optimal channel networks a framework for the study of river basin morphology optimal channel networks ocns are dendritic structures obtained by minimizing the local and global rates of energy dissipation in a continuously fed in space and time plane aggregation system reminiscent and based on the properties of the planform of three dimensional natural drainage networks geomorphological and fractal properties of ocns are known from earlier studies by the authors this paper explores further the structures derived by optimization of energy dissipation rates optimality of subnetworks and of basin shapes is investigated as a by product of competition for drainage a new perspective on the possible prediction of the width function of a basin network and hence of its hydrologic response is obtained by exploiting ocn techniques requiring only the definition of the outer boundaries of the basin the interplay between hillslope processes and the development of drainage networks is addressed aiming at the relative role and the mutual interrelations of geology and optimal organization in the structure of mature river basins also addressed is the issue of multiscaling and multifractality in the spatial organization of the network it is concluded that ocn approaches provide a comprehensive framework for the study of the morphology of geophysical structures

the role of h 2 o in rapid emplacement and crystallization of granite pegmatites resolving the paradox of large crystals in highly undercooled melts granite pegmatite sheets in the continental crust are characterized by very large crystals there has been a shift in viewing pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled liquids with this shift there has been a renewed debate about the role of h2o in the petrogenesis of pegmatites based on data on nucleation of minerals and new viscosity models for hydrous granite melts it is argued that h2o is the essential component in the petrogenesis of granite pegmatites h2o is key to reducing the viscosity of granite melts which enhances their transport within the crust it also dramatically reduces the glass transition temperature which permits crystallization of melts at hundreds of degrees below the thermodynamic solidus which has been demonstrated by fluid inclusion studies and other geothermometers published experimental data show that because h2o drastically reduces the nucleation rates of silicate minerals the minerals may not be able to nucleate until melt is substantially undercooled in a rapidly cooling intrusion nucleation starts at its highly undercooled margins followed by inward crystal growth towards its slower cooling hotter core delay in nucleation may be caused by competition for crystallization by several minerals in the near eutectic melts and by the very different structures of minerals and the highly hydrated melts once a mineral nucleates however it may grow rapidly to a size that is determined by the distance between the site of nucleation and the point in the magma at which the temperature is approximately that of the mineral s liquidus assuming components necessary for mineral growth are available along the growth path granite pegmatites are apparently able to retain h2o during most of their crystallization histories within the confinement of their wall rocks pegmatitic texture is a consequence of delayed nucleation and rapid growth at large undercooling both of which are facilitated by high h2o li b f and p contents in granite pegmatite melts without retention of h2o the conditions for pegmatitic textural growth may be difficult to achieve loss of h2o due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in rhyolites in contrast slow cooling within a large magma chamber promotes continuous exsolution of h2o from crystallizing magma growth of equant crystals and final solidification at the thermodynamic solidus these are the characteristics of normal granites that distinguish them from pegmatites

an analysis of an arctic channel network using a digital elevation model abstract drainage basins possess spatial patterns of similarity that can be characterized by universal qualities in the fractal dimension and the cumulative area distribution features called water tracks often drain hillslopes in basins with permafrost and impose significant control on the hydrologic response of watersheds we analyzed the arrangement of channel networks and water tracks in imnavait creek in northern alaska to determine if basins with permafrost possess the same universal characteristics as basins without permafrost using digital elevation models dems we explored the hillslope channel scaling regimes the spatial distribution of mass through the cumulative area distribution and the fractal characteristics of channel networks in the kuparuk river basin in northern alaska fractal analysis slope area analysis and field mapping suggest that water tracks are positioned on the hillslopes where channels should occur fully developed channel networks however possess certain universal characteristics in aggregation patterns that are manifested in a common cumulative area distribution imnavait creek possesses those universal characteristics only above the scale of the hillslope water track or when the drainage areas reach the main channels in the valley bottom our interpretation is that a rudimentary channel network formed on the hillslopes but never developed into a mature channel network because permafrost is limiting erosion consequently the undissected hillslopes are extensive given the dependence of permafrost on a cold climate a warming climate and subsequent degradation of permafrost may have significant impacts on the erosional development of channel networks in the arctic

self organized river basin landscapes fractal and multifractal characteristics in recent years a new theory of the evolution of drainage networks and associated landscapes has emerged mainly in connection with the development of fractal geometry and of self organized criticality soc concepts this theory has much improved our understanding of the mechanisms which determine the structure of natural landscapes and their dynamics of evolution in the first part of this paper the main ideas in the theory of landscape self organization are outlined and some remarkable features of the resulting structures are presented in the second part we apply theoretical tools developed in the context of multifractal fields to the study of the scaling properties of the field of elevations of soc landscapes we observe that such landscapes appear to be more complex than simple self affine fractals although in some cases a simple fractal framework may be adequate for their description we also show that multiple scaling may emerge as a result of heterogeneity in the field properties reflecting climate and geology

power law distributions of discharge mass and energy in river basins river networks constitute dissipative systems with many spatial degrees of freedom previous work by mandelbrot 1983 and bak et al 1987 1988 1990 suggests that such systems will follow power law distributions in their mass and energy characteristics it is shown that this is the case for river networks where the exponent in the distribution p x x x is approximately equal to 0 45 and 0 90 for discharge and energy respectively in the case of several networks analyzed in north america when these variables are calculated for each individual link throughout the drainage network an explanation of the values of is offered based on the fractal structure of rivers and on principles of energy expenditure in river basins

channel initiation and the problem of landscape scale since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape this threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head a field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth undissected hillslopes and the valley bottoms to which they drain this finding contradicts assertions that landscapes are scale independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization

formation of spinifex texture in komatiites an experimental study the formation of platy olivine spinifex the texture that characterizes komatiite lavas has long been enigmatic a major problem is that the dendritic morphology of the olivine resembles that of crystals grown in laboratory experiments at high cooling rates 50 c h but at the position where these textures form up to several meters below the komatiite flow top the cooling rate cannot have been greater than 1 5 c h we performed experiments that demonstrate that the platy habit of spinifex olivine or pyroxene is a consequence of slow cooling of ultramafic magma in a thermal gradient 7 35 c cm the charges were cooled at rates between 2 and 1428 c h and even at the low cooling rates the thermal gradient led to constrained growth and the development of preferentially oriented dendritic crystals with orphologies like those in natural platy spinifex textured lavas under these conditions olivine starts to crystallize at temperatures well below the equilibrium liquidus temperature 37 c dt 56 c depending on the composition of the starting material when the cooling rate is high the thermal gradient has a negligible effect on the texture and the crystals have a random orientation like that in the upper parts of komatiite flows

problems in automated recognition of valley features from digital elevation models and a new method toward their resolution valley recognition methods given in the literature fail to produce valley networks suitable for locating valley heads for one of two main reasons a they are based on the concept of higher than or b they are based on contributing area thresholds in this paper a new method for recognizing valley bottoms is presented this is based on two improvements of carroll s 1983 method and produces a network that reflects the topography well the network is used to locate valley heads once located the valley heads are delineated using criteria suggested by geomorphologists and hydrologists the resultant valley heads are generally well recognized although two problems are evident a there are a number of commission errors and b the valley heads recognized by the method are too small

a climatology of stratospheric polar vortices and anticyclones 1 united kingdom meteorological office global analyses from 1991 to 2001 are used to create a global climatology of stratospheric polar vortices and anticyclones new methodologies are developed that identify vortices in terms of evolving three dimensional 3 d air masses a case study illustrates the performance of the identification schemes during february and march of 1999 when a merger of anticyclones led to a stratospheric warming that split the arctic polar vortex the 3 d structure and temporal evolution of the arctic vortex and identified anticyclones demonstrates the algorithm s ability to capture complicated phenomena the mean geographical distribution of polar vortex and anticyclone frequency is shown for each season the frequency distributions illustrate the climatological location and persistence of polar vortices and anticyclones a counterpart to the aleutian high is documented in the southern hemisphere the australian high the temporal evolution of the area occupied by polar vortices and anticyclones in each hemisphere is shown as a function of potential temperature large polar vortex area leads to an increase in anticyclone area which in turn results in a decrease in the size of the polar vortex during northern winter and southern spring 9 years of daily anticyclone movement are shown on the 1200 k 36 km 4 hpa isentropic surface preferred locations of anticyclogenesis are related to cross equatorial flow and weak inertial stability regimes of traveling and stationary anticyclones are discussed index terms 3309 meteorology and atmospheric dynamics climatology 1620 3319 meteorology and atmospheric dynamics general circulation 3334 meteorology and atmospheric dynamics middle atmosphere dynamics 0341 0342 keywords polar vortex stratospheric anticyclones

scaling regimes of local slope versus contributing area in digital elevation models four scaling regimes are observed in digital elevation models dems when the average local slope is calculated for pixels grouped according to the values of the contributing area threshold criteria proposed by various researchers to identify the extent of the channel network are examined relative to the slope area scaling diagram the scaling response observed in dems is reproduced with a landscape evolution and channel network growth model originally developed by willgoose et al 1989 the threshold criterion proposed in this model is a useful tool to separate two different slope area scaling regimes observed in dem data

intraseasonal variability of the winter circulation in the southern hemisphere atmosphere abstract the intraseasonal variability of the southern hemisphere stratosphere and troposphere is studied using multilevel geopotential height data for nine winters 1979 87 the study uses empirical orthogonal function eof analysis of unfiltered data at five tropospheric and five stratospheric levels the four leading eofs at all tropospheric levels exhibit the patterns previously detected at 500 mb study of the corresponding principal components pcs at each level shows that the quasi stationary anomalies associated with the leading eofs are equivalent barotropic and exhibit no preference for early middle or late winter the five leading eofs in the stratosphere fall into two classes the first three eofs at all levels form the first class this class represents anomalies that are dominated by zonal wavenumber one wave 1 exhibit strong westward tilt with height and travel slowly eastward or remain stationary most cases of large persistent pc values for this class occur in early winter the fo

automatically derived catchment boundaries and channel networks and their hydrological applications abstract a method of generating drainage networks and catchment boundaries from gridded ground elevation data a digital terrain model is presented the method includes an algorithm for coping with localised low points in the data the paper suggests two objective ways of comparing the conventionally mapped with the automatically generated drainage network emphasis is placed on the way that this approach is capable of benefiting a variety of hydrological applications in particular the automatic estimation of flow statistics from catchment characteristics

hydrological characteristics of recurrent slope lineae on mars evidence for liquid flow through regolith and comparisons with antarctic terrestrial analogs abstract on the basis of their morphology and seasonal thermal characteristics recurrent slope lineae rsl on mars have been inferred to be a possible result of the flow of a liquid likely a saline brine through the upper portions of the martian regolith in this note we analyze repeat hirise imaging of rsl to show that the downslope growth rate of recurrent slope lineae is well fit by an elementary groundwater flow model that also describes the downslope propagation of antarctic water tracks terrestrial analogs to rsl and that the apparent permeability of rsl bearing slopes is consistent with the observed sandy regolith substrate

the importance of spatially heterogeneous erosivity and the cumulative area distribution within a basin evolution model abstract topography reflects the terrain composition and the forces to which it is exposed the elements that control this shape are intuitive precipitation wind soil composition geologic activity lithology and vegetation each of these elements varies naturally in space a model of basin evolution is shown to produce quantitatively different topography for different relative magnitudes of diffusive and incisive erosion processes spatial heterogeneities in erosivity also influence the simulated topography the calibration of this evolution model is compared to observed topography the calibration process ultimately hinges on quantification of the aggregation pattern of the basin through analysis of the cumulative area distribution the effect of spatially heterogeneous erosivity is shown to be crucially important to reproduce the cumulative area distribution of observed topography

comparison of single and multiple flow direction algorithms for computing topographic parameters in topmodel single flow direction sfd and multiple flow direction mfd algorithms were used to compute the spatial and statistical distributions of the topographic index used in the watershed model topmodel an sfd algorithm assumes that subsurface flow occurs only in the steepest downslope direction from any given point an mfd algorithm assumes that subsurface flow occurs in all downslope directions from any given point the topographic index in topmodel is in a tan 3 where in is the napierian logarithm a is the upslope area per unit contour length and tan 3 is the slope gradient the in a tan 3 distributions were computed from digital elevation model dem data for locations with diverse topography in arizona colorado louisiana nebraska north carolina oregon pennsylvania tennessee vermont and virginia the means of the in a tan 3 distributions were higher when the mfd algorithm was used for computation compared to when the sfd algorithm was used the variances and skews of the distributions were lower for the mfd algorithm compared to the sfd algorithm the differences between the mfd and sfd algorithms in the mean variance and skew of the in a tan 3 distribution were almost identical for the various dems and were not affected by dem resolution or watershed size topmodel model efficiency and simulated flow paths were affected only slightly when the in a tan 3 distribution was computed with the sfd algorithm instead of the mfd algorithm any difference in the model efficiency and simulated flow paths between the sfd and mfd algorithms essentially disappeared when the model was calibrated by adjusting subsurface hydraulic parameters

a new look for the southern hemisphere jet stream a new jet stream description defined as the geostrophic streamline of maximum average velocity is proposed an objective algorithm for detecting and tracking the jet has been developed tested and applied to the ncep ncar 200 hpa geopotential height in the southern hemisphere for the period 1958 2002 the results show the variability of the double character of the southern hemisphere jet with a marked seasonality during the warm season a single jet can be found around 40 s while autumn and winter are characterized by a clear double jet structure with a strong and dominant subtropical jet located around 30 s and a polar front jet progressively displaced toward southern latitudes and reaching 60 s by the end of the cold season in general a trend toward slower subtropical jets and stronger polar front jets has been detected during the study period the southern annular mode appears as a main modulator of the latitude and strength of the polar front jet influencing to a minor extent its subtropical counterpart the enso cycle strongly modifies the latitude and specially the strength of the subtropical jet affecting its preferred wavenumber as well nevertheless the effect of this oscillation seems fairly restricted in the pacific thus limiting the ability of this jet to drive the el nino teleconnections along the southern hemisphere the consistency of the results when compared with previous jet climatologies suggests that the new approach is a reliable jet tracking method thus providing a new tool to analyze climatic variability at hemispheric scales

endogenous growth in channelized komatiite lava flows evidence from spinifex textured sills at pyke hill and serpentine mountain western abitibi greenstone belt northeastern ontario canada spinifex textured sills i e veins characterized by komatiitic magmas that have intruded their own volcanic piles have long been recognized for instance in the early 1970s pyke and coworkers in their classic work at pyke hill in munro township noted that not all spinifex bearing ultramafic rocks formed as lava flows rather some were clearly emplaced as small dikes and sills several hypotheses have been proposed to explain spinifex textured sills intrusion into a cold host filter pressing or drainage of residual liquid however these do not satisfactorily explain the phenomenon field and petrographic observations at pyke hill and serpentine mountain demonstrate that spinifex bearing komatiite sills and dikes were emplaced during channel inflation processes when new magma was intruded into a cooler semi consolidated but permeable cumulate material komatiitic liquids were intruded into the olivine cumulate rocks near the boundary between the spinifex and the cumulate zones of well organized to organized komatiite flows spinifex textured sills are generally tabular in morphology stacked one above another with curviplanar contacts sub parallel to stratigraphy some sills exhibit complex digitated apophyses thinner sills typically have a random olivine spinifex texture similar though generally composed of coarser crystals to that of komatiite lava flows thicker sills exhibit more complex organization of their constituent crystals characterized by zones of random olivine spinifex overlying zones of organized coarse spinifex crystals similar to those found in lava flows they have striking coarse dendritic spinifex zones composed of very large olivine crystals up to several centimetres long and up to 1 cm wide that are not observed in lava flows typically at the sill margins the cumulate material of the host flow is composed of euhedral to subhedral olivine crystals that are larger than those distal to the contact many of these margin crystals have either concentric overgrowth shells or dendritic olivine overgrowths that grew from the cumulate sill contact toward the sill interior the dendrites grew on pre existing olivine cumulate at the contact in response to a sharp temperature gradient imposed by the intrusion of hot material whereas the concentric overgrowths formed as new melt percolated into the unconsolidated groundmass of the host flow cumulate material spinifex textured sills and dikes occur in well organized to organized flows that are interpreted to have formed by breakouts above and peripheral to lava pathways channels conduits as a result of inflation that accompanied voluminous komatiitic eruptions responsible for the construction and channelization of komatiitic flow fields the spinifex textured dikes and sills represent komatiitic lava that was originally emplaced into the channel roof during periods of episodic inflation that resulted in lava breakouts and was subsequently trapped in the roof rocks during periods of channel deflation accordingly the occurrence of spinifex textured sills and dikes may indicate proximity to and aid in the identification and delineation of lava channel ways that could potentially host ni cu pge mineralization within komatiitic lava flow fields

a physical explanation of an observed link area slope relationship an observed log log linear relationship between channel slope and contributing area is explained by the erosional physics that lead to catchment form it is postulated that tectonic uplift is in balance with the fluvial erosion down wasting that dominates catchment erosion and it is shown that this relationship results in the observed log log linear relationship at dynamic equilibrium in addition it has been observed that there are deviations from this log log linear relationship near the catchment divide with observed slopes being lower than those predicted from the relationship this is explained by noting that for small areas fluvial erosion effects are dominated by soil creep and rain splash modeled by diffusive physics the area at which this deviation from log log linearity occurs is that point on the hillslope at which diffusive physics like soil creep and rain splash begin to dominate fluvial erosion these predictions are confirmed by numerical simulations using a catchment evolution model

drainage networks from grid digital elevation models current algorithms that deduce the drainage network from a digital elevation model dem represented by a regular array of surface elevations share a fault unless the terrain is rugged the derived water channels tend to flow in parallel lines along preferred directions engendered by the sampling grid orientation we present a simple solution to the problem a second difficulty is the presence of noise that creates artificial pits we briefly describe a method which deals with pits in what we believe to be a more efficient manner for virtual memory environments than previous efforts our system has treated dems of nearly 9 000 000 pixels we show how depth first search of the resulting drainage network permits segmentation of the dem into basins by various criteria analysis of stream sediment anomaly dilution profiles improved hydrological models and other applications

recent large scale changes in the extratropical southern hemisphere atmospheric circulation abstract the zonal anomalies in the 500 hpa geopotential height field of the southern hemisphere are examined using the national centers for environmental prediction national center for atmospheric research reanalyses 1958 96 the zonal anomalies were constructed by removing the zonal mean from each grid point and principal components analysis was performed on the resulting spatial anomalies the principal components of the leading eof of the seasons march april may mam to september october november son indicate that a change has occurred in the extratropical large scale atmospheric circulation of the southern hemisphere it is estimated to have begun after 1975 and is limited to the southern late fall winter and early spring the elements of this change are a deeper pacific trough and higher indian ocean ridge at subtropical latitudes at higher latitudes the ridge over the pacific strengthened and shifted east while troughing occurred in the southwest atlantic weddell sea region consistent wi

the prediction of hillslope flow paths for distributed hydrological modelling using digital terrain models the accuracy of the predictions of distributed hydrological models must depend in part on the proper specification of flow pathways this paper examines some of the problems of deriving flow pathways from raster digital terrain data in the context of hydrological predictions using topmodel distributed moisture status is predicted in topmodel on the basis of spatial indices that depend on flow path definition the sensitivity of this index to flow path algorithm and grid size is examined for the case where the surface topography is a good indicator of local hydraulic gradients a strategy for the case where downslope subsurface flow pathways may deviate from those indicated by the surface topography is described with an example application