resistivity variations are linked to the presence of carbonate in the soils when soluble carbonates are present the resistivity of the infiltrated layer varies from 220 m dry state to less than 5 m wet state bringing about a decrease in apparent resistivity value for the 5m spacing in the absence of carbonate resistivity varies from 1500 to 180 m but produces an increase of the apparent resistivity value for the same spacing consequently we found time lapse apparent resistivity mapping to be an efficient way to delineate certain soil properties it also provided additional information about punctual observations however our results have led us to conclude that the 5 m inter electrode spacing is too large to monitor this type of shallow infiltration phenomenon and that the effect of temperature on resistivity should be considered when comparing maps over the period of a few months furthermore this type of survey should be controlled using electrical loggings in auger holes or electrical soundings in order to get a better understanding of in depth resistivity variations finally this survey indicated that deep infiltration processes are not occurring below the gully situated on the slope further studies are required downstream to identify the location of groundwater recharge in sahelian crystalline contexts

monitoring the impact of de icing salt on roadside soils with time lapse resistivity measurements monitoring systems along roads are needed to facilitate decisions on improving protection of water resources and decreasing the impact of road related pollutants on the roadside environment this paper presents a monitoring system using permanently installed electrodes and monthly measurements of resistivity at a motorway in sweden with heavy loads of de icing salt a significant increase in resistivity in the vadose zone with increasing distance from the road was shown in both sand and glacial till areas during the whole year by measuring temporal variations in a less affected environment further from the road a distinction could be made between more natural variations and variations due to de icing salt and melting of roadside snowbanks the highest resistivities occurred in october november and the lowest in january march while the more natural resistivities showed an opposing temporal variation the difference was up to 35 on a log scale in the sand area during the latter period hence the time lapse resistivity measurements clearly showed a strong influence of de icing salt on roadside soils and groundwater during winter and spring the measurement system and the analysis methods proved useful for monitoring both the spatial and seasonal variation in resistivity

ice sheet mass balance assessment attribution and prognosis contrary to prior expectations that warming would cause mass addition averaged over the greenland and antarctic ice sheets and over the next century the ice sheets appear to be losing mass at least partly in response to recent warming with warming projected for the future additional mass loss appears more likely than not

a comparison of recent elevation change estimates of the devon ice cap as measured by the icesat and envisat satellite altimeters we have used surface elevation measurements acquired by the ice cloud and land elevation satellite geoscience laser altimeter system glas and envisat radar altimeter 2 ra 2 satellite altimeters to assess the elevation change of the 13 700 km2 devon ice cap dic in arctic canada between 2002 and 2008 we present algorithms for the retrieval of elevation change rates over ice caps using data acquired from these satellites a comparison of glas elevation data to those acquired by the ra 2 shows reasonable agreement between the two instruments the root mean square elevation change difference was 56 cm and the correlation coefficient between the two data sets was 0 68 using only ra 2 elevation measurements which are spatially and temporally more continuous we determined the elevation change rate of the areas of the dic where the surface geometry allows the ra 2 retracker to maintain lock this includes most of the dic excluding large parts of the eastern half of the ice cap the elevation change rate was found to be insignificant given a statistical estimate of the measurement error 0 09 0 29 m a we also present an assessment of the regional variations of the dic elevation change including a significant 0 71 0 49 m a elevation change rate of the 1980 km2 western arm furthermore we present evidence of a localized 2 m drop in the surface elevation of the south croker bay glacier during summer 2007 this drop is apparent within both satellite data sets and we interpret this signal to reflect a sudden speedup of the glacier

radar response of firn exposed to seasonal percolation validation using cores and fdtd modeling we use ground penetrating radars gprs firn cores and electromagnetic finite difference time domain fdtd numerical modeling to characterize the gpr response to a frozen high arctic firn pack as a result of extensive summertime percolation the firn pack comprises a high fraction of ice layers lenses and vertical glands we show that the gpr response on the firn pack mainly depends on the following 1 the thickness of the ice layers 2 the distance between layers 3 the layer roughness and 4 the presence or absence of elliptical ice lenses using 3 d fdtd modeling we show that the gpr is not sensitive to typical ice glands which implies that the gpr underestimates firn heterogeneity such that firn stratigraphy in percolation and wet snow zones could be incorrectly interpreted as being better preserved than it actually is we find that thin ice layers 0 05 m or multiple thin ice layers give a strong response thicker ice layers typically give a weaker backscatter per unit area mainly due to the lack of interference of the reflections from the upper and lower interfaces but are due to their continuity easily trackable ice layers with a thickness comparable to the gpr wavelength give 180deg phase shifted upper and lower reflections and are in general separated by a band of low gpr response due to the lack of permittivity contrast within the ice layers despite the ice lenses relatively short horizontal correlation length as inferred from cores bands of high amplitude clutter caused by these features can be traced over several kilometers in gpr profiles

the application of gpr and electrical resistivity tomography as useful tools in detection of sinkholes in the cheria basin northeast of algeria sinkhole collapse is one of the main limitations in the development of karst areas especially where bedrock is covered by unconsolidated material studies of sinkhole formation have shown that sinkholes are likely to develop in cutter enlarged joint zones as a result of subterranean erosion by flowing groundwater ground penetrating radar gpr and electrical resistivity imaging or tomography restom are well suited to mapping sinkholes because of the ability of these two techniques for detecting voids and discriminating subtle resistivity variations nine gpr profiles and two dimensional electrical resistivity tomography have been applied with relative success to locate paleo collapses and cavities and to detect and characterize karst at two sinkhole sites near cheria city where limestone is covered by about 10 m of clayey soils the survey results suggest that gpr and restom are ideal geophysical tools to aid in the detection and monitoring of sinkholes and other subsurface cavities

landslides triggered factors analysed by time lapse electrical survey and multidimensional statistical approach abstract a temporal imagery of water circulation in a landslide by electrical resistivity tomography ert was conducted to identify spatially and temporally the coupling between rainfalls consecutive water inflows in a sliding mass and induced resistivity variations this work is based on a multi scale experimental approach applied on the vence landslide south eastern france which is characterised by a sandy clay sliding mass on a marly limestone substratum mostly controlled by high rainfall events at the landslide scale historical geological and geotechnical data combined with field investigations and the interpretation of three ert allowed the definition of a geometrical model of the landslide and the calibration of the resistivity values on the basis of these results a permanent time lapse ert survey was designed on a specific part of the landslide coupled with water level acquisition piezometric levels and rain fall events results covering a three month period are showed the statistical analyses of all the physical parameters measured during the three months of investigation show that the matrix of correlations highlighted strong correlations between the rainfall piezometric elevation and the resistivity these first results show an accurate answer resistivity piezometric elevation that can be associated as a precursor of the reactivation of the landslide

precipitation over greenland and its relation to the north atlantic oscillation the equation method based on an equivalent isobaric geopotential height in coordinates has been used to retrieve the precipitation over greenland this approach is designed to accurately represent the topographic effects of the greenland ice sheet on atmospheric motion and precipitation the 11 year mean precipitation from 1985 to 1996 over all of greenland is 376 mm yr 1 which is close to the long term mean precipitation of 346 mm yr 1 estimated from glaciological data the precipitation over all of greenland shows that the largest value in 1986 is 472 mm yr 1 and the smallest value in 1995 is 309 mm yr 1 the major interannual variability of the atmospheric circulation in the north atlantic can be represented by the variation of the north atlantic oscillation nao index which is most pronounced during winter it is found that if the nao index increases the total precipitation over greenland decreases and vice versa the correlation coefficient between these two series for 1985 1995 is 0 75 the mean precipitation over southern greenland where the majority of precipitation falls is more closely related to the nao index in winter and their correlation coefficient is 0 80 this relationship can be understood from the composite maps of sea level pressure and greenland precipitation for the high and low index months during months of high nao index values the icelandic low is strong during months of low nao index values the monthly mean low is located to the southwest of greenland over the labrador sea precipitation amounts over the southeast coast of greenland are about 100 mm larger during the low nao index months than the high nao index months precipitation over all of greenland during the low nao index months is higher there are significant downward trends in annual precipitation from 1985 1995 for all of greenland and its southern and central west coastal regions amounting to about 3 per year

mass balance of polar ice sheets recent advances in the determination of the mass balance of polar ice sheets show that the greenland ice sheet is losing mass by near coastal thinning and that the west antarctic ice sheet with thickening in the west and thinning in the north is probably thinning overall the mass imbalance of the east antarctic ice sheet is likely to be small but even its sign cannot yet be determined large sectors of ice in southeast greenland the amundsen sea embayment of west antarctica and the antarctic peninsula are changing quite rapidly as a result of processes not yet understood