a regional scale climate reconstruction of the last 4000 years from lakes in the nebraska sand hills usa abstract high resolution paleohydrological reconstructions were carried out in five shallow lakes in the nebraska sand hills across an east west transect in order to 1 determine whether long term droughts of the past 4000 years were spatially and temporally coherent across the region 2 distinguish local variation in climate or hydrology from regional patterns of change and 3 compare the paleolimnological results with the existing dune inferred drought records diatom inferred lake level was reconstructed for all sites and compared with other regional records alterations between high and low lake levels were frequent during the past 4000 years which suggests that shifts between dry and wet periods were prevalent across the sand hills extended multi decadal to centennial scale droughts were more common prior to 2000 years bp while the last two millennia were hydrologically more variable and climate conditions alternated on shorter timescales despite some discrepancies among the five records the paleohydrological reconstructions refine the holocene drought history of the nebraska sand hills particularly between 2200 and 4000 cal a bp many of the observed drought events are contemporaneous with severe droughts documented at sites in the northern great plains and rocky mountains lending support for the severity and regional significance of these events in western north america

site response in las vegas valley nevada from nts explosions and earthquake data we report site response in las vegas valley lvv from historical recordings of nevada test site nts nuclear explosions and earthquake recordings from permanent and temporary seismic stations our data set significantly improves the spatial coverage of lvv over previous studies especially in the northern deeper parts of the basin site response at stations in lvv was measured for frequencies in the range 0 2 5 0 hz using standard spectral ratios ssr and horizontal vertical spectral ratios hvr for the ssr measurements we used a reference site approximately nehrp b rock classification located on frenchman mountain outside the basin site response at sedimentary sites is variable in lvv with average amplifications approaching a factor of 10 at some frequencies we observed peaks in the site response curves at frequencies clustered near 0 6 1 2 and 2 0 hz with some sites showing additional lower amplitude peaks at higher frequencies the spatial pattern of site response is strongly correlated with the reported depth to basement for frequencies between 0 2 and 3 0 hz although the frequency of peak amplification does not show a similar correlation for a few sites where we have geotechnical shear velocities the amplification shows a correlation with the average upper 30 meter shear velocities v 30 we performed two dimensional finite difference simulations and reproduced the observed peak site amplifications at 0 6 and 1 2 hz with a low velocity near surface layer with shear velocities 600 750 m s and a thickness of 100 200 m these modeling results indicate that the amplitude and frequencies of site response peaks in lvv are strongly controlled by shallow velocity structure

microbial biomass and community structure in alkaline lakes of the nebraska sand hills usa abstract the many lakes present among the inactive dunes of the nebraska sand hills are unique ecosystems that formed under particular conditions of local and regional geomorphology hydrology and climate they offer an exceptional opportunity to study the inter relations between hydrology microbial diversity and biogeochemistry in comparatively closely spaced bodies of water ranging in ph from neutral to alkaline we analyzed for lipid biomarkers in the surficial sediments of six sand hills lakes including alkanes alkenes alkanols phytol sterols hopanoids tetrahymanol and phospholipid fatty acids overall these biomarkers exist in significantly lower concentrations in the alkaline lakes of the sand hills than in ordinary neutral lakes more specifically microbial functional groups can be identified in the lake sediments on the basis of the biomarkers for which we analyzed the presence of eukaryotic phototrophs is indicated by the detection of phytadienes phytol phytanol diatomsterol dinosterol and polyunsaturated fatty acids the detection of short chain wax esters suggests the presence of anoxygenic autotrophic sulfur bacteria purple and green sulfur bacteria the phospholipid fatty acid plfa profiles indicate the presence of sulfate reducing bacteria in the sediment tetrahymanol and polyunsaturated fatty acids in the sediments suggest the presence of bacterivorous ciliates and phototrophic sulfur bacteria which probably live at the chemocline our results reveal that there are unique and integrated ecosystems within the sand hills lakes that are defined by the close interactions between regional hydrology geochemistry and microbiology

a comparison and test of various site response estimation techniques including three that are not reference site dependent abstract we compare various site response estimation techniques using after shock data of the 1989 loma prieta earthquake collected in oakland california because of recent interest in comparing results from weak and strong motion to infer any nonlinearity and between direct s and coda waves we pay particular attention to the uncertainties first sediment to bedrock spectral ratio estimates between pairs of sites are compared with those obtained from various generalized inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously we find that the site amplification factors are very similar among these approaches but that the uncertainties can be significantly different depending on how the data are weighted we also examine and test three site response estimation techniques that do not rely on a reference site to estimate source and path effects the first involves a parameterized source and path effects inversion even when the bedrock data are excluded from consideration this approach is found to reveal the frequency dependence of site response at each of the sediment sites the second technique involves taking horizontal to vertical component spectral ratios receiver function type estimates of shear wave aftershock data these are also found to reveal the frequency dependence of site response at the sediment sites and the results for the bedrock site are relatively flat and near unity the third estimate is formed by taking horizontal to vertical component ratios of ambient seismic noise and these are shown to reveal the fundamental resonant frequency of the sediment sites unfortunately discrepancies exist among all of the site response estimates and with one dimensional predictions with respect to a frequency independent scaling factor nevertheless the highly frequency dependent character of site response is well constrained and the fact that non reference site dependent methods are capable of revealing this is promising for site specific hazard assessments in regions that lack adequate reference sites

a tale of two lakes a multi proxy comparison of lateglacial and holocene environmental change in cappadocia turkey individual palaeoenvironmental records represent a combination of regional scale e g climatic and site specific local factors here we compare multiple climate proxies from two nearby maar lake records assuming that common signals are due to regional scale forcing a new core sequence from nar lake in turkey is dated by varves and u th to the last 13 8 ka markedly dry periods during the lateglacial stadial at 4 3 3 7 and at 3 2 2 6 ka bp are associated with peaks in mg dolomite positive 18o elevated diatom inferred electrical conductivity an absence of laminated sediments and low quercus chenopod ratios wet phases occurred during the early mid holocene and 1 5 0 6 ka bp characterized by negative 18o calcite precipitation high ca sr ratios a high percentage of planktonic diatoms laminated sediments and high quercus chenopod ratios comparison with the record from nearby eski acigol shows good overall correspondence for many proxies especially for 18o differences are related to basin infilling and lake ontogeny at eski acigol which consequently fails to register climatic changes during the last 2 ka and to increased flux of lithogenic elements into nar lake during the last 2 6 ka not primarily climatic in origin in attempting to separate a regional signal from site specific noise two lakes may therefore be better than one

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

on the horizontal to vertical spectral ratio in sedimentary basins the horizontal to vertical spectral ratio hvsr has been used by many researchers to characterize local conditions in terms of the dynamic response of the soil one of its variants is that proposed by nakamura 1989 in which records of microtremors are used usually the analysis is aimed to obtain the predominant period of the site under study in this work we explore what can be achieved by using this method we study the response of different configurations under incident waves coming from an explosive source using the indirect boundary element method ibem we investigate two cases low and high velocity contrast holding constant the physical properties inside the basin and changing only the properties of the bedrock then we compute the seismic response using the horizontal sediment to bedrock spectral ratio sbsr at various locations on the free surface of the basins and compare it with the one calculated by the hvsr at the same locations the comparison shows that in general the predominant period computed with the hvsr is not the same as that obtained by the sbsr in all the locations on the other hand the hvsr approximation can reasonably well predict the fundamental local frequencies when the impedance contrast between the basin and the bedrock is low however hvsr cannot be used in sedimentary basins having a high impedance contrast with respect to the bedrock below

a local magnitude scale for new zealand earthquakes abstract new procedures have been used since 1977 for assigning local earthquake magnitudes in new zealand based on a slightly modified form of richter9s definition and derived from amplitude distance relations determined from new zealand earthquakes the magnitudes for all crustal earthquakes and for subcrustal earthquakes in or north of the main seismic region are determined using a formula for geometric spreading which is appropriate to head wave propagation for subcrustal earthquakes south of the main seismic region they are determined assuming body wave propagation inelastic attenuation needs to be taken into account for subcrustal earthquakes south of the main seismic region and when paths from crustal earthquakes pass through a region which includes the zone of recent volcanism in the center of the north island

temporal and spatial variability in dune reactivation across the nebraska sand hills usa the nebraska sand hills is a stabilized dune field on the great plains of north america although it is well known that this dune field like several others on the great plains last experienced widespread activity during the medieval climatic anomaly mca ad 900 1300 spatial variation in the timing and nature of drought development is poorly constrained to elucidate spatial trends in dune reactivation samples potentially representing mca activity across the sand hills were collected and dated using optically stimulated luminescence osl ages from the older part of the mca were obtained from eolian sediments in the northwestern sand hills while ages from later in the episode were obtained to the southeast suggesting a geographic trend in the timing of revegetation of the dunes near the end of the drought revegetation likely occurred to the northwest initially as a result of renewed moisture availability from a rising water table in the interdunes which serve as refugia for vegetation during times of drought vegetation then gradually spread to the southeastern sand hills an additional spatial trend in ages is apparent in the chronology of linear dune mobilization across the sand hills linear dunes in the northwest are superimposed on megadunes and originated during the last reactivation while linear dunes in the southeast are built around older cores of dunes and formed during several reactivations our geochronology reveals three episodes of eolian transport including the mca in the formation of linear dunes in the southeast

a ground motion transfer function matrix between two nearby rock and soil sites a system identification problem abstract standard single component spectral ratios used to estimate localized site effects may be generalized by using a 3 3 matrix transfer function between the three component motion observed at a reference rock site and that at a nearby soil site this complex matrix g matrix represents the complete three dimensional site response within the linear regime no prior knowledge of the geology at the soil site is required and g may be computed directly from the spectral responses of earthquakes recorded at both sites earthquakes from different hypocentral locations may be used but must be selected so that the body wave incidence at both stations is nearly vertical and must be time windowed to avoid surface waves g is calculated using a stochastic inverse of the observed response spectra and corresponding noise covariance for each response this method of analysis depends on the availability of a good quality reference rock site as do standard spectral ratio techniques that compare rock and soil site records we have applied this method to determine site effects to data recorded by strong ground motion seismographs near wellington north island new zealand magnitudes of the diagonal elements of g are similar to standard single component spectral ratios and large values of the off diagonal matrix elements are seen at some locations at these locations the maximum response is better estimated from the principal values of the transfer function matrix the orientation at which the maximum amplification up to 14 in one case occurs varies widely according to location

quantification of salt dust pathways from a groundwater fed lake implications for solute budgets and dust emission rates 1 emissions of salt dust from the shores of saline lakes significantly impact lake chemistry air quality transportation human health and climate quantitative methods for assessing these emissions however are still in the developmental stage we investigate salt pathways from groundwater to dust using an approach that takes advantage of opportune conditions at a groundwater fed saline lake in the nebraska sand hills region the mass of salt in the lakeshore surface crust and soil was measured as well as in the dust on the surrounding dune field these data together with information on the lake hydrology show that dust emission is an important mechanism controlling lake salinity even though a mere fraction of the salt crust is deflated each year under extant climatic conditions wind data collected at the lake site indicate high wind speeds capable of dust mobilization therefore the physical and chemical bonding of salts in the crust is offered as the primary limiting factor for dust emission rates

detailed evaluation of site response estimation methods across and along the sedimentary valley of volvi euro seistest using records from the euro seistest area near thessaloniki greece we studied empirically the amplification effects of a sedimentary valley one of the main advantages of this site is the very detailed knowledge of the near subsurface structure due to exhaustive geophysical and geotechnical measurements and data analyses jongmans et al 1998 the uniqueness of the seismological data set is the very dense network 31 stations were installed perpendicular and parallel to the valley axis with minimum interstation distances of 250 m the mean ampli fications along both axes estimated from 13 local events were imaged using the traditional spectral ratio technique variations of the mean amplifications due to the choice of different data subsets were tested by the use of up to 89 local events supplemented by four teleseismic events offering good low frequency resolution results obtained using the traditional spectral ratio technique were compared with results from a generalized inversion technique the h v ratio technique a coda wave technique and nakamura s technique advantages and disadvantages of each method are outlined especially from a practical point of view

a diatom record of late holocene climate variation in the northern range of yellowstone national park usa the changes in diatom species composition in a sediment core from crevice lake yellowstone national park spanning the past 2550 yr were used to reconstruct long term limnological and ecological conditions that may be related to late holocene climate variability planktic forms dominate the fossil diatom assemblages throughout this record but changes in species dominance indicate varying nutrient levels over time particularly phosphorus the changes in the nutrient concentrations in the lake were probably driven by changes in temperature and wind strength that affected the duration of water column mixing and thus the extent of nutrient recycling from deep waters prior to 2100 cal before present bp stephanodiscus minutulus and synedra tenera dominated suggesting long cool springs with extensive regeneration of phosphorus from the hypolimnion that resulted from isothermal mixing from 2100 to 800 cal bp these species were replaced by cyclotella michiganiana and cyclotella bodanica these species are characteristic of lower nutrient concentrations and are interpreted here to reflect warm summers with long periods of thermal stratification from 800 to 50 cal bp s minutulus dominated the diatom assemblage suggesting a return to lengthy mixing during spring the most dramatic late holocene changes in the fossil diatom assemblages occurred during the transition from the medieval period to the little ice age approximately 800 cal bp

spatial and temporal patterns of late quaternary eolian deposition eastern colorado u s a abstract eolian sediment covers about 60 of colorado east of the rocky mountains about 30 of the sediment is sand and 70 is loess initially flood plains were the principal sources of eolian sediment but during the holocene dunes formed from older eolian sand and alluvium on uplands since latest pleistocene time dominant dune forming winds have been northwesterly in the northern part of the region and southwesterly in the southern part at present sand sheets and dunes mainly parabolic types are stable and covered with vegetation in dunes sand is commonly 20 30 m thick but elsewhere averages

diatom inferred changes in effective moisture during the late holocene from nearshore cores in the southeastern region of the winnipeg river drainage basin canada the winnipeg river drainage basin wrdb within the boreal forest region of northwest ontario is a region that is expected to be negatively affected by climate warming inferences of droughts over the past two millennia from little raleigh lake were based on two nearshore sediment cores the core locations were from depths of 12 and 15 m and were based on sufficient nearshore sediment accumulation and distance from the modern benthic to planktonic diatom boundary where a distinct shift from dominance of benthic taxa changed to dominance of planktonic taxa in surficial sediments at 11 8 m diatom inferred depth was based on a model developed from 60 surficial sediments within the study lake depth inferences indicate that prolonged periods of aridity occurred from ad 950 to 1300 corresponds to medieval climate anomaly and from ad 1625 to 1750 aridity during little ice age we found that the core collected from a depth closer to the benthic to planktonic diatom boundary was more sensitive to changes in lake level than the deeper core where planktonic diatoms dominated the assemblage the inferred low water stands of the past two millennia are well outside of the range of the past 100 years suggesting that recent drought history may not be a good estimate of future extremes

loess record of dry climate and aeolian activity in the early to mid holocene central great plains north america thick holocene bignell loess sections provide new evidence for the timing of early to middle holocene aridity in the central great plains the immediate source of loess in these sections was dune fields just upwind based on thickness trends and grain size data thus periods of rapid loess accumulation indicate episodes of extensive dune activity under drier than present climate in typical bignell loess sections a thick zone of coarse textured loess with minimal pedogenic alteration is interpreted as a record of the most rapid sustained holocene loess accumulation optical ages indicate that loess in this zone was deposited beginning about 9000 10000 years ago at five study sites accumulation ended shortly after 6500 years ago at three sites but possibly earlier at the other two in two well dated sections the average rate of loess accumulation between 9000 10000 and 6500 years ago was about 1 6 3 5 times greater than in the late holocene thus we infer that the most extensive sustained holocene

a 10 000 year record of dune activity dust storms and severe drought in the central great plains dune fi elds and loess deposits of the great plains of north america contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought we present a 10 000 yr reconstruction of dune activity and dust production in the central great plains region based on 95 optically stimulated luminescence ages the integration of data from both eolian sand and loess is an important new aspect of this record clusters of ages defi ne episodes of extensive eolian activity which we interpret as a response to frequent severe drought at 1 0 0 7 ka and 2 3 4 5 ka with peaks centered on 2 5 and 3 8 ka sustained eolian activity occurred from 9 6 to 6 5 ka parts of this record may be consistent with hypotheses linking holocene drought to sea surface temperature anomalies in the pacifi c or atlantic oceans or to the el nino southern oscillation phenomenon but the record as a whole is diffi cult to reconcile with any of these hypotheses

aeolian sand depositional records from western nebraska landscape response to droughts in the past 1500 years the great plains is dominated by presently stabilized dune fields that are indicators of extreme drought in the late holocene this study focused on deciphering the timing of reactivation of dunes in western nebraska stratigraphy adjacent to dune dammed lakes reveals aeolian sand separated by palaeosols indicating mobilization of aeolian sand followed by landscape stability the chronology of aeolian sand depositional events is constrained using the luminescence based single aliquot regeneration method providing resolution to relate dune movement to tree ring and palaeolimnologic records of drought there are at least a six aeolian depositional events in the past 1500 years with apparent mean ages of 1390 130 670 70 470 40 240 40 140 20 and 70 10 yr all study sites show evidence for aeolian accumulation in the twentieth century potentially reflecting the 1930s drought significant aeolian activity is coincident with the tree ring identified sixteenth century megadrought indicating widespread landscape impacts

regional aridity in north america during the middle holocene increased aridity throughout the great plains and rocky mountain region during the middle holo cene has been documented from pollen records aeolian proxy variables in lake cores and active sand dune migration varve calibration provided by a continuously varved record of the holocene from a core from elk lake northwestern minnesota shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3 8 ka with peak aeolian activity at about 6 ka if aeolian influx to elk lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in minnesota dune fields then the varve calibration in elk lake provides precise time calibration of periods of peak aeolian activity in minnesota palaeowind studies from the minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest the same as the dominant wind direction in dune fields throughout the great plains and rocky mountains if the mid holocene aeolian act

effect of soil conditions on predicted ground motion case study from western anatolia turkey abstract we present a site effect study for the city of izmir western anatolia turkey local amplification was evaluated using state of practice tools ten earthquakes recorded at 16 sites were analysed using spectral ratios relative to a reference site horizontal to vertical spectral ratios and an inversion scheme of the fourier amplitude spectra of the recorded s waves seismic noise records were also used to estimate site effects the different estimates are in good agreement among them although a basic uncertainty of a factor of 2 seems difficult to decrease we used our site effect estimates to predict ground motion in izmir for a possible m6 5 earthquake close to the city using stochastic modelling site effects have a large impact on psv pseudospectral velocity where local amplification increases amplitudes by almost a factor of 9 at 1 hz relative to the firm ground condition our results allow identifying the neighbourhoods of izmir where hazard mitigation measurements are a priority task and will also be useful for planning urban development

dune dammed paleovalleys of the nebraska sand hills intrinsic versus climatic controls on the accumulation of lake and marsh sediments although running water is the dominant geomorphic agent on earth eolian processes can gain ascendancy in regions where the climate is arid vegetation is sparse and abundant sand is available for transport with climate change the boundaries between fluvial dominated and eolian dominated areas may shift although there have been few reports in the north american literature of river systems blocked by dune sand our work in the nebraska sand hills provides evidence of multiple episodes of such blockage events during prolonged arid intervals in latest pleistocene and middle holocene time eolian dune sand blocked two large valley systems in western nebraska these blockages raised the water table of the high plains aquifer as much as 25 m over an area of 7000 km2 and created over one thousand lakes wetlands far removed from the discharge points of the buried paleovalley system are strongly alkaline exceeding 250 000 mg l total dissolved solids tds relatively fresh 280 mg l tds flow through lakes are present at the distal end of the system where the gradient of the water table is steep and the cross section of the buried valley is large anomalously thick marsh and lake sediments accumulated in deep paleovalleys upstream of dune dams near the southern margin of the sand hills our cores and radiocarbon dates from blue and crescent lakes reveal their histories to be quite distinct from adjacent swan lake these differences are best explained by multiple blockage events our work explains why lakes are most abundant in the driest part of the sand hills it also provides another line of evidence for major dune activity in the sand hills region during holocene time and shows that factors other than regional climate specifically location height and hydraulic conductivity of dune dams can control the rise and fall of the ground water table and the chemistry of lakes

a 2200 yr record of hydrologic variability from foy lake montana usa inferred from diatom and geochemical data abstract a 2200 yr long high resolution 5 yr record of drought variability in northwest montana is inferred from diatoms and 18o values of bio induced carbonate preserved in a varved lacustrine core from foy lake a previously developed model of the diatom response to lake level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data high frequency decadal fluctuations in the de trended 18o record mirror variations in wet dry cycles inferred from banff tree rings demonstrating the sensitivity of the oxygen isotope values to changes in regional moisture balance low frequency multi centennial isotopic changes may be associated with shifts in the seasonal distribution of precipitation from 200 b c to a d 800 both diatom and isotope records indicate that climate was dry and lake level low with poor diatom preservation and high organic carbon nitrogen ratios subsequently lake level rose slightly although the climate was drier and more stable than modern conditions at a d 1200 lake level increased to approximately 6 m below present elevation after which the lake fluctuated between this elevation and full stage with particularly cool and or wetter conditions after 1700 the hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake s outlet spectral analysis of the 18o data indicates that severe droughts occurred with multi decadal 50 to 70 yr frequency

optical and radiocarbon ages of stacked paleosols and dune sands in the nebraska sand hills usa abstract optical ages for eolian sands from the nebraska sand hills indicate periods of extensive eolian activity at ca 115 25 840 70 2300 240 and 3560 340 a activity was also noted at single sampling locations at ca 6180 370 8430 510 and 13110 800 a many of these ages are similar to those noted by earlier authors optical ages from samples collected within paleosols indicate shorter and possibly less extensive periods of eolian activity at approximately 1220 150 1590 110 and possibly 1950 150 a during which the paleosol sands accumulated what was originally interpreted as a single 1 2 m thick paleosol is shown by optical dating to consist of three or more welded soils developed within eolian sands with optical ages of ca 3800 240 2740 240 1560 110 and possibly 1930 140 a each of which match eolian pulses recognized elsewhere scatter in some optical ages is attributable to intersection of sand filled rodent burrows extending in outcrop 1 5 m below the contact between paleosol and overlying topset beds a 5310 360 a optical age for one probable intersected burrow provides evidence for upward or lateral transport of older sands

an active heat tracer experiment to determine groundwater velocities using fiber optic cables installed with direct push equipment a new approach is developed to insert fiber optic cables vertically into the ground with direct push equipment groundwater temperatures may be measured along the cables with high spatial and temporal resolution using a distributed temperature sensing system the cables may be inserted up to depths of tens of meters in unconsolidated sedimentary aquifers the main advantages of the method are that the cables are in direct contact with the aquifer material the disturbance of the aquifer is minor and no borehole is needed this cost effective approach may be applied to both passive and active heat tracer experiments an active heat tracer experiment was conducted to estimate horizontal groundwater velocities in a managed aquifer recharge system in the netherlands six fiber optic cables and a separate heating cable were inserted with a 1 m spacing at the surface the heating cable was turned on for 4 days and temperatures were measured during both heating and cooling of the aquifer temperature measurements at the heating cable alone were used to estimate the magnitude of the groundwater velocity and the thermal conductivity of the solids the direction of the velocity and heat capacity of the solids were estimated by including temperature measurements at the other fiber optic cables in the analysis the latter analysis suffered from the fact that the cables were not inserted exactly vertical the three dimensional position of the fiber optic cables must be measured for future active heat tracer experiments

lake sediments record large scale shifts in moisture regimes across the northern prairies of north america during the past two millennia six high resolution climatic reconstructions based on diatom analyses from lake sediment cores from the northern prairies of north america show that shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia the predominant broad scale pattern seen at all sites is a major shift in moisture regimes from wet to dry or vice versa depending on location that occurred after a period of relative stability these large scale shifts at the different sites exhibit spatial coherence at regional scales the three canadian sites record this abrupt shift between anno domini 500 and 800 and subsequently conditions become increasingly variable all three u s sites underwent a pronounced change but the timing of this change is between anno domini 1000 and 1300 thus later than in all of the canadian sites the mechanisms behind these patterns are poorly understood but they are likely related to changes in the shape and location of the jet stream and associated storm tracks if the patterns seen at these sites are representative of the region this observed pattern can have huge implications for future water availability in this region

multidecadal drought and holocene climate instability in the rocky mountains time series analysis of a diatom inferred drought record suggests that holocene hydroclimate of the northern rocky mountains has been characterized by oscillation between two mean climate states the dominant climate state was initiated at the onset of the holocene ca 11 ka under this climate state drought was strongly cyclic recurring at frequencies that are similar to twentieth century multidecadal phase changes of the pacific decadal oscillation this pattern remained consistent throughout much of the midholocene continuing until ca 4 5 ka after this time the mean climate state changed and drought recurrence became unstable periods of cyclic drought alternated with periods of less predictable drought the timing of this shift in climate was coincident with widespread severe drought in the mid continent of north america overall the strongest periodicity in severe drought occurred during the mid holocene when temperatures in the northern rocky mountains were warmer than today

site response and attenuation in the puget lowland washington state simple spectral ratio ssr and horizontal to vertical h v site response estimates at 47 sites in the puget lowland of washington state document significant attenuation of 1 5 to 20 hz shear waves within sedimentary basins there amplitudes of the horizontal components of shear wave arrivals from three local earthquakes were used to compute ssrs with respect to the average of two bedrock sites and h v spectral ratios with respect to the vertical component of the shear wave arrivals at each site ssr site response curves at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 hz and decreasing spectral amplification with increasing frequency above 6 hz ssrs at nonbasin sites show a variety of shapes and larger resonance peaks we attribute the spectral decay at frequencies above the amplification peak at basin sites to attenuation within the basin strata computing the frequency independent depth dependent attenuation factor q s int from the ssr spectral decay between 2 and 20 hz gives values of 5 to 40 for shallow sedimentary deposits and about 250 for the deepest sedimentary strata 7 km depth h v site responses show less spectral decay than the ssr responses but contain many of the same resonance peaks we hypothesize that the h v method yields a flatter response across the frequency spectrum than ssrs because the h v reference signal vertical component of the shear wave arrivals has undergone a degree of attenuation similar to the horizontal component recordings correcting the ssr site responses for attenuation within the basins by removing the spectral decay improves agreement between ssr and h v estimates

lateral propagation effects observed at parkway new zealand a case history to compare 1d versus 2d site effects abstract the importance of 2d or 3d site effects has been shown by theoretical studies however there is a lack of observational evidence of site effects due to lateral heterogeneities we have evaluated 1d and 2d site effects in the data obtained by a dense seismograph array that operated for more than 2 months in the alluvial basin of parkway new zealand 1d site effects were evaluated using three independent techniques spectral ratios relative to a reference station spectral ratios of horizontal relative to vertical ground motion and a generalized inversion scheme site effects due to lateral heterogeneity were investigated in detail for two events using frequency wavenumber analysis our results show that it is not possible to separate 1d from 2d site effects using frequency domain techniques this explains the scatter obtained from standard 1d data analysis the 1d resonance peaks will be more or less contaminated with laterally propagating waves for different events

late holocene flooding and drought in the northern great plains usa reconstructed from tree rings lake sediments and ancient shorelines during the ad 1990s the waubay lakes complex in eastern south dakota experienced historically unprecedented high water levels property damage from this flooding led to an examination of the occurrence of past pluvial episodes and their relation to climate a 1000 year hydroclimate reconstruction was developed from local bur oak quercus macrocarpa tree ring records and lake sediment cores analysis of lake shoreline and drainage features provides late quaternary geomorphic context for this high resolution record tree ring width and shell geochemistry of the ostracode candona rawsoni show marked coherence indicating synchronous responses to moisture balance in vegetation and lake salinity geomorphic evidence suggests buffering of lake system expansion during pluvial periods by evaporative dynamics pluvial periods display a recurrence frequency of approximately 140 160 years over the past millennium prior to ad 1800 both lake highstands and droughts tended towards greater persistence than during the past two centuries frequency and timing of hydroclimatic oscillations show strong similarities to records from other sites in the northern great plains ngp of north and south dakota and incomplete coherence with records of southern manitoba

a general inversion scheme for source site and propagation characteristics using multiply recorded sets of moderate sized earthquakes abstract we derive an inversion scheme that fits the logarithms of seismic body wave spectra to 2 source shapes conditioned by anelastic attenuation with a frequency independent q the residuals from this fit are then projected onto the set of sources and sites with the source terms damped to estimate residual source spectra and site response spectra this two part inversion process is iterated until the square of the residuals summed over frequency for all recordings is minimized the absolute amplitudes of the source and site spectra are determined by fitting the site response spectra to estimates of the site amplification derived from geotechnical data we apply this inversion process to three sets of recordings of loma prieta aftershocks the first data set contains s waves recorded both in the marina district and at nearby hard rock stations we constrain the site response using the s wave velocity measured in a nearby borehole the absolute site amplifications for the marina stations are 15 to 20 at 1 hz and 10 at 2 5 hz the other two data sets comprise the p and s wave recordings from 28 aftershocks ranging in size from m d 2 1 to 4 6 obtained at accelerograph sites near the epicentral area the p and s wave attenuations determined from the inversions are q 414 and 380 while the average near site attenuations are t 0 012 and 0 026 sec respectively the average ratio of the s to p wave seismic moments is 1 1 0 1 the ratio of the p to s wave corner frequencies is 1 24 0 11 and the ratio of s to p wave radiated energy is 13 9 3 2 the brune stress drops for these aftershocks range from 8 to 800 bars the largest earthquakes exhibit the largest stress drops but there is no apparent dependence of stress drop on seismic moment for earthquakes with 10 19 m 0 21 dyne cm

a computational approach to quaternary lake level reconstruction applied in the central rocky mountains wyoming usa abstract sediment based reconstructions of late quaternary lake levels provide direct evidence of hydrologic responses to climate change but many studies only provide approximate lake elevation curves here we demonstrate a new method for producing quantitative time series of lake elevation based on the facies and elevations of multiple cores collected from a lake s margin the approach determines the facies represented in each core using diagnostic data such as sand content and then compares the results across cores to determine the elevation of the littoral zone over time by applying the approach computationally decisions are made systematically and iteratively using different facies classification schemes to evaluate the associated uncertainty after evaluating our assumptions using ground penetrating radar gpr we quantify past lake elevation changes precipitation minus evapotranspiration p et and uncertainty in both at lake of the woods and little windy hill pond wyoming the well correlated r 0 802 0 002 reconstructions indicate that water levels at both lakes fell at 11 300 8000 5500 and 4700 1600 cal yr bp when p et decreased to 50 to 250 mm yr differences between the reconstructions are typically small 10 24 mm yr since 7000 cal yr bp and the similarity indicates that our reconstruction method can produce statistically comparable paleohydrologic datasets across networks of sites

the choice of reference sites for seismic ground amplification analyses case study at parkway new zealand this article focuses on how close reference rock sites need to be to the sites of interest when using the standard spectral ratio technique to obtain consistent estimates of site amplifications reflecting behavior of seismic waves at the sites of interest taber and smith 1992 and chavez garcia et al 1999 obtained very different amplification factors for the parkway basin new zealand using reference sites only 2 km apart we analyze 34 earthquakes recorded by a dense 2d array of 19 soil sites in the basin and 4 rock sites fringing the basin spectral ratios are calculated separately relative to taber and smith9s reference site p01 2 km from the basin and to a hypothetical average rock site pr formed from the four local rock sites including reference site p25 of chavez garcia et al our spectral ratios of the soil sites relative to pr are remarkably consistent with a distribution of the dominant peaks at 8 4 2 0 at 1 7 0 3 hz in contrast the spectral ratios relative to p01 are much less consistent and the dominant peak ratios are widely scattered around two separate centers of 14 5 4 1 at 1 8 0 3 hz and 14 4 5 5 at 4 7 1 0 hz our results relative to the two references are similar to those obtained by chavez garcia et al and taber and smith respectively we find that extreme differences between the two are often associated with the occurrence of troughs in the amplitude spectra at p01 possibly due to topographic deamplification there relative to the parkway basin in some but not all earthquakes

middle and late holocene dune reactivation in the nebraska sand hills usa a combination of optical dating of aeolian sand and radiocarbon dating of palaeosols and interdune sediments has allowed some refinements to the chronostratigraphy of the nebraska sand hills our results directly confirm previous inferences that multiple phases of aeolian deposition including megabarchan dune reactivation occurred within the sand hills during the middle and late holocene optical dates on stratified aeolian barchan dune sediments from two localities provide ages ranging from c 6 ka to 300 years ago weakly developed palaeosols intercalated within the dune sediments and other forms of post depositional modification indicate periods of relative stability and reduced dune mobilization optical dates on the upper 12 15 m of two dune exposures indicate repeated possibly regionally correlative reactivations which occurred between c 400 500 and 200 300 years ago comparison of late holocene aeolian activity with local and regional palaeoclimatic proxy records suggests that dune reacti

holocene eolian sand deposition linked to climatic variability northern great plains canada the bigstick and seward sand hills are possibly two of the oldest dune fields within the late wisconsin glaciated regions of the northern great plains as with most northern great plains dune fields source sediments are former proglacial outwash sands thus holocene dune construction is primarily related to spatial temporal variations in surface cover and transport capacity rather than renewed sediment input however eolian landscape reconstructions on the northern great plains have been temporally constrained to recent periods of activity as older episodes of deposition are typically reworked by younger events in this study sediment cores from shallow lacustrine basins and interdune areas provide an improved record of holocene eolian sand deposition eolian sand accumulation in the interdunes and basins occurred between 150 and 270 years ago 1 9 and 3 0 ka 5 4 and 8 6 ka and prior to ca 10 8 ka these episodes of sand accumulation were bracketed by lacustrine deposition and soil formation whi

late quaternary eolian sand depositional record for southwestern kansas landscape sensitivity to droughts abstract eolian deposits and landforms are ubiquitous in western kansas particularly south of the arkansas river with the presence of a stabilized dune field stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late quaternary the oldest eolian deposits from ca 16 to 12 ka completely or partially bury high terraces with ages of ca 16 and 33 ka it is unknown whether these eolian deposits reflect regional aridity and or a change in sediment availability with the transition of the arkansas river from a single channel to a braided system there is pervasive evidence for episodic eolian erosion and deposition ca 9 8 to 6 3 ka generally coincident with loess deposition on upland surfaces olson c g nettleton w d porter d a brasher b r 1997 middle holocene aeolian activity on the high plains of west central kansas holocene 7 3 255 261 and thus it is inferred to reflect regional aridity sites within the dune field show a sequence of eolian sands and weak buried soils reflecting either dune migration or sand sheet accretion at ca 1490 430 380 320 180 and 70 yr ago which correspond well with continental scale droughts in the tree ring record eolian sand in the arkansas river dune field may be derived principally with variability in fluvial activity and in climate during the late pleistocene to the middle holocene with substantial reactivation of eolian systems during decadal scale drought variability in the past 2000 yr

holocene eolian activity in the minot dune field north dakota stabilized eolian sand is common over much of the great plains region of the united states and canada including a subhumid area of 1500 km2 near minot north dakota eolian landforms consist of sand sheets and northwest trending parabolic dunes dunes and sand sheets in the minot field are presently stabilized by a cover of prairie grasses or oak woodland stratigraphic studies and accelerator mass spectrometry radiocarbon dating of paleosols indicate at least two periods of eolian sand movement in the late holocene pedologic data suggest that all of the dune field has experienced late holocene dune activity though not all parts of the dune field may have been active simultaneously similar immobile element ti zr la ce concentrations support the interpretation that eolian sands are derived from local glaciofluvial and glaciolacustrine sediments however glaciolacustrine and glaciofluvial source sediments have high ca concentrations from carbonate minerals whereas dune sands are depleted in ca

patterns of hydroclimatic change in the rocky mountains and surrounding regions since the last glacial maximum abstract the paleohydrologic record of western north america since the last glacial maximum reveals a wide range of hydroclimatic variability in time and space to improve the understanding of abrupt hydroclimatic shifts and millennial scale hydrologic changes in the central rocky mountains we reconstruct the lake level histories of two small lakes in the beartooth and bighorn mountains in northern wyoming over the past 17 ka to do so we use ground penetrating radar gpr and sediment cores to track the elevations of shoreline sediments within the lakes through time we compare the stratigraphies with those from four other lakes in wyoming and colorado and find widespread evidence for a terminal pleistocene drought from 15 to 11 ka an early holocene humid period from 11 to 8 ka and mid holocene aridity from 8 to 5 5 ka the northern wyoming lakes also provide evidence of high levels in the pleistocene possibly before ca 15 ka and rapid hydroclimatic changes that may have correlated with heinrich event 1 ca 16 8 ka we place the changes in a broad context by summarizing and mapping water level changes from 107 additional previously studied lakes important patterns include 1 extensive drying across the western u s after 15 ka 2 sub regional differences during the pleistocene holocene transition 3 a north south contrast from 9 to 6 ka consistent with a northward shift in storm tracks as the influence of the laurentide ice sheet diminished and 4 rapid increases in effective moisture across much of western north america from 6 to 4 ka

hydrologic variation in the northern great plains during the last two millennia abstract reconstructions of lake water salinity at decadal resolution for the last 2000 yr are compared among three lakes in north dakota to infer regional patterns of drought the intersite comparisons are used to distinguish local variation in climate or hydrology from regional patterns of change at one lake diatom inferred salinity and lake water mg ca inferred from ostracode shell chemistry are coherent both in terms of direction and magnitude of change indicating that each is a robust technique for reconstructing lake water chemistry the data show that the last 2000 yr have been characterized by frequent shifts between high and low salinity suggesting shifts between dry and moist periods long intervals of high salinity suggest periods of multiple decades when droughts were intense and frequent thus indicating times when drought was more persistent than in the 20th century both the medieval period and little ice age were hydrologically complex and there is no clear evidence to suggest that either interval was coherent or unusual in effective moisture relative to long term patterns differences among the three sites may be attributed to variation in local hydrology and these differences emphasize the need for multiple sites in deriving regional climate interpretations from paleoecological data

what is a reference site many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a reference motion the critical assumption in these methods is that the surface rock site record reference is equivalent to the input motion at the base of the soil layers data collected in this study show that surface rock sites can have a site response of their own which could lead to an underestimation of the seismic hazard when these sites are used as reference sites data were collected from local and regional earthquakes on digital recorders both at the surface and in boreholes at two rock sites and one basin site in the san jacinto mountains southern california the two rock sites keenwild and pinon flat are located on granitic bedrock of the southern california peninsular ranges batholith the basin site garner valley is an ancestral lake bed with watersaturated sediments on top of a section of decomposed granite which overlies the competent bedrock ground motion is recorded simultaneously at the surface and in the bedrock at all three sites when the surface rock sites are used as the reference site i e the surface rock motion is used as the input to the basin the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 hz this underestimation by a factor of 2 to 4 depending on frequency and site results from the rock sites having a site response of their own above the 2 to 5 hz frequencies the near surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest even at sites that appear to be located on competent crystalline rock the bedrock borehole ground motion can be used as the reference motion but the effect of the downgoing wave field and the resulting destructive interference must be considered this destructive interference may produce pseudo resonances in the spectral amplification estimates if one is careful the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site

site effects and microzonation in the city of thessaloniki greece comparison of different approaches ten seismological stations were installed for 3 months in the city of thessaloniki greece to record earthquakes and ambient noise over areas of different geotechnical characteristics about 40 local earthquakes were recorded with a good signal to noise ratio in more than two stations we analyze the records with three different techniques a the classical spectral ratios to a reference station b the receiver functions and c the horizontal to vertical h v spectral ratios on noise recordings the results are compared in terms of predominant frequencies and amplification levels the three methods are found equally able to reveal the fundamental frequency regarding local site amplification they provide significantly different results with a general trend of the two h v ratio methods b and c to underestimate the amplification levels compared to that inferred from the classical spectral ratio technique a the results are then used in relation with the microzonation of the city of thessaloniki a good correlation is found between the local site amplification and the type and age of the local geological formation as well as with the depth of bedrock finally we compare our observations with the damage distribution in the city which was hit by a magnitude 6 5 earthquake in 1978 the amplification levels from spectral ratios obtained at each site exhibit a good agreement with the map of damage intensity

site amplification in the san fernando valley california variability of site effect estimation using the s wave coda and h v methods during the months that followed the 17 january 1994 m 6 7 northridge california earthquake portable digital seismic stations were deployed in the san fernando basin to record aftershock data and estimate site amplification factors this study analyzes data recorded on 31 three component stations from 38 aftershocks ranging from m 3 0 to m 5 1 and depths from 0 2 to 19 km site responses from the 31 stations are estimated from coda waves s waves and ratios of horizontal to vertical h v recordings for the coda and the s waves site response is estimated using both direct spectral ratios and a generalized inversion scheme results from the inversions indicate that the effect of qs can be significant especially at high frequencies site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other depending on the choice of the reference site the difference is reduced when an average of six rock sites is used as a reference site in addition when using this multi reference site the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the s waves however for nonrock sites the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the s waves the h v method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods the amplifications determined from the hiv method are however different from the amplifications determined from the other methods finally the stations were grouped into classes based on two different classifications general geology and a more detailed classification using a quaternary geology map for the los angeles and san fernando areas average site response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification

explosion earthquake and ambient noise recordings in a pliocene sediment filled valley inferences on seismic response properties by reference and non reference site techniques abstract explosion earthquake and ambient noise recordings were collected at the eastern edge of a pliocene sediment filled valley nearby the town of san casciano dei bagni tuscany central italy the anomalous amplification of amplitudes as high as a factor of 20 between sediment and hard rock sites shown by classical spectral ratios were compared with results obtained by using non reference site techniques like that of nakamura 1989 and its receiver function like extension the explosion data were collected in june 1990 as part of a crustal refraction experiment carried out in the monti vulsini volcanic area a linear array of 13 digital stations was deployed orthogonally to the eastern border of the radicofani graben 2 stations were set up on a bedrock outcrop whereas the remaining 11 were deployed on soft sediments 100 m apart along a straight line toward the center of the valley during a second part of the study two stations were left in the field to record local and regional seismicity acquiring data from more than 150 events from a seismic swarm that occurred in the monti vulsini volcanic area in february 1992 with a maximum magnitude m d 3 8 spectral ratios were computed on the 41 strongest events showing a seismic response consistent with that computed using explosion data at least in the 0 5 to 4 hz frequency band non reference site techniques were employed to analyze earthquake and noise data although it is clear that the horizontal vertical h v spectral ratios describe some intrinsic properties of sites noise and earthquake data give equivalent results in our case they failed in recognizing both resonances and amplification levels large amplifications shown by classical spectral ratios were probably due to basin induced surface waves and 3d resonances seismic waves recorded by a small 50m radius 2d array deployed within the valley provided instrumental evidence for the existence of waves diffracted locally on lateral heterogeneities

vadose zone lag time and potential 21st century climate change effects on spatially distributed groundwater recharge in the semi arid nebraska sand hills summary deep drainage of water below plant root zones potential groundwater recharge will become groundwater recharge gr after a delay or lag time in which soil moisture traverses the vadose zone before reaching the water table depending on the thickness of the vadose zone the magnitude of deep drainage and soil hydraulic properties lag times will vary broadly exceeding decades to centuries in semi arid and arid environments yet studies of future climate change impacts to gr have typically avoided focusing on impacts beyond 100 years and often neglect to consider lag effects caused by the vadose zone we investigate the effects of vadose zone lag time and potential 21st century climate change on the spatial distribution and timing of gr throughout the semi arid nebraska sand hills nsh region 50 000 km2 we propose a simple and rapid quantitative approach for assessment of the groundwater system response time to changes in climate understanding of such effects is needed for groundwater modeling analysis of climate change impacts on groundwater and the effective management and sustainability of future water resources lag time estimates are made using the pressure based vertical velocity of soil moisture changes equivalent to a kinematic wave approximation of richards equation the underlying assumptions unit hydraulic gradient and relatively slow changes in climate are supported by observations in the high plains aquifer region encompassing the nsh the analysis relies on four sources of input data including spatially distributed high resolution 1 1 km gr rates as the difference in 2000 2009 mean precipitation and evapotranspiration estimated using a novel modis based approach thickness of the vadose zone based on 30 m digital elevation models of the land surface and water table elevations statistically downscaled estimates of future 2010 2099 potential gr rates from two hydroclimate model projections wrcp cmip3 with opposing gr trends and estimates of hydraulic conductivity and vertical velocity of the vadose zone pressure changes using the van genuchten mualem equation we found that vadose zone lag times with a mean gr rate of 78 mm yr for 2000 2009 have an estimated spatial mean of 5 28 yrs vertical velocity of 5 68 m yr end of century lag times with mean potential gr rates of 128 and 61 mm yr for 2090 2099 have estimated spatial means of 3 78 and 6 55 yrs vertical velocities of 8 13 and 4 69 m yr for wet and dry scenarios respectively findings suggest that the highly conductive materials primarily sand deposits and high potential gr rates in the nsh lead to relatively quick drainage of vadose zone water on the order of several years less than a decade for the entire 21st century however considerable spatial variations of lag times exist due to gradients in potential gr rates and the highly variable vadose zone thickness in the nsh

large wind shift on the great plains during the medieval warm period spring summer winds from the south move moist air from the gulf of mexico to the great plains rainfall in the growing season sustains prairie grasses that keep large dunes in the nebraska sand hills immobile longitudinal dunes built during the medieval warm period 800 to 1000 years before the present record the last major period of sand mobility these dunes are oriented nw se and are composed of cross strata with bipolar dip directions the trend and structure of the dunes record a drought that was initiated and sustained by a historically unprecedented shift of spring summer atmospheric circulation over the plains moist southerly flow was replaced by dry southwesterly flow

early holocene limnological and climatic variability in the northern great plains information on the timing and direction of climatic and environmental change on a millennial scale exists for many regions of north america whereas little is known about decadal to centennial scale variability here we present a high resolution analysis of diatom inferred salinity from a site in the northern great plains to reconstruct multidecadal and centennial scale climatic patterns during the early holocene the diatominferred salinity indicates a transition from fresh to highly saline conditions between c 13 400 and 7700 cal yr bp which suggests a major shift in climate from wet to dry conditions the overall trend toward increasing salinity is interrupted by several freshwater intervals between c 9800 and 7950 cal yr bp which may be the result of an increase in the frequency of monsoonal flow from the gulf of mexico the early holocene is considered to be a time of rapid change in climate and vegetation within the holocene although rates of change in the moon lake diatom assemblages were hi

diatom indicators of peatland development at pogonia bog pond minnesota usa sediment cores were taken from the central pond and fringing mat of a sphagnum peatland located in the big woods vegetation type pollen analysis of the pond core spanning 12 000 cal bp to the pre

alluvial stratigraphic evidence for channel incision during the mediaeval warm period on the central great plains usa alluvial valley fills from tributaries to the upper republican river southwest nebraska usa provide soil and morpho stratigraphic evidence for an episode of channel incision that occurred between c 1100 and 800 1 4 cyr bp based on 11 new radiocarbon ages this local episode of channel incision correlates with other alluvial stratigraphic studies from the central great plains and demonstrates regionally synchronous fluvial behaviour proxy records of palaeohydrologic conditions in and around the great plains including lacustrine aeolian and geomorphological sources indicate that channel incision correlates with a multicentennial episode of common widespread drought temporally this drought episode corresponds to the period recognized in many regions as the mediaeval warm period mwp therefore this research demonstrates 1 a relationship in the semi arid central great plains between drought and increased fluvial incision and 2 a local regional geomorphic response to a particular episode of global climate change