Steep Topography Research Articles

Climate remained a major driving factor of vegetation dynamics over the past 600 years in Huangshan Mountain, Southeast China

Holocene vegetation change is a complex process driven by climate and human activity, distinguishing their relative contributions is important to understand and adapt to the changing planet. Southeast China has a long history of human occupation and the evergreen broadleaved forests of the region have been strongly affected by human activity and replaced widely by anthropogenic vegetation in the last millennium. Pines exist both in natural and anthropogenic vegetation, therefore the interrelationship between pine and other species offers a means of differentiating natural from anthropogenic vegetation changes. Both climate and population in southeast China have undergone substantial change during the late Holocene, but the response of vegetation at centennial scale has not been fully resolved. In this study, we present a palynological analysis of a sediment core obtained from a high-elevation wetland on the Huangshan Mountain, which is known for its remarkably steep topography. The core provides evidence of vegetation history since the onset of the Little Ice Age. Results show that Huangshan vegetation shifted from montane mixed forest to upper montane grassland at the beginning of the Little Ice Age and that, although forest recovery thereafter was gradual, the coexistence of forest and grassland has been maintained since 1800 CE. Climate is shown to be the main driver of Huangshan vegetation change over the last six centuries as a whole, although the expansion of pines into broadleaved forests under the pressure of population growth, commonly reported at other sites in southeast China, is also observed in the Huangshan record from 1800 CE onward. Nevertheless, vegetation on the mountain appears to have been less impacted by human activity than in other parts of the region, possibly as result of the unique topography of Huangshan that restricts its accessibility. In conclusion, the study reveals climate still played an important role in driving last-millennium vegetation change.

Increasing Salt Marsh Elevation Using Sediment Augmentation: Critical Insights from Surface Sediments and Sediment Cores

Sea-level rise is particularly concerning for tidal wetlands that reside within an area with steep topography or are constrained by human development and alteration of sedimentation. Sediment augmentation to increase wetland elevations has been considered as a potential strategy for such areas to prevent wetland loss over the coming decades. However, there is little information on the best approaches and whether adaptive management actions can mimic natural processes to build sea-level rise resilience. In addition, the lack of information on long-term marsh characteristics, processes, and variability can hamper development of effective augmentation strategies. Here, we assess a case study in a southern California marsh to determine the nature of the pre-existing sediments and variability of the site in relation to sediments applied during an augmentation experiment. Although sediment cores revealed natural variations in the grain size and organic content of sediments deposited at the site over the past 1500 years, the applied sediments were markedly coarser in grain size than prehistoric sediments at the site (100% maximum sand versus 76% maximum sand). The rate of the experimental sediment application (25.1 ± 1.09 cm in ~2 months) was also much more rapid than natural accretion rates measured for the site historically. In contrast, post-augmentation sediment accretion rates on the augmentation site have been markedly slower than pre-augmentation rates or current rates on a nearby control site. The mismatch between the characteristics of the applied sediment and thickness of application and the historic conditions are likely strong contributors to the slow initial recovery of vegetation. Sediment augmentation has been shown to be a useful strategy in some marshes, but this case study illustrates that vegetation recovery may be slow if applied sediments are not similar or at a thickness similar to historic conditions. However, testing adaptation strategies to build wetland elevations is important given the long-term risk of habitat loss with sea-level rise. Lessons learned in the case study could be applied elsewhere.

Evaluating driving behavior patterns during wildfire evacuations in wildland-urban interface zones using connected vehicles data

Wildfire risk is increasing all over the world, particularly in the western United States and the communities in wildland-urban interface (WUI) areas are at the greatest risk of fire. Understanding the driving behavior of individuals to evacuate fire-affected WUI areas is important as the evacuees may encounter low visibility and difficult driving conditions due to burning material and steep topography. This study investigates the driving behavior patterns of individuals during historical wildfire events in rural and urban areas with mandatory evacuation orders using a connected vehicle dataset. This dataset provides the geolocation and timestamp of vehicles’ hard-braking (HB) and hard-acceleration (HA) events. The comparative analysis of the data demonstrated that the reported HA & HB event patterns were consistent in representing the varying driving behavior in response to the changing driving conditions and the depiction of the temporal and spatial impact of the fire in all studied areas. Moreover, the HB event dataset located critical traffic congestion points and the HA event dataset revealed the hurried response of evacuees on the exiting routes as a result of short-notice evacuation. In addition, significant differences in driving behavior patterns were noticed between rural and urban areas.

Internal Solitary Waves within the Cold Tongue of the Equatorial Pacific Generated by Buoyant Gravity Currents

Abstract The equatorial cold tongue in the Pacific Ocean has been intensely studied during the last decades as it plays an important role in air–sea interactions and climate issues. Recently, Warner et al. revealed gravity currents apparently originating in tropical instability waves. Both phenomena have strong dissipation rates and were considered to play a significant role in cascading energy from the mesoscale to smaller horizontal scales, as well as to vertical scales less than 1 m. Here, we present Sentinel-3 satellite observations of internal solitary waves (ISWs) in the Pacific cold tongue near the equator, in a zonal band stretching from 210° to 265°E, away from any steep bottom topography. Within this band these waves propagate in multiple directions. Some of the waves’ characteristics, such as the distance between wave crests, crest lengths, and time scales, are estimated from satellite observations. In total we identify 116 ISW trains during one full year (2020), with typical distances between crests of 1500 m and crest lengths of hundreds of kilometers. These ISW trains appear to be generated by buoyant gravity currents having sharp fronts detectable in thermal infrared satellite images. A 2D numerical model confirms that resonantly generated nonlinear internal waves with amplitudes of O(10) m may be continuously initiated at the fronts of advancing gravity currents. Significance Statement Satellite imagery reveals the repeated occurrence of internal solitary waves in the near-equatorial region of the east Pacific, despite the absence of topography. These waves appear to be resonantly generated over the sheared Equatorial Undercurrent by gravity currents that propagate as frontal zones of 1000-km scale tropical instability waves, providing a physical link with viscous mixing scales.

Projected Changes in Mountain Precipitation Under CO2‐Induced Warmer Climate

AbstractMountains play a vital role in shaping regional and global climate, altering atmospheric circulation and precipitation patterns. To this end, identifying projected changes in mountain precipitation is significantly challenging due to topographic complexity. This study explains how mountain precipitation could respond to rising greenhouse gases. Using a series of century‐long fully coupled high‐resolution simulations conducted with the Community Earth System Model, we aim to disentangle future changes in mountain precipitation in response to atmospheric carbon dioxide (CO2) perturbations. Our research findings indicate that the warming observed in global mountains is more pronounced when compared to the mean warming rates experienced globally and in the ocean under elevated CO2. We identify five low‐latitude mountain ranges with elevation‐dependent precipitation response, including New Guinea, East Africa, Eastern Himalayas, Central America, and Central Andes. Those mountains are expected to have a mixture of increasing and decreasing precipitation in response to CO2‐induced warming, especially over the summit and steep topography. To elucidate the mechanisms controlling future changes in mountain precipitation, we propose “Orographic moisture omega feedback” in which an increase in low‐level relative humidity enhances local precipitation by strengthening the upward motion through moist processes for the wetting response and vice versa for the drying response. The effects of Mountain precipitation changes can be extended to hydrology and could lead to significant consequences for human societies and ecosystems.

The rise of New Guinea and the fall of Neogene global temperatures

The ~2,000-km-long Central Range of New Guinea is a hotspot of modern carbon sequestration due to the chemical weathering of igneous rocks with steep topography in the warm wet tropics. These high mountains formed in a collision between the Australian plate and ophiolite-bearing volcanic arc terranes, but poor resolution of the uplift and exhumation history has precluded assessments of the impact on global climate change. Here, we develop a palinspastic reconstruction of the Central Range orogen with existing surface geological constraints and seismic data to generate time-temperature paths and estimate volumes of eroded material. New (U-Th)/He thermochronology data reveal rapid uplift and regional denudation between 10 and 6 Mya. Erosion fluxes from the palinspastic reconstruction, calibrated for time with the thermochronological data, were used as input to a coupled global climate and weathering model. This model estimates 0.6 to 1.2 °C of cooling associated with the Late Miocene rise of New Guinea due to increased silicate weathering alone, and this CO2 sink continues to the present. Our data and modeling experiments support the hypothesis that tropical arc-continent collision and the rise of New Guinea contributed to Neogene cooling due to increased silicate weathering.

Stability assessment of desander cut slope of Seti Khola Hydropower Project

Due to steep topography in the Himalaya, opening space for the development of infrastructures involves excavation of hillside slopes which possesses certain degree of risk associated with failure. This paper aims to assess the stability of the rock slope excavated for the placement of desander basin of the Seti Khola Hydropower Project (22 MW) located in Lekhnath, Kaski district of Gandaki Province. The height of the cut slope is about 60 m and is the challenging part of the construction work of this project. The slope is cut in the rock mass belonging to highly schistose graphitic phyllite intercalated with meta-sandstone at the upper part of the slope. To conduct a detailed stability assessment, the discontinuities, and geometric characteristics have been evaluated by field mapping. A three dimensional (3D) geological model has been generated to represent the geometric properties of discontinuities and the cut slope incorporating failure plane. The mechanical characteristic of the intact rock is established through literature review whereas the rock mass properties have been assessed using different equations. Rock mass quality has been assessed by using Rock Mass Rating (RMR) method of rock mass classification. RMR values are then correlated into Slope Mass Rating (SMR) incorporating different factors of slopes as per the site conditions. Further, kinematic analysis has been carried out using geometric and discontinuity properties to evaluate the structurally controlled failure. The influence of groundwater and earthquake has been considered while determining the shear strength of sliding surface. Finally, stability assessments have been carried out empirically, analytically and two dimensional (2D) numerical modelling.

Seasonal variability of eddy kinetic energy in the Banda Sea revealed by an ocean model: An energy budget perspective

Seasonal eddy kinetic energy (EKE) variability in the Banda Sea during 1993–2014 is studied from an energy budget perspective, based on the outputs of Ocean Forecasting Australian Model version 3. High EKE is confined within the upper 300 m of the western Banda Sea with the largest intensity exceeding 3 × 103 J/m2 in the northwest monsoon (NWM) season. In this strong EKE region during NWM, eddies derive almost two thirds of their kinetic energy from the direct wind power input (WP), with additional contributions from the barotropic (BT) and baroclinic instability (BC) of the background flow. Both WP and BT modulate the EKE seasonality and drive the peak energy during NWM, while BC strengthens in the southeast monsoon (SEM) season because of the intensified baroclinicity of the upper circulation. The westerly wind bursts associated with the Madden-Julian Oscillation (MJO), together with steep topography, facilitate more cyclonic eddy generation events in the western Banda Sea during NWM. During SEM, EKE becomes relatively moderate across the Banda Sea but with a regional peak to the southwest of the Buru Island, which can be attributed to island wake effect. Over the Banda Sea, WP, BT and BC contribute 74% (42%), 14% (19%) and 12% (39%) of total energy to EKE during NWM (SEM), respectively. The majority of EKE generated is diverged horizontally by pressure work and dissipated by turbulent viscosity, with dissipation depleting the most. This study highlights the importance of both monsoon and MJO wind forcing in generating EKE variability along the pathway of the Indonesian Throughflow.

Seismic anisotropy indicates organized melt beneath the Mid-Atlantic Ridge aids seafloor spreading

Abstract Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0–80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, δt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, φ, with SKS δt that are much longer (1.7–3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates.

Pemetaan Daerah Potensi Longsor Menggunakan Analisis Fault Fracture Density Pada Data DEMNAS Di Wilayah Cisarua, Kabupaten Bandung Barat, Jawa Barat

The Cisarua area is located in the northern part of Bandung City and is one of the sub-districts in West Bandung Regency and has steep topography in several locations, giving rise to the potential for landslides. The eastern region is part of the Lembang Fault which is still active in the North Bandung area. The potential for landslides related to the geological structure in this area can be identified by looking at the lineament patterns of the valleys, the presence of lithological offsets and others. The geological structure that developed in the Cisarua area was identified based on the results of analysis of satellite imagery data and the National Digital Elevation Model (DEMNAS) to identify the presence of geological structures and determine the direction of the dominant force. This study aims to determine the distribution of potential landslides in the Cisarua area using the Fault Fracture Density (FFD) method. Linearity analysis is estimated using the Fault Fracture Density (FFD) method. This method produces an analysis based on the straightness of the ridges and valleys around Cisarua which is then processed using a rosette diagram and interpreted as the dominant lineament trending west-east and northeast- southwest, this dominant lineament corresponds to the direction of the Lembang Fault. The lineament density level value describes a weak zone in the rock which creates the potential for landslide hazard. The weak zone in red indicates that the study area has a high landslide hazard potential, which is red, and the light green color indicates that the study area has a low landslide hazard potential. The potential for landslides in the study area is spread more in areas close to regional geological structures such as faults when compared to other areas. This pattern indicates that the presence of geological structures has an influence on the occurrence of landslides in potential areas.

Techniques to embed channels in finite element shallow water equation models

Two new and novel techniques to efficiently represent channels in large-scale finite element shallow water equation models are introduced. The first enables the discontinuous representation of bathymetric depth by allowing nodes along a discontinuity to have two land surface elevations, one representing the bottom of the discontinuity and the other the top of the discontinuity. Elemental integration proceeds using the nodal depth corresponding to whether the element is on the upper or lower side of the discontinuity and enables efficient treatment of steep sided features in a two-dimensional horizontal discretization. The second technique consolidates nodal equations at paired nodes across a channel. Doing this eliminates cross-channel variability and consequently eliminates the across-channel Courant-Friedrichs-Lewy stability constraint on the model time step. Together the two techniques allow channels of various sizes as well as other instances of steep topography to be embedded seamlessly, efficiently and in a fully coupled manner within an otherwise two-dimensional horizontal spatial discretization. This new capability is especially useful in the interface zone subject to both coastal and hydrological influences as it effectively captures bi-directional channelized flow, bi-directional flow between the channel and the floodplain, and coupled flow when the channel and floodplain are fully submerged. This paper describes the methodological development and presents a series of tests that verify the performance of this novel solution approach.

Hydrometeorological observations from the Upper Kentucky headwaters flash floods, 26–29 July 2022

Abstract Online OnlyIn late July 2022, heavy rains resulted in localised flash flooding across the Upper Kentucky River. To quantify the scale of this event, we analysed hydrometeorological activity using radar, precipitation and stream gauge data for three unregulated watersheds within the headwater forks of the Kentucky River Basin. The July 2022 event resulted from warm‐season elevated thunderstorm activity generating record flooding for the Whitesburg branch of the headwaters. Steeper topography and barren surface cover may have also influenced flooding on a local scale. A better understanding of local flood responses may mitigate future impacts of these events.

Eight-Day Typhoon Quantitative Precipitation Forecasts in Taiwan by the 2.5 km CReSS Model, Part II: Reduced Control of Track Errors on Rainfall Prediction Quality for Typhoons Associated with Southwesterly Flow

Due to the enhancement by its steep mesoscale topography, the overall rainfall amount and distribution in Taiwan from typhoons, to a first degree, are determined by the storm track relative to the island. Therefore, the quality of typhoon quantitative precipitation forecasts (QPFs) from numerical models is often controlled by track errors, with better quality from those with smaller track errors. However, the present work demonstrates that in daily QPFs over Taiwan made by a cloud-resolving model during five seasons of 2012–2016, targeted for 84 days during 27 typhoons and at ranges of day one (0–24 h) to day eight (168–192 h), the control of track errors on QPF quality is reduced for typhoons associated with southwesterly flow, compared to those without, and decent QPFs could still be obtained with large track errors in some cases. Subsequently, the circumstances and reasons for good (or bad) QPFs in selected examples are further investigated to deepen our understanding of typhoon QPFs in Taiwan. Some common ingredients are found in three cases where good QPFs were produced at a longer range (day 7 or 8) without a good track: these typhoons passed near northern Taiwan and the southwesterly flow prevailed over much of the island during the accumulation period. Responsible for much of the rainfall in Taiwan, the southwesterly flow was reasonably captured, resulting in good QPFs. In another example where the typhoon moved across southern Taiwan, on the contrary, the rainfall was produced by the storm’s circulation, and the QPF was degraded without a good enough track prediction.

Analysis of reservoir slope deformation during initial impoundment at the Baihetan Hydropower Station, China

Reservoir slope deformation induced by impoundment poses a threat to the safety of high-arch dams, which has received widespread attention in recent years. With an arch dam height of 300 m class, Baihetan is the largest hydropower station under construction. However, the theory behind its reservoir slope deformation under complex hydrogeological conditions is still an open question. In this study, a numerical model was developed to describe the complex seepage field at the dam site before and after impoundment. Furthermore, the numerical simulation of reservoir slope deformation was conducted based on the effective stress principle. The monitoring data collected during the initial impoundment of Baihetan validated the numerical model. Based on that, the spatiotemporal deformation law of the reservoir slope and its main controlling factors were analyzed. The results indicate that the contraction of the upstream valley is mainly caused by the reduction of the effective stress of the reservoir slope, the high and steep topography formed by the extensive excavation, and the weakening of structural planes. The effective stress reduction caused by the bypass seepage, and the stress adjustment may be the main reason for the deformation of the downstream reservoir slope. In addition, structural planes distributed in the dam foundation play a particular control role in the deformation of the foundation slope. This study systematically characterized the impoundment-induced reservoir slope deformation features of Baihetan and discussed the possible deformation mechanism, providing an essential reference for evaluating the structural safety of the dam during operation.

The geographic environment and the frequency of falling: a study of mortality outcomes in elderly people in China

Falling has become the first and second cause of death due to injury among urban and rural residents in China. This mortality is considerably higher in the southern part of the country than in the North. We collected the rate of mortality due to falling for 2013 and 2017 by province, age structure and population density, taking topography, precipitation and temperature into account. 2013 was used as the first year of the study since this year marks the expansion of the mortality surveillance system from 161 counties to 605 counties making available data more representative. A geographically weighted regression was used to evaluate the relationship between mortality and the geographic risk factors. High levels of precipitation, steep topography and uneven land surfaces as well as a higher quantile of the population aged above 80 years in southern China are believed to have led to the significantly higher number of falling compared with that in the North. Indeed, when evaluated by geographically weighted regression, the factors mentioned found a difference between the South and the North with regard to falling of 81% and 76% for the years 2013 and 2017, respectively. Interaction effects were observed between geographic risk factors and falling that, apart from the age factor, could be explained by topographic and climatic differences. The roads in the South are more difficult to negotiate on foot, particularly when it rains, which increases the probability of falling. In summary, the higher mortality due to falling in southern China emphasizes the need to apply more adaptive and effective measures in rainy and mountainous region to reduce this kind of risk.

26,000 years of environmental evolution of an incised valley in a rocky coast (La Janda wetland, SW Iberia).

The La Janda wetland record allows to expand the knowledge of the evolution of a restricted estuary since the Last Glacial Maximum. Multiproxy analysis of the sediments (facies, geochemistry, pollen, fossils) reveals that this system exhibits a classic incised valley sequence but differences with the adjacent estuaries can be established. The La Janda sedimentary infill can be split in four main sequences: (1) the Falling Stage Systems Tract (>20.6 ka BP) and (2) the Lowstand Systems Tract (16.7–20.6 ka BP) deposits record fluvial sedimentation in a narrow and incised valley, here seminally identified in the Gulf of Cádiz estuarine system; and (3) the Transgressive Systems Tract (7–16.7 ka BP), ending with the Holocene marine flooding surface at 7 ka BP, is composed of estuarine and marine basin deposits infilling a still steep and narrow topography coupled with the fast retreat of the river mouths. The (4) Highstand Systems Tract (0–7 ka BP) is composed of estuarine basin deposits with fluvial and tidal currents deposits in a wide a gentle sloped estuarine valley and it records the transformation of the basin into a terrestrial one. Comparison to other areas in the Gulf of Cádiz allows to correlate the different estuaries and to distinguish between open and restricted ones by the development of spit systems during the Highstand Systems Tract. But further work is needed to clarify the stratigraphy of the present eustatic cycle in the Gulf of Cádiz as the scarcity of data does not allow to determine the evolution of its estuaries during the last fall and rise of sea level. Such knowledge is crucial to disentangle the role of climate and tectonics in the future evolution of our coasts and design measures to adapt to sea level rise.

Investigating the Influence of a Pre-Existing Shear Band on the Seismic Response of Ideal Step-like Slopes Subjected to Weak Motions: Preliminary Results

The assessment of slope susceptibility to seismically-induced displacements receives wide attention in the geotechnical earthquake engineering field, but the alteration of the seismic wave inside the slope and at the ground surface due to the presence of a shear band confining a quiescent landslide body is rarely investigated. This paper describes the preliminary results of the numerical analysis of two step-like FE models, reproducing a gentle slope and steep cutting subjected to weak earthquakes, thus focusing on seismic wave amplification processes only. The results show that the higher the thickness of the weakened zone, the higher the maximum value of the amplification factors predicted at the ground surface. For gentle slopes affected by a landslide body confined by a thick shear band, the highest amplification factors are expected in the longer period range of 0.7–1.1 s, while the highest level of amplification is achieved in the intermediate period interval of 0.4–0.8 s in the case of steep slopes. In addition, the parasitic vertical component of acceleration can be considerably amplified beyond the crest and at the toe of the slope for increasing band thickness, especially in the case of steep topography, for which the effects of the shear band morphology enhance those related to the topographic profile. Finally, the fundamental frequency of the sloping deposit is not particularly affected by the presence of the shear band, while the amplitude of the amplification function at the fundamental frequency is clearly related to its thickness.

The slow downwearing of Madagascar: Inferring patterns and controls on long‐term basin‐averaged erosion rates from in situ10Be at the catchment and regional level

AbstractCosmogenic nuclide analysis of river sediment provides insight into catchment‐wide erosion rates and dynamics. Here, we investigate spatial patterns and controls on 10Be‐inferred erosion rates in Madagascar, a moderately seismically active microcontinent surrounded by passive margins with locally steep topography and a climate that varies from humid tropical to semiarid. We use a compiled dataset of 99 detrital 10Be measurements, 63 of which are new, covering more than 30% of the country and a wide range of topographic, bioclimatic and geologic characteristics. Overall, 10Be erosion rates are low (2.4–51.1 mm kyr−1), with clear differences between regions. The lowest rates are measured on the central highlands ( 8 mm kyr−1), in the Alaotra–Ankay graben ( 11 mm kyr−1) and in the large north‐central catchments ( 11 mm kyr−1). Higher rates are found on the steep eastern escarpment ( 20 mm kyr−1), in the northwest ( 31 mm kyr−1) and in the southwest ( 29 mm kyr−1). A stepwise linear regression model identified elevation as the main factor associated with variations in 10Be erosion rates (lower rates for higher catchments). Random within‐between statistical models (REWB), on the other hand, indicated that the differences between different regions can be explained by differences in river concavity, seismic events and gully (lavaka) densities, whereas additional variation within regions is only linked to seismicity. We find no correlation between catchment or river steepness and 10Be‐inferred erosion rates. Our results indicate that in Madagascar, long‐term erosion rates are overall low and that simple topography‐based models do not explain variations in rates of landscape change inferred from 10Be concentrations in river sediment. We demonstrate that identifying different regions aids in interpreting spatial patterns of erosion rates and that REWB models can be a powerful tool in deciphering environmental controls on 10Be erosion rates.

Surface area change detection of snow/ice cover over Zemu Glacier of Sikkim Himalaya

This study assessed the snow/ice surface area changes over the Zemu Glacier in the Eastern Himalayas. Zemu is considered to be the largest glacier in the Eastern Himalayas located in the Sikkim State of India. Change detection in the snow/ice surface areal extent of the Zemu Glacier was delineated from the year 1945 using US Army Map Service-Topographical Sheets and Landsat imageries available from 1987 to 2020. The results obtained solely focus on surface-change delineation using remote sensing satellite data and GIS software. Landsat imagery of the years 1987, 1997, 2009, 2018, and 2020 was used for snow and ice pixel extraction. Normalized Difference Snow Index (NDSI), Snow Cover Index (S3), and new band ratio index were used to extract the pure snow and ice pixels, fresh snow, and debris-covered snow/ice area and pixels mixed with shadow to demarcate the surface area changes. Manual delineation was required and undertaken for better results. A slope raster image was generated from Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) data to delineate the slope and hill shade. The results illustrate that the snow/ice surface covering the glacier had an area of 113.5km2 in the year 1945 which was reduced to 78.31km2 in the year 2020 indicating a total snow/ice surface area loss of 31% in 75years. A loss of 11.45% was observed from 1945 to 1987 in the areal extent, while from 1987 to 2009, a decadal loss of approximately 7% was observed. The surface area loss of 8.46% from 2009 to 2018 led to infer the maximum loss of snow and ice over the glacier body at a rate of 0.94% per year. Between the years 2018 and 2020, the glacier lost 1.08% of its surface area. The Accumulation Area Ratio (AAR) considering the accumulation and ablation areas of the glacier indicated a gradual decline of the accumulation zone in the recent years. Global Land Ice Measurement from Space (GLIMS) program data with RGI version 6.0 was used as a reference to demarcate the areal extent of Zemu Glacier. The study gained above 80% of overall accuracy by generating a confusion matrix in ArcMap. The analysis of this seasonal snow/ice cover showed that there has been a drastic decline in the surface snow/ice cover area over the Zemu Glacier over the years 1987 to 2020 and NDSI; S3 analysis provided improved results to delineate snow/ice cover area over such steep terrain topography of Sikkim Himalaya.

Hydrological characteristics of extreme floods in the Klaserie River, a headwater stream in southern Africa

Climate change models for southern Africa predict less frequent, but more intense, rainfall events, and an increased frequency of tropical cyclones. With their steep topography and small catchments, headwater streams generate large floods following intense rainfall events. Large flooding events in headwater streams are under studied in southern Africa. In this paper, we explore flooding in the upper Klaserie River, Limpopo River System, South Africa to determine the flow distribution and flood frequency for the catchment. In addition, we determine the return level for a large, economically damaging, flood generated following the landfall of a sub-tropical depression in January 2012 and, attempt to identify rainfall patterns that resulted in similar floods. An annual hydrological cycle with summer maxima and winter minima for both rainfall and flow was identified. The flood frequency analysis demonstrated that the January 2012 flood had an estimated return level of 225 years. This flood had a peak flowrate exceeding 1200 m3s-1 in a system with an average daily flowrate of 1 m3s-1. Regression tree analysis showed that a two-day rainfall in excess of 240 was a predictor for four of the five largest floods. A two-day rainfall in excess of 400 mm distinguished the January 2012 flood from other floods. Non-stationarity analyses for the flow and rainfall data and a SWAT hydrological model are recommend for the upper Klaserie River to evaluate climate and land cover changes, and their relationship to the magnitude of the 2012 flood. Our study demonstrates that South African river monitoring data can be used to detect and characterize major floods, despite deficiencies in these data. Continuation of these monitoring programs is vital for river health monitoring and the detection of trends in floods resulting from human activities and climate change.