Terrestrial Drainage Research Articles

A novel approach to the classification of terrestrial drainage networks based on deep learning and preliminary results on solar system bodies

Several approaches were proposed to describe the geomorphology of drainage networks and the abiotic/biotic factors determining their morphology. There is an intrinsic complexity of the explicit qualification of the morphological variations in response to various types of control factors and the difficulty of expressing the cause-effect links. Traditional methods of drainage network classification are based on the manual extraction of key characteristics, then applied as pattern recognition schemes. These approaches, however, have low predictive and uniform ability. We present a different approach, based on the data-driven supervised learning by images, extended also to extraterrestrial cases. With deep learning models, the extraction and classification phase is integrated within a more objective, analytical, and automatic framework. Despite the initial difficulties, due to the small number of training images available, and the similarity between the different shapes of the drainage samples, we obtained successful results, concluding that deep learning is a valid way for data exploration in geomorphology and related fields.

Ice sheet and palaeoclimate controls on drainage network evolution: an example from Dogger Bank, North Sea

Abstract. Submerged landscapes on continental shelves archive drainage networks formed during periods of sea-level lowstand. The evolution of these postglacial drainage networks also reveals how past climate changes affected the landscape. Ice-marginal and paraglacial drainage networks on low-relief topography are susceptible to reorganisation of water supply, forced by ice-marginal rearrangement, precipitation and temperature variations, and marine inundation. A rare geological archive of climate-driven landscape evolution during the transition from ice-marginal (ca. 23 ka) to a fully submerged marine environment (ca. 8 ka) is preserved at Dogger Bank, in the southern North Sea. In this study, our analysis of high-resolution seismic reflection and cone penetration test data reveal a channel network over a 1330 km2 area that incised glacial and proglacial lake-fill sediments. The channel network sits below coastal and shallow marine sediments and is therefore interpreted to represent a terrestrial drainage network. When mapped out, the channel form morphology reveals two distinct sets. The first set comprises two low-sinuosity, wide (>400 m) channels that contain macroforms of braid and side bars. These channels are interpreted to have originated as proglacial rivers, which drained the ice-sheet margin to the north. The second set of channels (75–200 m wide, with one larger, ∼400 m wide) has higher sinuosity and forms a subdendritic network of tributaries to the proglacial channels. The timing of channel formation lacks chronostratigraphic control. However, the proglacial rivers must have formed as the ice sheet was still on Dogger Bank, before 23 ka, to supply meltwater to the rivers. Ice-sheet retreat from Dogger Bank led to reorganisation of meltwater drainage and abandonment of the proglacial rivers. Palaeoclimate simulations show a cold and dry period at Dogger Bank between 23 and 17 ka. After 17 ka, precipitation increased, and drainage of precipitation formed the second set of channels. The second set of rivers remained active until marine transgression of Dogger Bank at ca. 8.5–8 ka. Overall, this study provides a detailed insight into the evolution of river networks across Dogger Bank and highlights the interplay between external (climate) and internal (local) forcings in drainage network evolution.

Millennial and centennial variations in zircon U‐Pb ages in the quaternary indus submarine canyon

AbstractUse of deep‐water sediments in submarine fans to reconstruct changing erosion onshore is based on the premise of relatively simple transport between source and sink. However, debate continues regarding the degree of sediment buffering and recycling in the sediment transport process. In this study, we investigate the origin of sediment in the Indus Submarine Canyon since the Last Glacial Maximum (LGM; ~20 ka) using zircon U‐Pb dates. Zircon grains in the submarine canyon are resolvably different from those at the river mouth, at least before 6.6 ka, implying a disconnection between the river mouth and the canyon up to that time. Sand may be stored near the river mouth as sea level rose, while finer‐grained sediment was directly transferred into deeper water. Since 1 ka the upper canyon has shown big and rapid provenance changes, most notably a sharp increase in erosion from Nanga Parbat, whose influence is minor in the modern river. Such rapid changes imply a lack of buffering in the recent past. The modern river contrasts with sediments in the canyon in terms of its zircon U‐Pb age populations and may be influenced by significant anthropogenic impact on the terrestrial drainage basin, especially damming.

Control on Pleistocene shelf drainage by post-Eocene stratigraphy of the Gippsland Basin

The Jemmys Point Formation - uppermost member of the cool-water carbonate Seaspray Group - records a terrestrial drainage system that traversed the now submerged continental shelf of the Gippsland Basin during the latest ice age. A partial network of large sinuous channels that includes a few diffuse areas of arced broadening (probably recording local slack-water lacustrine conditions), has been imaged by aeromagnetic survey data and now large-scale 3D exploration seismic data. Channels are well resolved in narrow-band quadrature phase exploration seismic data by a surface probe that runs along the first zero crossing of the seismic wavetrain. This corresponds closely with the modern-day bathymetric surface when assuming a seawater acoustic velocity of 1,500 m/s.The stratigraphy of these channel features has not been ground-truthed despite the basin being a highly mature and productive petroleum province. In fact, it seems that their distribution is a ‘negative’ of the distribution of petroleum field areas. This fact leads to the consideration of two hypotheses: ongoing structural inversion of existing trapping structures at depth pushes channels away; or intervening zones of relatively high differential compaction subsidence pull channels towards them. No evidence was found to confirm the former hypothesis but this may be because the rate of local uplift does not outpace shelf-wide sedimentation (so cannot be resolved in seismic or seabed bathymetry data). By contrast, the loci of drainage channels appear to correspond well with regions of the thickest post-Eocene stratigraphy. This suggests that differential compaction subsidence has continued to hold the lowstand drainage system in place since a pre-cursor canyon head network was developed by the Mid-Miocene inversion maximum.

Allochthonous carbon is a major regulator to bacterial growth and community composition in subarctic freshwaters.

In the subarctic region, climate warming and permafrost thaw are leading to emergence of ponds and to an increase in mobility of catchment carbon. As carbon of terrestrial origin is increasing in subarctic freshwaters the resource pool supporting their microbial communities and metabolism is changing, with consequences to overall aquatic productivity. By sampling different subarctic water bodies for a one complete year we show how terrestrial and algal carbon compounds vary in a range of freshwaters and how differential organic carbon quality is linked to bacterial metabolism and community composition. We show that terrestrial drainage and associated nutrients supported higher bacterial growth in ponds and river mouths that were influenced by fresh terrestrial carbon than in large lakes with carbon from algal production. Bacterial diversity, however, was lower at sites influenced by terrestrial carbon inputs. Bacterial community composition was highly variable among different water bodies and especially influenced by concentrations of dissolved organic carbon (DOC), fulvic acids, proteins and nutrients. Furthermore, a distinct preference was found for terrestrial vs. algal carbon among certain bacterial tribes. The results highlight the contribution of the numerous ponds to cycling of terrestrial carbon in the changing subarctic and arctic regions.

Consequences of suppressing natural vegetation in drainage areas for freshwater ecosystem conservation: considerations on the new "Brazilian forest code"

The input of particulate and dissolved organic matter (POM and DOM, respectively) from terrestrial ecosystem drainage basins is an important energy and nutrient source in limnic food chains. Studies indicated that semi-deciduous seasonal forests located in drainage areas in Brazil have the potential to produce 7.5 - 10.3 Mg ha−1/year of POM. The global increase in vegetation destruction, such as forests, threatens this allochthonous resource and can have significant impacts on river and lake communities and food chains. Therefore, it is critical that exploitation and occupation protocols are updated to protect the transition areas between terrestrial and limnic ecosystems. This review highlights the existing knowledge of these ecosystem interactions and proposes responsible sustainable methods for converting the vegetation in drainage basins. This was based on Brazilian ecosystem data and the new "Brazilian Forest Code." This study also considers the importance of including flood tracks in permanently protected areas to improve Brazilian legislation and protect hydric resources.

Carbon burial in soil sediments from Holocene agricultural erosion, Central Europe

AbstractNatural and human‐induced erosion supplies high amounts of soil organic carbon (OC) to terrestrial drainage networks. Yet OC fluxes in rivers were considered in global budgets only recently. Modern estimates of annual carbon burial in inland river sediments of 0.6 Gt C, or 22% of C transferred from terrestrial ecosystems to river channels, consider only lakes and reservoirs and disregard any long‐term carbon burial in hillslope or floodplain sediments. Here we present the first assessment of sediment‐bound OC storage in Central Europe from a synthesis of ~1500 Holocene hillslope and floodplain sedimentary archives. We show that sediment storage increases with drainage‐basin size due to more extensive floodplains in larger river basins. However, hillslopes retain hitherto unrecognized high amounts of eroded soils at the scale of large river basins such that average agricultural erosion rates during the Holocene would have been at least twice as high as reported previously. This anthropogenic hillslope sediment storage exceeds floodplain storage in drainage basins <105 km2, challenging the notion that floodplains are the dominant sedimentary sinks. In terms of carbon burial, OC concentrations in floodplains exceed those on hillslopes, and net OC accumulation rates in floodplains (0.7 ± 0.2 g C m−2a−1) surpass those on hillslopes (0.4 ± 0.1 g C m−2a−1) over the last 7500 years. We conclude that carbon burial in floodplains and on hillslopes in Central Europe exceeds terrestrial carbon storage in lakes and reservoirs by at least 2 orders of magnitude and should thus be considered in continental carbon budgets.

Morphology of fluvial networks on Titan: Evidence for structural control

Although Titan’s surface shows clear evidence of erosional modification, such as fluvial incision, evidence for tectonism has been less apparent. On Earth, fluvial networks with strongly preferred orientations are often associated with structural features, such as faults or joints, that influence flow or erodibility. We delineated and classified the morphologies of fluvial drainages on Titan and discovered evidence of structural control. Fluvial networks were delineated both on synthetic aperture radar (SAR) images covering ∼40% of Titan from the Cassini Titan Radar Mapper up through T71 and on visible light images of the Huygens landing site collected by the Descent Imager/Spectral Radiometer (DISR). The delineated networks were assigned to one of three morphologic classes—dendritic, parallel or rectangular—using a quantitative terrestrial drainage pattern classification algorithm modified for use with Titan data. We validated our modified algorithm by applying it to synthetic fluvial networks produced by a landscape evolution model with no structural control of drainage orientations, and confirmed that only a small fraction of the networks are falsely identified as structurally controlled. As a second validation, we confirmed that our modified algorithm correctly classifies terrestrial networks that are classified in multiple previous works as rectangular. Application of this modified algorithm to our Titan networks results in a classification of rectangular for one-half of the SAR and DISR networks. A review of the geological context of the four terrestrial rectangular networks indicates that tensional stresses formed the structures controlling those terrestrial drainages. Based on the similar brittle response of rock and cryogenic ice to stress, we infer that structures formed under tension are the most likely cause of the rectangular Titan networks delineated here. The distribution of these rectangular networks suggests that tensional stresses on Titan may have been widespread.

Fluvial network analysis on Titan: Evidence for subsurface structures and west‐to‐east wind flow, southwestern Xanadu

Data of Titan's surface from the Cassini‐Huygens mission show inferred fluvial networks interpreted as products of liquid alkane flow. Using synthetic aperture radar (SAR) data, we delineated drainage networks, measured network parameters, and used these measurements in a simplified algorithm for classifying terrestrial drainage patterns. The results show a variety of patterns, indicating that a variety of factors control fluvial drainage on Titan. Drainage network patterns in southwestern Xanadu are classified as rectangular, suggesting control by a subsurface tectonic structural fabric. Link orientations also suggest that this subsurface tectonic fabric is oriented predominantly east‐west. Spatial variations in drainage networks are consistent with a west‐to‐east precipitation pattern, supporting inferences from aeolian dune morphology. These results illustrate how fluvial landform analysis can yield new information on both atmospheric and subsurface processes.

Projected changes in Arctic Ocean freshwater budgets

Arctic Ocean freshwater budgets are examined from 10 models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. This includes an analysis of sea ice transport and storage, ocean transport and storage, and net surface flux exchange. Simulated budgets for the late 20th century are compared to available observations, followed by an analysis of simulated changes from 1950 to 2050. The consistent theme over this period is an acceleration of the Arctic hydrological cycle, which is expressed as an increase in the flux of water passing through the hydrologic elements. Increased freshwater inputs to the ocean from net precipitation, river runoff, and net ice melt result. While generally attended by a larger export of liquid freshwater to lower latitudes, primarily through Fram Strait, liquid freshwater storage in the Arctic Ocean increases. In contrast, the export and storage of freshwater in the form of sea ice decreases. The qualitative agreement between models for which the only common forcing is rising greenhouse gas concentrations implicates this greenhouse gas loading as the cause of the change. Although the models perform quite well in their simulations of net precipitation over the Arctic Ocean and terrestrial drainage, they differ significantly regarding the magnitude of the trends and their representation of contemporary mean ocean and sea ice budget terms. To reduce uncertainty in future projections of the Arctic freshwater cycle, the climate models as a group require considerable improvement in these aspects of their simulations.

Holocene denudation of the northwest sector of Iceland as determined from accumulation of sediments on the continental margin

Radiocarbon‐dated marine cores, measurements of sediment density and seismic surveys were used to estimate the sediment and mass accumulation rates (m/kyr and kg/m2/kyr) in the troughs from the southwest to north‐central Iceland shelf (i.e. northwest sector of Iceland). The 3.5‐kHz seismic survey showed varying thicknesses of acoustically transparent sediment in the troughs, whereas the inter‐trough banks were largely devoid of sediment. The survey showed a pervasive reflector 1 to ≥ 60 m below the sea floor, which turned out to be Saksunarvatn tephra, dated at 10 180 ± 60 cal. yr BP. The 3.5‐kHz analogue data were digitized at 1‐min intervals and provided 1645 estimates of maximum sediment thickness and 979 estimates of sediment accumulation over the last 10200 cal. yr BP. Maximum sediment accumulation occurred in the mid‐troughs and not, as expected, in the fjords. The median sediment accumulation rate (SAR) based on the core data was 0.23 m/kyr, but was 0.77 m/kyr based on the seismic data: the difference is attributed to coring limitations. Based on the volume of offshore sediment and the contributing terrestrial drainage area, the Holocene denudation of northern Iceland (c. 50 000 km2) is calculated to have been between 0.02 to 0.05 m/kyr, substantially lower that the 1–3 m/kyr derived from the suspended sediment load of rivers from southern Iceland but in agreement with the rate of accumulation of Holocene glacial lacustrine sediments in central Iceland.

Automatic mapping of valley networks on Mars

Martian valley networks bear some resemblance to terrestrial drainage systems, but their precise origin remains an active research topic. A limited number of valley networks have been manually mapped from images, but the vast majority remains unmapped because standard drainage mapping algorithms are inapplicable to valleys that are poorly organized and lack spatial integration. In this paper, we present a novel drainage delineation algorithm specially designed for mapping the valley networks from digital elevation data. It first identifies landforms characterized by convex tangential curvature, and then uses a series of image processing operations to separate valleys from other features having a convex form. The final map is produced by reconnecting all valley segments along drainage directions. Eight test sites on Mars are selected and manually mapped for valley networks. The algorithm is applied to the test sites and delineated networks are compared to mapped networks using a series of quantitative quality factors. We have found a good agreement between delineated and mapped networks. In the process of comparing manual and delineated networks some shortcomings of manual mapping became apparent. We argue that delineated networks are indeed of better quality than the networks manually mapped from images. Although the algorithm has been developed to study Martian surface, it may also be relevant to terrestrial geomorphology.

Holocene denudation of the northwest sector of Iceland as determined from accumulation of sediments on the continental margin

Radiocarbon-dated marine cores, measurements of sediment density and seismic surveys were used to estimate the sediment and mass accumulation rates (m/kyr and kg/m2/kyr) in the troughs from the southwest to north-central Iceland shelf (i.e. northwest sector of Iceland). The 3.5-kHz seismic survey showed varying thicknesses of acoustically transparent sediment in the troughs, whereas the inter-trough banks were largely devoid of sediment. The survey showed a pervasive reflector 1 to ≥ 60 m below the sea floor, which turned out to be Saksunarvatn tephra, dated at 10 180 ± 60 cal. yr BP. The 3.5-kHz analogue data were digitized at 1-min intervals and provided 1645 estimates of maximum sediment thickness and 979 estimates of sediment accumulation over the last 10200 cal. yr BP. Maximum sediment accumulation occurred in the mid-troughs and not, as expected, in the fjords. The median sediment accumulation rate (SAR) based on the core data was 0.23 m/kyr, but was 0.77 m/kyr based on the seismic data: the difference is attributed to coring limitations. Based on the volume of offshore sediment and the contributing terrestrial drainage area, the Holocene denudation of northern Iceland (c. 50 000 km2) is calculated to have been between 0.02 to 0.05 m/kyr, substantially lower that the 1–3 m/kyr derived from the suspended sediment load of rivers from southern Iceland but in agreement with the rate of accumulation of Holocene glacial lacustrine sediments in central Iceland.

Ecotoxicological Effects of Combined UVB and Organic Contaminants in Coastal Waters: A Review

ABSTRACTOrganisms living in coastal waters are exposed to anthropogenic contaminants from terrestrial drainage, ice melting and maritime traffic and to enhanced UVB radiation (UVBR; 280–320 nm) caused by decreased concentrations of ozone in the stratosphere. This article reviews available information about the combined effects of UVBR and selected hydrosoluble contaminants potentially present in surface waters on marine species and especially on plankton community structure in high‐latitude coastal zones. Effects of UVBR on three selected pesticides (Atrazine, carbaryl and Acifluorfen) and possible induction of phototoxicity are reviewed. Most toxicological studies have been conducted under laboratory conditions with questionable relevance for coastal marine ecosystems. Similarly, photoactivation of polycyclic aromatic hydrocarbons (PAHs) has been closely examined and reported effects on aquatic species summarized. Experiments with field‐sampled communities demonstrated the complexity and the difficulty in determining the impact of multiple stressors on an aquatic ecosystem, even for ecosystems simplified by eliminating large grazers and fish. Nutrient status, specific composition and light history have influenced the different responses of planktonic assemblages exposed to enhanced UVBR and water‐soluble fraction (WSF) from crude oil or to tributyltin. Plankton assemblages subjected to changes in the ozone hole were physiologically stressed and more susceptible to WSF tox‐icity than communities from less enhanced UVBR‐impacted sites. A close relationship between phytoplankton assemblages and bacteria was observed in all experiments in mesocosms. A contaminant‐induced phytoplankton crash after a bloom event may release important carbon and nutrient sources for bacteria. The magnitude of phytoplanktonic mortality induced by a contaminant probably influenced how rapidly bacteria grew over time. The transition from a herbivorous food web to a microbial food web has significant ecological implications for carbon cycling and energy flow in pelagic systems. A high phytoplankton mortality implies a situation in which the potential for downward carbon export from surface waters is high. In contrast, high bacterial enrichment implies that the phytoplankton carbon is largely recycled in surface waters through a microbial loop and does not contribute significantly to sinking particle flux. The most ecologically relevant results were obtained with mesocosm studies using field‐collected communities. The enhancement of hydrocarbon toxicity in the presence of a high level of UVBR cannot be described as being a synergistic or an additive effect, because the WSF alone is not toxic and may even be beneficial by increasing bacterial activity. This is a case in which one stressor has the ability to modify another stressor to cause it to be toxic to target organisms. These abiotically induced interactions may be important for biological communities exposed to extreme conditions when physical, chemical or photochemical reactions modify the nature of environmental stressors before they interact with biological functions. The need for models on the impacts of multiple stressors on biodiversity and ecosystem functioning is emphasized.

Integrating Remote Sensing Approach with Pollution Monitoring Tools for Aquatic Ecosystem Risk Assessment and Managment: A Case Study of Lake Victoria(UGANDA)

Aquatic ecosystems around the world, lake, estuaries and coastal areas are increasingly impacted by anthropogenic pollutants through different sources such as agricultural, industrial and urban discharges, atmospheric deposition and terrestrial drainage. Lake Victoria is the second largest lake in the world and the largest tropical lake. Bordered by Tanzania, Uganda, and Kenya, it provides a livelihood for millions of Africans in the region. However, the lake is under threat from eutrophication, a huge decline in the number of native fish species caused by several factors including loss of biodiversity, over fishing and pollution has been recently documented. Increasing usage of pesticides and insecticides in the adjacent agricultural areas as well as mercury contamination from processing of gold ore on the southern shores are currently considered among the most emergent phenomena of chemical contamination in the lake. By the application of globally consistent and comprehensive geospatial data-sets based on remote sensing integrated with information on heavy metals accumulation and insecticides exposure in native and alien fish populations, the present study aims at assessing the environmental risk associated to the contamination of the Lake Victoria water body on fish health, land cover distribution, biodiversity and the agricultural area surrounding the lake. By the elaboration of Landsat 7 TM data of November 2002 and Landsat 7 TM 1986 we have calculated the agriculture area which borders the Lake Victoria bay, which is an upland plain. The resulting enhanced nutrient loading to the soil is subsequently transported to the lake by rain or as dry fall. The data has been inserted in a Geographical information System (ARCGIS) to be upgraded and consulted. Heavy metals in fish fillets showed concentrations rather low except for mercury being higher than others as already described in previous investigations. In the same tissue, cholinesterases activity (ChE) as an indicator of insecticides exposure showed significant differences among fish species in both activity and sensitivity of selected inhibitor insecticides. This integrated approach aims at identifying and quantifying selected aquatic environmental issues which integrated with monitoring techniques such as contaminant concentrations and biological responses to insecticides exposure in fish populations will provide a scientific basis for aquatic zones management and assist in policy formulations at the national and international levels.

Ecotoxicological Effects of Combined UVB and Organic Contaminants in Coastal Waters: A Review

Organisms living in coastal waters are exposed to anthropogenic contaminants from terrestrial drainage, ice melting and maritime traffic and to enhanced UVB radiation (UVBR; 280-320 nm) caused by decreased concentrations of ozone in the stratosphere. This article reviews available information about the combined effects of UVBR and selected hydrosoluble contaminants potentially present in surface waters on marine species and especially on plankton community structure in high-latitude coastal zones. Effects of UVBR on three selected pesticides (Atrazine, carbaryl and Acifluorfen) and possible induction of phototoxicity are reviewed. Most toxicological studies have been conducted under laboratory conditions with questionable relevance for coastal marine ecosystems. Similarly, photoactivation of polycyclic aromatic hydrocarbons (PAHs) has been closely examined and reported effects on aquatic species summarized. Experiments with field-sampled communities demonstrated the complexity and the difficulty in determining the impact of multiple stressors on an aquatic ecosystem, even for ecosystems simplified by eliminating large grazers and fish. Nutrient status, specific composition and light history have influenced the different responses of planktonic assemblages exposed to enhanced UVBR and water-soluble fraction (WSF) from crude oil or to tributyltin. Plankton assemblages subjected to changes in the ozone hole were physiologically stressed and more susceptible to WSF toxicity than communities from less enhanced UVBR-impacted sites. A close relationship between phytoplankton assemblages and bacteria was observed in all experiments in mesocosms. A contaminant-induced phytoplankton crash after a bloom event may release important carbon and nutrient sources for bacteria. The magnitude of phytoplanktonic mortality induced by a contaminant probably influenced how rapidly bacteria grew over time. The transition from a herbivorous food web to a microbial food web has significant ecological implications for carbon cycling and energy flow in pelagic systems. A high phytoplankton mortality implies a situation in which the potential for downward carbon export from surface waters is high. In contrast, high bacterial enrichment implies that the phytoplankton carbon is largely recycled in surface waters through a microbial loop and does not contribute significantly to sinking particle flux. The most ecologically relevant results were obtained with mesocosm studies using field-collected communities. The enhancement of hydrocarbon toxicity in the presence of a high level of UVBR cannot be described as being a synergistic or an additive effect, because the WSF alone is not toxic and may even be beneficial by increasing bacterial activity. This is a case in which one stressor has the ability to modify another stressor to cause it to be toxic to target organisms. These abiotically induced interactions may be important for biological communities exposed to extreme conditions when physical, chemical or photochemical reactions modify the nature of environmental stressors before they interact with biological functions. The need for models on the impacts of multiple stressors on biodiversity and ecosystem functioning is emphasized.

An intense terminal epoch of widespread fluvial activity on early Mars: 1. Valley network incision and associated deposits

We present evidence that a final epoch of widespread fluvial erosion and deposition in the cratered highlands during the latest Noachian or early to mid‐Hesperian was characterized by integration of flow within drainage networks as long as 4000 km and trunk valley incision of 50 to 350 m into earlier Noachian depositional basins. Locally deltaic sediments were deposited where incised valley systems debouched into basins. Large alluvial fans of sediment deposited from erosion of alcoves in steep crater walls probably formed contemporaneously. The depth of incision below Noachian surfaces correlates strongly with the gradient and the total valley length, suggesting consistent regional hydrology. Estimated discharges from channel dimensions indicate flow rates equivalent to mean annual floods in terrestrial drainage basins of equivalent size. Such high flow rates imply either runoff directly from precipitation or rapid melting of accumulated snow. Development of duricrusts on the Noachian landscape may have contributed to focusing of late‐stage erosion within major trunk drainages. This late‐stage epoch of intense fluvial activity appears to be fundamentally different than the fluvial environment prevailing during most of the Noachian Period, which was characterized by widespread fluvial erosion of highlands and crater rims, deeply infilling crater floors, and intercrater basins through ephemeral fluvial activity, and development of local rather than regionally integrated drainage networks.

Modeling evidence for recent warming of the Arctic soil thermal regime

Daily soil temperature and thaw depth for the entire Arctic terrestrial drainage area are simulated using a one‐dimensional heat transfer model with phase change. Analyses of temperature trends at the soil surface and at 2 m depth are presented for the 23‐year time period 1980 through 2002. Soil warming is simulated for all permafrost regions, but is most pronounced (0.044°C/yr) at the surface in the continuous permafrost region. Trends for most recent years (1994–2002) are about three times higher. Active layer depth increases significantly for parts of Alaska and northern Canada, and southern and eastern Siberia. As assessed for the major river drainages, the most dramatic active layer deepening occurs in the Yenisey basin.

A model study of circum‐Arctic soil temperatures

AbstractA one‐dimensional heat transfer model with phase change is applied to the entire Arctic terrestrial drainage area. The main forcing parameters are reanalysed surface air temperature that was topography‐enhanced, and snow depth, derived from passive microwave satellite data. We present results based on daily fields of soil temperature, ranging from the soil surface down to 14 m depth. The horizontal grid cell resolution is 25 km × 25 km. Model results are compared to several long series of soil temperature measurements from Alaska and Siberia, with a relatively good match of the the annual cycle at different depths. We show time series of soil temperature at different depths for the 22‐year period 1980 through 2001, and both seasonal and regional analyses are included. A trend analysis reveals positive trends for all permafrost regions in response to positive trends in air temperature, with the strongest warming trend in regions of continuous permafrost. A slight cooling trend is only found for the topmost soil layers in regions of seasonally frozen ground at the southern margins of the Arctic drainage domain. Copyright © 2004 John Wiley & Sons, Ltd.

Martian geomorphology from fractal analysis of drainage networks

An alternative approach to conventional planetary terrain analysis is proposed. Martian terrains, represented by topography based on the Mars Orbiter Laser Altimetry (MOLA) data, are represented as a series of drainage basins, regardless of the historical presence or absence of actual fluid flow. A fractal analysis of each drainage network, computationally extracted from an underlying terrain, yields a network descriptor, a list of four numbers that describes the traits of a network. Network descriptors are used for a quantitative characterization and classification of Martian surfaces. We have extracted and analyzed 387 drainage networks from 74 Martian locations, representing all major epochs and geological units, to investigate whether their network descriptors are capable of differentiation between different epochs and geological units. We have found that our approach can distinguish morphologically different terrains, but only in a statistical sense. In particular, the method could be used to measure the degree of surface cratering and thus the age of the surface. In addition, for surfaces that are not heavily cratered the method is capable of distinguishing between different geological units. We have found no global trends in the character of Martian drainage networks; their network descriptors show no systematic dependence on location or elevation. Our analysis reveals that all Noachian surfaces grouped in the Npld geological unit have very similar network descriptors, regardless of prominence and integration of their visible fluvial features. The comparison between statistical description of Npld and terrestrial drainage networks shows overall similarities, but also some marked differences. Because of these differences, the notion of sustained rainfall on Mars during the Noachian epoch is inconsistent with our findings, but a limited rainfall is compatible with our results.