a theoretical and numerical study of density currents in nonconstant shear flows the previous idealized two fluid model of a density current in constant shear is extended to the case where the inflow shear is confined to the low levels the analytical solution is determined by the conservation of mass momentum vorticity and energy it is found that a low level shear acts in a similar manner to a uniform vertical shear in controlling the depth of a steady state density current when the shear enhances the low level flow against the density current propagation the current is deeper than half of the domain depth time dependent numerical experiments are conducted for a variety of parameter settings including various depths and strengths of the shear layer the numerical results agree closely with the theoretical analyses numerical experiments are also performed for a case where the initial depth of the density current is set to be comparable to the low level shear which is much shallower than that given by the steady state solution the circulation at the density current head remains shallow and is nonsteady in this case whereas the time averaged flow still exhibits a deep jump updraft pattern that is close to the theoretical solution suggesting the applicability of the theoretical results to even more transient flows the simulated flow features are discussed in terms of balanced and unbalanced dynamics and in the context of forcing and uplifting at the frontal zone in long lived convective systems here the term balance refers to a flow configuration that satisfies the steady state solution of the idealized theoretical model

wavelet based regularization of the extracted topographic index from high resolution topography for hydro geomorphic applications high resolution topography e g 1 m digital elevation model dem from light detection and ranging lidar offers opportunity for accurate identification of topographic features of relevance for hydrologic and geomorphologic modelling yet the computation of some derived topographic properties such as the topographic index ti is characterized by daunting challenges that hamper the full exploration of topography based models particular problems for example arise when a distributed or semi distributed rainfall runoff model is applied to high resolution dems indeed the characteristic dependency between landscape resolution and the computed ti distribution results in the formation of un physical unconnected saturated zones which in turn cause unrealistic representations of rainfall runoff dynamics in this study we present a methodology based on a multi resolution wavelet transformation that by means of a soft thresholding scheme on the wavelet coefficients filters the noise of high resolution topography to construct regularized sets of locally smoother topography on which the ti is computed while the methodology needs a somewhat arbitrary definition of the wavelet coefficients threshold our study shows that when the information content entropy of the ti distribution is used as a filtering efficiency metric a critical threshold automatically emerges in the landscape reconstruction the methodology is demonstrated using 1 m lidar data for the elder creek river basin in california while the proposed case study uses a topmodel approach the methodology can be extended to different topography based models and is not limited to hydrological applications copyright 2012 john wiley sons ltd

a hillslope scale experiment to measure lateral saturated hydraulic conductivity 1 one of the most challenging parameters in hillslope and watershed scale distributed hydrologic models is the lateral saturated hydraulic conductivity ks in this paper we present a methodology to determine the hillslope scale lateral ks above a moderately deep sloping restrictive layer in an 18 35 m hillslope plot using perched water level measurements and drain tile outflow data the hillslope scale lateral ks was compared to small scale ks measured with small soil cores and the guelph permeameter our results show that small scale ks measurements underestimate the actual hillslope scale ks the hillslope scale ks measurements were 13 7 4 1 and 3 2 larger than small soil core measurements in the a b and e horizons respectively we argue that the gap between small scale and hillslope scale ks within the same porous medium is foremost a measurement problem data analysis provided the ks distribution with depth showing a sharp decrease in ks within the first 0 1 m of the soil and an exponential decline in ks below 0 1 m the distribution of ks with depth was best described by a double exponential relationship overall results indicate the importance of macroporosity perhaps of biological origin in determining ks at a hillslope scale

digital terrain modelling a review of hydrological geomorphological and biological applications the topography of a catchment has a major impact on the hydrological geomorphological and biological processes active in the landscape the spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems furthermore topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology digital elevation models are the primary data used in the analysis of catchment topography we describe elevation data sources digital elevation model structures and the analysis of digital elevation data for hydrological geomorphological and biological applications some hydrologic models that make use of digital representations of topography are also considered

numerical simulations of an observed gravity current and gravity waves in an environment characterized by complex stratification and shear abstract numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic two dimensional dry primitive equation model data from one of the most complete documentations to date of gravity waves associated with a gravity current presented in an earlier study are used both to prescribe the gravity current s environment and for validation of the simulated gravity current and its associated gravity waves these comparisons indicate that the gravity current observed by a doppler wind profiler and sodars was well simulated in terms of depth density contrast and propagation speed and that the model produced a variety of gravity waves similar in many ways to these observed because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations e g wavelengths were not observed the validated simulations are used to test these tentative hypotheses the simulatio

conceptualizing lateral preferential flow and flow networks and simulating the effects on gauged and ungauged hillslopes 1 one of the greatest challenges in the field of hillslope hydrology is conceptualizing and parameterizing the effects of lateral preferential flow our current physically based and conceptual models often ignore such behavior however for addressing issues of land use change water quality and other predictions where flow amount and components of flow are imperative dominant runoff processes like preferential subsurface flow need to be accounted for in the model structure this paper provides a new approach to formalize the qualitative yet complex explanation of preferential flow into a numerical model structure we base our examples on field studies of the well studied maimai watershed new zealand we then use the model as a learning tool for improved clarity into the old water paradox and reasons for the seemingly contradictory findings of lateral preferential flow of old water where applied line sources of tracer appear very quickly in the stream following application we evaluate the model with multiple criteria including ability to capture flow hydrograph composition and tracer breakthrough we generate output ensembles with different pipe network geometries for model calibration and validation analysis surprisingly the range of runoff response among the ensembles is narrow indicating insensitivity to specific pipe placement our new model structure shows that high transport velocities for artificial line source tracers can be reconciled with the dominance of preevent water during runoff events even when lateral pipe flow dominates response the work suggests overall that preferential flow can be parameterized within a process based model structure via the structured dialog between experimentalist and modeler

reconnaissance soil gas radon survey over the faulted crystalline area of ile ife nigeria tropical rain forest areas are well known for problems with geological mapping because of the dense vegetation and thick overburden the application of soil gas radon measurements was carried out over known and suspected fault zones in the basement terrains of ile ife southwestern nigeria radon concentration in the area ranged from 25 6 to 6 227 8 pci l with distinct contrasts and overlap in concentrations among soil groups derived from various rock types radon concentrations in the soil exhibit a strong dependence on the local geology and reflect the bedrock geochemistry relatively high radon soil gas concentrations of between 4 000 and 6 227 8 pci l were found directly on the axis of the known and suspected strike slip faults the use of the radon 222 technique confirms its application as a useful tool in geological and structural mapping in basement terrains largely covered by relatively thick overburdens

on the kriging of water table elevations using collateral information from a digital elevation model in unconfined aquifers flowing under topographic gradients the water table is a subdued replica of the ground surface above this principle is the basis for using detailed collateral or secondary information from digital elevation models to supplement sparse observations from water wells in the mapping of phreatic surfaces data from dem derived secondary variables are incorporated into the estimation of water table elevations using the geostatistical method known as kriging with an external drift ked two different ked models are proposed based on the choice of secondary variable in the first water table elevation is expressed as the sum of a deterministic trend given by topographic elevation and a residual random component representing depth to water table in the second depth to water table is expressed as a linear function of a deterministic trend given by the topmodel topographic index and a residual random error the relationship between water table depth and topographic index is derived from simplified groundwater dynamics and forms the basis of topmodel type rainfall runoff models the two ked models are applied to the mapping of water table elevations in the oak ridges moraine orm an unconfined aquifer near toronto canada results show that ked with topographic elevation as external drift is the more robust of the two models despite its strong theoretical basis the second model yields kriged water table elevations that are not always physically plausible in part this is because field observations of water table depth do not verify the predicated relationship with topographic index in large parts of the study area however this relationship may be valid in other cases and at other spatial scales in such cases the second model would provide a very powerful approach for mapping water table elevations and for calibrating distributed parameters of the topmodel equations on water well observations

on the use of the boussinesq equation for interpreting recession hydrographs from sloping aquifers 1 the method of recession analysis proposed by brutsaert and nieber 1977 remains one of the few analytical tools for estimating aquifer hydraulic parameters at the field scale and beyond in the method the recession hydrograph is examined as dq dt f q where q is aquifer discharge and f is an arbitrary function the observed function f is parameterized through analytical solutions to the one dimensional boussinesq equation for unconfined flow in a homogeneous and horizontal aquifer while attractive in its simplicity as originally presented it is not applicable to settings where slope is an important driver of flow or where hydraulic parameters vary greatly with depth we compare analytical solutions to the linearized one dimensional boussinesq equation for a sloping aquifer to numerical solutions of the full nonlinear equation the behavior of the nonlinear boussinesq equation is also assessed when the aquifer is heterogeneous wherein the lateral saturated hydraulic conductivity k varies as a power law with height z above the impermeable layer k zn n constant 0 all of the analytical solutions differ in key aspects from the nonlinear solution when plotted as dq dt f q and thus are inappropriate for a brutsaert and nieber type analysis however new analytical solutions for a sloping aquifer are derived empirically from the numerical simulations that are applicable during the late period of recession when the recession curve converges to dq dt aqb where b 2n 1 n 1 and a is a function of the dimensions and hydraulic properties of the aquifer

how to model shallow water table depth variations the case of the kervidy naizin catchment france the aim of this work is threefold 1 to identify the main characteristics of water table variations from observations in the kervidy naizin catchment a small catchment located in western france 2 to confront these characteristics with the assumptions of the topmodel concepts and 3 to analyse how relaxation of the assumptions could improve the simulation of distributed water table depth a network of piezometers was installed in the kervidy naizin catchment and the water table depth was recorded every 15 min in each piezometer from 1997 to 2000 from these observations the kervidy naizin groundwater appears to be characteristic of shallow groundwaters of catchments underlain by crystalline bedrock in view of the strong relation between water distribution and topography in the bottom land of the hillslopes however from midslope to summit the water table can attain a depth of many metres it does not parallel the topographic surface and it remains very responsive to rainfall in particular hydraulic gradients vary with time and are not equivalent to the soil surface slope these characteristics call into question some assumptions that are used to model shallow lateral subsurface flow in saturated conditions we investigate the performance of three models topmodel a kinematic model and a diffusive model in simulating the hourly distributed water table depths along one of the hillslope transects as well as the hourly stream discharge for each model two sets of parameters are identified following a monte carlo procedure applied to a simulation period of 2649 h the performance of each model with each of the two parameter sets is evaluated over a test period of 2158 h all three models and hence their underlying assumptions appear to reproduce adequately the stream discharge variations and water table depths in bottom lands at the foot of the hillslope to simulate the groundwater depth distribution over the whole hillslope the steady state assumption topmodel is quite constraining and leads to unacceptable water table depths in midslope and summit areas once this assumption is relaxed kinematic model the water table simulation is improved a subsequent relaxation of the hydraulic gradient diffusive model further improves water table simulations in the summit area while still yielding realistic water table depths in the bottom land

influence of the synoptic scale flow on sea breezes observed during cape abstract mean sea breeze characteristics were determined by analyzing a number of sea breeze events during offshore parallel and onshore flow regimes during the convection and precipitation electrification experiment cape it was observed that offshore flow cases exhibited the widest and relatively strongest radar detected thin lines the thin line reflectivity values steadily increased during the day in contrast a thin line was detected only during late afternoon on parallel flow days while no easily identifiable thin line was observed during onshore flow days the gradients of temperature and moisture as measured by a surface meteorological station during sea breeze passage were strongest and weakest during offshore and onshore flow days respectively in addition the moisture and temperature gradients across the leading edge of the sea breeze steadily increased during the day and were strongest during late afternoon using dual doppler techniques the detailed kinematic structure of the sea

interaction between turbulent flow and sea breeze front over urban like coast in large eddy simulation turbulent flow and its interaction with a sea breeze front sbf over an urban like coast with a regular block array were investigated using a building resolving computational fluid dynamics model it was found that during daytime with an offshore ambient flow streaky turbulent structures tended to grow within the convective boundary layer cbl over a warm urban surface ahead of the sbf the structures were organized as streamwise streaks at an interval of a few hundred meters which initiated at the rooftop level with strong wind shear and strengthens in the cbl with moderate buoyancy the streaks then interacted with the onshore propagating sbf as it made landfall the sbf which was initially characterized as a shallow and quasi linear feature over the sea developed three dimensional structures with intensified updrafts at an elevated frontal head after landfall frontal updrafts were locally enhanced at intersections where the streaks merged with the sbf which greatly increased turbulent fluxes at the front the frontal line was irregular because of merging tilting and transformation effects of vorticity associated with streaky structures inland penetration of the sbf was slowed by the frictional effect of urban like surfaces and turbulent flow on land the overall sbf intensity weakened after the interaction with turbulent flow these findings aid understanding of local weather over coastal cities during typical sea breeze conditions

green and ampt infiltration into soils of variable pore size with depth most expressions for infiltration rely upon the assumption of vertical uniformity in soil texture and hydraulic properties we present an extension of the results of beven 1982 1984 for infiltration and lateral flow in soils with decreasing permeability with depth unlike beven we base the derivations upon joint changes in soil properties based on an overall change in characteristic pore size via miller scaling a set of very simple expressions for the time rate of infiltration are obtained using a green and ampt approach for soils with permeability that decreases with depth following linear power law and exponential relationships

aircraft observations of sea breeze frontal structure detailed aircraft observations of sea breeze frontal structure and dynamics are presented for two cases of well defined sea breeze fronts near the east coast of england in the first case the sea breeze was advancing into a well mixed convective boundary layer with strong turbulence and an offshore breeze of around 3 m s 1 in the second case the sea breeze was penetrating into a convective boundary layer characterized by weaker turbulence and an offshore wind speed of 2 3 m s 1 indeed during the course of the measurements a stable internal boundary layer was forming in the early evening cross sections of the frontal structure are derived from aircraft traverses at a range of heights along a fixed line normal to the coast a typical head like structure is observed in both cases with a region of strong mixing immediately seawards of the head an approximately exponential fall off in dissipation rate with distance seaward of the leading edge of the front is found heat and momentum fluxes are used to derive turbulent kinetic energy tke budgets for the mixing region and indicate that turbulence in this mixing region is maintained by tke generated by strong shear at the top of the cold air inflow in the first case the shear production of tke is almost twice that of the second case suggesting that the presence of ambient turbulence in the convective boundary layer has a significant effect upon the frontal dynamics in the second case as the turbulence decayed a curious wave like structure appeared behind the leading edge of the front the waves have wavelengths of 1 3 km and it is unlikely that they were caused by kelvin helmholtz instability it is suggested that the waves might be a solitary wave train emerging as the sea breeze interacts with a low level stable layer forming in the early evening

the effect of latitude on the sea breeze abstract long term five day integrations of a nonlinear numerical model of the sea breeze at the equator 20 n 30 n and 45 n indicate the importance of latitude on the sea breeze circulation during the hours of strong heating when friction is largest and the static stability is smallest a local sea breeze frontal circulation develops in a similar way at all four latitudes evaluation of the terms in the circulation theorem indicates the dominance of the solenoid term horizontal pressure gradient force associated with the strong temperature contrast during this period during the rest of the period however the pressure gradient and frictional forces weaken the static stability increases and the coriolis force is dominant except at the equator therefore quite different circulations evolve at the different latitudes at the equator the absence of the coriolis force results in a sea breeze at all times at the other latitudes the coriolis force is responsible for producing the large scale lan

generalization of topmodel for a power law transmissivity profile keywords hydrologie reference hydram article 1997 003doi 10 1002 sici 1099 1085 199707 11 9 3 0 co 2 uview record in web of science record created on 2005 10 11 modified on 2016 08 08

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

observations of atmospheric density currents using a tethered balloon borne turbulence probe system a tethered kite balloon with multiple instrumented probes clamped to the tethering cable is used to investigate atmospheric density current fronts passing through three sites at varying distances from the coast the system allows high frequency measurements of the three components of wind temperature and humidity to be made simultaneously at up to ten levels as the front passes through this enables a two dimensional visualization of the current and its circulation to be constructed as a result the structure of some of the wide variety of atmospheric density currents is revealed by direct observation the internal circulations and the effects of varying environments on the currents and their movement are described together with turbulence statistics one current was associated with what may have been a coastal trapped disturbance and in one case two opposing sea breezes appear to have collided inland two currents penetrating into surface inversions were dissociated from the surface one of these was travelling downwind and formed a strong near surface jet associated wave and bore structures were found to be travelling in layers above the currents in two cases in one case environmental air was found to intrude into the head region and convect internally while in another air from the head region extruded into the environmental flow the measurement array enabled the different terms of the dynamic equations to be evaluated and their relative contribution assessed a remarkable correlation is found between thermal and dynamic variables a study of the internal flux balance of daytime density currents shows that the internal flow plays a significant part in maintaining the capping inversion of the current eddies were observed at interfacial regions in which richardson numbers were significantly greater than 0 25 confirming some modelling studies frontal slopes in most cases are in accord with theoretical values but in one downwind case the slope is considerably less than predicted the inland currents are found to be deeper than coastal currents which may explain the increase in speed observed later in the day water vapour frontal structures are generally less marked than thermal fronts

a numerical study on sea land breezes as a gravity current kelvin helmholtz billows and inland penetration of the sea breeze front abstract a two dimensional nonhydrostatic compressible and dry numerical model was developed by using high spatial resolution and an appropriate turbulence model fine structure and dynamics of the sea breeze head were investigated kelvin helmholtz kh instability was found to occur in the foremost part of the head consequently kh billows were generated there and grew in amplitude while travelling backward relative to the advancing front along the zero velocity boundary the kh billows entrained the upper air into the sea breeze the resultant turbulent mixing and wave perturbations induced a top friction force acting on the sea breeze head structure of the kh billow and the wavelike motion induced by it near the ground were also investigated and compared with the results obtained in laboratory density currents linear theories and observations the kh instability khi did not occur at all stages of the sea breeze and this resulted in a diurnal variation of the fine structure and dynamics of

on topographic controls of soil hydraulic parameter scaling at hillslope scales 1 most upscaling efforts for soil hydraulic parameters developed thus far have opted to ignore the effect of topography in their derivation of effective parameter values this approach which considers a flat terrain with no lateral flows is reasonable as long as the coarser support dimensions are of the order of a few hundred meters in such a scenario the upscaled characteristics of the parameters are governed predominantly by the texture and structure of the soil in the domain however when upscaling fine scale hydraulic parameter data to much larger extents hillslope scales and beyond topography plays a bigger role and can no longer be ignored efforts to model hydrologic processes and phenomena with particular emphasis on those occurring in the unsaturated zone are conducted at various scales we present here a study to isolate the influence of topographic variations on the effective upscaled soil hydraulic parameters under different hillslope configurations the power averaging operator algorithm was used to aggregate fine scale soil hydraulic parameters to coarser resolutions hydrologic scenarios were simulated using hydrus 3d for four different topographic configurations under different conditions to test the validity of the upscaled soil hydraulic parameters the outputs from the simulations fluxes and soil moisture states were compared across multiple scales for validating the effectiveness of the upscaled soil hydraulic parameters it was found that the power averaging algorithm produced reasonably good estimates of effective soil hydraulic parameters at coarse scales further a probable threshold dimension beyond which the topography dominates the soil hydraulic property variation was analyzed on the basis of only the topography the scaling algorithm was able to capture much of the variation in soil hydraulic parameters required to generate equivalent flows and soil moisture states in a coarsened domain

delineation of contamination plume around oxidation sewage ponds in southwestern nigeria integrated surface electrical resistivity and electromagnetic em surveys were conducted in a hard rock terrain of southwestern nigeria in the vicinity of active oxidation sewage treatment ponds the aim was to detect soil contamination due to the spread of sewage effluent locate possible leachate plumes and conductive lithologic layers and access the risk of groundwater pollution in the vicinity of the sewage ponds dipole dipole resistivity profiling and very low frequency vlf data were acquired at 10 m intervals along five 200 m long east west geophysical traverses resistivity sections obtained revealed four subsurface geologic layers comprised of lateritic clay clayey sand sand weathered fractured bedrock and competent bedrock a distinct low resistivity zone corresponding to the contamination plume labeled b was delineated from all the resistivity sections this low zone extends into the weathered bedrock and possibly suggests contamination of this layer the filtered real component of the processed vlf data detected three distinct anomaly zones that are representative of fractured zones filled with conductive fluids and or lithologic boundaries that possibly serve as conduits for the movement of contaminated effluents the results obtained from the two methods suggest possible contamination of the subsurface soil layers and groundwater in the vicinity of the sewage ponds the existence of this contaminated plume poses a serious threat to the ecosystem and health of the people living in the vicinity of the sewage ponds

a comparison of simplified methods for routing topographically driven subsurface flow a series of numerical experiments are used to evaluate the accuracy of two approaches for modeling topographically driven subsurface flow suitable for inclusion in spatially distributed hydrologic models the first is an explicit grid cell by grid cell approach and the second is a statistical dynamic method widely used in topmodel both approaches were compared with analytical solutions to the kinematic wave equation for flow down an inclined plane of constant slope using a power law transmissivity function the hillslope discharge and water table profiles simulated by the explicit method were in good agreement with the analytic solution in all test cases the statistical dynamic method converged on the correct steady state solution but failed to reproduce accurately transient conditions the two algorithms were then compared using a 30 m dem representation of the mahantango pa basin and hourly precipitation measured at the site the agreement in discharge between the two methods improved with increasing soil thickness surface conductivity and power law exponent however the percent root mean squared difference rmsd in hourly discharge was above 20 percent in all cases the statistical dynamic model was then calibrated to discharge simulated by the explicit method calibration was most effective for soils with low power law exponents generally producing discharge hydrographs in closemore agreement for soils with linear transmissivity profiles however even under these conditions there are large discrepancies between local water table depths simulated by the two models the level of agreement is best during dry conditions and decreases during wetter conditions as the number of cells with surface soil saturation increases less

the sea breeze and gravity current frontogenesis the development of a sea breeze circulation into a sea breeze front in often observed late in the afternoon measurements of horizontal temperature and humidity profiles in a number of sea breeze circulation made using an instrumented light aircraft are presented on each of the four days for which data are presented an initial weak horizontal temperature gradient extending over a distance of 5 to 10 kilometres developed later in the into a sharp front only a few hundred metres across the data are discussed in terms of simple gravity current frontogenesis models applied to the development of sharp sea breeze front from an initial diffuse transition between the air over the land sea examination of the equations for frontogenesis and the result of some previous laboratory experiments on gravity current frontogenesis lead to the proposal that given sufficient contrast between land and sea temperatures the formation of a sea breeze front depends on the balance between convergent horizontal winds which act to generate a front and turbulent convective mixing over the land which tends to prevent its formation the differences in driving force wind convergence and turbulent intensity are discussed for each day on which data are available and are shown to be consistent with the theory proposed

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

geological hazards of sw natib volcano site of the bataan nuclear power plant the philippines the sw sector of mount natib a potentially active volcano in the bataan volcanic arc in western luzon is the site of a mothballed nuclear power plant that members of the national legislature have proposed to activate detailed geological fieldwork was conducted to assess the capability of the volcano and to identify any volcanic hazards it might pose to the nuclear plant the nearest eruptive centre is 5 5 km away from the plant sw natib volcano is underlain by lava flows lahar deposits and at least six pyroclastic density current pdc deposits three directly underlying the nuclear reactor facility a fault trending n308e is aligned with the lubao fault a capable fault ne of the volcanic edifice radon emissions at the traces of these faults are high and comparable to those at known active faults an associated thrust fault at the nuclear site cuts through lahars up to the ground surface the results presented here can be used for general hazard preparedness of local communities and may assist the government to decide whether or not to recommission the nuclear power plant

digital elevation model networks demon a model of flow over hillslopes for computation of contributing and dispersal areas current algorithms for computing contributing areas from a rectangular grid digital elevation model dem use the flow routing model of o callaghan and mark 1984 which has two major restrictions 1 flow which originates over a two dimensional pixel is treated as a point source nondimensional and is projected downslope by a line one dimensional moore and grayson 1991 and 2 the flow direction in each pixel is restricted to eight possibilities we show that large errors in the computed contributing areas result for any terrain topography divergent convergent or planar we present a new model called digital elevation model networks demon which avoids the above problems by representing flow in two dimensions and directed by aspect demon allows computation of both contributing and dispersal areas demon offers the main advantage of contour based models e g moore et al 1988 the representation of varying flow width over nonplanar topography while having the convenience of using rectangular grid dems

radon soil gas as a geological mapping tool case study from basement complex of nigeria in an effort to quantify the geogenic radon soil gas potential and appraise the use of radon technique as a geological mapping tool in a crystalline basement rock terrain of ile ife nigeria radon measurement concentration were made using a radon detector instrument eda rd 200 that measures radon isotopes by a scintillator cell coupled to a photomultiplier tube the data were collected from soils derived from three different lithologic rock units the observed values were then correlated with the geology of the area significant differences in the radon soil gas concentrations among the three geologic units were observed granite gneiss has the highest concentration followed by grey gneiss and mica schist in that order the geometric mean gm concentration of radon 222 measured in soils directly overlying the three different rock types were 301 4 pci l for granite gneiss 202 8 pci l for the grey gneiss and 199 4 pci l for mica schist conversely the average values for radon 220 averaged 1510 0 815 4 and 733 0 pci l for granite gneiss grey gneiss and mica schist rocks respectively statistical t test 0 05 results indicated that there was no significant difference in the geometric mean of radon soil gas measured between low and medium potential zones however significant differences were found between the low and high radon potential zones and between the medium and high zones the low concentrations of radon soil gas emission observed in this study is explained in terms of the seasonal variation due to thermal convection fluid movement while the radon concentrations were found to be controlled by the lithology and geochemistry of the underlying bedrock

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

non normal growth of kelvin helmholtz eddies in a sea breeze the generalized stability of a sea breeze front is analyzed using a two dimensional model the objective is to understand the mechanisms leading to the shedding of eddies behind the sea breeze front as seen in observations laboratory experiments and numerical models regions with ri 1 4 the energy source for optimal growth is the vertical shear of the mean horizontal wind the vertical shear in the upper part of the front and the horizontal shear in the lower part the growth begins with vertical advection by the perturbation velocity of the mean flow momentum located in the upper part of the front perturbations eventually propagate away from the localized shear area and a feedback mechanism is needed for this growth to be sustained this feedback occurs through temperature anomalies in the upper part of the front inducing pressure gradient anomalies in the lower part these gradients lead to a growing vertical wind component and this vertical wind component then enters the upper part of the front which reinforces the extraction of energy thereby closing the feedback loop and leading to both normal mode instability and in the stable regime large non normal growth we find that both the instability and the non normal growth are vulnerable to parameter changes that weaken this feedback loop

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development

a numerical and analytical study of atmospheric undular bores summary in this paper the formation and structure of an atmospheric undular bore is examined with the aid of a numerical model and theoretical analysis the numerical model is first used to simulate a density current propagating into an unstratified environment a detailed comparison of the model results with existing theory shows that for a given pressure head the current can travel at a greater speed than that predicted by irrotational theory when the density current is allowed to propagate into a low level stable layer it is found that for certain parameter regimes an undular bore forms ahead of the current the results for two different types of stable layers are examined for a layer with a step profile of potential temperature comparison is made with both classical bore theory and the theory of density currents in two layer fluids and encouraging agreement is found when a linear profile is specified in the stable layer it is found that energy conserving solutions can be obtained the numerical fields are then compared with measurements of the morning glory an undular bore observed in north eastern australia and again agreement is good finally preliminary results of model simulations with stratification above the stable layer are discussed