188 research outputs found
Climatic and geologic controls on suspended sediment flux in the Sutlej River Valley, western Himalaya
The sediment flux through Himalayan rivers directly impacts water quality and is important for sustaining agriculture as well as maintaining drinking-water and hydropower generation. Despite the recent increase in demand for these resources, little is known about the triggers and sources of extreme sediment flux events, which lower water quality and account for extensive hydropower reservoir filling and turbine abrasion. Here, we present a comprehensive analysis of the spatiotemporal trends in suspended sediment flux based on daily data during the past decade (2001â2009) from four sites along the Sutlej River and from four of its main tributaries. In conjunction with satellite data depicting rainfall and snow cover, air temperature and earthquake records, and field observations, we infer climatic and geologic controls of peak suspended sediment concentration (SSC) events. Our study identifies three key findings: First, peak SSC events (â„ 99th SSC percentile) coincide frequently (57â80%) with heavy rainstorms and account for about 30% of the suspended sediment flux in the semi-arid to arid interior of the orogen. Second, we observe an increase of suspended sediment flux from the Tibetan Plateau to the Himalayan Front at mean annual timescales. This sediment-flux gradient suggests that averaged, modern erosion in the western Himalaya is most pronounced at frontal regions, which are characterized by high monsoonal rainfall and thick soil cover. Third, in seven of eight catchments, we find an anticlockwise hysteresis loop of annual sediment flux variations with respect to river discharge, which appears to be related to enhanced glacial sediment evacuation during late summer. Our analysis emphasizes the importance of unconsolidated sediments in the high-elevation sector that can easily be mobilized by hydrometeorological events and higher glacial-meltwater contributions. In future climate change scenarios, including continuous glacial retreat and more frequent monsoonal rainstorms across the Himalaya, we expect an increase in peak SSC events, which will decrease the water quality and impact hydropower generation
Measuring decadal vertical land-level changes from SRTM-C (2000) and TanDEM-X (âŒâ2015) in the south-central Andes
In the arctic and high mountains it is common to measure vertical changes of
ice sheets and glaciers via digital elevation model (DEM) differencing. This
requires the signal of change to outweigh the noise associated with the
datasets. Excluding large landslides, on the ice-free earth the land-level change
is smaller in vertical magnitude and thus requires more accurate DEMs for
differencing and identification of change. Previously, this has required
meter to submeter data at small spatial scales. Following careful
corrections, we are able to measure land-level changes in gravel-bed channels
and steep hillslopes in the south-central Andes using the SRTM-C (collected
in 2000) and the TanDEM-X (collected from 2010 to 2015) near-global 12â30 m
DEMs. Long-standing errors in the SRTM-C are corrected using the TanDEM-X as
a control surface and applying cosine-fit co-registration to remove
âŒâ1â10 pixel (âŒâ3 m) shifts, fast Fourier transform (FFT) and filtering to
remove SRTM-C short- and long-wavelength stripes, and blocked shifting to
remove remaining complex biases. The datasets are then differenced and
outlier pixels are identified as a potential signal for the case of gravel-bed
channels and hillslopes. We are able to identify signals of incision and
aggradation (with magnitudes down to âŒâ3 m in the best case) in two
â>â100 km river reaches, with increased geomorphic activity downstream of
knickpoints. Anthropogenic gravel excavation and piling is prominently
measured, with magnitudes exceeding ±5 m (up to â>â10 m for large
piles). These values correspond to conservative average rates of 0.2Â to
>â0.5 m yrâ1 for vertical changes in gravel-bed rivers. For
hillslopes, since we require stricter cutoffs for noise, we are only able to
identify one major landslide in the study area with a deposit volume of
16 ± 0.15 × 106 m3. Additional signals of change can be
garnered from TanDEM-X auxiliary layers; however, these are more difficult to
quantify. The methods presented can be extended to any region of the world
with SRTM-C and TanDEM-X coverage where vertical land-level changes are of
interest, with the caveat that remaining vertical uncertainties in primarily
the SRTM-C limit detection in steep and complex topography.</p
Atmospheric dynamics of extreme discharge events from 1979 to 2016 in the southern Central Andes
During the South-American Monsoon season, deep convective systems occur at the eastern flank of the Central Andes leading to heavy rainfall and flooding. We investigate the large- and meso-scale atmospheric dynamics associated with extreme discharge events (> 99.9th percentile) observed in two major river catchments meridionally stretching from humid to semi-arid conditions in the southern Central Andes. Based on daily gauge time series and ERA-Interim reanalysis, we made the following three key observations: (1) for the period 1940â2016 daily discharge exhibits more pronounced variability in the southern, semi-arid than in the northern, humid catchments. This is due to a smaller ratio of discharge magnitudes between intermediate (0.2 year return period) and rare events (20 year return period) in the semi-arid compared to the humid areas; (2) The climatological composites of the 40 largest discharge events showed characteristic atmospheric features of cold surges based on 5-day time-lagged sequences of geopotential height at different levels in the troposphere; (3) A subjective classification revealed that 80% of the 40 largest discharge events are mainly associated with the north-northeastward migration of frontal systems and 2/3 of these are cold fronts, i.e. cold surges. This work highlights the importance of cold surges and their related atmospheric processes for the generation of heavy rainfall events and floods in the southern Central Andes.Fil: Castino, F.. Universitat Potsdam; AlemaniaFil: Bookhagen, B.. Universitat Potsdam; AlemaniaFil: de la Torre, Alejandro. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad Austral. Facultad de IngenierĂa. Departamento de Ciencias BĂĄsicas; Argentin
Topology and seasonal evolution of the network of extreme precipitation over the Indian subcontinent and Sri Lanka
Peer reviewedPublisher PD
India Summer Monsoon and Spatial Erosion Variability in the Arun Valley, Eastern Nepal
Abstract HKT-ISTP 2013
B
Fluvial Sediment Aggradation and Incision in NW Sub-Himalaya
Abstract HKT-ISTP 2013
A
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Topology and seasonal evolution of the network of extreme precipitation over the Indian subcontinent and Sri Lanka
This paper employs a complex network approach to determine the topology and evolution of the network of extreme precipitation that governs the organization of extreme rainfall before, during, and after the Indian Summer Monsoon (ISM) season. We construct networks of extreme rainfall events during the ISM (June-September), post-monsoon (October-December), and pre-monsoon (March-May) periods from satellite-derived (Tropical Rainfall Measurement Mission, TRMM) and rain-gauge interpolated (Asian Precipitation Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources, APHRODITE) data sets. The structure of the networks is determined by the level of synchronization of extreme rainfall events between different grid cells throughout the Indian subcontinent. Through the analysis of various complex-network metrics, we describe typical repetitive patterns in North Pakistan (NP), the Eastern Ghats (EG), and the Tibetan Plateau (TP). These patterns appear during the pre-monsoon season, evolve during the ISM, and disappear during the post-monsoon season. These are important meteorological features that need further attention and that may be useful in ISM timing and strength prediction
Determining the optimal grid resolution for topographic analysis on an airborne lidar dataset
Digital elevation models (DEMs) are a gridded representation of
the surface of the Earth and typically contain uncertainties due to data
collection and processing. Slope and aspect estimates on a DEM contain
errors and uncertainties inherited from the representation of a
continuous surface as a grid (referred to as truncation error; TE) and from
any DEM uncertainty. We analyze in detail the impacts of TE and propagated
elevation uncertainty (PEU) on slope and aspect.
Using synthetic data as a control, we define functions to quantify both TE
and PEU for arbitrary grids. We then develop a quality metric which captures
the combined impact of both TE and PEU on the calculation of topographic
metrics. Our quality metric allows us to examine the spatial patterns of
error and uncertainty in topographic metrics and to compare calculations on
DEMs of different sizes and accuracies.
Using lidar data with point density of âŒ10 pts mâ2 covering
Santa Cruz Island in southern California, we are able to generate DEMs and
uncertainty estimates at several grid resolutions. Slope (aspect) errors on
the 1 m dataset are on average 0.3â (0.9â) from TE and
5.5â (14.5â) from PEU. We calculate an optimal DEM resolution
for our SCI lidar dataset of 4 m that minimizes the error bounds on
topographic metric calculations due to the combined influence of TE and PEU
for both slope and aspect calculations over the entire SCI. Average slope
(aspect) errors from the 4 m DEM are 0.25â (0.75â)
from TE and 5â (12.5â) from PEU. While the smallest grid
resolution possible from the high-density SCI lidar is not necessarily
optimal for calculating topographic metrics, high point-density data are
essential for measuring DEM uncertainty across a range of resolutions.</p
Active deformation in the Pamir â Tian Shan collision zone, NW China
Abstract HKT-ISTP 2013
A
Impact of transient groundwater storage on the discharge of Himalayan rivers
International audienceIn the course of the transfer of precipitation into rivers, water is temporarily stored in reservoirs with different residence times such as soils, groundwater, snow and glaciers. In the central Himalaya, the water budget is thought to be primarily controlled by monsoon rainfall, snow and glacier melt, and secondarily by evapotranspiration. An additional contribution from deep groundwater has been deduced from the chemistry of Himalayan rivers, but its importance in the annual water budget remains to be evaluated. Here we analyse records of daily precipitation and discharge within twelve catchments in Nepal over about 30 years. We observe annual hysteresis loops--that is, a time lag between precipitation and discharge--in both glaciated and unglaciated catchments and independent of the geological setting. We infer that water is stored temporarily in a reservoir with characteristic response time of about 45 days, suggesting a diffusivity typical of fractured basement aquifers. We estimate this transient storage capacity at about 28km3 for the three main Nepal catchments; snow and glacier melt contribute around 14km3yr-1, about 10% of the annual river discharge. We conclude that groundwater storage in a fractured basement influences significantly the Himalayan river discharge cycle
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